WEAK HYDROGEN-BONDING .2. THE HYDROGEN-BONDING NATURE OF SHORT C-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-PI CONTACTS - CRYSTALLOGRAPHIC, SPECTROSCOPIC AND QUANTUM-MECHANICAL STUDIES OF SOME TERMINAL ALKYNES

In a number of crystalline terminal alkynes, the alkyne C-H groups form short intermolecular contacts with other alkyne or with aromatic,moieties. A particularly short C-H ... phenyl contact is found in (+/-)-3-phenylbut-1-yn-3-ol, with an H ... midpoint separation of only 2.51 Angstrom. Crystallographic, spectroscopic:and computational evidence is given to show that these contacts represent hydrogen-bond-type interactions. Quantum chemical calculations indicate hydrogen bond energies in the range 1-2.2 kcal mol(-1). Interconnected contacts such as C=C-H ... C=C-H ... C=C-H exhibit a pronounced cooperativity effect, with energy increases of similar to 0.5 kcal mol(-1) compared with isolated contacts. Based on these results, the role of the terminal alkyne group in cooperative hydrogen bond networks is described as topologically paralleling that of hydroxy groups.