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| RAY'S PERSONAL CHOICE |
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Research projects are often started in a wild burst of enthusiasm, but
later, in the cold light of day, the initial confidence may wane. Here
is Ray’s choice of those topics that appear to have stood the test of
time. Full papers have been selected in preference to initial
communications. The numbering scheme and titles match those of the main
publication list.
2) Cobalt Nuclear Resonance Spectra. In general, Ramsey’s theory
of chemical shielding involves a summation over all excited electronic
wavefunctions, but in the special case of cobalt-III complexes there is a
very low-lying excited state (which determines the colour of these
complexes). This term dominates all other contributions to the cobalt-59
chemical shifts, which therefore show a very good linear relationship
with respect to the UV-visible wavelength, thus corroborating Ramsey’s
theory.
5) Frequency Control of an Oscillator by Nuclear Magnetic Resonance.
Robert Pound employed a super-regenerative oscillator to detect radar
responses from aircraft. This pulse-modulated oscillator normally runs
incoherently with the phase of each new oscillation determined by
circuit noise. But when the device detects a radar echo, this triggers
subsequent oscillations in a coherent mode. This simple (double-triode)
circuit can also detect NMR signals because the oscillator locks onto
the NMR free precession frequency, faithfully following any variations
of the applied magnetic field.
15) The Relative Signs of Geminal and Vicinal Proton Spin Coupling Constants.
In a coupled three-spin (AMX) proton system, the A and X regions both
consist of J(AX) doublets split again by the coupling to M, but the
assignment of these doublets remains ambiguous. The relative signs of
J(AM) and J(MX) are determined by selectively irradiating the low-field A
doublet and observing which of the two X doublets is decoupled. This
experiment demonstrated that geminal and vicinal couplings have opposite
signs, whereas Karplus predicted that both should be positive.
18) Use of Weak Perturbing Magnetic Fields in Nuclear Magnetic Double Resonance.
An early indication that NMR spectra might be manipulated in new and
useful ways. Weak irradiation of a single line of a high-resolution
spectrum generates new splittings on lines that share a common energy
level (connected transitions). How well these splittings are resolved
depends on the relative configuration of the irradiated and observed
transitions – either progressive (poorly resolved) or regressive (well
resolved). Jeener has asserted that this paper prompted him to think
about the possibilities of two-dimensional NMR.
23) Assignment of NMR Spectra with the Aid of Double Quantum Transitions.
First practical application of double-quantum (DQ) transitions in NMR.
The first step in computer analysis of strongly-coupled proton spectra
requires assignment of the observed transitions to an energy-level
diagram, accomplished by detecting the DQ transitions. Then the
iterative fitting the experimental spectrum converges rapidly, even with
arbitrary starting parameters. Sample: trivinylphosphine.
38) Selective Determination of Relaxation Times in High Resolution NMR.
Application of a ‘soft’ frequency-selective pulse in the form of a weak
audiofrequency modulation sideband, controlled by a simple mechanical
gating system. Pulse durations of the order of hundreds of
milliseconds. NMR signals can be observed during the pulses. Studies of
spin-lattice and spin-spin relaxation times in proton spectra, one line
at a time.
43) Spin-Lattice Relaxation of High Resolution NMR Spectra of Carbon-13.
The advent of Fourier transform NMR allowed new ‘hard-pulse’ methods to
supersede the ‘soft-pulse’ technique. This note describes the first
Fourier transform measurements of carbon-13 spin-lattice relaxation of
six sites monitored simultaneously. Triggered a flurry of investigations
of relaxation of carbon-13 in natural abundance samples.
45) High Resolution Study of NMR Spin Echoes: 'J-Spectra'.
Fourier transformation of spin echo modulation generates a ‘J-spectrum’
where the scalar couplings can be measured with high precision. A proton
resonance of 0.028 Hz (full width) is recorded in the J-spectrum
3-bromothiophene-2-aldehyde. Hints at the possibility of two-dimensional
spectroscopy, where the indirect dimension maps spin-spin multiplet
structure.
46) Fourier Transform Study of NMR Spin-Lattice Relaxation by 'Progressive Saturation'.
Describes a simple scheme for determining spin-lattice relaxation with a
pulse scheme that operates in a steady-state regime. Variation of the
pulse rates affects the observed NMR intensities by changing the balance
between pulse excitation and spin-lattice relaxation. No modification
of the spectrometer is required.
