STABILIZATION OF 3-MEMBERED RINGS BY PROTONATION - THE CYCLIC GLOBAL MINIMUM OF HP2O+, THE PROTONATED PHOSPHORUS ANALOG OF NITROUS-OXIDE

Second-row analogues of N2O have been the subjects of several recent studies. Reported here are ab initio theoretical geometries and vibrational frequencies for the N2O analogue P2O. These results were obtained at higher levels of theory than previously reported. We also report ab initio theoretical results for the isomers of protonated P2O. Although the second-row atoms strongly affect the electronic structure and lower the relative energy of the cyclic isomer, the linear isomer remains the global minimum for the P2O molecule. However, it is found that protonation of P2O reverses the relative energies of the cyclic and linear isomers. Protonated (either phosphorus- or oxygen-protonated) cyclic P2O is predicted to be lower in energy than protonated linear P2O by about 11 kcal mol-1. This leads us to postulate cyclic structures for isoelectronic species, including P2CH2 and P2NH, and to quantitatively predict a cyclic structure for P2F+, based on ab initio results.