Vibrational frequencies in hydrogen-bonded and non-hydrogen-bonded CH groups

In the present review, the parameters governing the frequencies in hydrogen-bonded CH and in non-bonded CH groups are discussed. The red-shifts in CH groups bonded to anions and the blue shifts in hydrogen bonds involving hydrogen peroxide depend both on the changes in occupation of the σ∗(CH) 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 CH groups, suggesting their similar nature. In contrast, blue shifts of CH groups are mainly determined by a decrease in hyperconjugation, leading to a decrease in the occupation of the σ∗(CH) orbitals.