The potential energy surfaces of protonated N2, P2, and PN are explored at RHF, MP2, and CISD levels. Stationary structures were not only optimized but also characterized by computation of their vibrational frequencies at each of these levels. Model dependencies of potential energy surface characteristics, geometries, vibrational frequencies, relative isomer stabilities, and proton affinities are studied in a systematic fashion. The potential energy surface of HP2+ exhibits dramatic model dependencies and it is clarified by a scan of the potential energy surface as a function of the H-P-P angle at the CISD level. Geometries and vibrational frequencies of the most stable isomers also are reported at the CISD(full)/6-31 IG(dfp) level. Accurate scale factors for bond lengths and vibrational frequencies determined for the neutral diatomics X=X can be applied successfully to the protonated species as well. End-on protonation of N2, edge-on protonation Of P2, and N-protonation of PN are favored, and our best estimates for the proton affinities are 116.3, 161.2, and 194.2 kcal/mol, respectively. Proton affinities parallel the increase of the polarizabilities (N2 < NP < P2) but the polarizabilities perpendicular to the bond axis, alpha(perp), are significantly small than alpha(para) and their ratio alpha(para)/alpha(perp) is not related to the isomer preference energies. For protonated N2 and P2, the isomer preference is closely related to the X2 bond length per se and the structural preferences of the protonated systems thus reflect the same factors that also determine the X=X bond lengths in the neutral diatomics. The proton is atypically electrophilic compared to carbenium ions and the carbenium ion affinities of N2, P2, and PN are generally much smaller than proton affinities. While the potential energy surfaces of protonated and methylated N2 and PN are qualitatively similar, the respective derivatives of P2 differ significantly.
