An ab initio study of the structural and vibrational properties of the C3H6–HCN, C2H4–HCN and C2H2–HCN hydrogen-bonded complexes

Atomic charges from IR intensity parameters: theory, implementation and application

“…Working on the expressions of the APT components as function of ECCF (or EOP), it is possible to extract local charge parameters and in particular to obtain the set of the atomic charges. 1 This task was accomplished in the past for a selected number of molecules and atoms [25,32,35], and it allowed a good description of several intra-and intermolecular chemical/physical properties [25,32,35,[47][48][49][50][51][52][53]. Anyway, severe limitations (such as the intrinsic difficulties related to absolute intensity determination, the presence of several overlapping bands, and the need of a good force field for the calculation of vibrational eigenvectors) hinder the direct determination of APT (or ðoM =oR k Þ 0 ) and thus of ECCF from experimental IR intensities.…”

An ab initio study of the C2H2HF, C2H(CH3) HF and C2(CH3)2HF hydrogen-bonded complexes

“…Thus, this prevents a direct comparison between calculated and experimental intensity ratios. Nowadays, on the other hand, it is well known that this increase in the H X (X = F, Cl, CN, NC and CCH) stretching intensity is due to the charge-flux term [9,10,12], which is strongly affected by complexation, in contrast to what happens with the hydrogen charge. The latter is always positive whereas the charge-flux is slightly negative in the free molecule and becomes quite positive after complexation.…”

“…For example, Andrews et al [7] have performed high-resolution FT-IR matrix isolation studies to identify a hydrogen-bonded complex for C 2 H 2 HX with C 2v symmetry through comparison of the relatively small shifts for the C C stretching frequencies and relatively larger shifts for in-plane fundamental bending frequencies in the acetylene moieties and it was obtained a loss of degeneracy in all observed bending modes. On the other hand, theoretical calculations [8][9][10][11][12][13] have been particularly useful to estimate binding energies, H-bond lengths and structural (electrical and vibrational) changes that take place in the isolated molecules after complexation. Moreover, theoretical calculations have been successful in predicting the new low-frequency vibrational modes, which, in general, show very weak intensities and, therefore, are difficult to characterize experimentally.…”