Diazonium ions are reactive intermediates in deamination reactions pertinent to chemical carcinogenesis. While methyldiazonium ion has been shown to exist as a short-lived intermediate in the gas phase and in solution, benzyldiazonium ions have never been observed, and the reaction intermediates in deaminations of benzyl systems remain a matter of debate. We therefore studied the benzylcation-dinitrogen interaction by ab initio methods; several important conclusions resulted. Analysis of the potential energy surface at the level MP4(sdtq)/6-31 G*/ MP 2(full)/6-31 G* + ΔVZPE(MP 2(full)/6-31 G*) revealed that a classical "benzyldiazonium ion" does not exist. The interaction of N2 with benzyl cation M-1 results in an electrostatically bound complex 2C with a long C-N distance (2.935 Å) as the most stable structure. A covalently bound planar benzyldiazonium ion 2A with a "normal" C-N bond length (1.514 Å) is the transition-state structure for automerization of the electrostatic complex with concomitant rotation about the exocyclic C-C bond. The potential energy surface characteristics result from the highly efficient π-dative Ph → CH2 bonding in M-1; this is clearly demonstrated in its structure and that of its transition state for rotational automerization TS-1 by the very high activation barrier for rotation (47.6 kcalmol-1!) and by the gradient vector fields of the total electron densities of conformers of 1 and 2. The rotational barrier for 1 is reduced to 27.9 kcalmol-1 in the N2 complex 2, and the potential energy surface characteristics of benzyldiazonium ion essentially facilitate the N2-catalyzed rotational isomerization of benzyl cation. The benzyldiazonium ion system shows for the first time that the interaction of a donor molecule with a carbenium ion with a valence LUMO can lead to the formation of an electrostatic complex as opposed to dative bond formation. Dative σ-bond formation between N2 and the CH2 carbon of 1 is energetically not competitive with dative Ph → CH2 π;-bond formation.