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 the ory (MP2). At both levels the 6-31G basis set has been used. The esti mation of relative stability of these tautomeric forms depends largely on the calculation level. The total energies of the cyanides calculat ed by the MP2 method are 25-30 kcal mol-(1) lower than those of the co rresponding isocyanides. The oxo-tautomeric forms containing four-coor dinate phosphorus are 15-25 kcal mol(-1) more stable than the three-co ordinate phosphorus aci-derivatives. The internal rotation potential c urves of the aci-forms are characterized by a deep minimum for the tra ns-arrangement of the methoxy group and phosphorus lone electron pair. Two additional less clearly pronounced minima are located symmetrical ly on both sides of the weak maximum, which corresponds to the cis-arr angement. The equilibrium ore-form structures have a staggered configu ration of the methyl group with respect to the phosphorus atom bonds.