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 3 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 chara cteristic infrared spectroscopic features of weak hydrogen bonds, This provides direct evidence that the range of C-H ...pi interactions don ated by sufficiently acidic C-H groups extends beyond van der Waals se paration, In two of the crystal structures, the C-H ...pi interactions form interconnected systems C=C-H ... C=C-H ... C=C-H.