Add time:07/11/2019         Source:sciencedirect.com

The equilibrium structures of 1H-, 2H-, and 3H-diazaphospholes and the 2-formyl and 4-amino derivatives of the 2H-isomer have been evaluated by ab initio gradient computation at the Hartree—Fock level. Split valence shell basis sets were used with d functions on the phosphorus atom and with additional tests of the effect of adding polarization functions to the amino nitrogen atom basis. All geometry parameters were fully optimized. The 2H-isomer is found to be most stable, with the 1H- and 3H-forms higher in energy by 7.7 and 45 kcal mol−1, respectively. The 1H- and 2H- isomers are planar with extensive delocalization of the ring π electrons, particularly in the 2H-form. The 3H-isomer has a pyramidal arrangement around the phosphorus atom. The formyl substituent is found to have two stable conformations, both coplanar with the ring, with the more stable form having the carbonyl group eclipsing the PN ring bond. The formyl group produces a marked reduction in electron delocalization in the ring with all of the formally single bonds becoming longer and the formally double bonds becoming shorter. A form with the formyl group perpendicular to the ring is approximately 15 kcal mol−1 higher in energy than the most stable conformation. The amino group has only one minimum-energy conformation with the nitrogen lone pair essentially perpendicular to the ring plane and the amine group out of the plane by an angle similar to that found in aniline. Comparisons are made with available experimental evidence from X-ray and electron diffraction, NMR spectroscopy, and study of reaction products.

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