Gareth A Morris

Publications (8)4.28 Total impact

• Article: Selective excitation in Fourier transform nuclear magnetic resonance. 1978.

  Gareth A Morris, Ray Freeman
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  ABSTRACT: The applications of frequency-selective excitation methods in Fourier transform NMR are discussed, and a simple technique is described for selective excitation of a narrow frequency region of a high-resolution NMR spectrum in a Fourier transform spectrometer. A regular sequence of identical radiofrequency pulses of small flip angle exerts a strong cumulative effect on magnetizations close to resonance with the transmitter frequency or one of a set of equally spaced sidebands separated by the pulse repetition rate. All other magnetizations precess through an incomplete number of full rotations between pulses, and are caught by successive pulses at an ever changing phase of their precession, which destroys the cumulative effect. The motion of the various nuclear magnetization vectors may be described pictorially according to the Bloch equations, neglecting relaxation during the pulse sequence. A general theory is presented for selective or "tailored" excitation by an arbitrary modulation of the radiofrequency transmitter signal. It confirms earlier conclusions that the frequency-domain excitation spectrum corresponds to the Fourier transform of the transmitter modulation pattern, provided that the NMR response remains linear. The excitation spectra calculated for the selective pulse sequence by these two alternative approaches show good agreement within their respective limitations. A number of practical applications of selective excitation are explored, including solvent peak suppression, the detection of partial spectra from individual chemical sites, selective studies of relaxation and slow chemical exchange, and holeburning or localized saturation.
  Journal of Magnetic Resonance 12/2011; 213(2):214-43. · 2.14 Impact Factor
• Article: The 'DANTE' experiment.

  Ray Freeman, Gareth A Morris
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  ABSTRACT: The selective excitation scheme known as 'DANTE' emerged from a confluence of several ideas for new NMR experiments, some more fanciful than others. DANTE offers a simple and effective way to restrict excitation to a very narrow frequency band, usually that of a single resonance line. Initially applied to the study of individual proton-coupled carbon-13 spin multiplets, the method has been extended to water presaturation, relaxation measurements, and chemical exchange studies. Through the imposition of a magnetic field gradient it offers a simple method to enhance resolution by restricting the effective volume of the sample. Multiple DANTE excitation (with Hadamard encoding) can speed up multidimensional spectroscopy by orders of magnitude. Applied to magnetic resonance imaging, the DANTE sequence has been used to superimpose a rectangular grid onto a cardiac image, permitting motional distortions to be monitored in real time.
  Journal of Magnetic Resonance 09/2011; 213(2):244-6. · 2.14 Impact Factor
• Source

  Available from: Ray Freeman

  Article: Enhancement of nuclear magnetic resonance signals by polarization transfer

  Gareth A. Morris, Ray Freeman
  04/2002;
• Article: "Tailored detection" of nuclear magnetic resonance signals: application to the assignment of carbon-13 spectra

  Gareth A. Morris, Ray Freeman
  04/2002;
• Article: Proton-coupled carbon-13 J spectra in the presence of strong coupling. II

  Geoffrey Bodenhausen, Ray Freeman, Gareth A Morris, David L Turner
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  ABSTRACT: Proton-coupled carbon-13 NMR spectra have been studied by a technique which involves Fourier transformation of spin echo modulation. When the protons are strongly coupled, the “proton flip” version of this experiment generates a new form of carbon-13 spectrum which is quite distinct from the multiplet structure of the conventional NMR spectrum. The transition frequencies and intensities are exactly symmetrical about the center frequency and some lines have negative intensities. Experimental spectra from the C2 site in pyridine are presented and analyzed by means of an iterative program derived from LAOCOON 111. The resulting proton-carbon coupling constants are in excellent agreement with those obtained by analysis of the conventional spectrum. The new program may also be used to calculate the echo modulation in strongly coupled homonuclear spin systems, and the echo modulation induced in heteronuclear systems by strongly coupled nonresonant spins.
  Journal of Magnetic Resonance (1969).
• Article: NMR spectra of some simple spin systems studied by two-dimensional Fourier transformation of spin echoes

  Geoffrey Bodenhausen, Ray Freeman, Gareth A Morris, David L. Turner
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  ABSTRACT: Phase-modulated spin echoes were generated by a 90-180 Deg pulse sequence applied to some strongly coupled proton spin systems (AB, ABX, and ABC), and the modulation was analyzed by 2-dimensional Fourier transformation. Projection of the resulting 2-dimensional spectra onto the F1 axis leads to a J spectrum, whereas projection onto the F2 axis gives the conventional NMR spectrum. Anal. solns. are presented which correctly predict the shape of the AB and ABX J-spectra, and an iterative numerical anal. were used to fit the ABC case. Projections onto the F1-axis are predicted to have exact symmetry about F1 = 0, and this is borne out by expt., whereas the spin multiplets of the conventional high-resoln. spectrum in general lack symmetry. Certain peaks in the 2-dimensional spectra are predicted to have neg. intensities, although no spin population inversion is involved. Exptl. spectra from AB, ABX, and ABC spin systems illustrate these effects, and strong lines of neg. intensity are detected unequivocally. [on SciFinder (R)]
• Article: A simple approach to single-channel quadrature detection

  Geoffrey Bodenhausen, Ray Freeman, Gareth A Morris, Reinhard Niedermeyer, David L. Turner
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  ABSTRACT: The modification described by A. G. Redfield and S. D. Kunz (1975) for a dual-channel NMR quadrature detection scheme is used in a single-channel spectrometer by stepping the actual ref. phase of the receiver by 90 Deg between adjacent sample points according to an analogous 4-step cycle. The resulting free induction signal is transformed and phase-cor. by the std. software of a conventional single-channel spectrometer. The modification provides 4 selectable phases of 0, 45, 90, and 135 Deg at a frequency of 1.125 MHz by division of the 4.5-MHz master clock frequency with bistable multivibrators. Applications of this scheme are described: (1) the proton-decoupled 13C spectrum of mesitylene; and (2) a 2-dimensional J spectrum of 13C in enriched MeI obtained in the F2 frequency dimension. [on SciFinder (R)]
• Article: A simple pulse sequence for selective excitation in Fourier transform NMR

  Geoffrey Bodenhausen, Ray Freeman, Gareth A Morris
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  ABSTRACT: A pulse-repetition rate set to q = 2np radians, i.e., Dfn = n/t, where q = degrees, n = an integer, f = frequency, and t = sec, provides, for a very narrow band of frequencies near the q = 2np conditions, a magnetization vector in which the steps caused by pulses outweigh those caused by precession and which induces a max. NMR signal which is close to the pure absorption-mode condition. Selective excitation by this simple pulse sequence was illustrated by added multiplet subspectra in the Fourier transform 13C NMR of Me2C:CHNMe2. [on SciFinder (R)]