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Conformers of β-Vinylethanol:  Internal Hydrogen Bonding to π-Bonded System

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journal contribution
posted on 1999-04-24, 00:00 authored by Jonathan G. Crofts, Ronald D. Brown, Peter D. Godfrey
In a combined experimental spectroscopic and theoretical ab initio MP2/6-311++G(d,p) study of β-vinylethanol a new (second known) conformer (y) has been detected via heated preexpansion free jet microwave spectroscopy. The new species was found to have rotational constants A = 19928.6325(24), B = 2172.37374(91), C = 2144.28416(76) MHz and electric dipole moment components μa = 0.01284(37), μb = 1.262(11), and μc = 0.486(43) D. Species y has been identified with the theoretically predicted extended conformer CAA, having dihedral angles of ∠CCCC = −113.9°, ∠CCCO = 183.1°, ∠CCOH = 181.5° by comparison of experimental data with the ab initio predicted rotational constants and electric dipole moment components for the seven stable conformers of β-vinylethanol. A similar analysis for the first known conformer (x) originally detected by Marstokk and Møllendal in a previous conventional microwave spectroscopy study identifies this species with the theoretically predicted gauche conformer CMG, having dihedral angles of ∠CCCC = −107.4°, ∠CCCO = 64.3°, ∠CCOH = −49.7°. From an ab initio exploration of the energy barriers of the conformational potential hypersurface it has been shown that the two energetically comparable but undetected gauche conformers CMA and CMM would relax in the jet expansion to the lower energy detected species CMG and that similarly the two undetected extended conformers CAG and CAM of slightly higher energy than CAA would relax to the detected species CAA. Relaxation of the extended conformers to the lower energy gauche CGM is precluded by high intervening energy barriers. The observed energy difference between CAA and CGM is consistent with the stabilizing effect of an internal hydrogen bond in CGM linking the hydroxyl hydrogen to the π-electron cloud of the vinyl group. Bonding appears to be directed approximately along an axis perpendicular to the vinyl plane and passing through the nonterminal sp2-carbon atom.