Weak hydrogen bonding. Part 2. The hydrogen bonding nature of short C–H ⋯π contacts: crystallographic, spectroscopic and quantum mechanical studies of some terminal alkynes

Abstract

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 A. 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 CC–H ··· CC–H ··· CC–H exhibit a pronounced cooperativity effect, with energy increases of ∼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.