Nuclear quantum effects in gas-phase 2-fluoroethanol.

Torsional motions along the FCCO and HOCC dihedrals lead to the five unique conformations of 2-fluoroethanol, of which the conformer that is along both dihedrals has the lowest energy. In this work, we explore how nuclear quantum effects (NQEs) manifest in the structural parameters of the lowest energy conformer, in the intramolecular free energy landscape along the FCCO and HOCC dihedrals, and also in the infrared spectrum of the title molecule, through the use of path integral simulations. We have first developed a full dimensional potential energy surface using the reaction surface Hamiltonian framework.

Wavepacket dynamical study of H-atom tunneling in catecholate monoanion: the role of intermode couplings and energy flow.

We present a study of H-atom tunneling in catecholate monoanion through wavepacket dynamical simulations. In our earlier study of this symmetrical double-well system [, 2022, , 10887], a limited number of transition state modes were identified as being important for the tunneling process. These include the imaginary frequency mode , the CO scissor mode , and the OHO bending mode .

Multidimensional H-atom tunneling in the catecholate monoanion.

We present the catecholate monoanion as a new model system for the study of multidimensional tunneling. It has a symmetrical O-H double-well structure, and the H atom motion between the two wells is coupled to both low and high frequency modes with different strengths. With a view to studying mode-specific tunneling in the catecholate monoanion, we have developed a full (33) dimensional potential energy surface in transition state (TS) normal modes using a Distributed Gaussian Empirical Valence Bond (DGEVB) based approach.

Excited-state proton transfer in a 2-aminopyridine dimer: a surface hopping study.

We present a theoretical investigation of the excited-state intermolecular proton transfer process in a 2-aminopyridine dimer. Previous experimental and theoretical studies on this doubly hydrogen bonded system have attributed an ultrafast 50 fs timescale to the process at low excitation wavelengths and have shown that it involves access to the charge transfer (CT) states of the dimer. We have carried out a trajectory-based surface hopping study of the proton transfer process.

N-H photodissociation dynamics of electronically excited aniline: a three dimensional time-dependent quantum wavepacket study.

We have simulated the dynamics of 1πσ* state-mediated nonadiabatic N-H bond dissociation in photo-excited aniline (C6H5NH2). A three electronic state diabatic model potential, involving the ground, 1ππ*, and 1πσ* diabatic states, and focussing on the NH2 degrees of freedom alone is constructed using XMS-CASPT2 energies. Using a kinetic energy operator in the polyspherical framework, wavepacket dynamics in three vibrational modes, viz.

Alkyl hydrogen atom abstraction reactions of the CN radical with ethanol.

We present a study of the abstraction of alkyl hydrogen atoms from the β and α positions of ethanol by the CN radical in solution using the Empirical Valence Bond (EVB) method. We have built separate 2 × 2 EVB models for the H and H reactions, where the atom transfer is parameterized using ab initio calculations. The intra- and intermolecular potentials of the reactant and product molecules were modelled with the General AMBER Force Field, with some modifications.

Isotopic fractionation in proteins as a measure of hydrogen bond length.

If a deuterated molecule containing strong intramolecular hydrogen bonds is placed in a hydrogenated solvent, it may preferentially exchange deuterium for hydrogen. This preference is due to the difference between the vibrational zero-point energy for hydrogen and deuterium. It is found that the associated fractionation factor Φ is correlated with the strength of the intramolecular hydrogen bonds.

Effect of quantum nuclear motion on hydrogen bonding.

This work considers how the properties of hydrogen bonded complexes, X-H⋯Y, are modified by the quantum motion of the shared proton. Using a simple two-diabatic state model Hamiltonian, the analysis of the symmetric case, where the donor (X) and acceptor (Y) have the same proton affinity, is carried out. For quantitative comparisons, a parametrization specific to the O-H⋯O complexes is used.

A multi-sheeted three-dimensional potential-energy surface for the H-atom photodissociation of phenol.

Extending the previous work of Lan et al. [J. Chem.

