Andrei M. Tokmachev

Hydrogen-Bond Networks in Water Clusters (H2O)20: An Exhaustive Quantum-Chemical Analysis

Water aggregates allow for numerous configurations due to different distributions of hydrogen bonds. The total number of possible hydrogen-bond networks is very large even for medium-sized systems. We demonstrate that targeted ultra-fast methods of quantum chemistry make an exhaustive analysis of all configurations possible. The cage of (H(2)O)(20) in the form of the pentagonal dodecahedron is a common motif in water structures. We calculated the spatial and electronic structure of all hydrogen-bond configurations for three systems: idealized cage (H(2)O)(20) and defect cages with one or two hydrogen bonds broken. More than 3 million configurations studied provide unique data on the structure and properties of water clusters. We performed a thorough analysis of the results with the emphasis on the cooperativity in water systems and the structure-property relations.

Hydrogen-bond networks in finite ice nanotubes

An exhaustive analysis of all H‐bond networks for finite elements of ice nanotubes formed by up to 32 water molecules (3,660,732 configurations in total) is performed. The results constitute a unique database and demonstrate the H‐bond network formation and changes with the growth of the ice nanotube. The statistical analysis shows that H‐bonds can be classified according to their structural positions, and there are remarkable dependencies of the cooperativity energy and bond lengths on the systems morphology. The study of low‐energy configurations supports the conclusion about the ferroelectric order in ice nanotubes with odd numbers of water molecules in the ring.

Photostability via Sloped Conical Intersections: A Computational Study of the Pyrene Radical Cation

The photophysics of the pyrene radical cation, a polycyclic aromatic hydrocarbon (PAH) and a possible source of diffuse interstellar bands (DIBs), is investigated by means of hybrid molecular mechanics-valence bond (MMVB) force field and multiconfigurational CASSCF and CASPT2 ab initio methods. Potential energy surfaces of the first three electronic states D 0, D 1, and D 2 are calculated. MMVB geometry optimizations are carried out for the first time on a cationic system; the results show good agreement with CASSCF optimized structures, for minima and conical intersections, and errors in the energy gaps are no larger than those found in our previous studies of neutral systems. The presence of two easily accessible sloped D 1/D 2 and D 0/D 1 conical intersections suggests the pyrene radical cation is highly photostable, with ultrafast nonradiative decay back to the initial ground state geometry predicted via a mechanism similar to the one found in the naphthalene radical cation.