MOLECULAR-ORBITAL STUDY OF THE CONFORMATIONAL CONSEQUENCES OF STABILIZING AND DESTABILIZING ORBITAL INTERACTIONS IN HYDRAZINE, DIPHOSPHINE, AND AMINOPHOSPHINE

Ab initio SCF-MO calculations have been performed at the 4-31G level for the rotational and pyramidal inversional processes in hydrazine, diphosphine, and aminophosphine. Extensive geometry optimization has been carried out in each case. Combination of the present results with related data from the literature leads to the following conclusions: (1) only the gauche conformation is a minimum on the rotational curve of hydrazine; (2) both gauche and anti conformations are minima on the rotational curve of diphosphine; (3) the dihedral angle in gauche diphosphine is 76° (4) the pyramidal inversion barrier of hydrazine is slightly higher than that of ammonia; (5) the pyramidal inversion barrier of diphosphine is significantly lower than that of phosphine; (6) the “bisected” conformation is the transition state for pyramidal inversion in both hydrazine and diphosphine, but it is the ground state of aminophosphine; (7) nitrogen adopts a pyramidal configuration in the rotational transition state of aminophosphine; (8) the nitrogen inversion barrier is lower in aminophosphine than in ammonia; (9) the phosphorus inversion barrier is higher in aminophosphine than in phosphine. All of these observations are examined by a quantitative perturbational molecular orbital (PMO) analysis of the 4-31G wave functions, which takes into account both the stabilizing two-orbital two-electron and the destabilizing two-orbital four-electron orbital interactions between two AH2fragments or between an AH2and a BH2fragment. Useful insights into the factors responsible for the static and dynamic stereochemical properties of these molecules have thus been achieved. In particular, it has been possible to define, more clearly than in our previous work, the interrelationships between the PMO methodology and more classical concepts such as steric effects, dipoledipole interactions, and electronegativity effects. © 1979, American Chemical Society. All rights reserved.