Novel intermolecular C-H pi interactions: An ab initio and density functional theory study

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 ...pi interactions. Extensive ab initio and density functional theory calculations have been performed on a variety of model systems displaying a T-shaped C-H ...pi motif. The interaction of ethyne, C2H2, with a variety of small proton donor molecules (HCN, CH4-nCln, n = 0-3) is invariably found to be weak (Delta E(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 pi 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 ...pi interactions in the gold ethynide arise directly as a consequence of the electron-donating properties of the AuPR(3) fragment and are fundamentally different to the much weaker C-H ...pi interactions in purely organic systems.