A theoretical study of blue-shifting hydrogen bonds in π weakly bound complexes

Abstract

In this work, a theoretical study about vibrational blue-shifting hydrogen bonds in π weakly bound complexes formed by acetylene (C 2H 2···HCF 3), ethylene (C 2H 4···HCF 3), cyclopropene (C 3H 4···HCF 3), tetrahedrene (C 4H 2···HCF 3), and fluoroform (HCF 3) is presented. In these systems, the formation of the (π···H) interaction occurs through the charge transfer from hydrocarbons to fluoroform via contact between their π bonds and hydrogen atoms, respectively. By taking into account calculations performed at the B3LYP/6-311++G(d,p) level of theory, geometry results indicate a shortening of H–C bond of HCF 3, where in infrared vibrational analysis, this structural observation is known as a blue-shifting stretch mode. In other words, it was observed that the H–C stretch frequency is shifted to upward wavenumbers accompanied by a reduction on the absorption intensity. Energetically, these π complexes are weakly bound because their intermolecular energies are very low, varying from −1.3 kJ mol −1 to −4.7 kJ mol −1. Moreover, a theoretical explanation for blue-shifts on H–C bonds of the fluoroform was presented through the evaluation of the ChelpG atomic charges, by which the quantification of charge transfer was used in order to justify the strengthening on (π···H) hydrogen bond as follows: C 2H 2···HCF 3 > C 2H 4···HCF 3 > C 3H 4···HCF 3 > C 4H 2···HCF 3.