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. The vibrational energy levels of the one-dimensional ground state adiabatic potential of the model are used to make quantitative comparisons with a vast body of condensed phase data, spanning a donor-acceptor separation (R) range of about 2.4 − 3.0Å, i.e., from strong to weak hydrogen bonds. The position of the proton (which determines the X-H bond length) and its longitudinal vibrational frequency, along with the isotope effects in both are described quantitatively. An analysis of the secondary geometric isotope effect, using a simple extension of the two-state model, yields an improved agreement of the predicted variation with R of frequency isotope effects. The role of bending modes is also considered: their quantum effects compete with those of the stretching mode for weak to moderate Hbond strengths. In spite of the economy in the parametrization of the model used, it offers key insights into the defining features of H-bonds, and semi-quantitatively captures several trends.

The photoinduced hydrogen elimination reaction in thiophenol via the conical intersections of the dissociative (1)πσ∗ excited state with the bound (1)ππ∗ excited state and the electronic ground state has been investigated with ab initio electronic-structure calculations and time-dependent quantum wave-packet calculations. A screening of the coupling constants of the symmetry-allowed coupling modes at the (1)ππ∗-(1)πσ∗ and (1)πσ∗-S(0) conical intersection shows that the SH torsional mode is by far the most important coupling mode at both conical intersections. A model including three intersecting potential-energy surfaces (S(0), (1)ππ∗, (1)πσ∗) and two nuclear degrees of freedom (SH stretch and SH torsion) has been constructed on the basis of ab initio complete-active-space self-consistent field and multireference second-order perturbation theory calculations. The nonadiabatic quantum wave-packet dynamics initiated by optical excitation of the (1)ππ∗ and (1)πσ∗ states has been explored for this three-state two-coordinate model. The photodissociation dynamics is characterized in terms of snapshots of time-dependent wave packets, time-dependent electronic population probabilities, and the branching ratio of the (2)σ/(2)π electronic states of the thiophenoxyl radical. The dependence of the timescale of the photodissociation process and the branching ratio on the initial excitation of the SH stretching and SH torsional vibrations has been analyzed. It is shown that the node structure, which is imposed on the nuclear wave packets by the initial vibrational preparation as well as by the transitions through the conical intersections, has a profound effect on the photodissociation dynamics. The effect of additional weak coupling modes of CC twist (ν(16a)) and ring-distortion (ν(16b)) character has been investigated with three-dimensional and four-dimensional time-dependent wave-packet calculations, and has been found to be minor.

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. In a result consistent with aqueous phase infrared experiments [Bergström, P. A.; Lindgren, J.; Kristiansson, O. J. Phys. Chem. 1991, 95, 8575-8580], the CT and OH red shift in NO3(-) x 6 H2O are found to be smaller than those for all of the six-water halide complexes, despite the presence of three H-bonding sites. The inability of the nitrate anion to effect substantial CT lies in the preservation of the pi-system being energetically favored over charge localization and enhancement of the strengths of the multiple H-bonds.

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. Changes in intensities of the nu(1) and 2nu(4) bands upon simple alterations of the dipole moment expansion are described.