Uncommon hydrogen bonds between a non-classical ethyl cation and π hydrocarbons: a preliminary study

This study undertakes a theoretical investigation into uncommon hydrogen bonds between the ethyl cation (C2H5 (+)) and pi hydrocarbons. Firstly, it considers the hyperconjugation effect of the ethyl cation, in which the non-localized hydrogen (H+) is taken to be a pseudoatom bound to the carbons of the methyl groups. The goal of the research is to use this electronic phenomenon to gain a better understanding of the (H+center dot center dot center dot pi) and (H+center dot center dot center dot p-pi) hydrogen bonds, which are considered uncommon because they are formed through the interaction of the H+ of the ethyl cation with the pi bonds of the acetylene (C2H2) and ethene (C2H4), as well as with the pseudo-pi bond of the cyclopropane (C3H6). In view of this, B3LYP/6-311++G(d,p) calculations were used to determine the geometries of the C2H5 (+)center dot center dot center dot C2H2, C2H5 (+)center dot center dot center dot C2H4, and C2H5 (+)center dot center dot center dot C3H6 hydrogen-bonded complexes. Deformations of the bond lengths and bond angles of these systems were analyzed geometrically. Examination of the stretch frequencies and absorption intensities of the (H+center dot center dot center dot pi) and (H+center dot center dot center dot p-pi) hydrogen bonds has revealed red-shifts in pi and p-pi bonds. After structural modeling and vibrational characterization, analysis of the charge transfer following the ChelpG approach and subsequently quantification of the hydrogen bond energies (basis sets superpostition error and zero point vibrational energies being considered) were used to predict the strength of the (H+center dot center dot center dot pi) and (H+center dot center dot center dot p-pi) hydrogen bonds. In addition, the molecular topography was estimated using the quantum theory of atoms in molecules (QTAIM). QTAIM was chosen because of a desire to understand the (H+center dot center dot center dot pi) and (H+center dot center dot center dot p-pi) hydrogen bonds chemically on the basis of the quantity of charge density and interpretation of Laplacian fields.