Open-shell species: A challenge to electronic structure theory
Our research is focused on theoretical modeling of open-shell species. Since chemical transformations involve bond-breaking, radicals and diradicals are often encountered as reaction intermediates or transition states. Therefore, they play a central role in mechanistic understanding of processes important in the environment, synthetic chemistry, material science, biochemistry, etc. Since these open-shell species are often very reactive and short-lived, their experimental observations are difficult. That is why electronic structure theory is a valuable tool for studying their properties.
Radiation-induced or oxidative damage of DNA involves ionization of nucleic acid bases producing radical cations.
Modulated by local interactions (h-bonding, pi-stacking, electrostatics), the hole migrates along the strand causing a variety of structural changes. Using EOM-IP-CCSD, we characterized the effect of non-covalent interactions on ionization energies of the bases.
We develop new theoretical tools that enable accurate and efficient description of properties of open-shell species, such as their energies, spectroscopy, and reactivity. This involves development of new ab initio methods within equation-of-motion coupled-cluster formalism, as well as creating interfaces between electronic structure theory and spectroscopy modeling for more direct comparison with experimental measurements.
To study molecules in realistic environments (e.g., chromophores
in solutions or in proteins) we employ Effective Fragment Potential
method and hybrid QM/MM approach.
In equation-of-motion coupled-cluster approach, problematic open-shell states are described as "excitations"
from a well-behaved closed-shell reference wave function.
The amplitudes R are found by diagonalization of similarity-transformed Hamiltonian exp(-T)Hexp(T).
Among important contributions from our group are: the Spin-Flip method, calculation of
Dyson orbitals for open-shell and electronically excited states, a variety of properties calculations.
Efficient C++ computer codes developed in Krylov's group are integrated in the Q-Chem electronic structure package.
By using new methodology and in collaboration with excellent experimental groups at USC and around the world, we study a variety of fascinating systems ranging from the species relative to atmospheric chemistry and combustion to such biologically important systems as green fluorescent protein chromophores and building blocks of DNA. A common theme in these studies is characterization of bonding in open-shell compounds and understanding the fundamental rules that governs their electronic structure and their interaction with light.
pi-stacking interactions reduce ionization energies of nucleobases by as much as 0.3-0.4 eV.
The effect is due to hole delocalization and can be explained within simple molecular orbital framework.
Above: The molecular orbitals and ionization energies of thymine and its pi-stacked dimer.
Note that ionization also changes bonding from non-covalent to covalent, which induces significant structural changes.
Using a variety of quantum chemistry tools, we characterized the electronically excited and ionized states of green fluorescent protein and
discovered that the bright absorbing state is embedded in photodetachment continuum. To understand the effect of environment on electronic properties,
we also develop methods for including solvent in ab initio calculations.
More details can be found on Krylov's group website and
iOpenShell. Some of the essential concepts of our research are explained in these shorts films: "Shine a light" and "Laser"
See Dr. Krylov Website for publication list