Molecular Geometry of Benzaldehyde and Salicylaldehyde:  A Gas-Phase Electron Diffraction and ab Initio Molecular Orbital Investigation

The molecular geometries of benzaldehyde and salicylaldehyde have been determined by gas-phase electron diffraction and ab initio molecular orbital calculations at the MP2(FC)/6-31G* level. Several parameter differences from the molecular orbital calculations were incorporated as constraints in the electron diffraction analysis of salicylaldehyde. Some selected bond lengths ( r g ) and angles obtained in the electron diffraction analyses are as follows: benzaldehyde (C−H) mean 1.095 ± 0.005 Å; (C−C) mean (benzene) 1.397 ± 0.003 Å; C 2 −C 7 1.479 ± 0.004 Å; C O 1.212 ± 0.003 Å; C 2 −C 7 O 123.6 ± 0.4°; the benzene ring is undistorted within experimental error; salicylaldehyde (C−H) mean 1.090 ± 0.011 Å; (C−C) mean (benzene) 1.404 ± 0.003 Å; C 1 −C 2 1.418 ± 0.014 Å; C−O 1.362 ± 0.010 Å; O−H 0.985 ± 0.014 Å; C 2 −C 13 1.462 ± 0.011 Å; C O 1.225 ± 0.004 Å; C 2 −C 13 O 123.8 ± 1.2°; C 2 −C 1 −O 120.9 ± 1.1°. All the data are consistent with planar equilibrium structures for both molecules. The barrier to formyl group torsion is estimated to be appreciably higher for salicylaldehyde (at least 30 kJ/mol) than for benzaldehyde (at least 20 kJ/mol). There is intramolecular hydrogen bonding in the salicylaldehyde molecule of comparable strength with that in o -nitrophenol. The hydrogen bond is characterized by the following observed/calculated distances: O···H(−O) 1.74(2)/1.80 Å and O···O 2.65(1)/2.68 Å. The structural changes in the rest of the molecule, as compared with the parent benzaldehyde and phenol molecules, are consistent with resonance-assisted hydrogen bonding similar to the o -nitrophenols. These changes include a lengthening of the C O bond (0.013 Å), a shortening of the exocyclic C−C bond (0.020 Å), a lengthening of the ring C−C bond between the substituents (0.017 Å), and a shortening of the hydroxy C−O bond (0.022 Å).