TAUTOMERIC AND CONFORMATIONAL STABILITIES OF METHYLPHOSPHONIC DICYANIDE, METHOXYDICYANOPHOSPHINE, AND THEIR ISOCYANO ANALOGS - AN AB-INITIO CALCULATION

The relative energies and structural parameters of the equilibrium forms and the potential functions of internal rotation of methylphosphonic dicyanide, CH3(=O)(CN)(2), methoxydicyanophosphine, CH3OP(CN)(2), and their isocyano analogs, CH3P(= O)(NC)(2) and CH3OP(NC)(2), have been calculated at the RHF/6-31G* level. The total energy of the more stable oxo forms CH3P(=O)(CN)(2) and CH3P(=O)(NC)(2) are 10-20 kcal mol(-1) lower than the energies of the aci forms CH3OP(CN)(2) and CH3 OP(NC)(2). The relative stabilities of the cyano and isocyano isomers are almost the same in the case of the oxo forms, but for the aci forms the energies of the cyano isomers are 8 kcal mol(-1) lower than those of the isocyano isomers. The potential curves for internal rotation in the aci forms are characterized by a deep minimum corresponding to the trans arrangement of the methyl group and the lone pair of electrons on the phosphorus atom. Two less pronounced minima are symmetrically situated with respect to relative maximum corresponding to the transition cis form. The potential curves of internal rotation in the oxo form possess three minima corresponding to staggered configurations of the methyl group and phosphorus atom bonds. The energy characteristics and geometrical parameters of the studied molecules are compared with known data for similar compounds.