The hydrogen-bond C–H donor and π-acceptor characteristics of three-membered rings

Abstract

Crystallographic results, retrieved from the Cambridge Structural Database, show that the C--H protons of cyclopropane, aziridine and oxirane form C—H...O (particularly C—H...O—C) hydrogen bonds. The frequency of formation and geometrical characteristics of these bonds indicate a bond-strength ordering: Csp 1—H...O > C(ring)—H...O ≃ Csp 2—H...O > Csp 3—H...O, which is in excellent agreement with the well known ethylenic properties of C(ring)—H and with residual δ+ charges calculated for these systems. There is some evidence to suggest that C=C—H in cyclopropene, known to be a highly acidic H, forms stronger hydrogen bonds than C—H in saturated three-membered rings. Crystallographic data have also been used to provide geometrical evidence for the formation of O,N—H...π(ring) bonding to three-membered rings, proposed on the basis of spectroscopic data [Joris, Schleyer & Gleiter (1968). J. Am. Chem. Soc. 90, 327–336]. The two modes of H...π(ring) binding suggested there, viz. `edge-on' approach of H to a ring C—C bond and `face-on' approach towards the ring centroid, are found to be dominant in crystallographic observations of this novel hydrogen bond.