Jean-Claude Duplan

Scalar relaxation of the second kind for 29Si in SiCl4 and SiHCl3

Abstract In this paper we examine the problem of determining coupling constants from second-kind scalar relaxation time measurements. A simple statistical analysis is proposed in order to point out the influence ofthe cross-correlation coefficient between the effects of two quadrupolar nuclei upon the transverse relaxation of a spin- 1 2 nucleus. The adiabatic linewidths of 29Si in SiCl4 and SiHCl3 are analyzed in order to point out the difficulties of the method in this case.

BROADBAND ANALYTICAL DECOUPLING IN HIGH-RESOLUTION NMR SPECTRA

A data processing method, based on double integral calculations, to obtain decoupled high‐resolution NMR spectra in the weak scalar coupling approximation is described. Data are recorded from conventional spin echo experiments using different values of the delay of the echo. The collected echoes are processed in the time domain in such a way that resonances appear as single lines at their chemical shift frequencies in the resulting spectrum. The method described is a two‐step post‐acquisition treatment. In the first step, broadband decoupling of the spectrum is achieved. The second processing step suppresses the modulation of the amplitude by the scalar coupling J. Finally the spectrum obtained may be presented in the pure absorption mode without J‐modulation. This technique, designed by analytical decoupling, improves the signal‐to‐noise ratio and facilitates the interpretation of complicated spectra. Intensities are not modulated by scalar coupling and they can be employed for quantitative analysis. The method was applied to proton NMR spectra of diethyl ether and 6‐vinyl‐1,3‐dioxane 1,3. It may be also applied to heteronuclear spin systems and an example of a broadband decoupled carbon‐13 NMR spectrum of a mixture of diethyl ether and ethanol is given.

Digital programming of relaxation studies on a cw spectrometer

Abstract We have built a low-cost programmable impulse sequences generator for NMR. The general block diagrams of the logic hardwired program card, the available standard pulse sequences, and the very simple operating mode are presented. This programmer has been designed for audiofrequency pulses on a cw spectrometer and permits accumulative data for low-intensity high-resolution selective relaxation studies.

Overhauser effect on saturated lines of an AX spins system: 13C−1H in chloroform

When saturating fields are successively applied at resonance frequencies of an AX spins system, the populations of the energy levels change towards new equilibrium values. The transient decay of a saturated line is used, in the case of chloroform, to obtain information about the relaxation of carbon‐13.

Measurements of Long‐Range Homonuclear Coupling Constants

Coupling constants in proton systems provide access to useful structural information. Several methods have been proposed to measure these constants in high‐resolution spectra, but many of them are not well suited when the coupling constants are comparable to the spectral linewidth. In such a case the measurement of the apparent splitting, obtained from conventional NMR or from two‐dimensional correlation spectroscopy (COSY), can cause a miscalculation of the true coupling constant value. In this work, data processing for extracting small coupling constants is described. Signals are obtained from spin‐echo experiments and analysed in the time domain in such a way that couplings are apparently multiplied by n+1, where n is positive. Small coupling constants in 4‐methyl‐1,3‐dioxane were obtained by this method.

Pyrrolidine conformation: A low-temperature dielectric relaxation study

Abstract The 4.2 K dielectric relaxation rates of pyrrolidine, several of its derivatives and other secondary amines absorbed in polyethylene and polypropylene are compared and it is concluded that pyrrolidine prefers the C s (“envelope”) conformation in which the CNC bond angle is smallest and the nitrogen inversion barrier highest, 3-methyl-3-propylpyrrolidine also prefers this conformation, though less markedly. Incorporation into a bicyclic structure can cause the pyrrolidine ring to be distorted beyond the range of forms observed in the monocyclic species.