AB-INITIO STUDY OF MOLECULAR-STRUCTURE AND INTERNAL-ROTATION IN METHYLDICYANOPHOSPHINE AND METHYLDIISOCYANOPHOSPHINE - THE RAMAN-SPECTRUM OF METHYLDICYANOPHOSPHINE AND ITS INTERPRETATION WITH THE USE OF SCALING OF AB-INITIO FORCE-FIELDS

The equilibrium geometric parameters and structures of transition stat es of internal rotation for the molecules of methyldicyanophospine MeP (CN)(2) and its isocyano analog MeP(NC)2 were calculated by the RHF an d MP2 methods with the 6-31G and 6-31G** basis sets. At the MP2 level , the total energy of cyanide is similar to 35 kcal mol(-1) lower than that of isocyanide and the barriers to internal rotation of methyl gr oup for MeP(CN)2 and MeP(NC)2 are 2.2 and 2.7 kcal mol(-1) respectivel y. For both molecules, the one-dimensional ab initio potential functio ns of internal rotation approximated by a truncated Fourier series wer e used to determine the frequencies of torsional transitions by solvin g direct vibrational problems for a non-rigid model. The Raman spectru m of crystalline MeP(CN)2 was recorded in the range 3500-50 cm(-1). Th e vibrational spectra of this compound were interpreted by scaling ab initio force fields calculated by the RHF and MP2 methods. The vibrati onal spectrum of methyldiisocyanophosphine was predicted with the use of the obtained scale factors.