MINDO-SCF-MO-berechnung der cis- trans-isomerisierung von N-methyl- und N-phenylazomethin

Abstract

MINDO/2-SCF-MO calculations for the ground state properties of N-methyl- and N-phenyl-azomethin have been carried out. The calculated rotation barrier for the methyl group in N-methyl-azomethin was 0·8 kcal/mol, the eclipsed conformation being most stable. The calculated rotation barrier about the CN bond in the protonated methylazomethin was 27·9 kcal/mol. MINDO/1-SCF-MO treatment for the N-inversion barrier of the unprotonated species yielded 13·00 kcal/mol. Similar MINDO/2 calculations for N-phenylazomethin yielded 4·0 kcal/mol for the rotation barrier of the phenyl ring around the CN= bond, the perpendicular conformation of the ring to the CNC plane being most stable. For the corresponding N protonated derivative the value 27·3 kcal/mol was calculated for the rotation barrier around the CN bond. MINDO/1 treatment yielded an inversion barrier of 14·0 kcal/mol for N-phenylazomethin. It was found that, the cis-trans isomerization of such azomethin compounds proceeds via an inversion mechanism rather than a CN bond rotation, in agreement with the experimental evidence. A PMO treatment of the phenyl ring's conformation in N-phenylazomethin showed that its perpendicular conformation is within 6·6 kcal/mol more stable than the coplanar structure.