A Perturbative Model of Frequency Shifts Revisited—Application to Hydrogen, Halogen, Pnicogen, Chalcogen and Tetrel Bonds in Model Binary Complexes

ABSTRACT A computational study of binary hydrogen‐bonded Z 3 C─H…YW/NH 3 (Z = F, Cl, and Br; YW = BF, CO, N 2 , OC, and FB) and σ‐hole bonded F─X(A)…YW/NH 3 (F─X(A) = F─Cl, F─SH, F─PH 2 , and F─CH 3 ; X = Cl, P, S, and C) complexes was undertaken in order to examine and compare the shifts of their respective C─H and F─X harmonic stretching frequencies, as well as the relative IR intensities of these vibrational modes, on complex formation. A perturbation theory model for frequency shifts, derived many years ago, was reformulated such that the frequency shift is partitioned into the sum of two distinct contributions, one dependent on the second derivative of the interaction energy with respect to C─H (F─X) bond length extension, and the other dependent on the C─H (F─X) bond length change. Generally, the shifts predicted by the perturbative model agree well with standard ab initio computations, but at much lower computational cost, for the hydrogen‐bonded halomethanes and the σ‐hole bonded complexes, except for the strongly bound FCl…BF dimer, where the integrity of the constituent monomers appears to have been lost. The relationship between the relative IR intensity and numerically evaluated permanent dipole moment and induced dipole moment derivatives (with respect to C─H (F─X) displacement) was also explored and found to be useful in rationalizing the energetic trends for both the hydrogen‐bonded and σ‐hole bonded complexes.