Investigation of NQR Parameters on the Tetrazole-Azide Tautomeric Equilibria: A DFT Study

• Published in: Journal of the Mexican Chemical Society 2006-12, Vol.50 (4), p.143-148
• Main Authors: Monajjemi, Majid, Honarparvar, Bahareh, Monajemi, Hadieh
• Format: Article
• Language: Portuguese
• Subjects: Chemistry, Multidisciplinary
• ISSN: 1870-249X

A theoretical study was undertaken to reinvestigate whether NQR parameters such as quadrupole coupling constant, asymmetry factor, and NQR frequencies obtained from electric field gradient tensor can provide information to infer about the substituent effect on tautomeric forms of tetrazole-azide equilibria in a gas phase. For this purpose, 14N-NQR parameters of 5-R -tetrazolo [1, 5-a] pyridines and its corresponding tautomer, on the tetrazole-azide isomerization, were calculated at the B3LYP/6-31++G** level of theory and compared with each other, where R= H, CH3, OH, Cl, OCH3, NO2. The results reveal that NQR parameters of nitrogen atoms of these compounds are highly sensitive to substituent effects. The influence of these groups on the transition state (TS) was also studied. Moreover, according to some plotted graphs reasonable relationship has been found between the values of the Hammett constant (σp) of mentioned substituents and their 14N-NQR parameter data. 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 , relative to the unsubstituted compound. The TS has a close resemblance to the tetrazole structure. Also, the energy differences between tautomeric forms, the graphs of σp versus energy values, and the graph of Hammett-constants versus Eact as well as the graph of rN3-N4 versus Eact have been analyzed. Finally, we have focused on some critical points in the plotted graphs of both E±Q (J) versus v±Q (MHz) and E±Q (J) versus η.