THEORETICAL APPROACH TO THE STRUCTURE AND HYPERFINE COUPLING-CONSTANTS OF NONRIGID RADICALS - THE CASE OF H2NO

The structure and EPR parameters of dihydronitrosyl radical H2NO have been investigated by highly correlated ab-initio methods. The relative stabilities of planar and pyramidal structures have been analyzed in detail, taking also into account the effect of small-amplitude vibrations perpendicular to the inversion motion. Vibrational averaging of hyperfine coupling constants has been computed by a quantum-mechanical treatment based on the vibrational adiabatic zero curvature approximation. The general picture emerging from this study, substantiated by several checks, consists in a quasi-planar molecule with a nearly free inversion motion for out-of-plane angles as large as 30-degrees. Due to compensation of different terms, vibrational averaging gives results very close to those obtained from a static treatment at an out-of-plane angle of about 20-degrees. An equally important outcome of this work is the introduction of a general and reliable ab-initio strategy for the study of magnetic properties in nonrigid radicals.