# Chemical Physics

## Current research reports and chronological list of recent articles..

The international scientific journal Chemical Physics welcomes experimental and theoretical papers in the molecular, biological and materials sciences.

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## Chemical Physics - Abstracts

The role of near resonance electronic energy transfer on the collisional quenching of NO (A2Σ+) by C6H6 and C6F6 at low temperature

Publication date: 14 February 2018
Source:Chemical Physics, Volume 501

Author(s): Joshua D. Winner, Niclas A. West, Madison H. McIlvoy, Zachary D. Buen, Rodney D.W. Bowersox, Simon W. North

We present measurements of the collisional fluorescence quenching cross section of NO (A2Σ+) by benzene and hexafluorobenzene at a series of temperatures using a custom built slow flow cell and a pulsed de Laval nozzle test cell. The measurements show a deviation from the common harpoon mechanism of collisional fluorescence quenching, specifically the large quenching cross section of 147 Å2 at 300 K measured for benzene which is not predicted by the harpoon mechanism due to the low electron affinity of benzene (−1.5 eV). We speculate that the quenching of NO (A2Σ+) by benzene and hexafluorobenzene occurs through near resonance electronic energy transfer from the NO A-state to the A-state of the quencher via a pseudo-emission process. The predicted T−1/3 dependence of the cross section for this model is consistent with the experimental observations.

Datum: 22.01.2018

Molecular dynamics simulation of the thermosensitivity of the human connexin 26 hemichannel

Publication date: 26 January 2018
Source:Chemical Physics, Volume 500

Connexin hemichannels mediate cytoplasm and extracellular milieu communication by exchanging a variety of cytoplasmic molecules and ions. These hemichannels can be regulated by external stimuli such as mechanical stress, applied voltage, pH and temperature changes. Although there are many studies on structures and functions of connexin 26 in contexts of pH, ion concentration and voltage, employing computational methods, no such study has been performed so far involving temperature changes. In this study, using molecular dynamics simulation, we investigate thermosensitivity of the human Connexin 26 hemichannel. Our results show that the channel approaches a structurally closed state at lower temperature compared to higher temperature. This is in fair agreement with experimental results that indicate channel closure at lower temperature. Furthermore, our MD simulation results show that some regions of connexin 26 hemichannel are more sensitive to temperature compared to other regions. Whereas the intercellular half of the channel does not show any considerable response to temperature during the simulation time accessible in this study, the cytoplasmic half approaches a closed structural state at lower temperature compared to the higher temperature. Specifically, our results suggest that the cytoplasmic loop, the cytoplasmic half of the second transmembrane helix, and the N-terminus helix play a dominant role in temperature gating.

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Datum: 22.01.2018

Infrared insights into the effect of cholesterol on lipid membranes

Publication date: Available online 21 December 2017
Source:Chemical Physics

Author(s): P. Stevenson, A. Tokmakoff

Utilizing a combination of time-resolved Infrared (IR) spectroscopies, we are able to probe the effects of a small biological molecule (cholesterol) on the dynamics of lipid membranes from picoseconds to milliseconds. By monitoring the ultrafast dynamics of the system with multi-dimensional IR spectroscopy, we are able to resolve the influence cholesterol has on the electric field fluctuations inside the membrane. We use temperature-jump (T-jump) spectroscopy to extend our experimentally-observable time window beyond the vibrational lifetime of our probe and find that the presence of cholesterol introduces significant inhomogeneity into the relaxation rates of the system. This approach provides new insight into the interaction between cholesterol and lipids, while also demonstrating the utility of this combined IR spectroscopic toolbox for studying multiscale dynamics.

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Datum: 22.01.2018

Stretched-to-compressed-exponential crossover observed in the electrical degradation kinetics of some spinel-metallic screen-printed structures

Publication date: 14 February 2018
Source:Chemical Physics, Volume 501

Author(s): V. Balitska, O. Shpotyuk, M. Brunner, I. Hadzaman

Thermally-induced (170 °C) degradation-relaxation kinetics is examined in screen-printed structures composed of spinel Cu0.1Ni0.1Co1.6Mn1.2O4 ceramics with conductive Ag or Ag-Pd layered electrodes. Structural inhomogeneities due to Ag and Ag-Pd diffusants in spinel phase environment play a decisive role in non-exponential kinetics of negative relative resistance drift. If Ag migration in spinel is inhibited by Pd addition due to Ag-Pd alloy, the kinetics attains stretched exponential behavior with ∼0.58 exponent, typical for one-stage diffusion in structurally-dispersive media. Under deep Ag penetration into spinel ceramics, as for thick films with Ag-layered electrodes, the degradation kinetics drastically changes, attaining features of two-step diffusing process governed by compressed-exponential dependence with power index of ∼1.68. Crossover from stretched- to compressed-exponential kinetics in spinel-metallic structures is mapped on free energy landscape of non-barrier multi-well system under strong perturbation from equilibrium, showing transition with a character downhill scenario resulting in faster than exponential decaying.

Datum: 22.01.2018

Magnetic modulation of the unbraiding dynamics of pairs of DNA molecules to model the system as an intermittent oscillator

Publication date: 6 March 2018
Source:Chemical Physics, Volume 502

Author(s): Carlos J. Martínez-Santiago, Edwin Quiñones

We present a new method to measure mechanical properties of braided DNA molecules constrained between a superparamagnetic dumbbell and a glass surface. The molecules were braided using magnetic tweezers and subsequently allowed to unbraid in presence of a weak static magnetic field. For braids with high catenation numbers, the initial stages of the unbraiding process were not affected by the field. However, after releasing some energy, the static field introduced a sinusoidal perturbation on the system and the unbraiding plots showed oscillatory behavior. We demonstrated that in this regime, the system behaves as a phase oscillator. Adopting this model, we estimated the magnitudes of the torque generated by the DNA molecules and that exerted by the field. The modulated signal is sensitive to changes in the torque exerted by DNA. Torque calculations involved the average of several cycles of the modulated signal.

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Datum: 22.01.2018

Recognition of anions using urea and thiourea substituted calixarenes: A density functional theory study of non-covalent interactions

Publication date: 14 February 2018
Source:Chemical Physics, Volume 501

Author(s): Mohd Athar, Mohsin Y. Lone, Prakash C. Jha

Designing of new calixarene receptors for the selective binding of anions is an age-old concept; even though expected outcomes from this field are at premature stage. Herein, we have performed quantum chemical calculations to provide structural basis of anion binding with urea and thiourea substituted calixarenes (1, 2, and 3). In particular, spherical halides (F, Cl, Br) and linear anions (CN, N3 , SCN) were modelled for calculating binding energies with receptor 1, 2 and 3 followed by their marked IR vibrations; taking the available experimental information into account. We found that the thiourea substitutions have better capability to stabilize the anions. Results have suggested that the structural behaviour of macrocyclic motifs were responsible for displaying the anion binding potentials. Moreover, second order “charge transfer” interactions of n‐σ NH and n‐σ OH type along the H‐bond axisplayed critical role in developing hydrogen bonds. The present work also examines the role of non-covalent interactions (NCI) and their effects on thermodynamic and chemical-reactivity descriptors.

