A THEORETICAL AND EXPERIMENTAL-STUDY OF THE INTRINSIC BASICITIES OF METHYLDIAZOLES

The gas-phase basicities of pyrazole, all possible methylpyrazoles (13 derivatives), imidazole, and a selected set of methylimidazoles (8 derivatives) have been determined by ICR. These experimental values and literature data on methylpyrroles and methylpyridines are discussed with the aid of ab initio calculations. STO-3G fully optimized geometries of neutral molecules and their corresponding cations are found to be necessary for proper quantitative evaluation of the effects of the methyl substituents. An examination of the 4-31G//STO-3G energies shows that azolium ions are more sensitive than azoles to substituent effects. Of the four available positions for methyl substitution, those a to the basic center shown an extra stabilization due to methyl hydrogen-nitrogen lone-pair interactions. When the substituent is at position 1 (TV-methyl derivatives) the effect is quite different, due to a partial loss of hyperconjugation. As predicted earlier from preliminary theoretical calculations, the corresponding gas-phase and aqueous basicities are linearly related within four separate families (that is, for N-H and V-CH3 pyrazoles and imidazoles). The aqueous solution attenuation factors are fixed (4.1), having a value similar to that for methylpyridines (3.5). Tautomerism of 3(5)-methylpyrazoles and 4(5)-methylimidazoles is discussed by use of theoretical and experimental values. The proton affinities of pyrazole, imidazole, and their methyl derivatives cover a 25 kcal mol-1 2range (from 212.7 to 237.6). The latter value is for 1,2,4,5-tetramethylimidazole, which is only 5 kcal mol-1 less basic than 1,8-bis(dimethylamino)naphthalene (proton sponge). This illustrates the utility of polyalkyl substitution to obtain very strong gas-phase basicity. © 1990, American Chemical Society. All rights reserved.