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

Abstract

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 µ a ) 0.01284(37), µ b ) 1.262(11), and µ c ) 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 sp 2 -carbon atom.

Intramolecular OH⋯π interactions in alkenols and alkynols

“…33 Subsequent theoretical investigations suggest that the lowest energy conformer of PA is not stabilized by an OHÁ Á Áp intramolecular hydrogen bond. 34 For AA, there have been gas electron diffraction studies, 35 microwave studies, [35][36][37][38] theoretical investigations, 35,[39][40][41][42][43][44] Nuclear Magnetic Resonance (NMR), 45 Raman, 43 photoelectron 44 and IR spectral studies. 11,36,[41][42][43]45,46 Only some of these previous investigations offer an opinion on the existence of an OHÁ Á Áp intramolecular hydrogen bond in AA.…”

“…34 For AA, there have been gas electron diffraction studies, 35 microwave studies, [35][36][37][38] theoretical investigations, 35,[39][40][41][42][43][44] Nuclear Magnetic Resonance (NMR), 45 Raman, 43 photoelectron 44 and IR spectral studies. 11,36,[41][42][43]45,46 Only some of these previous investigations offer an opinion on the existence of an OHÁ Á Áp intramolecular hydrogen bond in AA. 11,36,[38][39][40][41]45 Oki and Iwamura state that there is an interaction between the OH-group and the p-electrons, but they do not go as far as calling the interaction a hydrogen bonding interaction.…”

“…11,36,[41][42][43]45,46 Only some of these previous investigations offer an opinion on the existence of an OHÁ Á Áp intramolecular hydrogen bond in AA. 11,36,[38][39][40][41]45 Oki and Iwamura state that there is an interaction between the OH-group and the p-electrons, but they do not go as far as calling the interaction a hydrogen bonding interaction. 11 Bacon et al states that it is probable that the OHÁ Á Áp intramolecular hydrogen bond is not the main factor in the conformation of the most stable conformer of AA, as the electron repulsion between the oxygen lone pairs and the p-cloud is considerable.…”

“…Some of the previous studies have been carried out in jet-cooled conditions, which can prevent the observation of higher energy conformers. 36,37,48,49,54 We assign absorption peaks in the experimental spectra using local mode anharmonic oscillator calculations with explicitly correlated coupled cluster potential energy and dipole moment curves. 7,56…”

Detection of a Higher Energy Conformer of 2-Phenylethanol by Millimeter-Wave Spectroscopy

“…The most stable conformers of 1,2-disubstituted ethanes such as PEA (involving the benzene ring), histamine (involving the imidazole ring), and 2-vinylethanol (involving the vinyl group) appear to be stabilized by a nonclassical hydrogen bond. This bond forms via an interaction between a hydrogen in the amino or hydroxyl substituent group and the π-electron cloud of the other substituent.…”