Page 268 - Strategies and Applications in Quantum Chemistry From Molecular Astrophysics to Molecular Engineer

Vibrational Modulation Effects on EPR Spectra

V. BARONE, A. GRAND, C. MINICHINO and R. SUBRA
Dipartimento di Chimica, Universita Federico II,Via Mezzocannone 4, 80134 Napoli, Italy
SESAM, CEN Grenoble, BP 85X, F-38041 Grenoble, France

1. Introduction

Hyperfine coupling constants provide a direct experimental measure of the distribution of
unpaired spin density in paramagnetic molecules and can serve as a critical benchmark for
electronic wave functions [1,2]. Conversely, given an accurate theoretical model, one can
obtain considerable information on the equilibrium structure of a free radical from the
computed hyperfine coupling constants and from their dependence on temperature. In this
scenario, proper account of vibrational modulation effects is not less important than the use
of a high quality electronic wave function.

Semirigid molecules can be described in terms of normal modes by well known
perturbative treatments [3]. This approach is, however, ill-adapted to treat large amplitude
vibrations, in view of their strong curvilinear character and of poor convergency in the
Taylor expansion of the potential [4]. These situations demand, especially in the case of
lareg (i.e. containing more than four atoms) molecules, some separation between the active
large amplitude motions (LAM) and the "spectator" small amplitude ones. On these
grounds, the influence of vibrational effects on EPR parameters has been studied at the ab-
initio level for a series of radicals [5-14], using different basis sets, correlation expansions,
and treatments of vibrational averaging. In our opinion the key limitation of these
approaches is their lack of generality. In fact, the use of global internal coordinates and of
analytical kinetic energies leads to quite complicated formalisms specific to a reduced class
of systems [12-15], unless oversimplified metrics are used [11,13,14]. We have recently
proposed a general numerical procedure [16] to treat the nuclear motion taking into the
proper account the variation of the reduced mass along any kind of curvilinear LAM. Here
we apply this approach to the radicals CH 3 and CF 3, whose inversion motion is governed
by quite different potential wells. In order to focus attention on general trends, avoiding
specific technical details, we have used a standard polarised basis set (6-311G**) and
treatment of correlation (MP2). The more so as for localized pseudo radicals, this level of
theory appears completely adequate and readily applicable to large systems [17],

2. Methods

All the electronic calculations were performed with the GAUSSIAN/90 [18] and
GAUSSIAN/92 [19] codes and the vibrational studies by the DiNa package [16]. Electronic
wave functions were generated by the Unrestricted Hartree-Fock (UHF) formalism,
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Y. Ellinger and M. Defranceschi (eds.), Strategies and Applications in Quantum Chemistry, 251–260.
© 1996 Kluwer Academic Publishers. Printed in the Netherlands.