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 formalde hyde 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 Gaussia n basis set, augmented with polarization and diffuse functions (6-31+G*). These dimers are distinguished by the relative orientations of t he monomer planes (coplanar, parallel or perpendicular) and dipoles (c ollinear, parallel or antiparallel). Three of the dimers were found to be genuine minima on the potential energy surface. These are a C-s st ructure with its dipoles antiparallel and its molecular planes perpend icular; a coplanar species of C-2h symmetry with antiparallel monomer dipoles; and a C-2v adduct with the dipoles collinear and the molecula r planes perpendicular. The infrared spectra (wavenumbers and intensit ies) of these three dimers were computed, and the wavenumber shifts an d intensity changes on dimerization were derived. The perturbations of these spectroscopic properties are compared with those observed for f ormaldehyde isolated in argon and nitrogen matrices at cryogenic tempe ratures, and the most probable structure is deduced in the light of al l the available spectroscopic evidence. (C) 1997 Elsevier Science B.V.