DIDEHYDROPYRIDINES (PYRIDYNES) - AN ABINITIO STUDY

We report the first ab initio studies on the six DHP (didehydropyridine) isomers at the RHF, ROHF, and GVB levels, obtained with a 3-21G basis set. The geometries of all DHPs are fully optimized at each level of theory to determine their relative stability sequence as 2,4-(S) > 2,3-(S) almost-equal-to 2,4-(S) > 3,5-(S) almost-equal-to 2.6-(T) almost-equal-to 2,5-(S), where (S) and (T) represent singlet and triplet, respectively. At the RHF level the hexagonal structures of the 2,n-DHPs (n = 3, 4, 5, 6) are severely distorted (for n = 3, 4, 6) or broken (for n = 5) by strong N1-C2 bonding, facilitated through a delocalized MO between the nitrogen lone pair orbital and its adjacent radical (C2) lobe orbital. At the GVB level the limited electron correlation between the two radical electrons lowers the RHF energies of the DHPs by 30-48 kcal/mol and the optimized geometries are much less strained hexagonal structures because the two radical electrons and the nitrogen lone pair tend to be confined to different regions. Only 2,6-DHP has a triplet ground state; it may be explained in terms of hindered through-space interaction by the nitrogen lone pair orbital, which stands between the two radical centers.