H. D. W. Hill

Fourier Transform Study of NMR Spin–Lattice Relaxation by “Progressive Saturation”

When a nuclear spin system is subjected to a repetitive sequence of strong radiofrequency pulses, a steady state is established where there is a dynamic balance between the effect of the pulses and spin relaxation. Under certain readily satisfied pulse conditions, the deviation of the intensity of the free induction signal from its thermal equilibrium value is an exponential function of the pulse interval with time constant equal to the spin–lattice relaxation time. The determination is unaffected by spin–spin relaxation provided that the interval between pulses is long enough to permit all transverse components of magnetization to be eliminated, and provided precautions are taken to inhibit spin‐echo formation. Through Fourier transformation of the transient response, high resolution spectra with many component resonances may be studied, and the spin–lattice relaxation times of the individual lines determined. The technique lends itself particularly well to repeated accumulation of the transient signal f...

Phase and intensity anomalies in fourier transform NMR

Abstract Carbon-13 Fourier transform NMR spectra are reported which exhibit gross discrepancies in signal amplitude and phase that vary in a cyclic manner as a function of the frequency offset from resonance. These anomalies occur when the interval between pulses is made short in comparison with the spin-spin relaxation times, and they are associated with the establishment of a steady-state response where there are finite macroscopic transverse components of magnetization at the end of the pulse interval. The regular pulse sequence has the property of refocussing the isochromatic magnetization vectors that have been dispersed by field inhomogeneity, giving rise to a spin echo at the time of the next pulse. These echoes and the phase and intensity effects may be masked by short-term instabilities in the field/frequency ratio, or by incoherence introduced by heteronuclear noise decoupling. A simple device is proposed which effectively suppresses these anomalies by introducing a small random delay in the timing of the radiofrequency pulses.

Fourier synthesized excitation of nuclear magnetic resonance with application to homonuclear decoupling and solvent line suppression

A method is described for exciting a nuclear magnetic resonance spectrum with a radio frequency source having any desired frequency spectrum. The frequency spectrum of the source is first specified and then Fourier synthesized to define a function which is used to modulate a radio frequency carrier. The NMR spectrum is obtained by Fourier transforming the response of the spin system to this excitation. The technique is applied to the problem of suppressing a strong solvent line while simultaneously observing all other resonance lines in a spectrum and to homonuclear decoupling while observing a complete spectrum.

Dipolar contribution to NMR spin-lattice relaxation of protons

Dipole‐dipole interactions between protons in a glucopyranose derivative have been studied by means of the nuclear Overhauser effect and by spin‐lattice relaxation experiments. The Overhauser enhancement measurements utilize a new pulse technique which permits the saturating field to have any specified spectral density function so that finite frequency bands in the NMR spectrum can be irradiated. By this method, the dipolar interactions between the various pairs of protons are evaluated. In contrast, the dipolar relaxation of a chosen proton by all other protons in the sample has been determined by comparing the inversion‐recovery rates after selective and nonselective 180° pulses. The relaxation measurements indicate that the relaxation in this molecule is purely dipolar in origin. The Overhauser enhancement measurements of one proton are used to determine the contribution to its relaxation from two neighboring protons and, from these results, information about the geometry of the pyranose ring is evaluated.

High‐Resolution Study of NMR Spin Echoes: “J Spectra”

The spin–spin relaxation of individual lines in a high‐resolution NMR spectrum has been studied by a technique which extracts an essentially “monochromatic” component from the Carr–Purcell spin‐echo response by means of a narrow‐band filter. The resulting selectivity significantly reduces the complexity of the modulation of the spin‐echo amplitude due to homonuclear spin–spin coupling, and further simplification may be achieved by double irradiation experiments. Echo modulation has been used as a precise measure of the 0.051‐Hz long‐range coupling in 3‐bromothiophene‐2‐aldehyde, normally hidden by magnet inhomogeneity. It is proposed that modulation and spin–spin relaxation effects are best analyzed in the general case by calculating the Fourier transformation of the envelope of the spin‐echo peaks. The general term suggested for this new mode of presentation is the “spin‐echo spectrum.” It consists of a set of resonance responses with widths determined by the relevant spin–spin relaxation times, at freue...

An adaptive scheme for measuring NMR spin-lattice relaxation times

Abstract A method is proposed for reducing the time required to determine spin-lattice relaxation times of individual lines in a high resolution NMR spectrum. The steady-state response of the spins to a repetitive sequence of 90° pulses is measured at two different pulse repetition rates, and the relaxation times derived from the ratio of the intensities of a given line in the two Fourier transformed spectra. The technique has been made the basis of an automatic program which adapts the pulse rate parameter in order to optimize the determination of the relaxation times. Applied to the carbon-13 spectrum of cortisone acetate the proposed method gives results in good agreement with those obtained by the widely used inversion-recovery technique. The spin-lattice relaxation times found for the methyl, methylene, methine and quaternary carbon atoms of cortisone acetate are found to fall into four quite distinct ranges.

Utilization of chirp frequency modulation with 180°-phase modulation for heteronuclear spin decoupling

Abstract Heteronuclear decoupling with Chirp modulation in conjunction with 180°-phase alternation is demonstrated to be a more efficient decoupling method than 180°-phase alternation alone. A theory that predicts the results of Chirp modulation alone is developed, and it is used to explain the results of decoupling with Chirp modulation and 180°-phase alternation. This method allows for efficient spin decoupling with as little as 2 W of decoupling power.

Artifacts in quadrature detection

Abstract Two different sampling schemes for quadrature detection are in common use. In the first, data from both channels are sampled simultaneously at regular intervals. In the second, the data are sampled alternately from each channel. While both sampling schemes produce similar results for frequencies below the Nyquist frequency, the results are quite different for frequencies above the Nyquist frequency which are aliased into the chosen spectral width. The effects of aliasing are described for both sampling schemes, in both one- and two-dimensional spectra.

Spinning sidebands in two-dimensional spin-echo spectra

Two-dimensional Fourier transformation of spin echoes produces a spectrum in 2 frequency dimensions which can have enhanced resolving power. In such spectra 2 sets of anomalous spinning sidebands are obsd., having F1 coordinates equal to 1/2 the spinner frequency. A theor. treatment is presented which accounts for this behavior, showing that the anomalous sidebands are a consequence of the 180 Deg refocusing pulse and that they disappear when this pulse is omitted. qConventionalq spinning sidebands are also obsd., corresponding to modulation at the spinning rate in both time dimensions; they have half the intensity of the anomalous sidebands and are inverted in phase with respect to the center-band signal. [on SciFinder (R)]

Real-time multirate FIR digital filters in RF transient spectroscopy applications and their implementation on TMS320-C30

The application of multirate finite-impulse-response (FIR) filtering to radio-frequency (RF) transient spectroscopy is reported. The implementation issues of these multirate filters for sampling rate reduction (decimation) and sampling rate increase (interpolation) in this application are discussed. These implementation issues include the transient response of the digital filters and the aliasing noise due to the decimation process in the passband and the stopband of the digital decimating filters. All implementation measurements reported were performed on a TMS320-C30 DSP (digital signal processor) chip.<<ETX>>