André Grand

Vibrational modulation effects on the hyperfine coupling constants of fluoromethyl radicals

A general quantum‐mechanical protocol for the study of nonrigid free radicals has been applied to the series CH3, CH2F, CHF2, and CF3. 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 quite 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.

STRUCTURE AND ESR FEATURES OF GLYCINE RADICAL IN ITS ZWITTERIONIC FORM

Abstract Extensive post Hartree-Fock calculations are reported for the geometrical structure and hyperfine parameters of the carbon-centered glycine radical in its zwitterionic form. Vibrational averaging effects connected to inversion at the radical center are significant for C α and H α atoms. The good agreement between computed and experimental hyperfine splittings confirms the nature of one of the radicals obtained from irradiation of glycine crystals. On the other hand, the ESR spectrum obtained in aqueous solution cannot be due to the zwitterionic form.

Theoretical investigation of the EPR spectrum of the cyclopropyl radical

Abstract A general quantum-mechanical protocol recently introduced for the theoretical study of non-rigid radicals has been used in the investigation of the structure, inversion rate, and EPR spectrum of the cyclopropyl radical. The results are in close agreement with experiment and can be interpreted in terms of the interplay between the variation of the potential energy and the different properties along the inversion coordinate.

Dependence of the vibrational frequencies and intensities on the configuration of polyacrylonitrile: an ab initio study on model oligomers

Abstract Using the oligomer approach, RHF calculations are carried out to investigate the influence of configurational effects on the IR spectra of polyacrylonitrile (PAN). In a first step, test calculations are performed on propionitrile using the STO-3G, 3-21G, 6-31G ** and 6-31+G * bases, in order to assess the reliability of calculated vibrational frequencies and IR intensities. Then, isotactic and syndiotactic acrylonitrile oligomers (up to hexamers) are investigated at the RHF/3-21G level, in the all-trans conformation. In line with previous experimental work, we find that the frequencies are not significantly affected by the chain configuration, therefore we focus on the intensities. The pattern in the 1400-1200 cm −1 region, corresponding to deformations of the methylene and methine groups, appear as a source of information on the configuration of the chain. However, it is in fair agreement with the experimental spectra only for the syndiotactic case.

Ab initio hartree-fock Raman spectra of polyacrylonitrile

Abstract Because of the recent progress in near-infrared Fourier transform (n.i.r.-FT) Raman spectroscopy, it has become possible to circumvent the fluorescence problem which precludes recording Raman spectra of many important systems, such as some polymers widely used in industry. Being especially concerned with polyacrylonitrile (PAN), we have carried out preliminary ab initio SCF calculations in order to assess how the Raman intensities depend on the molecular structure of this material. The results reported are expected to be useful for future n.i.r.-FTRaman studies of PAN in order to interpret the differences that should be observed between various samples.

ESR features of the bicyclobutyl radical revisited. A counterintuitive ordering of short- and long-range isotropic hyperfine coupling constants

Abstract A quantum mechanical protocol for the study of flexible open-shell systems has been applied in a reinvestigation of the isotropic hyperfine coupling constants of the bicyclobutyl radical. Our computations indicate that the most stable structure of the radical corresponds to the exo-form and is characterized by a strongly pyramidal radical center. The hyperfine coupling constants computed for this form are in good agreement with experiment, but indicate that the accepted assignment of H α and H γ endo protons should be reversed. The energy difference between the exo and endo form is sufficiently large to avoid thermal averaging of observables at reasonable temperatures. Vibrational averaging effects in the potential well corresponding to the exo minimum are significant for the C α atom, but do not alter results for H atoms. A striking outcome of this study is the demonstration that stereo-electronic can lead to the counterintuitive results that long range couplings are larger than α ones.

Vibrational Modulation Effects on EPR Spectra

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.

Uhf Calculations of the g Tensor in Metal-Carbonyles Radicals

An experimental and theoretical study of the transient Fe(CO)⋅/4 radical is presented. This radical, isoelectronic with Co(CO)4, is obtained for the first time in solution, by y-irradiation of dilute Fe(CO)5 at 77 K. The general features of its ESR spectrum are similar to those of Co(CO)4, i.e., a nearly axial g tensor (g∥ = 2.0039, g⊥ = 2.0707), but it was impossible to get further information on 13C couplings. On the basis of the experimental results for Co(CO)4, we present a theoretical study by both Extended Huckel and Xα-methods for Fe(CO)⋅/4 in trigonal pyramidal geometry.