47) Phase and Intensity Anomalies in Fourier Transform NMR.
Fourier transform spectrometers often operate with interpulse intervals
comparable with the spin-spin relaxation times. This sets up a
steady-state regime that causes anomalies in the amplitude and phase of
the detected signals. Short pseudo-random delays between pulses
suppresses these effects. Recently applied to problems in fast magnetic
resonance imaging.
71) Double Fourier Transformation in High Resolution NMR. Nuts
and bolts of the methodology of two-dimensional NMR. The ‘phase twist’
line-shape (combining absorption and dispersion contributions) is
described for the first time. The ‘phantom’ and ‘ghost’ artifacts are
analyzed, and the shapes of two-dimensional Lorentzian and Gaussian
responses are illustrated.
73) Suppression of Artifacts in Two-Dimensional J-Spectroscopy.
Describes a phase cycle to suppress artifacts (phantoms and ghosts)
generated by imperfections of radiofrequency pulse sequences used in
two-dimensional NMR. Phase cycling has later been extended to most
complex NMR sequences, only superseded when pulsed field gradient
methods became available.
79) Selective Excitation in Fourier Transform Nuclear Magnetic Resonance.
First comprehensive description of the DANTE experiment, where
selective irradiation is achieved by a repetitive sequence of hard
pulses of small flip angle. Excitation is at the transmitter frequency
and at sidebands separated by multiples of the pulse repetition rate.
86) Enhancement of NMR Signals by Polarization Transfer. The
INEPT pulse sequence, where the signals from nuclear species of low
intrinsic sensitivity (carbon-13 or nitrogen-15) are enhanced by
transfer of polarization from protons. Now an integral part of many
complex pulse sequences.
87) NMR Population Inversion using a Composite Pulse. First
description of a composite radiofrequency pulse [90º(X) 180º(Y) 90º(X)]
designed to compensate the imperfections of a single 180º pulse. Later
became an important element in broadband composite decoupling sequences.
106) An NMR Technique for Tracing out the Carbon Skeleton of an Organic Molecule.
The two-dimensional INADEQUATE experiment for samples with carbon-13 in
natural abundance. Signals from coupled pairs of carbon-13 spins are
separated from the much stronger signals from isolated arbon-13 spins by
momentary creation of double-quantum coherence. The evolution
dimension is used to separate the individual coupled subspectra as a
function of the double quantum frequencies.
110) Composite Pulse Decoupling. Broadband heteronuclear
decoupling employing composite spin inversion pulses. The protons
follow cyclic trajectories at a rate fast compared with the coupling
constant. Composite inversion elements R are arranged in a magic cycle R
R R’ R’, where R’ is the phase-inverted counterpart of R. Fore-runner
of a host of broadband decoupling schemes.
112) Investigation of Complex Networks of Spin-Spin Coupling by Two-Dimensional NMR.
Comprehensive analysis of Jeener’s COSY experiment. Shows how to
employ time-domain weighting functions to emphasize correlation peaks at
the expense of diagonal peaks, and how to determine relative signs of
coupling constants by inspection. Demonstrates the utility of intensity
contour plots. Can be used to generate proton spectra without
proton-proton splittings.
118) Echoes and Antiechoes in Coherence Transfer NMR: Determining the Signs of Double-Quantum Frequencies.
A theoretical treatment of the INADEQUATE pulse sequence indicates that
if the final ‘read’ pulse differs from 90º the observed signal contains
components derived from both the real and imaginary parts of the
double-quantum coherence. This introduces phase modulation during
evolution, permitting the sign of the double-quantum frequency to be
determined.
126) Simplification of NMR Spectra by Filtration through Multiple-Quantum Coherence.
The sensitivity of a N-quantum coherence to the relative phases of the
radiofrequency pulses allows separation of signals derived from
different orders N. A two-dimensional proton spectrum of a mixture of
two-spin, three-spin and four-spin systems is filtered to show only the
four-spin spectrum.
130) Evaluation of a New Broadband Decoupling Sequence: WALTZ-16.
A new spin inversion pulse 90º(X) 180º(–X) 270º(X) proves to be
remarkably insensitive to imperfections in the phase or amplitude of the
radiofrequency pulses. Employed in a supercycle (WALTZ-16) it achieves a
decoupling bandwidth equal to twice the intensity of the decoupling
field (γB2/2π).
