Structural studies of mesogen benzalazine derivatives. Part I. Semiempirical MO calculations (CNDO/2,MNDO) on intramolecular H-bond and vicinal interactions

The molecular geometry of benzalazine derivatives was studied by spectroscopic and semiempirical MO (MNDO, CNDO/2) methods. The existence of a strong intramolecular H-bond (7-10 kcal mol-1) was determined by 1H NMR spectroscopy in molecules having a hydroxy group in the ortho position with reference to the diazine central bridge. A study of the H-bonds using the MNDO method proved to be unsatisfactory, whereas the results obtained by the CNDO/2 method were in excellent agreement with experimental values. Both theoretical calculations (CNDO/2) and experimental evidence (UV and 1H NMR) show that the existence of a first H-bond weakened the force of a second in benzalazines. © 1987.