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Datum: 22.01.2018

NO3 full-dimensional potential energy surfaces and ground state vibrational levels revisited

Publication date: Available online 12 January 2018
Source:Chemical Physics

Author(s): Alexandra Viel, Wolfgang Eisfeld

A new full-dimensional (6D) diabatic potential energy surface (PES) model is presented representing the five lowest PESs corresponding to the $X ̃$ $2 A 2 ′ , A ̃$ $2 E ′ ′$, and $B ̃$ $2 E ′$ electronic states of the nitrate radical (NO3). It is based on high-level ab initio calculations of roughly 90 000 energy data over a wide range of nuclear configurations and represents the energies with a root mean-squares (rms) error of about 100 $cm - 1$. An accurate dipole surface was developed for the $X ̃$ stateas well. The new PES model is used to re-investigate the infra-red (IR) spectrum corresponding to the electronic ground state by full quantum dynamics simulations. Vibrational eigenstates, IR transition probabilities, and isotopic shifts are computed and analyzed. Levels up to 2000 $cm - 1$ are obtained and show good to excellent agreement with known experimental values. Some larger deviations are observed and discussed as well. The new results are in agreement with previous theoretical studies that the disputed $ν 3$ fundamental corresponds to a frequency of roughly 1022 $cm - 1$ and that the prominent experimental feature observed at 1492 $cm - 1$ is due to the $3 1 4 1$ ($e ′$) combination mode. Observed discrepancies in the IR intensities may be explained by coupling to the $B ̃$ statewhich is also analysed by diabatic decomposition of the eigenstates.

Datum: 22.01.2018

Analysis of paramagnetic 3d ions (Cr3+ and Fe3+) centers in fluoroelpasolite Cs2NaGaF6 crystal by both DFT and SPM calculations

Publication date: 14 February 2018
Source:Chemical Physics, Volume 501

Author(s): D. Erbahar, Y. Emül, M. Açıkgöz

The structural properties of 3d ions (Cr3+ and Fe3+) centers in Cs2NaGaF6 have been investigated by performing density functional theory (DFT) and semi-empirical superposition model (SPM) calculations. The local geometry (i.e. equilibrium ligand distance and angles between the host cation ion (Cs+, Na+, or Ga3+) and F ligands) for pure, Cr3+ doped and Fe3+ doped structures have been determined after a fully relaxed geometry optimization at each lattice site. The previous experimental zero-field splitting (ZFS) parameters (ZFSPs) have been analyzed by SPM calculations using the obtained structural properties from DFT. The combination of two methods provide us to confirm the main result of the previous electron magnetic resonance and optical investigations stating that transition metal (TM) ions substitute for octahedral sites instead of being distributed randomly in the lattice of Cs2NaGaF6.

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Datum: 22.01.2018

CO oxidation on AlAu nano-composite systems

Publication date: 6 March 2018
Source:Chemical Physics, Volume 502

Author(s): C. Rajesh, C. Majumder

Using first principles method we report the CO oxidation behaviour of AlAu nano-composites in three different size ranges: Al6Au8, Al13Au42 and a periodic slab of AlAu(1 1 1) surface. The clusters prefer enclosed structures with alternating arrangement of Al and Au atoms, maximising Auδ− Alδ+ bonds. Charge distribution analysis suggests the charge transfer from Al to Au atoms, corroborated by the red shift in the density of states spectrum. Further, CO oxidation on these nano-composite systems was investigated through both Eley – Rideal and Langmuir Hinshelwood mechanism. While, these clusters interact with O2 non-dissociatively with an elongation of the OO bond, further interaction with CO led to formation of CO2 spontaneously. On contrary, the CO2 evolution by co-adsorption of O2 and CO molecules has a transition state barrier. On the basis of the results it is inferred that nano-composite material of AlAu shows significant promise toward effective oxidative catalysis.

Datum: 22.01.2018

Editorial Board

Publication date: 26 January 2018
Source:Chemical Physics, Volume 500

Datum: 22.01.2018

Binding affinity of the L-742,001 inhibitor to the endonuclease domain of A/H1N1/PA influenza virus variants: Molecular simulation approaches

Publication date: 26 January 2018
Source:Chemical Physics, Volume 500

Author(s): Hung Nguyen, Hoang Linh Nguyen, Huynh Quang Linh, Minh Tho Nguyen

The steered molecular dynamics (SMD), molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) and free energy perturbation (FEP) methods were used to determine the binding affinity of the L-742,001 inhibitor to the endonuclease domain of the A/H1N1/PA influenza viruses (including wild type (WT) and three mutations I79L, E119D and F105S for both pH1N1 PA and PR8 PA viruses). Calculated results showed that the L-742,001 inhibitor not only binds to the PR8 PAs (1934 A influenza virus) better than to the pH1N1 PAs (2009 A influenza virus) but also more strongly interacts with the WT endonuclease domain than with three mutant variants for both pH1N1 PA and PR8 PA viruses. The binding affinities obtained by the SMD, MM-PBSA and FEP methods attain high correlation with available experimental data. Here the FEP method appears to provide a more accurate determination of the binding affinity than the SMD and MM-PBSA counterparts.

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Datum: 22.01.2018

Diffusion of Brownian particles in a tilted periodic potential under the influence of an external Ornstein–Uhlenbeck noise

Publication date: 26 January 2018
Source:Chemical Physics, Volume 500

Author(s): Zhan-Wu Bai, Wei Zhang

The diffusion behaviors of Brownian particles in a tilted periodic potential under the influence of an internal white noise and an external Ornstein–Uhlenbeck noise are investigated through numerical simulation. In contrast to the case when the bias force is smaller or absent, the diffusion coefficient exhibits a nonmonotonic dependence on the correlation time of the external noise when bias force is large. A mechanism different from locked-to-running transition theory is presented for the diffusion enhancement by a bias force in intermediate to large damping. In the underdamped regime and the presence of external noise, the diffusion coefficient is a monotonically decreasing function of low temperature rather than a nonmonotonic function when external noise is absent. The diffusive process undergoes four regimes when bias force approaches but is less than its critical value and noises intensities are small. These behaviors can be attributed to the locked-to-running transition of particles.

