Vibrational spectra and calculation of the noemal vibrations of 1,3-dimethyl-1,1,3,3-tetrachlorodisiloxane

Abstract

We have recorded the IR spectra of DMTChDS in the gaseous, liquid, and crystalline states and the Raman spectra in the liquid and crystalline states (Fig. I). The frequencies and forms of the normal vibrations of this molecule have been calculated in order to provide a more reliable interpretation of the vibrational spectra. On account of the lack of data concerning the geometry of this molecule, data on hexachlorodisiloxane (HChDS) and hexamethy~disiloxaneo(HMDS) from [2, 3] were employed. The following parameters were used: RSi 0 1.60 A, RSi C 1.88 A, RSiCI 2.01 ~, RCH 1.09 ~, SiOSi 146 ~ The angles at the Si atoms were assumed to be tetrahedral. Retarded internal rotation around the Si--O bonds is possible in DMTChDS as a result of which this molecule can exist in the form of several conformers of C~, Cs, C2~ and C~ v symmetry. In carrying out the calculations it was assumed that one of the SiC1 or SiC bonds of the CH3CI2Si-groups lies on the SiOSi plane in a trans-position with respect to the SiO bond (Fig. 2) althongh in [2] it was concluded that there is some deviation from the planar structure of the trans-chain in HChDS. We carried out calculations for all four conformers using the one and the same force constant matrix which was set up using data on the force fields obtained in [4] for HChDS and H}fDS. No additional refinement of the force field was carried out. The results of the calculations are shown in Table i. The calculation enabled one to assign the experimental frequencies using the calculated forms of the vibrations. There is no doubt concerning the assignment of tNe frequencies of 2990 and 2918 cm -I to the ~(CH)as and ~(CH) s valence vibrations respectively and the frequencies of 1405 and 1270 cm -I to the ~(CH3)as and ~(CH3)s deformation vibrations. The absorption bands in the 760-850 cm -I region are assigned to p(CH3) and, moreover, the vibration with a frequency at around 800 cm -: is very strongly mixed with v(SiO) s. The antisymmetric v(SiO)as vibration appears in the spectrum as a strong broad band the position of which varies somewhat upon passing from the liquid to the crystal (1095 cm -~ in the liquid and 1145 cm -: in the crystal) which may be associated with an increase in the SiOSi angle in the crystal in comparison with this angle in the liquid [5-7]~ Two frequencies, 680 and 645 cm -~, may be assigned to the ~(SiO) s vibration. The first only appears in the IR spectrum as a very weak band which is most probably a combination frequency or an overtone and it is therefore more reasonable to assign the weak band at 645 cm -~ in the IR and Raman spectra to ~(SiO) s. It is partially polarized in the Raman spectrum. The weak, depolarized band in the Raman spectrum at 755 cm -~ is assigned to the v(SiC) valence vibration. The assignment of the valence vibrations of the SIC12 groups, the vibrational frequencies of which are shown in Table i, also does not present any significant difficulty. The assignment of the deformation vibrations of the CH3CI2Si-fragment is considerably more difficult. These vibrations are very strongly mixed with one another and any specific assignments can only be made provisionally (Table i).