Novel intermolecular C–H ⋯π it interactions: an ab initio and density functional theory study

Abstract

The recent characterisation of short contacts between chloroform solvate molecules and the C–C triple bond of gold ethynides has prompted a theoretical investigation of the strength of C–H ⋯π interactions. Extensive ab initio and density functional theory calculations have been performed on a variety of model systems displaying a T-shaped C–H ⋯π motif. The interaction of ethyne, C₂H₂, with a variety of small proton donor molecules (HCN, CH_{4 –n}Cl_n, n= 0–3) is invariably found to be weak (ΔE_int < 10 kJ mol^–1). Replacement of the two acetylenic protons with more electron-donating sodium atoms increases the electron density in the C–C π bond and results in a substantial increase in the interaction with the proton donor. The calculated interaction energies rise to as much as 60 kJ mol^–1 in the case of C₂Na₂⋯ CHCl₃. The interaction of CHCl₃ with a model gold ethynide, H₃PAuCCAuPH₃, is intermediate between these two extremes, ab initio and density functional calculations both giving estimates of ca. 25 kJ mol^–1 comparable to a reasonably strong hydrogen bond. The unusually strong C–H ⋯π interactions in the gold ethynide arise directly as a consequence of the electron-donating properties of the AuPR₃ fragment and are fundamentally different to the much weaker C–H ⋯π interactions in purely organic systems.