Datum: 22.01.2018

Exploration for the stabilities of CHN7 and CN7−: A theoretical study on the formation and dissociation mechanisms

Publication date: 6 March 2018
Source:Chemical Physics, Volume 502

Author(s): Tao Yu, Ying-Zhe Liu, Wei-Peng Lai

CHN7 and CN7 are meta-stable species. In order to study on the relationship between thermodynamic and kinetic stabilities, the potential energy surfaces of CHN7 and CN7 were scanned at the B3LYP/aug-cc-pVDZ level. After the analysis of potential energy surfaces, the optimum pathways were got to conclude the dissociation and formation mechanisms. The dissociation barriers of 5-azido-1H-tetrazole and 5-azido-2H-tetrazole are about 150 kJ mol−1. They are sufficient to keep the two azidotetrazoles stable. The reaction between cyanogen azide and azide anion cannot produce azidotetrazolate anion, but produce the linear CN7 with a lower barrier. The reaction between cyanogen azide and hydrazoic acid preferentially produce 5-azido-1H-tetrazole. The decyclization barriers of 1H-tetrazolo[1,5-d]tetrazole and tetrazolo[1,5-d]tetrazolate anion are 44.7 and 81.6 kJ mol−1, respectively. The deprotoned anion is more available than the neutral compound. Heptaazacubane and heptaazacubanide anion with cubic geometries are highly unstable.

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Datum: 22.01.2018

Adsorption studies of trimethyl amine and n-butyl amine vapors on stanene nanotube molecular device – A first-principles study

Publication date: 14 February 2018
Source:Chemical Physics, Volume 501

Author(s): R. Bhuvaneswari, V. Nagarajan, R. Chandiramouli

The stanene nanotube is designed and used for the detection of trimethyl amine (TMA) and n-butyl amine (n-BA) vapors, which is investigated using first-principles study. The electronic properties of bare stanene nanotube and the adsorption properties of TMA and n-BA molecules are studied using density functional theory with non-equilibrium Green’s function. Moreover, the device density of states shows the shift in the peak maxima upon adsorption of TMA and n-BA molecules on to the stanene nanotube. The variation in the flow of electron is noticed upon adsorption of TMA and n-BA molecules in the transmission spectrum of stanene nanotube. I-V characteristics clearly confirm the variation in the current upon adsorption of TMA and n-BA molecules. The findings of the study clearly suggest that the stanene nanotube molecular device can be used for the detection of trace levels of TMA and n-BA molecules present in the atmosphere.

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Datum: 22.01.2018

Graphical abstract TOC continued

Publication date: 26 January 2018
Source:Chemical Physics, Volume 500

Datum: 22.01.2018

Stark shift of impurity doped quantum dots: Role of noise

Publication date: 14 February 2018
Source:Chemical Physics, Volume 501

Author(s): Sk. Md. Arif, Aindrila Bera, Anuja Ghosh, Manas Ghosh

Present study makes a punctilious investigation of the profiles of Stark shift (SS) of doped GaAs quantum dot (QD) under the supervision of Gaussian white noise. A few physical parameters have been varied and the consequent variations in the SS profiles have been monitored. The said physical parameters comprise of magnetic field, confinement potential, dopant location, dopant potential, noise strength, aluminium concentration (only for $Al x Ga 1 - x As$ alloy QD), position-dependent effective mass (PDEM), position-dependent dielectric screening function (PDDSF), anisotropy, hydrostatic pressure (HP) and temperature. The SS profiles unfurl interesting features that heavily depend upon the particular physical quantity concerned, presence/absence of noise and the manner (additive/multiplicative) noise enters the system. The study highlights feasible means of maximizing SS of doped QD in presence of noise by suitable adjustment of several control parameters. The study deems importance in view of technological applications of QD devices where noise plays some prominent role.

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Datum: 22.01.2018

Molecular mechanism of melting of a helical polymer crystal: Role of conformational order, packing and mobility of polymers

Publication date: 6 March 2018
Source:Chemical Physics, Volume 502

Author(s): Ramesh Cheerla, Marimuthu Krishnan

The molecular mechanism of melting of a superheated helical polymer crystal has been investigated using isothermal-isobaric molecular dynamics simulation that allows anisotropic deformation of the crystal lattice. A detailed microscopic analysis of the onset and progression of melting and accompanying changes in the polymer conformational order, translational, and orientation order of the solid along the melting pathway is presented. Upon gradual heating from room temperature to beyond the melting point at ambient pressure, the crystal exhibits signatures of premelting well below the solid-to-liquid melting transition at the melting point. The melting transition is manifested by abrupt changes in the crystal volume, lattice energy, polymer conformation, and dynamical properties. In the premelting stage, the crystal lattice structure and backbone orientation of the polymer chains are retained but with the onset of weakening of long-range helical order and interchain packing of polymers perpendicular to the fibre axis of the crystal. The premelting also marks the onset of conformational defects and anisotropic solid-state diffusion of polymers along the fibre axis. The present study underscores the importance of the interplay between intermolecular packing, interactions, and conformational dynamics at the atomic level in determining the macroscopic melting behavior of polymer crystals.

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Datum: 22.01.2018

Reaction mechanism and kinetics of the degradation of terbacil initiated by OH radical – A theoretical study

Publication date: 14 February 2018
Source:Chemical Physics, Volume 501

Author(s): S. Ponnusamy, L. Sandhiya, K. Senthilkumar

The reaction of terbacil with OH radical is studied by using electronic structure calculations. The reaction of terbacil with OH radical is found to proceed by H-atom abstraction, Cl-atom abstraction and OH addition reactions. The initially formed alkyl radical will undergo atmospheric transformation in the presence of molecular oxygen leading to the formation of peroxy radical. The reaction of peroxy radical with other atmospheric oxidants, such as HO2 and NO radicals is studied. The rate constant is calculated for the H-atom abstraction reactions over the temperature range of 200–1000 K. The results obtained from electronic structure calculations and kinetic study show that the H-atom abstraction reaction is more favorable. The calculated lifetime of terbacil is 24 h in normal atmospheric OH concentration. The rate constant calculated for H-atom abstraction reactions is 6 × 10−12, 4.4 × 10−12 and 3.2 × 10−12 cm3molecule−1s−1, respectively which is in agreement with the previous literature value of 1.9 × 10−12 cm3molecule−1s−1.

Datum: 22.01.2018

Collective effect of light-induced and natural nonadiabatic phenomena in the dissociation dynamics of the NaI molecule

Publication date: Available online 2 January 2018
Source:Chemical Physics

Author(s): András Csehi, Gábor J. Halász, Ágnes Vibók

Natural and light-induced nonadiabatic effects are ubiquitous in many photochemical and photophysical processes. Here we study the interplay between them when they are present simultaneously in a molecular system. Our showcase example is the $NaI$ molecule. Solving the time-dependent nuclear Schrödinger equation the photodissociation rate and the angular distribution of the molecular photofragments are calculated and discussed at several resonant laser energies and intensities. Obtained results clearly demonstrate that the dissociation rate and the angular distribution of the $NaI$ photofragments can be considered as clear fingerprint of a collective effect of light-induced and natural nonadiabatic phenomena.

