Hyperconjugation in adjacent OO bonds: Remarkable odd/even effects

We report a theoretical analysis of the hyperconjugation effect in the case of adjacent oxygen-oxygen bonds. With this aim, quantum chemistry calculations for helical structures of hydrogen polyoxides HOnH have been carried out at the B3LYP (n = 3-10) and QCISD ( n = 3-6) levels using the 6-311+G(2df,2p) basis set. Hyperconjugation is estimated using second-order perturbation theory with natural bond orbitals analysis. Calculations predict a remarkable chain parity effect due to delocalized hyperconjugation interactions. In particular, in even-n chains, there is a clear systematic alternation of long and short O-O bonds and O-O distances do not become uniform as the chain length increases. (C) 2009 Elsevier B. V. All rights reserved.