Structure, energy, vibrational spectrum, and Bader's analysis of π•••H hydrogen bonds and H-δ•••H+δ dihydrogen bonds

In this paper, the intermolecular structural study asserted by the vibrational analysis in the stretch frequencies of hydrogen bonds (pi center dot center dot center dot H) and dihydrogen bonds (H-delta center dot center dot center dot H+delta) have definitively been revisited by means of calculations carried out by Density Functional Theory (DFT) and topological parameters derived from the classic treatise of the Quantum Theory of Atoms in Molecules (QTAIM). As a matter of fact the pi center dot center dot center dot H hydrogen bond is formed between the hydrofluoric acid and the C C bond of the acetylene, but the QTAIM calculations revealed a distortion in this interaction due to the formation of the ternary complex C2H2 center dot center dot center dot 2(HF). Although the pi bonds of ethylene (C2H4), propylene (C2H3(CH3)), and t-butylene (C2H2(CH3)(2)) are considered proton acceptors, two hydrogen-bond types-pi center dot center dot center dot H and C center dot center dot center dot H-can be observed. Over and above the analysis of the pi hydrogen bonds, theoretical arguments also were used to discuss the red-shifts in the stretch frequencies of the binary dihydrogen complexes formed by BeH2 center dot center dot center dot HX with X = F, Cl, CN, and CCH. Although a vibrational blue-shift in the stretch frequency of the H-C bond of HCF3 due to the formation of the BeH2 center dot center dot center dot HCF3 dihydrogen complex was obtained, unmistakable red-shifts were detected in LiH center dot center dot center dot HCF3, MgH2 center dot center dot center dot HCF3, and NaH center dot center dot center dot HCF3. Moreover, the alkali-halogen bonds were identified in relation to the formation of the trimolecular systems NaH center dot center dot center dot 2(HCF3) and NaH center dot center dot center dot 2(HCCl3). At last, theoretical calculations and QTAIM molecular integrations were used to study a novel class of dihydrogen-bonded complexes (mC(2)H(5)(+)center dot center dot center dot nMgH(2) with m = 1 or 2 and n = 1 or 2) based in the insight that MgH2 can bind with the non-localized hydrogen H+delta of the ethyl cation (C2H5+). In an overview, QTAIM calculations were applied to evaluate the molecular topography, charge density, as well as to interpret the shifted frequencies either to red or blue caused by the formation of the hydrogen bonds and dihydrogen bonds.