Datum: 22.01.2018

A molecular electron density theory study of the chemo- and regioselective [3 + 2] cycloaddition reactions between trifluoroacetonitrile N-oxide and thioketones

Publication date: 14 February 2018
Source:Chemical Physics, Volume 501

Author(s): Saeedreza Emamian, Tian Lu, Luis. R. Domingo, Leily Heidarpoor Saremi, Mar Ríos-Gutiérrez

The [3 + 2] cycloaddition (32CA) reaction between trifluoroacetonitrile N-oxide (NO 7) and 2,2,4,4-tetramethyl-3-thioxocyclobutan-1-one (THK 12) as well as the self-dimerization of NO 7 as a competitive pathway were studied within the Molecular Electron Density Theory (MEDT) using several DFT functionals together with def2-TZVP basis set. Taking CCSD(T)/TZVP activation energies as reference, among the employed functionals, just the B2PLYP-D3(BJ) one is able to portray complete predominance of 32CA reaction over self-dimerization process in excellent agreement with the experimental outcomes. Analysis of the global reactivity indices permits to characterize NO 7 and THK 12 as a strong electrophile and a strong nucleophile, respectively, while analysis of the Parr functions allows explaining the chemo- and regioselectivity observed experimentally. The electron localization function (ELF) analysis of several points along the IRC profile associated with the energetically most favorable reaction channel permits to establish a non-concerted two-stage one-step molecular mechanism for this polar pmr-type 32CA reaction.

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Datum: 22.01.2018

On the solvation of hydronium by carbon dioxide: Structural and infrared spectroscopic study of (H3O+)(CO2)n

Publication date: 14 February 2018
Source:Chemical Physics, Volume 501

Author(s): Jianpeng Yang, Xiangtao Kong, Ling Jiang

Hydronium (H3O+) is the smallest member of protonated water. In this work, we use quantum chemical calculations to explore the solvation of H3O+ by adding one CO2 molecule at a time. The effect of stepwise solvation on infrared spectroscopy, structure, and energetics has been systematically studied. It has been found that the first solvation shell of H3O+ is completed at n = 6. Besides the hydrogen-bond interaction, the $C CO 2 — O CO 2$ intermolecular interaction is also responsible for the stabilization of the larger clusters. The transfer of the proton from H3O+ onto CO2 with the formation of the OCOH+ moiety is not observed in the early stage of solvation process. Calculated IR spectra suggest that vibrational frequencies of H-bonded OH stretching would afford a sensitive probe for exploring the early stage solvation of hydronium by carbon dioxide. IR spectra for the (H3O+)(CO2) n (n = 1–7) clusters could be measured by the infrared photodissociation spectroscopic technique and thus provide a vivid physical picture about how carbon dioxide solvates the hydronium.

Datum: 22.01.2018

Publication date: 26 January 2018
Source:Chemical Physics, Volume 500

Author(s): Martina Zámečníková, Pavel Soldán

Radiative lifetimes of the ro-vibrational bound states of $HeLi +$($b 3 ∑ +$) are calculated quantum-mechanically when both bound-bound and bound-free processes are taken into account. The calculations are restricted to the initial states with low rotational quantum numbers. For the rotationless molecular ion, the shortest lifetime is $1.30 × 10 - 6 s$ of the ground vibrational state. With increasing vibration excitations the radiative lifetimes slightly increase, but keep below $10 - 5 s$ for the next 17 vibrational states, then they start to increase more rapidly up to $2.41 × 10 - 2 s$ for the highest vibrational state. The radiative lifetimes also tend to slightly prolong with increasing rotational excitations but stay on the same magnitude for the low rotational quantum numbers.

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Datum: 22.01.2018

Correlation between the size and the magnetic properties of Ag2+ clusters loaded on ceria surface and their catalytic performance in the total oxidation of propylene. EPR study

Publication date: 6 March 2018
Source:Chemical Physics, Volume 502

Three different types of Ag2+ ions (“a”, “b”, and “c”) have been identified and examined by electron paramagnetic resonance (EPR) on 10% wt Ag/CeO2 prepared by impregnation method. One of them, Ag2+ (b), behaves differently than the two others, Ag2+ (a) and Ag2+ (c), under redox atmospheres. The fact that, in reducing conditions (vacuum, propylene, hydrogen, and carbon black), Ag2+ (a) and Ag2+ (c) species were more easily reduced than Ag2+ (b) ones, could not explain the catalytic performance and stability of this latter species compared to the first ones in the reaction of total oxidation of propylene. The EPR technique evidenced that Ag2+ (b) species form, upon propene oxidation, a cluster. This cluster is composed of two parallel electron spins (dimer) and three nuclear spins (trimer). It seems that before propylene oxidation, Ag2+ (b) clusters were ferromagnetic. This ferromagnetic character of Ag2+ (b) species may explain their better catalytic performance, in propylene oxidation, than those of Ag2+ (a) and Ag2+ (c) ones.

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Datum: 22.01.2018

Excited state properties of 2′-hydroxychalcone analogues functionalized with a diene moiety studied by steady state and laser flash photolysis

Publication date: 26 January 2018
Source:Chemical Physics, Volume 500

Author(s): Yukino Shinozaki, Minoru Yamaji, Tatsuo Arai

2′-Hydroxychalcone (HC) analogues 1 and 2 having a diene part tethering the phenyl and naphthyl chromophores, respectively, were prepared, and their photochemical and photophysical properties were studied. Fluorescence from these compounds was absent in solution and the solid state. Based on the results obtained upon steady state and laser flash photolyses, compound 2 was found to be substantially stable on photoirradiation without undergoing intersystem crossing to the triplet state whereas compounds 1 showed transient absorption due to the triplet tautomer. The deactivation processes in the excited states were discussed by considering energetic reaction diagrams for the corresponding tautomers and isomers.

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Datum: 22.01.2018

Influence of acceptor on charge mobility in stacked π-conjugated polymers

Publication date: 14 February 2018
Source:Chemical Physics, Volume 501

Author(s): Shih-Jye Sun, Miroslav Menšík, Petr Toman, Alessio Gagliardi, Karel Král

We present a quantum molecular model to calculate mobility of π-stacked P3HT polymer layers with electron acceptor dopants coupled next to side groups in random position with respect to the linear chain. The hole density, the acceptor LUMO energy and the hybridization transfer integral between the acceptor and polymer were found to be very critical factors to the final hole mobility. For a dopant LUMO energy close and high above the top of the polymer valence band we have found a significant mobility increase with the hole concentration and with the dopant LUMO energy approaching the top of the polymer valence band. Higher mobility was achieved for small values of hybridization transfer integral between polymer and the acceptor, corresponding to the case of weakly bound acceptor. Strong couplings between the polymer and the acceptor with Coulomb repulsion interactions induced from the electron localizations was found to suppress the hole mobility.

