Quantum-Chemical Study of the Structure and Reactivity of Pyrazol-5-ones and Their Thio and Seleno Analogs: VII. Electronic and Steric Structure of Radicals Derived from 4-Hydroxyimino-3-methyl-4,5-dihydropyrazol-5-ones

Abstract

The structure of iminoxyl radicals formed by electrochemical oxidation of 4-hydroxyimino-3-methyl-4,5-dihydropyrazol-5-ones was characterized by nonempirical (ROHF and UHF with wide-range variation of basis sets), UB3LYP 6-31G*//UB3LYP 6-31G*, and semiempirical calculations (AM1 and PM3 involving annihilation procedures). The radicals were found to exist as two configurational isomers Z and E, the former being more stable by 6.0-7.5 kJ/mol. All heavy atoms in the radicals lie in one plane. The experimental hyperfine coupling constants are reproduced on a quantitative level only in terms of nonempirical calculations with inclusion taken of electron correlation (UB3LYP), while the effect of geometry optimization level is not strong. Both nonempirical and semiempirical calculations based on the unrestricted Hartree-Fock methods reveal qualitative similarity of all the examined radicals.