During the last few years we have been working in biologically related systems, in which proton transfer plays a crucial role. Our aim is to investigate these processes (in enzymes, vitamins, etc.) from a theoretical point of view.
Specifically, we employ semiclassical methods such as variational transition state theory with multidimensional tunneling corrections (VTST/MT) and the approximate instanton method (AIM).

The figure shows a model used by us to calculate thermal rate constants and kinetic isotope effects for proton transfer in carbonic anhidrase II.

The purpose of these studies is the interpretation and simulation of the experimental results obtained in other laboratories using either cross molecular beams or photon initiated techniques.

The photodissociation of formic acid at 248 and 193 nm was investigated by classical trajectory and RRKM calculations using an interpolated potential energy surface, iteratively constructed using the B3LYP/aug-cc-pVDZ level of calculation. Several sampling schemes in the ground electronic state were employed to explore the possibility of conformational memory in formic acid. The CO/CO₂ branching ratios obtained from trajectories initiated at the cis and at the trans conformers are almost identical to each other and in very good accordance with the RRKM results. In addition, when a specific initial excitation that simulates more rigorously the internal conversion process is used, the calculated branching ratio does not vary with respect to those obtained from cis and trans initializations. This result is at odds with the idea of conformational memory in the ground state proposed recently for the interpretation of the experimental results. It was also found that the calculated CO vibrational distributions after dissociation of the parent molecule at 248 nm are in agreement with the experimental available data. This work was done in collaboration with C.M. Estevez, I. Borges Jr. and F. J. Aoiz and J. F. Castillo.

Adittionally, non-adiabatic direct dynamics calculations were carried out to investigate the radical and molecular decomposition channels of formic acid. The calculations show, in agreement with experiment, that the HCO + OH dissociation channel accounts for 70% of the product yields. In addition, the molecular eliminations of CO and CO₂ are minor channels, with a CO/CO₂ branching ratio that depends on what isomer is initially excited to the S₁ state. This result is also in qualitative agreement with experimental results in an Ar matrix environment. This work was done in collaboration with Persico´s group