VIBRATIONAL ANALYSES OF THE TETRATHIOSQUARATE ION BASED ON AB-INITIO MOLECULAR-ORBITAL AND DENSITY-FUNCTIONAL CALCULATIONS - EFFECT OF THE JAHN-TELLER DISTORTION IN THE EXCITED ELECTRONIC-STATE ON RAMAN INTENSITIES

The vibrational force field of the tetrathiosquarate ion (C4S42-) is a nalyzed on the basis of the observed infrared and Raman spectra and ab initio molecular orbital (MO) and density functional (DF) calculation s, Definite assignments are made for the main infrared and Raman bands , The intensity pattern in the observed Raman spectrum is reasonably w ell reproduced by the ab initio MO calculations at the third-order Mol ler-Plesset perturbation level and by the DF calculations. Electron co rrelation is seen to have a strong effect on the Raman intensities of bands which have a significant contribution of the CS stretch. The rel ationship between the Raman intensities and the Jahn-Teller distortion in the excited electronic state (the intermediate state in the Raman process) is examined by calculating the optimized structures at the st ationary points (the rectangular, rhombic and 'deformed rhombic' forms ) on the potential energy surface of that state. It is shown that thes e optimized structures an consistent with the character of the non-tot ally symmetric modes, which give rise to strong Raman bands, It is sug gested that the appearance of a band arising from the first overtone o f an out-of-plane mode in the observed pre-resonance Raman spectrum or iginates from the character of the rhombic form as a saddle point for deformation in an out-of-plane direction.