Why open-shells?

Our research is focused on theoretical modeling of open-shell molecules. 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.

Since a chemical bond is formed by a pair of electrons, bond breaking usually results in a formation of two radicals. Many reactions proceed through the so-called diradical transition states, e. g., isomerization around a double bond (basic step in the chemistry of vision). Ring-opening reactions often involve diradical intermediates. Other important examples include reaction centers in enzymes, organometallic compounds, photochemical processes, molecular magnets, and more.

Rotation around a double C-C bond

Rotation around a double C-C bond

Cyclobutane ring opening

Cyclobutane ring opening

Angewandte Chemie 2004-43/6 cover In addition to their practical importance, open-shell species are of great fundamental interest. For example, the interactions between the radical centers often result in unusual bonding patterns that have distinct spectroscopic and chemical signatures. In the case of these fascinating species, our intuition often fails to predict an outcome of the competition between the Aufbau principle and Hund's rule, the standard molecular orbital occupancy guidelines. Moreover, we have found examples when both of the above fundamental rules fail! Characterization of bonding in open-shell compounds and understanding the fundamental rules that governs electronic structure in open-shell species is a major theme of our research.

Contrary to the standard occupancy guidelines, such as Hund's rule or the Aufbau principle, three unpaired electrons are coupled anti-ferromagnetically in the 5-dehydro-m-xylylene (DMX) triradical. This system is the first example of an organic molecule with an open-shell doublet ground state and has been characterized in a joint experimental and theoretical study by Slipchenko, Munsch, Wenthold, and Krylov. The singly occupied orbitals in the triradical are shown on the cover picture, highlighting the melding of the theoretical and computational results.

We address these questions by performing electronic structure calculations, that is, by solving the Schrodinger equation using powerful computers. Despite the impressive progress in hardware and electronic structure methodology, the theoretical modeling of open-shells still remains major challenge for electronic structure theory due to electronic (near) degeneracies inherent in these species. Recently, we have introduced a novel approach, the Spin-Flip (SF) method that provides an accurate, robust, and efficient tool for studying bond-breaking, diradicals, triradicals, etc. Recent applications of the SF method include calculations of structures, spectroscopy, and thermochemistry of diradicals and triradicals. We also continue methodological developments in order to extend the SF method to tackle more extensive degeneracies, larger systems, and to enable calculations of properties such as non-adiabatic and spin-orbit couplings.