C-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-C HYDROGEN-BONDING INVOLVING YLIDES

Ab initio calculations on nitrogen and phosphorous ylides, and their c omplexes with methane and acetylene, are reported, A hydrogen bond str ength of 35 kJ mol(-1) is found between H3N-CH2 and acetylene at the M P2/6-311 ++ G(3d,3p) level; even with methane as the proton donor some stabilisation (ca. 5 kJ mol(-1)) is predicted, Complexes of H3P-CH2 a re found to be rather less stable (19 kJ mol(-1) with acetylene). Topo logical analysis of the charge density indicates a fundamental differe nce in the bonding between H3N-CH2 and H3P-CH2, with significant doubl e bond character only in the latter, Atoms in molecules decomposition identifies the source of the hydrogen bond strength to be stabilisatio n of C in the N ylides and stabilisation of P in the P ylides. The eff ects of methyl and phenyl substituents on the ylidic carbon are invest igated: methyl substitution enhances basicity slightly, but phenyl sub stitution decreases it dramatically. There is some suggestion that thi s may be a steric as well as an electronic effect.