Weak hydrogen bonding .5. Experimental evidence for the long-range nature of C C-H center dot center dot center dot pi interactions: Crystallographic and spectroscopic studies of three terminal alkynes

In the crystal structures of three terminal alkynes, long C=C-H ... C=C contacts with H ... C separations in the range 2.7-3.1 Angstrom are observed, Despite the long distances, these contacts possess the characteristic infrared spectroscopic features of weak hydrogen bonds, This provides direct evidence that the range of C-H ...pi interactions donated by sufficiently acidic C-H groups extends beyond van der Waals separation, In two of the crystal structures, the C-H ...pi interactions form interconnected systems C=C-H ... C=C-H ... C=C-H.