ANALYSIS OF THE REMARKABLE DIFFERENCE IN THE STABILITIES OF METHYLDIAZONIUM AND ETHYLDIAZONIUM IONS

A remarkable difference of 30.7 kcal/mol has been determined for the dediazoniation enthalpies of methyldiazonium ion (1) (DELTA-H = 42.2 kcal/mol) and ethyldiazonium ion (2) (DELTA-H = 11.5 kcal/mol) at a theoretical level that is shown to well reproduce the experimental methyl cation affinity of N2. Only a small part (5.6 kcal/mol) of this stability difference may be accounted for by the production of the nonclassical ethyl cation in the dissociation of 2. The intrinsic differences in the CN linkages in 1 and 2 are analyzed in a variety of ways. Electron density analysis at correlated levels confirms that diazonium ions are best described as carbocations and that they are inadequately described by the commonly accepted Lewis notations; the overall electron transfer from N2 to the carbocation on formation of the diazonium ions is small. A new method is described for the evaluation of electrostatic contributions to bonding based on atom-centered charges and atomic dipole moments. This method together with an analysis of the binding energies in terms of fragment stabilities reveals the two major reasons for the large difference in the binding energies: the reduction of the electrostatic contribution to CN binding in 2 compared to 1 and the comparatively larger destabilization of the diazo group in 2 with regard to free N2. CN bonding in both diazonium ions results because of stabilization of the hydrocarbon fragment and despite the destabilization of the diazo group. The structural and topological features of the CN linkages in 1 and 2 are similar, and their consideration alone cannot account for the large difference in their dissociation energies. These results emphasize that, in general, the characterization of the electron density in the bonding region alone is not sufficient to fully characterize all of the bond properties.