Vibrational frequencies in hydrogen-bonded and non-hydrogen-bonded C-H groups

In the present review, the parameters governing the frequencies in hydrogen-bonded C-H and in non-bonded C-H groups are discussed. The red-shifts in C-H groups bonded to anions and the blue shifts in hydrogen bonds involving hydrogen peroxide depend both on the changes in occupation of the sigma*(C-H) orbitals and the changes in hybridization of the C at the bonded H. The coefficients of these two parameters do not greatly differ for both blue- and red-shifted hydrogen-bonded C-H groups, suggesting their similar nature. In contrast, blue shifts of C-H groups are mainly determined by a decrease in hyperconjugation, leading to a decrease in the occupation of the sigma*(C-H) orbitals. (c) 2007 Elsevier B. V. All rights reserved.