The Raman,Infrared and Proton NMR Spectra and the Structures of Dimethyl Chalcogenides

The vibrational bands of dimethyl chalcogenides (CH₃-X-CH₃) (X = O,S,Se,Te) have been assigned on the basis of C_2V symmetry by most of previous workers.^1–13) On the other hand, some ones have assigned D^3d to these molecules ^14–15) and to structurally related compounds; SiH₃-O-SiH₃ ¹⁶), CH₃-Hg-CH₃ ¹⁷), CF₃-Hg-CF₃ ¹⁸) and Cl₃Si-O-SiCl₃ ¹⁹). The symmetry of ethane(CH₃-CH₃) is obviously D_3d ²⁰⁾, because the vibrational spectra which indicate the lack of coincidence between Raman and infrared frequencies can be explained based upon D_3d symmetry, and the bent c c should theoretically be impossible. The CH₃-X-CH₃ molecules are considered as a derivative in which CH₃ group is connected to CH₃ in opposite direction along the z axis and x atom is inserted between the two CH₃ groups. Consequently, for a linear C-X-C model, the vibrational spectra of CH₃-X-CH₃ ^14,15) should be similar to those of CH₃-CH₃, except being added C-O-C stretching and bending bands. The obtained spectra of CH₃-X-CH₃ ^14,15) show a marked correspndence to that of ethane, and indicate that the rule of mutual exclusion holds between Raman and infrared frequencies. Therefore the symmetry of CH₃-X-CH₃ Seems to be D_3d, But mst of the previom papers ^1–13) have assigned C_2v to CH₃-X-CH₃ as mentioned above.