Competing hydrogen bonding in methoxyphenols: The rotational spectrum of o-vanillin

The conformational preferences of o-vanillin have been investigated in a supersonic jet expansion using Fourier transform microwave (FT-MW) spectroscopy. Three molecular conformations were derived from the rotational spectrum. The two most stable structures are characterized by a moderate O-H center dot center dot center dot O=C hydrogen bond between the aldehyde and the hydroxyl groups, with the methoxy side chain either in plane (global minimum a-cis-trans) or out of plane (a-cis-gauche) with respect to the aromatic ring. In the third conformer the aldehyde group is rotated by ca. 180 degrees, forming a O-H center dot center dot center dot O hydrogen bond between the methoxy and hydroxyl groups (s-trans-trans). Rotational parameters and relative populations are provided for the three conformations, which are compared with the results of ab initio (MP2) and density-functional (B3LYP, M05-2X) theoretical predictions. (C) 2011 Elsevier Inc. All rights reserved.