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 Å. 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.