Theoretical analysis and experimental study of the spatial structure and isomerism of acetone azine and its cyclization to 3,5,5-trimethyl-4,5-dihydro-1H-pyrazole

An ab initio (RBF/6-31G* and MP2/6-31G*) study has been undertaken to investigate the spatial structure and relative stability of the acyclic isomers of acetone azine (acetone N-(1-methylethylidene)hydrazone (CH3)(2)C=N-N=C(CH3)(2)), differing in the position of multiple bonds, and of the cyclic isomer 3,5,5-trimethyl-4,5-dihydro-1H-pyrazole. In the series of structures under study, the latter possesses the greatest thermodynamic stability so that formation of acyclic isomers such as 1-isopropenyl-2-isopropyldiazene, acetone N-isopropenylhydrazone, and 1,2-diisopropenylhydrazine from acetone azine is thermodynamically unfavorable. For each structure, stable conformations have been found, and the internal rotation barriers of molecules relative to the N-N, C-N, and C-C single bonds have been estimated. The noncoplanar gosh conformer of acetone azine was found to possess the greatest thermodynamic stability. The acetone azine molecule is considered in comparison with model hydrazine, butadiene, and vinylamine molecules. Acetone azine is conformationally preferable not only because of steric factors, but also due to the interaction of the lone electron pair (LEP) of nitrogen atoms with adjacent N=C multiple bonds.