Incipient nucleophilic attack as a probe for the electronic structure of diazonium ions. An analysis of neighboring-group interactions in .beta.-(carboxyvinyl)diazonium ions

Crystal structures of diazonium ions with nucleophilic neighboring groups exhibit distortions that have commonly been interpreted by postulating an ''incipient nucleophilic attack'' of the proximate nucleophile on N(alpha). We have recently challenged the assumption that the charge distribution is correctly represented by the most important Lewis structure R-N+ = N and propose here an alternative explanation of these structural features thereby providing a crucial link between the theoretically derived bonding model and experimental data. The rotamers of 3-diazonium propenoic acid and their zwitterions are examined in this context. The role of the atomic first moments for a correct appreciation of the anisotropy of the electron density distribution within the atomic basins and for an adequate description of electrostatic interaction between neighboring groups is discussed. A method is described for the evaluation of neighboring group interactions based on integrated atomic charges, first moments and quadrupole moments. It is found that the electrostatic interactions of the neighboring groups in the cis isomers correlate with the nucleophilicity of the proximate nucleophile and that the differences in the neighboring group interactions of geometrical isomers correlate with the cis preference energies.