A PBE hybrid functional study of blue-shifting and red-shifting hydrogen bonds in pi hydrocarbons

Abstract

This study examines a selected group of π hydrocarbon complexes, represented by C2H4···HCF3, C2H2···HCF3, C2H4···HCF3 and C2H2···HCF3, from a theoretical point of view. From BPBE/6-311++G(d,p) calculations, the geometrical results of these complexes revealed an elongation and shortening of the H—C bond lengths of chloroform (HCCl3) and fluoroform (HCF3), respectively. In terms of the infrared spectrum, the analysis of stretch frequencies revealed that the variations in the H—C modes are essentially recognized as red and blue-shifting modes. For the purposes of understanding the two vibrational phenomena of the π hydrocarbon complexes studied here, PBE/6- 311++G(d,p) calculations were carried out and partitioning of atomic charges derived from the ChelpG algorithm were also used. A theoretical justification of red- and blueshift effects was drawn up using charge-transfer analysis, which is manifested in the π bonds of acetylene and ethylene to chloroform (H—CCl3) and fluoroform (H—CF3), respectively. Finally, a further debate regarding the distinct polarizability power of chloroform and fluoroform is presented, concluding that, in comparison with fluoroform, chloroform possesses the requisite features for conventional proton donors and a redshift is therefore observed in the C2H4···HCCl3 and C2H2···HCCl3 complexes.