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

Crystallographic results, retrieved from the Cambridge Structural Database, show that the C-H protons of cyclopropane, aziridine and oxirane form C-H .circle circle 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 similar or equal to 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 delta+ 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 ...pi(ring) bonding to three-membered rings, proposed on the basis of spectroscopic data [Joris, Schleyer & Gleiter (1968). J. Am. Chem. Sec. 90, 327-336]. The two modes of H ...pi(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.