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 ofab initio force fields

The equilibrium geometric parameters and structures of transition states of internal rotation for the molecules of methyldicyanophospine MeP(CN)2 and its isocyano analog MeP(NC)2 were calculated by the RHF and MP2 methods with the 6–31G* and 6–31G** basis sets. At the MP2 level, the total energy of cyanide is −35 kcal mol−1 lower than that of isocyanide and the barriers to internal rotation of methyl group for MeP(CN)2 and MeP(NC)2 are 2.2 and 2.7 kcal mol−1, respectively. For both molecules, the one-dimensionalab initio potential functions of internal rotation approximated by a truncated Fourier series were used to determine the frequencies of torsional transitions by solving direct vibrational problems for a non-rigid model. The Raman spectrum of crystalline MeP(CN)2 was recorded in the range 3500–50 cm−1. The vibrational spectra of this compound were interpreted by scalingab initio force fields calculated by the RHF and MP2 method. The vibrational spectrum of methyldiisocyanophosphine was predicted with the use of the obtained scale factors.