Radical hydrogen bonding: origin of stability of radical-molecule complexes.

Natural bond orbital analysis is used to investigate the nature of hydrogen bonding in a series of binary open-shell complexes involving hydroperoxy radical HO(2)...X and analogous closed-shell H(2)O(2)...X complexes (where X = H(2)O, H(2)O(2), HONO, HONO(2), CH(3)OH, HCOOH, CH(3)COOH, and H(2)SO(4)) in order to elucidate and identify the electronic factors responsible for the strength of radical hydrogen bonds. Results from this study suggest that the radical species strongly alters the strength of the characteristic n --> sigma(*) donor-acceptor interaction in the hydrogen bonding. This interaction is found to contribute to the unusually strong binding in radical-molecule complexes. These findings have important new ramifications for our fundamental understanding of radical hydrogen bonds.