Laser Raman spectra of bis(2,4-pentanedionato)dimethyltin(IV) in oriented single crystals and solutions: the molecular structure in solution

Abstract

In a study of the structure of (CH3)2Sn(acac)2(Hacac = acetylacetone), Raman spectra have been recorded with benzene solutions, non-oriented crystalline samples, and with oriented single crystals. Far infrared spectra also are reported for non-oriented samples. Factor group analyses in C2h5 symmetry are carried out for the skeletal and ligand vibrations. The crystal spectra are assigned in terms of the factor group species and in most cases also in terms of the symmetry species of the molecular modes. The frequencies of the skeletal vibrations are almost identical for the crystal where (CH3)2Sn(acac)2 exists as a centrosymmetric molecule with trans methyl groups and the benzene solution. Intermolecular interactions between the two molecules in the unit cell are very weak as correlation splittings must be less than 1 cm-1. Relative values were determined for the Raman crystal tensor components of the symmetric (SnC2) stretching mode from single crystal intensity measurements. By appropriate transformation, these were used to obtain the relative values of the molecular tensor for this mode. From these data, the depolarization ratio for a non-oriented sample was computed to be p = 0.19 ± 0.02, ps = 0.10 ± 0.02. With the benzene solution, the observed values were 0.18 ± 0.02, 0.11 ± 0.01. The relative values of the components of the derived polarizability tensor are similar to those of (CH3)2SnF2 which also has trans methyl groups. Using the Wolkenstein approximation of bond polarizabilities, it is estimated that the decrease in anisotropy brought about by a trans to cis isomerization upon dissolution should cause a decrease in p of 38 per cent, in marked contrast to the experimental results. Both the frequency and intensity data suggest that (CH3)2Sn(acac)2 has the same trans structure in solution and in the crystal, although the cis structure has been assigned to the molecule in solution on the basis of dipole moment data. Possible origins of the reported orientation dipole moment of 2.95 D for the molecule in benzene solution are discussed. © 1973.