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Datum: 22.01.2018

Tolman’s length and limiting supersaturation of vapor

Publication date: 26 January 2018
Source:Chemical Physics, Volume 500

Author(s): Nikolay V. Alekseechkin

The classical Kelvin formula for the equilibrium vapor pressure over a droplet of radius $R$ is extended to small radii and vapor non-ideality, from where the limiting supersaturation condition is obtained by relating the point $R = 0$ to the value of limiting (spinodal) supersaturation of vapor. The analysis of different dependences of the Tolman length on radius, $δ ( R )$, obeying this condition suggests that (i) the value of $δ ( 0 )$ is positive and the function $δ ( R )$ decreases with increasing radius; (ii) the curvature effect (the dependence of surface tension on radius) in the nucleation region is determined by the value of $δ ( 0 )$. At the same time, this effect is weakly sensitive to the form of the function $δ ( R )$ and insensitive to its asymptotic value $δ ∞$ .

Datum: 22.01.2018

Predicting glass-to-glass and liquid-to-liquid phase transitions in supercooled water using classical nucleation theory

Publication date: 26 January 2018
Source:Chemical Physics, Volume 500

Author(s): Robert F. Tournier

Glass-to-glass and liquid-to-liquid phase transitions are observed in bulk and confined water, with or without applied pressure. They result from the competition of two liquid phases separated by an enthalpy difference depending on temperature. The classical nucleation equation of these phases is completed by this quantity existing at all temperatures, a pressure contribution, and an enthalpy excess. This equation leads to two homogeneous nucleation temperatures in each liquid phase; the first one (Tn- below Tm) being the formation temperature of an “ordered” liquid phase and the second one corresponding to the overheating temperature (Tn+ above Tm). Thermodynamic properties, double glass transition temperatures, sharp enthalpy and volume changes are predicted in agreement with experimental results. The first-order transition line at TLL = 0.833 × Tm between fragile and strong liquids joins two critical points. Glass phase above Tg becomes “ordered” liquid phase disappearing at TLL at low pressure and at Tn+ = 1.302 × Tm at high pressure.

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Datum: 22.01.2018

S⋯N chalcogen bonded complexes of carbon disulfide with diazines. Theoretical study

Publication date: 26 January 2018
Source:Chemical Physics, Volume 500

Author(s): Wiktor Zierkiewicz, Jindřich Fanfrlík, Mariusz Michalczyk, Danuta Michalska, Pavel Hobza

Carbon disulfide complexes with diazine (pyridazine, pyrimidine or pyrazine) have been studied by density functional BLYP-D3 and ab initio CCSD(T) methods. All possible conformers of these complexes have been found. In the chalcogen bonded complexes, the CCSD(T)/cc-pvtz calculated interaction energies (ΔE) range between −0.89 and −2.19 kcal mol−1. These complexes are more stable than those stabilized by hydrogen bond. The linear correlation between the ΔE and the most negative values of the electrostatic potential surfaces (Vs,min) on the nitrogen atom of the diazines has been found. According to the symmetry-adapted perturbation theory (SAPT) analysis, in the chalcogen bonded complexes among all of the attraction forces the electrostatic component is the most important one, while in the hydrogen bonded and stacking complexes the dispersion contribution is the leading term. Moreover, the Natural Bond Orbitals (NBO), AIM and Noncovalent Interaction Index (NCI) analyses have been performed.

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Datum: 22.01.2018

Complementary experimental-simulational study of surfactant micellar phase in the extraction process of metallic ions: Effects of temperature and salt concentration

Publication date: 14 February 2018
Source:Chemical Physics, Volume 501

Author(s): Alan Gustavo Soto-Ángeles, María del Rosario Rodríguez-Hidalgo, César Soto-Figueroa, Luis Vicente

The thermoresponsive micellar phase behaviour that exhibits the Triton-X-100 micelles by temperature effect and addition of salt in the extraction process of metallic ions was explored from mesoscopic and experimental points. In the theoretical study, we analyse the formation of Triton-X-100 micelles, load and stabilization of dithizone molecules and metallic ions extraction inside the micellar core at room temperature; finally, a thermal analysis is presented. In the experimental study, the spectrophotometric outcomes confirm the solubility of the copper-dithizone complex in the micellar core, as well as the extraction of metallic ions of aqueous environment via a cloud-point at 332.2 K. The micellar solutions with salt present a low absorbance value compared with the micellar solutions without salt. The decrease in the absorbance value is attributed to a change in the size of hydrophobic region of colloidal micelles. All transitory stages of extraction process are discussed and analysed in this document.

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Datum: 22.01.2018

Computational study of red- and blue-shifted CH⋯Se hydrogen bond in Q3CH⋯SeH2 (Q = Cl, F, H) complexes

Publication date: 26 January 2018
Source:Chemical Physics, Volume 500

Author(s): Pragya Chopra, Shamik Chakraborty

This work presents CH⋯Se hydrogen bonding interaction at the MP2 level of theory. The system Q3CH⋯SeH2 (Q = Cl, F, and H) provides an opportunity to investigate red- and blue-shifted hydrogen bonds. The origin of the red- and blue-shift in CH stretching frequency has been investigated using Natural Bond Orbital analysis. A large amount of electron density is being transferred to the σCH orbital in red-shifted Cl3CH⋯SeH2. Electron density transfer in the blue-shifted F3CH⋯SeH2 is primarily to the remote fluorine atoms. Further, due to polarization of the CH bond, the contradicting effects of rehybridization and hyperconjugation are important. The extent of hyperconjugation reigns predominant in explaining the nature of the CH⋯Se hydrogen bond in Q3CH⋯SeH2 complexes as the hydrogen bond acceptor remain same in this investigation. Red- and blue-shift in Q3CH⋯SeH2 (Q = Cl and F) complexes is best described by pro-improper hydrogen bond donor concept.

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Datum: 22.01.2018

Effect of different substitution position on the switching behavior in single-molecule device with carbon nanotube electrodes

Publication date: 26 January 2018
Source:Chemical Physics, Volume 500

Author(s): Jingjuan Yang, Xiaoxiao Han, Peipei Yuan, Baoan Bian, Yixiang Wang

We investigate the electronic transport properties of dihydroazulene (DHA) and vinylheptafulvene (VHF) molecule sandwiched between two carbon nanotubes using density functional theory and non-equilibrium Green’s function. The device displays significantly switching behavior between DHA and VHF isomerizations. It is found the different substitution position of F in the molecule influences the switching ratio of device, which is analyzed by transmission spectra and molecular projected self-consistent Hamiltonian. The observed negative differential resistance effect is explained by transmission spectra and transmission eigenstates of transmission peak in the bias window. The observed reverse of current in VHF form in which two H atoms on the right side of the benzene ring of the molecule are replaced by F is explained by transmission spectra and molecule-electrode coupling with the varied bias. The results suggest that the reasonable substitution position of molecule may improve the switching ratio, displaying a potential application in future molecular circuit.

