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Issue 29, 2011
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Conformational behaviour, hydrogen bond competition and intramolecular dynamics in vanillin derivatives: acetovanillone and 6-hydroxy-3-methoxyacetophenone

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Abstract

The conformational landscape of the structural isomers acetovanillone (apocynin, AV) and 6-hydroxy-3-methoxyacetophenone (HMAP) has been investigated in a supersonic jet using Fourier transform microwave spectroscopy. Two conformers have been detected in the jet-cooled expansion for each molecule (s-cis and s-trans in AV; s-trans and a-trans for HMAP), differing in the relative orientation of the acetyl and methoxy groups. Both molecules are stabilized by O–H⋯O or O–H⋯O[double bond, length as m-dash]C hydroxyl intramolecular hydrogen bonds, either constraining the local conformations of the methoxy group in AV, or that of the acetyl group in HMAP. Internal rotation splittings have been observed in both conformers of each molecule, originated by the acetyl group, that yield information on the influence of the intramolecular hydrogen bonds on the methyl torsion. The similar internal rotation barriers in both molecules (6.6 and 7.4 kJ mol−1 in AV; 7.3 and 7.0 kJ mol−1 in HMAP) suggest that the acetyl torsion is only slightly affected by intramolecular hydrogen bonding. The absence of torsional tunnellings due to the methoxy group indicates torsional barriers above 10.2 and 8.9 kJ mol−1 for AV conformers, 10.1 and 10.4 kJ mol−1 for HMAP. Conformational ratios and relative free energies have been estimated from relative intensity measurements of the spectral lines. Ab initio (MP2) and density functional calculations using the recent M05-2X empirical functional have been used to aid the experimental work in describing the structures, internal rotation barriers and isomerization potentials.

Graphical abstract: Conformational behaviour, hydrogen bond competition and intramolecular dynamics in vanillin derivatives: acetovanillone and 6-hydroxy-3-methoxyacetophenone

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Publication details

The article was received on 05 Nov 2010, accepted on 03 Jun 2011 and first published on 27 Jun 2011


Article type: Paper
DOI: 10.1039/C0CP02418J
Citation: Phys. Chem. Chem. Phys., 2011,13, 13310-13318

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    Conformational behaviour, hydrogen bond competition and intramolecular dynamics in vanillin derivatives: acetovanillone and 6-hydroxy-3-methoxyacetophenone

    E. J. Cocinero, F. J. Basterretxea, P. Écija, A. Lesarri, J. A. Fernández and F. Castaño, Phys. Chem. Chem. Phys., 2011, 13, 13310
    DOI: 10.1039/C0CP02418J

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