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.