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28 Mar 2013

Volume 138, Issue 12, Articles (12xxxx)

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J. Chem. Phys. 138, 124701 (2013); http://dx.doi.org/10.1063/1.4794685 (9 pages)

Luying Wang, Randall S. Dumont, and James M. Dickson
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back to top Liquids, Glasses, and Crystals

Global sampling of the photochemical reaction paths of bromoform by ultrafast deep-UV through near-IR transient absorption and ab initio multiconfigurational calculations

S. K. Pal, A. S. Mereshchenko, E. V. Butaeva, P. Z. El-Khoury, and A. N. Tarnovsky

J. Chem. Phys. 138, 124501 (2013); http://dx.doi.org/10.1063/1.4789268 (19 pages)

Online Publication Date: 22 March 2013

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Ultrafast deep-ultraviolet through near infrared (210–950 nm) transient absorption spectroscopy complemented by ab initio multiconfigurational calculations offers a global description of the photochemical reaction pathways of bromoform following 255-nm excitation in methylcyclohexane and acetonitrile solutions. Photoexcitation of CHBr3 leads to the ground-state iso-CHBr3 product in a large quantum yield (∼35%), formed through two different mechanisms: concerted excited-state isomerization and cage-induced isomerization through the recombination of the nascent radical pair. These two processes take place on different time scales of tens of femtoseconds and several picoseconds, respectively. The novel ultrafast direct isomerization pathway proposed herein is consistent with the occurrence of a conical intersection between the first excited singlet state of CHBr3 and the ground electronic state of iso-CHBr3. Complete active space self-consistent field calculations characterize this singularity in the vicinity of a second order saddle point on the ground state which connects the two isomer forms. For cage-induced isomerization, both the formation of the nascent radical pair and its subsequent collapse into ground-state iso-CHBr3 are directly monitored through the deep-ultraviolet absorption signatures of the radical species. In both mechanisms, the optically active (i.e., those with largest Franck-Condon factors) C−Br−Br bending and Br−Br stretching modes of ground-state iso-CHBr3 have the largest projection on the reaction coordinate, enabling us to trace the structural changes accompanying vibrational relaxation of the non-equilibrated isomers through transient absorption dynamics. The iso-CHBr3 photoproduct is stable in methylcyclohexane, but undergoes either facile thermal isomerization to the parent CHBr3 structure through a cyclic transition state stabilized by the polar acetonitrile medium (∼300-ps lifetime), and hydrolysis in the presence of water.
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33.20.Lg Ultraviolet spectra
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
82.30.Qt Isomerization and rearrangement
31.15.ae Electronic structure and bonding characteristics
31.15.xr Self-consistent-field methods
33.20.Ea Infrared spectra

A thermodynamic derivation of the reciprocal relations

N. Kocherginsky and M. Gruebele

J. Chem. Phys. 138, 124502 (2013); http://dx.doi.org/10.1063/1.4793258 (6 pages)

Online Publication Date: 25 March 2013

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Starting with the continuity and Smoluchowski equations, we write the mass flux for a system out of equilibrium in terms of the physicochemical potential μg. μg is a coarse-grained analog of the chemical potential in the presence of forces that drive the system out of equilibrium. The expression for flux in terms of μg allows for a macroscopic derivation of the Onsager reciprocal relations for the case of transport by diffusion and drift in single or multi-component systems, without recourse to microscopic fluctuations or equations of motion. Transport coefficients for any time reversal-invariant properties now are expressed in terms of only partial molar derivatives and mobilities (diffusion coefficients). The thermodynamic derivation cannot treat time reversal.
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05.60.-k Transport processes
05.70.-a Thermodynamics

Hyperfine coupling of the hydrogen atom in high temperature water

Kirill Nuzhdin and David M. Bartels

J. Chem. Phys. 138, 124503 (2013); http://dx.doi.org/10.1063/1.4795005 (8 pages)

Online Publication Date: 26 March 2013

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The hyperfine coupling constant of the hydrogen atom has been measured in pressurized liquid water up to 300 °C. The reduced constant Awater/Avacuum is 0.9939 at room temperature, and decreases to a minimum of 0.9918 at 240 °C. The reduced constant then increases at higher temperature. The g-factor is 2.002244(10) at room temperature and decreases to 2.00221(1) at 240 °C. The change in g-factor is proportional to the change in hyperfine coupling. The behavior below 110 °C is in excellent agreement with a previously proposed model in which the H atom is confined to a harmonic solvent cage, and vibrations within the cage mix “p-type” character into the wavefunction, resulting in Awater/Avacuum < 1. The harmonic model breaks down above 130 °C. We demonstrate that a classical binary collision model using approximate partial molar volume information can recover the observed minima near 240 °C.
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31.30.Gs Hyperfine interactions and isotope effects
31.70.Dk Environmental and solvent effects
32.10.Fn Fine and hyperfine structure

Water proton configurations in structures I, II, and H clathrate hydrate unit cells

Fumihito Takeuchi, Masaki Hiratsuka, Ryo Ohmura, Saman Alavi, Amadeu K. Sum, and Kenji Yasuoka

