Abstract

MONAJJEMI, Majid; HONARPARVAR, Bahareh  and  MONAJEMI, Hadieh. Investigation of NQR Parameters on the Tetrazole-Azide Tautomeric Equilibria: A DFT Study. J. Mex. Chem. Soc [online]. 2006, vol.50, n.4, pp.143-148. 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 (σ_p) of mentioned substituents and their ¹⁴N-NQR parameter data. In general, it is observed that groups which stabilize the azide isomer lower the E_act of the isomerization reaction, and conversely, those that favor the tetrazole form, raise the E_act , 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 σ_p versus energy values, and the graph of Hammett-constants versus E_act as well as the graph of r_N3-N4 versus E_act 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.

Keywords : Tetrazole; Tautomeric equilibrium; ¹⁴N-NQR parameters; DFT; Hammett Constant.

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