The length of a CC single bond joining two 1,2-dihydropyridine rings in 6,7,12a,12b-tetrahydrodipyrido[1,2-a:2′,1′-c]pyrazine and its congeners

Abstract

We have carried out full geometry optimizations using the MNDO MO method on conformers of the titled molecule and those of its congeners. It is found that a dl form is more stable than the corresponding meso form by 1.2–5.7kcalmol −1. Further, the CC single bond which joins the two 1,2-dihydropyridine rings is found to be unusually long, especially for the meso conformer of the higher members, ca. 1.60 Å. A perturbational MO analysis reveals that a through-bond interaction is operative between the two non-conjugated C=C-C=C-N π systems via the CC single bond. Besides, the C atoms of the CC single bond are shown to carry large positive charges, and a fairly good linear correlation is found between the CC single bond distance and the magnitude of the net atomic charge. The origin of bond elongation common to the meso and dl conformers of the present molecules is taken as the combined effects due to the throughbond and coulombic interactions.