Theoretical approach to the structure and hyperfine coupling constants of nonrigid radicals: the case of dihydronitrosyl radical

The structure and EPR parameters of dihydronitrosyl radical H2NO were investigated by highly correlated ab-initio methods. The relative stabilities of planar and pyramidal structures were analyzed in detail, taking into account the effect of small-amplitude vibrations perpendicular to the inversion motion. Vibrational averaging of hyperfine coupling consts. was computed by quantum-mech. treatment based on the vibrational adiabatic zero curvature approxn. The general picture emerging from this study, substantiated by several checks, consists in a quasi-planar mol. with a nearly free inversion motion for out-of-plane angles as large as 30°. 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°. 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.