EXPERIMENTAL AND THEORETICAL-STUDY OF LI+ AFFINITIES OF METHYLDIAZOLES

The gas-phase Li+ affinities of a wide set of methyldiazoles were obtained by ion cyclotron resonance techniques. Simultaneously, Hartree-Fock calculations at the 3-21G level have been performed to investigate the structure and stability of these Li+ complexes. A topological analysis of the Laplacian of the electronic charge density reveals that the nature of the N-Li linkage is markedly ionic and therefore noticeably different from the N-H linkage in the corresponding protonated species. However, both experimental and theoretical results show that methyl substituent effects on Li+ binding energies are practically additive. A reasonably good linear relationship between the free energies of adduct formation for Li+ vs H+ is found for this set of compounds, with the slope of this correlation being about 2, i.e., similar to that found for unsubstituted azoles. The basicity trends along this family of compounds can be easily rationalized in terms of ion-dipole and polarization interactions. A quantitative analysis of the contributions of these two terms is offered. © 1990 American Chemical Society.