J.B. Robert

EPR spectroscopy at very high field

Abstract A very high field EPR spectrometer, which can operate at magnetic fields up to 19 T, has been constructed. The first EPR spectra of a nitroxide free radical in solution are reported. A discussion of the applications of high frequency EPR (corresponding to the far infrared) is presented.

Possible observation of parity nonconservation by high-resolution NMR

In achiral conditions, a difference in the NMR spectra of the d, l forms of a chiral molecule should be a manifestation of the never observed parity nonconservation in molecules. The involved NMR parameters depend on the never measured nuclear-spin-dependent PNC potential. Calculations performed with a relativistically parameterized extended Huckel method show a difference of a few mHz in the metal resonance frequency of enantiomers, containing Pt or Pb. A discussion of the various possible limiting factors shows that such a difference could be observed.

Calculation of the 1J(PP) angular dependence in P2H4

Abstract Using the results obtained from an MO SCF ab initio calculation on P 2 H 4 in four different conformations, the 1 J (PP) NMR coupling constants have been calculated. The 1 J (PP) values are highly dependent upon the rotational angle φ (−238.0 Hz in the eclipsed conformation, 10.9 Hz in the staggered one). The shape of the theoretical 1 J (PP) plot seems to be in good agreement with experimentally measured 1 J (PP) values.

Structure, 31P and 13C chemical shift anisotrophy of (CH3)3P, (CH3)3PO, (CH3)3PS and (CH3)3PSe as determined by liquid crystal NMR spectroscopy

Abstract The 13 P and 13 C spectra of the triply 13 C labelled molecules (CH 3 ) 3 P, (CH 3 ) 3 PO, (CH 3 ) 3 PS and (CH 3 ) 3 PSe oriented in a nematic phase are reported. The Cue5f8Pue5f8C bond angles have been measured. The 13 P chemical shift tensor shows a large anisotropy except in the case of (CH 3 ) 3 P. The abnormal large value observed for the Pue5fbSe bond length suggests a large anisotropy of the 1 J (Pue5f8Se) spin coupling.

31P Solid state NMR geometrical effects on the chemical shift tensor

Abstract The principal components of the 31 P chemical shift tensor σ of four cyclic organophosphorus compounds of different size, where the phosphorus atoms have the same chemical environment are reported from solid state NMR studies. The σ tensors show a large anisotropy. The asymmetry parameter η shows a linear variation as a function of the intracyclic bond angle around the phosphorus.

Parity non conservation: NMR parameters in chiral molecules

The formula giving the parity-violating electroweak interaction contribution to the magnetic shielding tensor has been applied to enantiomers containing thallium. Using an extended Hückel relativistically parameterized method a chemical shift difference close to 1 mHz at 11.7 T is calculated in two couples of enantiomers. Such a difference is slightly lower than the extreme line width which may be actually reached in ultra-high resolution NMR.

Molecular Organization or Motional Changes with Temperature in Liquid Quinoline

At a same temperature Tt ~ 290 K, the saturated heat capacity, and the 13C, 14N and 2H NMR spin lattice relaxation times of quinoline show a discontinuity in the slope of their temperature dependence. This is tentatively interpreted as a continuous change in the liquid organization which takes place between Tm (melting temperature) and Tt.

Very high field and multifrequency ESR study of a coal sample

Abstract The ESR spectra of a coal sample have been obtained at very high fields (8.837 T, 12.805 %, 15.715 T). The linewidth values, as compared to those obtained at lower field, allow one to assign numerical values to the g factor dispersion and to the spin-exchange frequency.

13C NMR relaxation time of fullerene as a function of magnetic field

Abstract The 13 C spin—lattice relaxation time of C 60 in carbon disulfide was measured at four different frequencies, 50.288, 75.432, 100.576 and 150.864 MHz. A good linear fit of T −1 as a function of B 2 0 is obtained. The small value of the intercept T −1 1 at B = 0 equal to 3.51 × 10 −4 s −1 shows that the chemical shift anisotropy is the dominant term to the relaxation rate over all the frequency range considered. The correlation time τ c (16 ps) obtained shows that the C 60 motion may be described through the Stokes—Einstein—Debye model.

Changes in the anisotropy of quinoline molecular reorientation in the liquid state as probed by 13C and 14N nuclear magnetic resonance relaxation times

At the same temperature T t (290 K), the derivatives with respect to the temperature of the 13C T 1 spin-lattice relaxation times, of the 14N spin-spin relaxation time and of the saturated specific heat of quinoline show a large variation. The calculations of the rotational diffusion coefficients from the nuclear magnetic resonance data show that the molecular motion is more anisotropic below T t than above. A large change is observed at T t for the energy barrier of the rotation around one of the molecular axes. This may be interpreted as due to a change in molecular organization at T t.