Vibrational modulation effects on the hyperfine coupling constants of fluoromethyl radicals

Abstract

A general quantum-mechanical protocol for the study of nonrigid free radicals has been applied to the series CH 3 , CH 2 F, CHF 2 , and CF 3 . Electronic structures have been computed by highly correlated ab initio methods and vibrational modulation effects have been studied by a nonrigid invertor Hamiltonian. The effect of small amplitude vibrations perpendicular to the inversion motion has been taken into account by an adiabatic model. The results are in close agreement with experiment, and can be interpreted guite straightforwardly in terms of the interplay between the potential energy and the property hypersurfaces. This allows a more dynamically based analysis of hyperfine coupling constants extensible to large, nonrigid radicals.

Variational treatment of the vibrational Hamiltonian for NH3 and H2NO

“…The calculated inversion barrier was 50.1 cm Ϫ1 and the energy minimum occurred at ϭ 22.3°. These numbers are in satisfying agreement with the results of Barone et al 10 Their QCISD(T) results with the 6-311G** basis set gave (interpolating the results from their Table 7): R NH ϭ 1.015 Å, R NO ϭ 1.263 Å, and HNH ϭ 117.6°for C s (E ϭ Ϫ130.855301), and R NH ϭ 1.0128 Å, R NO ϭ 1.262 Å, and HNH ϭ 119.43°(E ϭ Ϫ130.85507) for C 2v . The minimum was at ϭ 22.0°and the inversion barrier was 51.3 cm Ϫ1 .…”

“…This adiabatic approximation has been used in particular by Barone et al 10 in a study of H 2 NO [with ⌬V(s) neglected], to which we will refer later.…”

“…Note that the 271.2 cm Ϫ1 of 1 is in good agreement with the 267 cm Ϫ1 given in ref. 10 for the out-of-plane frequency. It is seen here again that the zero order Hamiltonian approximation is a very good approximation except for the ZPE.…”

Complete structural and magnetic characterization of biological radicals in solution by an integrated quantum mechanical approach: Glycyl radical as a case study

“…34 We have previously shown that the effect of this motion can be effectively evaluated using the distinguished coordinate ͑DC͒ approach, 61,62 in which all the geometrical parameters are optimized at selected value of the out-ofplane displacement of the H ␣ atom. 12,63 The energy profile corresponding to this displacement has been computed by the 6-31ϩG(d,p) basis set, which provides an harmonic frequency of 558 cm Ϫ1 and an anharmonicity constant of 15 cm Ϫ1 . The energy profile and the corresponding vibrational levels are sketched in Fig.…”

Multireference perturbation CI IV. Selection procedure for one-electron properties

“…Full line ® variational results, dashed line ® EN perturbation results, dotted line ® MPB perturbation results. Horizontal full line ®``experimental'' value (28 Gauss[16], see text) The same asFig. 3but with the aimed + property selection procedure.…”