INVESTIGATION OF UV SPECTRA OF ISOMERIC NITROPYRAZOLES BY THE SEMIEMPIRICAL AM1 (CI) METHOD

The absorption bands in the UV spectra of isomeric nitropyrazoles were assigned by the calculations in the semiempirical AML (CI) approximat ion. The long-wave absorption of nitropyrazoles is caused by pi-->pi and n(0)-->pi transitions. The charge-transfer band is the most inten se. The pi-->pi transitions undergo a considerable bathochromic shift in the deprotonation. The first ionization potential (PI) of the 4-ni tropyrazole anion was estimated from the empirical dependence of the e nergy of the excited pi-state on PI of alkyl-substituted 4-nitropyrazo les. The PI of the 4-nitropyrazole anion is 3 eV lower than that of a neutral molecule. This is evidence for a substantial destabilization o f the boundary pi-orbital in the heterolytic cleavage of the N-H bond. The analysis of the UV and NMR spectra of 3(5)-nitropyrazole confirms the viewpoint that the 3-nitro tautomer predominates in solution.