The hydrogen-bond C-H donor and ∏ -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: Csp¹-H...O > C(ring)-H...O Csp²-H...O > Csp³-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∏ (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...∏ (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.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-H···O > C(ring)-H···O Csp-H···O > Csp-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∏ (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···∏ (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.