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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—HO—C) hydrogen bonds. The frequency of formation and geometrical characteristics of these bonds indicate a bond-strength ordering: Csp1—HO > C(ring)—HO ≃ Csp2—HO > Csp3—HO, 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—Hπ(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 Hπ(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.
O (particularly C—HO—C) hydrogen bonds. The frequency of formation and geometrical characteristics of these bonds indicate a bond-strength ordering: Csp1—HO > C(ring)—HO ≃ Csp2—HO > Csp3—HO, 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—Hπ(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 Hπ(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.
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