Ab initio investigation of tautomeric stability, molecular structure, and internal rotation of methylphosphonic dicyanide, methoxydicyanophosphine, and their isocyano analogs

The equilibrium geometric parameters and structures of the transition states of internal rotation for MeP(O)(CN)(2), MeOP(CN)(2), and their isocyano analogs, MeP(O)(NC)(2) and MeOP(NC)(2), have been calculated by the ab initio SCF method and with inclusion of electron correlation effects according to the second-order Moller-Plesset perturbation theory (MP2). At both levels the 6-31G* basis set has been used. The estimation of relative stability of these tautomeric forms depends largely on the calculation level. The total energies of the cyanides calculated by the MP2 method are 25-30 kcal mol-(1) lower than those of the corresponding isocyanides. The oxo-tautomeric forms containing four-coordinate phosphorus are 15-25 kcal mol(-1) more stable than the three-coordinate phosphorus aci-derivatives. The internal rotation potential curves of the aci-forms are characterized by a deep minimum for the trans-arrangement of the methoxy group and phosphorus lone electron pair. Two additional less clearly pronounced minima are located symmetrically on both sides of the weak maximum, which corresponds to the cis-arrangement. The equilibrium ore-form structures have a staggered configuration of the methyl group with respect to the phosphorus atom bonds.