Effect of the 5-Substituent on the Tetrazole–Azide Isomerization in Tetrazolo[1,5-a]pyridines by Ab Initio Calculations

The effect of substituents CH3, OH, Cl, OCH3, NO2 and COOH at the 5-position in tetrazolo[1,5-a]pyridines, on the tetrazole–azide equilibrium, was investigated at the 6-31G∗∗/MP2 level of theory. For both, the parent and 5-CH3 compounds, the tetrazole forms are largely favored, while the other substituents-OH, Cl, OCH3, and NO2-stabilize the azide isomer. With the COOH group, the tetrazole and azide forms are predicted to be in equilibrium in nearly equal amounts. The influence of these groups on the transition state (TS) was also studied. In general it is observed that, groups which stabilize the azide isomer, lower the Eact of the isomerization reaction, and conversely, those that favor the tetrazole form, raise the Eact of this reaction, relative to the unsubstituted compound. The TS has a close resemblance to the tetrazole structure.