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Datum: 22.01.2018

Theoretical and experimental study of electron-deficient core substitution effect of diketopyrrolopyrrole derivatives on optoelectrical and charge transport properties

Publication date: 26 January 2018
Source:Chemical Physics, Volume 500

Author(s): Guodong Ding, Asif Mahmood, Ailing Tang, Fan Chen, Erjun Zhou

Three new diketopyrrolopyrrole based compounds with Acceptor-Donor-Acceptor-Donor-Acceptor (A-D-A-D-A) skeletons were designed and synthesized through varying the electron-deficient core from diphenylquinoxaline (DP-Qx), thieno[3,4-c]pyrrole-4,6-dione (DP-TPD) to 2-dodecyl-6,7-diphenyl-2H-[1,2,3]triazole[4,5-g]quinoxaline (DP-TQx). We have calculated and studied the effect of central acceptor units on electronic, optical and non-optical properties. As well as, we have predicted the charge transport properties. Results indicate that change of central acceptor unit remarkably affects the molecular electronic, optical and non-optical properties. And the molecular band gap and UV/vis adsorption spectra are significantly changed. It should be noted that Compound 3 with 2-dodecyl-6,7-diphenyl-2H-[1,2,3]triazole[4,5-g]quinoxaline as core show superior non-optical properties as compare to other compounds. Our study here indicate that inserting the strong electron-deficient moieties improves intramolecular charge transfer (ICT) and charge transport properties dramatically.

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Datum: 22.01.2018

Isosteric heat of water adsorption and desorption in homoionic alkaline-earth montmorillonites

Publication date: 14 February 2018
Source:Chemical Physics, Volume 501

Author(s): M. Belhocine, A. Haouzi, G. Bassou, T. Phou, D. Maurin, J.L. Bantignies, F. Henn

The aim of the present work is to study by means of thermodynamic measurements and Infrared spectroscopy, the effect of the interlayer cations on the adsorption–desorption of water in the case of a montmorillonite exchanged with alkaline-earth metals. For the first time, the net isosteric heat of water adsorption and desorption is determined from isotherms recorded at three temperatures. The net isosteric heat is a very useful parameter for getting more insights into the sorption mechanism since it provides information about the sorption energy evolution which can be complementary to that obtained from structural or gravimetric measurements. The homoionic montmorillonite samples are prepared from purification and cationic exchanged in aqueous solution of the raw material, i.e. the reference SWy-2 Wyoming material. XRD at the dry state and elemental chemical analysis confirm that the treatment does not deteriorate the clay structure and yield the expected homoionic composition. The sorption isotherms measured at various temperatures show that the nature of the interlayer, i.e. exchangeable, cation changes the adsorbed/desorbed amount of water molecules for a given water relative pressure. The total amount of water adsorbed at $P P ∘$ = 0.5 follows the cation sequence Ca ∼ Mg&gt;Ba while the sorption isosteric heats follow a slightly different sequence, i.e. Ca &gt; Mg&gt;Ba. This discrepancy between the adsorption and desorption heat is due to the higher irreversibility of water sorption process in the Ca exchanged montmorillonite. Finally, analysis of the IR spectra recorded at room temperature and under a primary vacuum reveals that the amount of adsorbed water follows the same sequence as that of the isosteric heat of adsorption and shows the coexistence of liquid-like and solid-like water confined in the interlayer space.

Datum: 22.01.2018

Quasirelativistic potential energy curves and transition dipole moments of NaRb

Publication date: 26 January 2018
Source:Chemical Physics, Volume 500

Author(s): M. Wiatr, P. Jasik, T. Kilich, J.E. Sienkiewicz, H. Stoll

We report on extensive calculations of quasi-relativistic potential energy curves and, for the first time, transition dipole moments including spin-orbit and scalar-relativistic effects of the NaRb molecule. The calculated curves of the $0 +$, $0 -$, 1, 2 and 3 molecular states correlate for large internuclear separation with the fourteen lowest atomic energies up to the Na($3 s 2 S 1 / 2$) + Rb($7 s 2 S 1 / 2$) atomic limit. Several new features of the potential energy curves have been found.

Datum: 22.01.2018

Graphical abstract TOC

Publication date: 26 January 2018
Source:Chemical Physics, Volume 500

Datum: 22.01.2018

Graphical abstract TOC

Publication date: 26 January 2018
Source:Chemical Physics, Volume 500

Datum: 22.01.2018

6.2 μm spectrum and 6-dimensional morphed potentials of OC-H2O

Publication date: 14 February 2018
Source:Chemical Physics, Volume 501

Author(s): Luis A. Rivera-Rivera, Blake A. McElmurry, Kevin W. Scott, Sean D. Springer, Robert R. Lucchese, John W. Bevan, Igor I. Leonov, Laurent H. Coudert

Rovibrational transitions associated with tunneling states in the ν 5 (water bending) vibration of the OC-H2O complex have been recorded using a supersonic jet mode-hop free quantum cascade laser spectrometer at 6.2 μm. Analysis of the resulting spectra is facilitated by incorporating fits of previously recorded microwave and submillimeter data accounting for Coriolis coupling. The theoretical basis of morphing a 5-D frozen monomers potential was initially developed and then extended to two 6-D morphed potentials. A combination of these spectroscopic results and previous rovibrational data for the ν 5 vibration in OC-D2O are then used to generate a 6-D morphed potential surface for the intermolecular and the water bending vibrations. An alternative 6-D morphed potential of the intermolecular and the ν 3 (CO stretching) vibrations was also generated. These determined morphed potentials then formed the basis for modeling the dynamics of the complex and prediction of accurate intermolecular rovibrational frequencies of the complex.

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Datum: 22.01.2018

NMT – a new individual ion counting method: comparison to a Faraday cup

Publication date: Available online 17 January 2018
Source:Chemical Physics

Author(s): Michael Burton, Boris Gorbunov

Two sample detectors used to analyze the emission from Gas Chromatography (GC) columns are the Flame Ionization Detector (FID) and the Electron Capture Detector (ECD). Both of these detectors involve ionization of the sample molecules and then measuring electric current in the gas using a Faraday cup. In this paper a newly discovered method of ion counting, Nanotechnology Molecular Tagging (NMT) is tested as a replacement to the Faraday cup in GCs. In this method the effective physical volume of individual molecules is enlarged up to 1 billion times enabling them to be detected by an optical particle counter. It was found that the sensitivity of NMT was considerably greater than the Faraday cup. The background in the NMT was circa 200 ions per cm3, corresponding to an extremely low electric current ∼10-17 A.

