Evidence for blue-shifting and red-shifting effects in the C2H4···HCF3, C2H3(CH3)···HCF3 and C2H2(CH3)2···HCF3 complexes: π and improper-π hydrogen bonds

A theoretical study of blue and red vibrational shifts on stretching frequencies of the C2H4···HCF3, C2H3(CH3)···HCF3 and C2H2(CH3)2···HCF3 hydrogen-bonded complexes is presented here. The B3LYP/6-311++G(d,p) level of theory was used to determine the optimized geometries of these complexes, for which a detailed analysis of the harmonic spectra and related parameters was performed. The hydrogen bond energies and charge transfer amounts were quantified for the purpose of characterizing the hydrogen bonds. Although the π centers of ethylene (C2H4), propylene (C2H3(CH3)) and t-butylene (C2H2(CH3)2) are proton acceptors, two hydrogen bond types were observed: π···H and C···H. The inherent calculations of the Quantum Theory of Atoms in Molecules (QTAIM) were essential for characterizing the charge density, and, especially, locating Bond Critical Points (BCP) in the CC and C-H bonds of the hydrocarbons and fluoroform (HCF3) respectively, but also in the π···H and C···H hydrogen bonds. Finally, although HCF3 is known to be a typical blue-shift infrared proton donor, this study presents red-shifts ones related to the C-H stretching frequencies.