138) Gaussian Pulses. Selective radiofrequency irradiation with
pulses having a rectangular envelope suffers from undesirable
sinc-function wiggles. Shaping with a Gaussian function greatly
improves the excitation envelope in the frequency domain.
151) Pulsed Field Gradients in NMR. An Alternative to Phase Cycling.
Scheme to replace the ubiquitous phase cycle with suitably matched
pairs of pulsed field gradients, thus saving time and complication. An
extension is employed to filter signals that pass through triple-quantum
coherence.
152) Computer Optimized Decoupling Scheme for Wideband Applications and Low-Level Operation (CODSWALLOP).
A remarkable improvement in decoupling bandwidth of WALTZ-16 is
achieved by relaxing the usual settings of the pulse-length parameters,
and by accepting slightly larger residual splittings in the decoupled
spectrum. Non-linear numerical optimization employed. Decoupling
efficiency is more than doubled.
189) Band-Selective Radiofrequency Pulses. Simulated annealing
used to design soft radiofrequency ‘BURP’ pulses that excite uniformly
in pure absorption over a predefined frequency band with negligible
excitation outside that band. There are versions for excitation
(E-BURP), spin inversion (I-BURP), refocusing (RE-BURP) and universal
rotation (U-BURP).
195) Accurate Measurement of Coupling Constants by J-Doubling. A
free induction decay containing a modulation term cos(πJt) is
multiplied by a function sin(πJ*t) where J* is varied. When J* = J,
the transformed spectrum shows a splitting of 2J, and the integral of
the absolute magnitude of the spectral intensities reaches a minimum,
allowing J to be determined with high accuracy.
200) User-Friendly Selective Pulses. This ‘spin pinging’
experiment acts on magnetization prepared along the Y axis of the
rotating frame, alternatively rotating it about the X and Y axis by
means of a soft 180° pulse. Pure absorption-mode signals are excited.
Can be extended to two-dimensional spectroscopy where the detection
bands are restricted in both dimensions.
240) Adiabatic Pulses for Wideband Inversion and Broadband Decoupling.
First description of sausage-shaped ‘WURST’ stretched adiabatic pulses.
Supersedes previous schemes for broadband heteronuclear decoupling,
achieving very broad decoupling bandwidths at low radiofrequency
intensity.
264) An Implementation of the Deutsch-Josza Algorithm on a Three-Qubit NMR Quantum Computer.
First practical example of the use of a high-resolution NMR spectrum
for quantum computation. Two or more line-selective radiofrequency
inversion pulses are applied simultaneously to a three-spin proton
system. The experiment distinguishes between constant and balanced
functions.
281) Hadamard NMR Spectroscopy. Review of Hadamard encoding for
speeding up multidimensional NMR, using prior knowledge of the chemical
shifts. Permits fast recording of multidimensional spectra of selected
sites in globally enriched proteins, as if specific isotopic enrichment
had been used.
285) The Projection-Reconstruction Technique for Speeding up Multidimensional NMR Spectroscopy.
Faster multidimensional NMR by restricting acquisition to a limited set
of plane projections at different inclinations, relying on Bracewell’s
slice/projection theorem. Several schemes for the reconstruction stage
are described.
293) Hyperdimensional NMR Spectroscopy. Describes a method for
deriving any of the N(N–1)/2 two-dimensional correlation spectra from a
coupled N-dimensional spin system (Agitoxin, N = 10). Low-dimensional
spectra are combined on a hypothetical N-dimensional scaffold.
301) SPEED: Single-point evaluation of the evolution dimension.
Contrary to common belief, the evolution domain of a two-dimensional
experiment need not be explored comprehensively point-by-point, provided
there is prior knowledge of the chemical shifts. Measurement at a
single time-point in the evolution dimension is then sufficient to
derive the complete two-dimensional spectrum.
308) Molecular structure from a single NMR sequence (fast PANACEA).
Three standard sequences (INADEQUATE, HSQC, HMBC) are combined into a
single measurement by exploiting new multiple receiver technology.
Hadamard encoding is used to speed up the INADEQUATE feature.
Establishes the molecular structure of menthol in less than a minute.
Ray Freeman; 7 December 2012.
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| Ray Freeman |
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