Intramolecular forces: the molecular structure and conformation of 2-cyclopropylpyridine and 4-cyclopropylpyridine

The molecular structure and conformation of 2-cyclopropylpyridine and 4-cyclopropylpyridine have been studied experimentally by gasphase electron diffraction as well as by ab initio quantum chemical calculations. 2-Cyclopropylpyridine appears exclusively in the bisected form, with the cyclopropyl group pointing towards the nitrogen atom. The 4- cyclopropylpyridine molecules are, however, present as a mixture of conformers, dominated by the bisected one (71.0(6.5)%) along with the perpendicular conformer. The energies of the two cyclopropylpyridines are being compared to the energy of cyclopropylbenzene. The presence of the nitrogen atom is generally found to lower the energy of these molecules. When the orbitals of the cyclopropyl group may interact with the nitrogen lone-pair electrons, as in one of the bisected 2-cyclopropylpyridine conformers, this has a profound effect on the stability of the molecule, resulting in a lowering of the energy by ca. 3 kcal molK1. The expected shortening of the distal cyclopropyl C–C bond length is observed in 2-cyclopropylpyridine, while the calculated difference between vicinal and distal C–C bonds had to be assumed in the experimental study of 4-cyclopropylpyridine. q 2004 Elsevier B.V. All rights reserved