Datum: 22.01.2018

Possible influence of the Kuramoto length in a photo-catalytic water splitting reaction revealed by Poisson–Nernst–Planck equations involving ionization in a weak electrolyte

Publication date: Available online 10 January 2018
Source:Chemical Physics

Author(s): Yohichi Suzuki, Kazuhiko Seki

We studied ion concentration profiles and the charge density gradient caused by electrode reactions in weak electrolytes by using the Poisson–Nernst–Planck equations without assuming charge neutrality. In weak electrolytes, only a small fraction of molecules is ionized in bulk. Ion concentration profiles depend on not only ion transport but also the ionization of molecules. We considered the ionization of molecules and ion association in weak electrolytes and obtained analytical expressions for ion densities, electrostatic potential profiles, and ion currents. We found the case that the total ion density gradient was given by the Kuramoto length which characterized the distance over which an ion diffuses before association. The charge density gradient is characterized by the Debye length for 1:1 weak electrolytes. We discuss the role of these length scales for efficient water splitting reactions using photo-electrocatalytic electrodes.

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Datum: 22.01.2018

Angle Dependence in Slow Photon Photocatalysis using TiO2 Inverse Opals

Publication date: Available online 10 January 2018
Source:Chemical Physics

Author(s): Mariano Curti, Gonzalo Zvitco, María Alejandra Grela, Cecilia B. Mendive

The slow photon effect was studied by means of the photocatalytic degradation of stearic acid over TiO2 inverse opals. The comparison of the degradation rates over inverse opals with those obtained over disordered structures at different irradiation angles showed that the irradiation at the blue edge of the stopband leads to the activation of the effect, evidenced by an improvement factor of 1.8±0.6 in the reaction rate for irradiation at 40°. The rigorous coupled-wave analysis (RCWA) method was employed to confirm the source of the enhancement; simulated spectra showed an enhancement in the absorption of the TiO2 matrix that composes the inverse opal at a 40° irradiation angle, owing to an appropriate position of the stopband in relation to the absorption onset of TiO2.

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Datum: 22.01.2018

Monolayer adsorption of noble gases on graphene

Publication date: 14 February 2018
Source:Chemical Physics, Volume 501

Author(s): Sidi M. Maiga, Silvina M. Gatica

We report our results of simulations of the adsorption of noble gases (Kr, Ar, Xe) on graphene. For Kr, we consider two configurations: supported and free-standing graphene, where atoms are adsorbed only on one or two sides of the graphene. For Ar and Xe, we studied only the case of supported graphene. For the single-side adsorption, we calculated the two-dimensional gas-liquid critical temperature for each adsorbate. We determined the different phases of the monolayers and constructed the phase diagrams. We found two-dimensional incommensurate solid phases for krypton, argon and xenon, and a two-dimensional commensurate solid phase for krypton. For double side adsorption of Kr, we do not see evidence of an ordering transition driven by the interlayer forces.

Datum: 22.01.2018

Direct application of the phase estimation algorithm to find the eigenvalues of the Hamiltonians

Publication date: Available online 4 January 2018
Source:Chemical Physics

The eigenvalue of a Hamiltonian, $H$, can be estimated through the phase estimation algorithm given the matrix exponential of the Hamiltonian, $exp ( - i H )$. The difficulty of this exponentiation impedes the applications of the phase estimation algorithm particularly when $H$ is composed of non-commuting terms. In this paper, we present a method to use the Hamiltonian matrix directly in the phase estimation algorithm by using an ancilla based framework: In this framework, we also show how to find the power of the Hamiltonian matrix-which is necessary in the phase estimation algorithm-through the successive applications. This may eliminate the necessity of matrix exponential for the phase estimation algorithm and therefore provide an efficient way to estimate the eigenvalues of particular Hamiltonians. The classical and quantum algorithmic complexities of the framework are analyzed for the Hamiltonians which can be written as a sum of simple unitary matrices and shown that a Hamiltonian of order $2 n$ written as a sum of L number of simple terms can be used in the phase estimation algorithm with $( n + 1 + logL )$ number of qubits and $O ( 2 a nL )$ number of quantum operations, where a is the number of iterations in the phase estimation. In addition, we use the Hamiltonian of the hydrogen molecule as an example system and present the simulation results for finding its ground state energy.

Datum: 22.01.2018

Anharmonic vibrational spectroscopy calculations using the ab initio CSP method: Applications to H2CO3, (H2CO3)2, H2CO3-H2O and isotopologues

Publication date: Available online 26 December 2017
Source:Chemical Physics

Author(s): Lior Sagiv, Barak Hirshberg, R. Benny Gerber

The anharmonic frequencies of the O–H and the C–O stretching modes of carbonic acid are calculated using the ab initio classical separable potentials approximation. In this approach, ab initio molecular dynamics simulations are used to determine an effective classical potential for each of the normal modes of the system. The frequencies are calculated by solving the time-independent Schrödinger equation for each mode using time-averaged potentials. Hydrogen-bonded carbonic acid dimer, carbonic acid–water complex and isotopologues of these systems were also studied, showing good agreement with experiments. Useful insights are obtained by Fourier transforming the effective potentials, which relates the computed quantum frequencies to the classical dynamics of the system. This approach is illustrated for the systems studied, including for the challenging anharmonic isotope effect. In conclusion, the ab initio CSP approximation yields results in good accord with experiments, and the method provides interpretations in terms of the dynamics of vibrational motions.

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Datum: 22.01.2018

Direct observation of the change in transient molecular structure of 9,9′-bianthryl using a 10 fs pulse UV laser

Publication date: Available online 28 December 2017
Source:Chemical Physics

Author(s): Sena Hashimoto, Atsushi Yabushita, Takayoshi Kobayashi, Kotaro Okamura, Izumi Iwakura

Photoexcitation of 9,9′-bianthryl has been considered to produce a locally excited (LE) state and then a charge transfer (CT) state by intramolecular charge transfer (ICT) in polar solvents. In contrast, the CT state is considered to be unstable in non-polar solvents and triggers a dipole flip-flop between the two anthracene rings. The central C–C bond between the two anthracene rings is considered to twist during relaxation in the LE state and CT state, and this was visualized in the present work using an ultrashort ultraviolet (UV) pulse. The C–C stretching frequency was recovered in a period of ca. 500 fs, which implies that the C–C bond undergoes repeated torsion back and forth in the electronic excited state. Investigation of samples in a polar solvent and a non-polar solvent indicated that ICT does not affect the torsion around the central C–C bond.

