Stationary structures on the potential energy surfaces of fluoro- (1), hydroxy- (2), and aminodiazonium ions (3) and of N2O, HN3, and CH2N2 were determined and characterized at the RHF/6-31G* and the MP2(full)/6-31G* levels, and reaction energies for automerization, dediazoniation, and deprotonation were determined up to full fourth order of Moller-Plesset theory with fully polarized triple valence basis sets. Geometries, IR spectra, relative isomer stabilities, and the activation energies for automerization are discussed. The stabilities of the diazonium ions with regard to dediazoniation and deprotonation were determined as well, and cation nitrogen affinities and proton affinities of the conjugated bases are reported. The electronegativity of the X group is used as the ordering principle, and effects of electron correlation are discussed throughout. The model dependency of relative energies and results of our prior higher level studies of methyldiazonium ion (5) and of available experimental structural, infrared, and energetic data suggest that this theoretical level is appropriate. At the correlated level, the ring structures of 1-3 are local minima and intermediates in the two-step automerizations of the more stable open structures. Electron correlation affects the potential energy surface of 1 drastically by changing the mode of automerization and that of 3 in a subtle but significant manner by changing the symmetry of the automerization pathway. Moreover, the C2-upsilon symmetric open structure of 3 is found to be a transition-state structure for inversion of the most stable minimum of 3 with its pyramidal NH2 group and overall C(s) symmetry, suggesting that H2N-N pi-bonding is less important than had previously been thought. In general, increasing X electronegativity in XNN+ stabilizes the open structures more than the bridged structures and the symmetrically bridged structures more than the asymmetrically bridged structures. Thus, relative isomer stabilities, activation energies for isomerization, and cation nitrogen affinities all increase with increasing X electronegativity. The proton affinities of N2O, HN3, and CH2N2 increase with decreasing X electronegativity; 2 is more acidic than 3 and 3 is more acidic than 5. We found, surprisingly, that C2-upsilon CH2N2 is not the minimum at the correlated level, but the inversion transition-state structure of C(s) CH2N2 with its slightly pyramidalized CH2 group and consequences are discussed. Basic concepts of orbital correlation, molecular graphs, and other topological properties show that the ring bonding is dominated by the a-symmetric interaction between X and N2. Graphical analysis of the functions DELTA-rho = rho(MP2) - rho(RHF) show a general tendency of electron correlation to change T-shaped molecular graphs into ring graphs but not as the result of increased b-type interactions. This type of bonding together with MO theoretical perturbation analysis explains the unexpected stereochemistry of the ring-opening reaction.
