Diazonium Ions. A Theoretical Study of Pathways to Automerization, Thermodynamic Stabilities, and Topological Electron Density Analysis of the Bonding


Equilibrium geometries and transition-state structures for automerization and thermodynamic stabilities toward loss of N2 and N scrambling are reported for the parent alkyl-, alkenyl-, and alkynyldiazonium ions [RN2]+ (R = methyl (1), vinyl (2), and ethynyl (3)). The automerizations of 1 and 2 involve essentially complete disconnection, rotation, and reconnection of N2, but N scrambling in 3 occurs within a bound ion-molecule complex via a two-step process. The degree of unsaturation affects the binding energies in an unexpected fashion; they increase in the order 2 < 1 < 3. This finding has led to important conclusions regarding the electronic structures of vinyl cations. Topological electron density of the classical vinyl cation shows CC-π-density localization at the CH group and a large positive charge for the CH2 group. This result is supported by structural features of β-disubstituted vinyl cations, and it provides a consistent explanation for the lability of 2, for the higher stability of β-(di)substituted alkenyldiazonium ions, and for the Cβ-SN2t-type chemistry of the latter. Density integration shows small charges ( < +0.16) for the diazo groups in 1-3. Electron density accumulation in the CN-bonding region, strong internal polarization of N2, and radial expansion of the density in the NN-bonding region are common features of the electron density distributions of 1-3. These features are explained with a bonding model invoking synergetic σ-donation from N2 to the positively charged hydrocarbon fragment and π-back-donation of comparable magnitude. Implications are discussed regarding the site of nucleophilic attack, the electronic structures of heterosubstituted diazonium ions, and the possibility of stabilizing P2 in diphosphonium ions. Of theoretical interest are the occurrences of nonnuclear (3,-3) and noncage (3,+3) critical points in CC triple bonds. The crucial role of the curvature λ3 is emphasized as a parameter for the partitioning of multiple bonds with low polarities.



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© 1989 American Chemical Society (ACS), All rights reserved.

Publication Date

01 Nov 1989