Ab initio calculations of the structural, energetic and vibrational properties of some hydrogen bonded and van der Waals dimers .3. The formaldehyde dimer

The structures, energies and atomic charges of five dimers of formaldehyde have been examined by means of ab initio molecular orbital theory, using the Gaussian-90 computer program at the second-order level of Moller-Plesset perturbation theory and the 6-31G split valence Gaussian basis set, augmented with polarization and diffuse functions (6-31++G**). These dimers are distinguished by the relative orientations of the monomer planes (coplanar, parallel or perpendicular) and dipoles (collinear, parallel or antiparallel). Three of the dimers were found to be genuine minima on the potential energy surface. These are a C-s structure with its dipoles antiparallel and its molecular planes perpendicular; a coplanar species of C-2h symmetry with antiparallel monomer dipoles; and a C-2v adduct with the dipoles collinear and the molecular planes perpendicular. The infrared spectra (wavenumbers and intensities) of these three dimers were computed, and the wavenumber shifts and intensity changes on dimerization were derived. The perturbations of these spectroscopic properties are compared with those observed for formaldehyde isolated in argon and nitrogen matrices at cryogenic temperatures, and the most probable structure is deduced in the light of all the available spectroscopic evidence. (C) 1997 Elsevier Science B.V.