SPECTROSCOPIC AND STRUCTURAL EVIDENCE FOR THE HYDROGEN-BOND NATURE OFC-C-H...C=C CONTACTS IN ETHYNYL STEROIDS

Structural and IR-spectroscopic evidence is given that directed contac ts from terminal alkynes to C=C double bonds possess the essential cha racteristics of weak hydrogen bonds. The contacts are directed at the center of the a-bond rather than at one of the individual C-atoms. The contact distances from H to the center of the C=C bond are typically 2.8 Angstrom, with the shortest distances being similar to 2.5 Angstro m. The interaction is of pronounced long-range nature and can be detec ted in the infrared spectrum even with a long H...pi distance of almos t 3.0 Angstrom. The sample used is mainly composed of ethynyl steroids belonging to the progestine family. (C) 1998 Elsevier Science B.V.