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Datum: 22.01.2018

Insight into the new excited-state intramolecular proton transfer (ESIPT) mechanism of N,N′-bis(salicylidene)-p-phenylenediamine (p-BSP)

Publication date: 14 February 2018
Source:Chemical Physics, Volume 501

Author(s): Jiaojiao Hao, Yang Yang

A new excited-state intramolecular proton transfer (ESIPT) mechanism of N,N′-bis(salicylidene)-p-phenylenediamine (p-BSP) including the single and double ESIPT processes has been proposed using the time-dependent density functional theory (TDDFT) method, which was clearly different from the one proposed previously (Ziółek et al., J. Phys. Chem. A. 2009, 113, 2854). By analyzing the bond lengths, bond angles and IR vibrational spectra, we determined that two intramolecular hydrogen bonds (IHBs) (O1–H2⋯N3 and O4–H5⋯N6) of p-BSP were strengthened in the S1 state, which could facilitate the ESIPT process. Additionally, intramolecular charge transfer based on the frontier molecular orbitals (MOs) and the map of the electron density difference between the S0 and S1 states demonstrated the possibility of ESIPT reaction. To reveal the detailed ESIPT mechanism, we constructed potential energy surfaces (PESs) in the S0 and S1 states, and thus determined that the single and double ESIPT processes coexisted in the S1 state.

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Datum: 22.01.2018

Photochemical dynamics of a trimethyl-phosphine derivatized [FeFe]-hydrogenase model compound

Publication date: Available online 23 December 2017
Source:Chemical Physics

Author(s): Rachel L. Meyer, Annette D. Zhandosova, Tara M. Biser, Edwin J. Heilweil, Christopher J. Stromberg

Though there have been many studies on photosensitizers coupled to model complexes of the [FeFe]-hydrogenases, few have looked at how the models react upon exposure to light. To extract photoreaction information, ultrafast time-resolved UV/visible pump, IR probe spectroscopy was performed on Fe2(μ-S2C2H4)(CO)4(PMe3)2 (2b) dissolved in heptane and acetonitrile and the photochemical dynamics were determined. Excitation with 532 and 355 nm light produces bleaches and new absorptions that decay to half their original intensity with time constants of 300 ± 120 ps and 380 ± 210 ps in heptane and acetonitrile, respectively. These features persist to the microsecond timescale. The dynamics of 2b are assigned to formation of an initial set of photoproducts, which were a mixture of excited-state tricarbonyl isomers. These isomers decay into another set of long-lived photoproducts in which approximately half the excited-state tricarbonyl isomers recombine with CO to form another complex mixture of tricarbonyl and tetracarbonyl isomers.

Datum: 22.01.2018

Effect of oligomer length on vibrational coupling and energy relaxation in double-stranded DNA

Publication date: Available online 19 December 2017
Source:Chemical Physics

Author(s): Gordon Hithell, Paul M. Donaldson, Gregory M. Greetham, Michael Towrie, Anthony W. Parker, Glenn A. Burley, Neil T. Hunt

The effect of oligomer length on the vibrational mode coupling and energy relaxation mechanisms of AT-rich DNA oligomers in double- and single-stranded conformations has been investigated using two-dimensional infrared spectroscopy. Vibrational coupling of modes of the DNA bases to the symmetric stretching vibration of the backbone phosphate group was observed for oligomers long enough to form duplex-DNA structures. The coupling was lost upon melting of the duplex. No significant effect of oligomer length or DNA secondary structure was found on either the timescale for vibrational relaxation of the base modes or the mechanism, which was consistent with a cascade process from base modes to intermediate modes, some of which are located on the deoxyribose group, and subsequently to the phosphate backbone. The study shows that vibrational coupling between base and backbone requires formation of the double-helix structure while vibrational energy management is an inherent property of the nucleotide.

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Datum: 22.01.2018

The Ag+ state falls below 3Ag- at carotenoid-relevant conjugation lengths

Publication date: Available online 21 December 2017
Source:Chemical Physics

Author(s): Elliot J. Taffet, Gregory D. Scholes

Spectroscopy of all-trans linear polyenes is characterized by a one-photon electronic transition to a bright S2 state and two-photon transition to a lower-lying dark S1 state. We apply ab initio quantum chemistry to distinguish additional diffuse dark singlet states. Building on a systematic implementation of CASSCF and DMRGSCF coupled to strongly-contracted NEVPT2, we report the excitation energies for a series of polyenes from butadiene to C16H18 at optimized S0, S1 and S2 geometries. Ionic/covalent state inversion within each manifold is identified at seven conjugated double bonds, making the $B u -$ a dark intermediate state at its optimized geometry and the $A g +$ the S3 state at the S0 optimized geometry. The relevance of these results to the function of carotenoids in photosynthetic light harvesting is discussed.

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Datum: 22.01.2018

A DFT study of the electronic structures and optical properties of (Cr, C) co-doped rutile TiO2

Publication date: 14 February 2018
Source:Chemical Physics, Volume 501

Author(s): Hao Chen, Xuechao Li, Rundong Wan, Sharon Kao-Walter, Ying Lei

To get an effective doping model of rutile TiO2, we systematically study geometrical parameters, density of states, electron densities, dielectric functions, optical absorption spectra for the pure, C mono-doping, Cr mono-doping and (Cr,C) co-doping rutile TiO2, using density functional calculations. We find that a C doped system presents higher stability under Ti-rich condition, while Cr doped and (Cr,C) co-doped systems are more stable under O-rich condition. For (Cr,C) co-doping situation, the imaginary part of the dielectric function reflects the higher energy absorption efficiency for incident photons. Moreover, co-doping system exhibits much bigger red-shift of optical absorption edge compared with Cr/C single doping systems, because of the great reduction of the direct band gap. The calculated optical absorption spectra show that the (Cr,C) co-doping rutile TiO2 has higher photocatalytic activity in the visible light region.

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Datum: 22.01.2018

A computational investigation on the photo-isomerization of 2,4,6-octatriene and its UV–visible spectrum

Publication date: Available online 16 December 2017
Source:Chemical Physics

The dynamical processes following photoexcitation of all-trans 2,4,6-octatriene to the two lowest singlet excited states are investigated theoretically, from two different points of view. The S1 -S0 photoisomerization is characterized with a focus on structural aspects (stationary points and reactive modes), while for the UV–visible spectrum, arising from excitation to the S2 state, nonadiabatic quantum dynamics calculations with four planar degrees of freedom are performed. The underlying electronic structure data are obtained from CASSCF and MS-CASPT2 ab initio computations, the quantum dynamical calculations rely on the Multiconfiguration Time-Dependent Hartree (MCTDH) method. The observed envelope of the UV–visible spectrum is well reproduced and the substantial broadness of the spectral features related to a nonradiative S2-S1 transition proceeding within few tens of femtoseconds. The vertical excitation energy from the Ag to the Bu state shows a red-shift (∼0.2 eV with the cc-pVTZ basis set) with reference to the all-trans hexatriene system. This affects the S2-S1 nonradiative decay, while the methylation of the terminal carbon atoms seems to have a minor influence on the photo-isomerization path.

Datum: 22.01.2018

Category: Current Chemistry Research

Last update: 04.01.2018.