J. Chem. Phys. 138, 124504 (2013); http://dx.doi.org/10.1063/1.4795499 (12 pages)

Online Publication Date: 27 March 2013

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Position and orientation of water protons need to be specified when the molecular simulation studies are performed for clathrate hydrates. Positions of oxygen atoms in water are experimentally determined by X-ray diffraction analysis of clathrate hydrate structures, but positions of water hydrogen atoms in the lattice are disordered. This study reports a determination of the water proton coordinates in unit cell of structure I (sI), II (sII), and H (sH) clathrate hydrates that satisfy the ice rules, have the lowest potential energy configuration for the protons, and give a net zero dipole moment. Possible proton coordinates in the unit cell were chosen by analyzing the symmetry of protons on the hexagonal or pentagonal faces in the hydrate cages and generating all possible proton distributions which satisfy the ice rules. We found that in the sI and sII unit cells, proton distributions with small net dipole moments have fairly narrow potential energy spreads of about 1 kJ/mol. The total Coulomb potential on a test unit charge placed in the cage center for the minimum energy/minimum dipole unit cell configurations was calculated. In the sI small cages, the Coulomb potential energy spread in each class of cage is less than 0.1 kJ/mol, while the potential energy spread increases to values up to 6 kJ/mol in sH and 15 kJ/mol in the sII cages. The guest environments inside the cages can therefore be substantially different in the sII case. Cartesian coordinates for oxygen and hydrogen atoms in the sI, sII, and sH unit cells are reported for reference.
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34.20.Gj Intermolecular and atom-molecule potentials and forces
31.50.-x Potential energy surfaces
33.15.Bh General molecular conformation and symmetry; stereochemistry

Non-monotonic size dependence of diffusion and levitation effect: A mode-coupling theory analysis

Manoj Kumar Nandi, Atreyee Banerjee, and Sarika Maitra Bhattacharyya

J. Chem. Phys. 138, 124505 (2013); http://dx.doi.org/10.1063/1.4796232 (8 pages)

Online Publication Date: 28 March 2013

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We present a study of diffusion of small tagged particles in a solvent, using mode coupling theory (MCT) analysis and computer simulations. The study is carried out for various interaction potentials. For the first time, using MCT, it is shown that only for strongly attractive interaction potential with allowing interpenetration between the solute-solvent pair the diffusion exhibits a non-monotonic solute size dependence which has earlier been reported in simulation studies [P. K. Ghorai and S. Yashonath, J. Phys. Chem. B 109, 5824–5835 (2005)10.1021/jp046312w]. For weak attractive and repulsive potential the solute size dependence of diffusion shows monotonic behaviour. It is also found that for systems where the interaction potential does not allow solute-solvent interpenetration, the solute cannot explore the neck of the solvent cage. Thus these systems even with strong attractive interaction will never show any non-monotonic size dependence of diffusion. This non-monotonic size dependence of diffusion has earlier been connected to levitation effect [S. Yashonath and P. Santikary, J. Phys. Chem. 98, 6368 (1994)10.1021/j100076a022]. We also show that although levitation is a dynamic phenomena, the effect of levitation can be obtained in the static radial distribution function.
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66.10.C- Diffusion and thermal diffusion
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1H relaxation dispersion in solutions of nitroxide radicals: Influence of electron spin relaxation

D. Kruk, A. Korpała, A. Kubica, J. Kowalewski, E. A. Rössler, and J. Moscicki

J. Chem. Phys. 138, 124506 (2013); http://dx.doi.org/10.1063/1.4795006 (15 pages)

Online Publication Date: 29 March 2013

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The work presents a theory of nuclear (1H) spin-lattice relaxation dispersion for solutions of 15N and 14N radicals, including electron spin relaxation effects. The theory is a generalization of the approach presented by Kruk et al. [J. Chem. Phys. 137, 044512 (2012)]10.1063/1.4736854. The electron spin relaxation is attributed to the anisotropic part of the electron spin–nitrogen spin hyperfine interaction modulated by rotational dynamics of the paramagnetic molecule, and described by means of Redfield relaxation theory. The 1H relaxation is caused by electron spin–proton spin dipole-dipole interactions which are modulated by relative translational motion of the solvent and solute molecules. The spectral density characterizing the translational dynamics is described by the force-free-hard-sphere model. The electronic relaxation influences the 1H relaxation by contributing to the fluctuations of the inter-molecular dipolar interactions. The developed theory is tested against 1H spin-lattice relaxation dispersion data for glycerol solutions of 4-oxo-TEMPO-d16-15N and 4-oxo-TEMPO-d16-14N covering the frequency range of 10 kHz–20 MHz. The studies are carried out as a function of temperature starting at 328 K and going down to 290 K. The theory gives a consistent overall interpretation of the experimental data for both 14N and 15N systems and explains the features of 1H relaxation dispersion resulting from the electron spin relaxation.
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33.35.+r Electron resonance and relaxation
34.20.Gj Intermolecular and atom-molecule potentials and forces
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Pw Fine and hyperfine structure
33.20.Sn Rotational analysis
33.25.+k Nuclear resonance and relaxation
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