C–H · · · C Hydrogen bonding involving ylides

Abstract

Ab initio calculations on nitrogen and phosphorous ylides, and their complexes with methane and acetylene, are reported. A hydrogen bond strength of 35 kJ mol^–1 is found between H₃N–CH₂ and acetylene at the MP2/6-311++G(3d,3p) level; even with methane as the proton donor some stabilisation (ca. 5 kJ mol^–1) is predicted. Complexes of H₃P–CH₂ are found to be rather less stable (19 kJ mol^–1 with acetylene). Topological analysis of the charge density indicates a fundamental difference in the bonding between H₃N–CH₂ and H₃P–CH₂, with significant double bond character only in the latter. Atoms in molecules decomposition identifies the source of the hydrogen bond strength to be stabilisation of C in the N ylides and stabilisation of P in the P ylides. The effects of methyl and phenyl substituents on the ylidic carbon are investigated: methyl substitution enhances basicity slightly, but phenyl substitution decreases it dramatically. There is some suggestion that this may be a steric as well as an electronic effect.