THEORETICAL INVESTIGATION OF THE TORSIONAL POTENTIAL OF 2,2'-BIPYRIMIDINE

The conformational behaviour of 2,2'-bipyrimidine has been studied by means of ab initio computations making use of basis sets of increasing flexibility. Rigid and flexible geometry computations give similar torsional potentials, but results in close agreement with electron diffraction data are obtained only with a large split-valence basis set augmented by polarization functions on nitrogen atoms. A comparison with the conformational behaviour of biphenyl indicates that replacement of ortho-CH groups by nitrogen reduces at the same time conjugative (attractive) and non-bonded (repulsive) interactions between the two rings, leading to a similar equilibrium conformation.