Vibrational symmetry breaking of NO3- in aqueous solution: NO asymmetric stretch frequency distribution and mean splitting.

We apply a solute-solvent approach to a theoretical study of vibrational symmetry breaking in aqueous NO(3)(-) solution. Experimental infrared and Raman spectra have shown that the NO asymmetric stretches, which are degenerate for the isolated anion, are split by 35-60 cm(-1) in dilute solution. As an initial step to calculating the spectra, we have computed the distribution of energies, or the "static spectrum", and the resulting mean splitting of the two NO asymmetric stretch eigenstates in an aqueous milieu.

Vibrational relaxation of OH and CH fundamentals of polar and nonpolar molecules in the condensed phase.

Studies of vibrational energy flow in various polar and nonpolar molecules that follows the ultrafast excitation of the CH and OH stretch fundamentals, modeled using semiclassical methods, are reviewed. Relaxation rates are calculated using Landau-Teller theory and a time-dependent method, both of which consider a quantum mechanical solute molecule coupled to a classical bath of solvent molecules. A wide range of decay rates are observed, ranging from 1 ps for neat methanol to 50 ps for neat bromoform.

Charge transfer and OH vibrational frequency red shifts in nitrate-water clusters.

A theoretical study of charge transfer (CT) characteristics in nitrate (NO3(-)) anion-water complexes is presented, together with those for the halides, F-, Cl-, and Br-, for comparison. The relation between the vibrational frequency red shifts of the hydrogen (H)-bonded OH stretches and CT from the anion to the water molecule, established in previous work for the one-water complexes of the halides, is studied for both the one- and six-water nitrate complexes and is extended to the six-water case for the halides. In NO3(-) x H2O, the water molecule receives about as much charge as that in Br- x H2O.

Relaxation of the CH stretch in liquid CHBr3: solvent effects and decay rates using classical nonequilibrium simulations.

This article addresses two questions regarding the decay of the CH stretch in liquid CHBr3. The first is whether the initial steps of the relaxation primarily involve energy redistribution within the excited molecule alone. Gas phase quantum mechanical and classical calculations are performed to examine the role of the solvent in this process.

Time scales and pathways of vibrational energy relaxation in liquid CHBr3 and CDBr3.

Molecular dynamics simulations are used in conjunction with Landau-Teller, fluctuating Landau-Teller, and time-dependent perturbation theories to investigate energy flow out of various vibrational states of liquid CHBr3 and CDBr3. The CH stretch overtone is found to relax with a time scale of about 1 ps compared to the 50 ps rate for the fundamental. The relaxation pathways and rates for the CD stretch decay in CDBr3 are computed in order to understand the changes arising from deuteration.

Vibrational relaxation of the CH stretch fundamental in liquid CHBr3.

In continuation of our work on haloforms, the decay of CH stretch excitation in bromoform is modeled using molecular dynamics simulations. An intermolecular force field is obtained by fitting ab initio energies at select CHBr3 dimer geometries to a potential function. The solvent forces on vibrational modes obtained in the simulation are used to compute relaxation rates.

Combination of perturbative and variational methods for calculating molecular spectra: calculation of the upsilon=3-5 CH stretch overtone spectrum of CHF3.

Highly excited states of the CHF3 molecule belonging to the third, fourth, and fifth Fermi polyad are calculated using a combination of the Van Vleck perturbation theory and a variational treatment. The perturbation theory preconditions the Hamiltonian matrix by transforming away all couplings except those between nearly degenerate states. This transformation is implemented so that eigenvalues can be found with significantly smaller matrices than that which would be needed in the original normal mode representation.

Combined perturbative-variational investigation of the vibrations of CHBr(3) and CDBr(3).

A full dimensional vibrational treatment of CHBr(3) and CDBr(3) using Van Vleck perturbation theory followed by a variational calculation is presented. The calculation of a force field, and its adjustment for better match with experiment, is discussed. The computed eigenstates and spectral features are compared to experiment.