An FT NMR spectrum is obtained by applying a pulse at the Larmor frequency to a sample in a magnetic field. The precession of the spins induces a voltage in the receiver coil which is recorded as a function of time. The Fourier transform of the time dependent signal is the NMR spectrum. What happens if you do not provide any pulses? You might think that you would not observe a signal - but this is not the case. Even without any pulses there is sufficient noise present to allow incoherent precession of the nuclear spins. This precession can be measured and indeed produce an NMR spectrum. This is demonstrated in the figure below. The bottom trace shows a conventional 300 MHz 1H NMR spectrum of ethyl acetate collected with one scan using a 30° pulse. The top trace was collected on the same sample by adding 256 single scan magnitude spectra using no pulses whatsoever. Although very weak, one can clearly see the NMR spectrum of ethyl acetate.
Mathematical treatment of adiabatic fast passage pulses for the computation of nuclear spin relaxation rates in proteins with conformational exchange
Mathematical treatment of adiabatic fast passage pulses for the computation of nuclear spin relaxation rates in proteins with conformational exchange
Abstract Although originally designed for broadband inversion and decoupling in NMR spectroscopy, recent methodological developments have introduced adiabatic fast passage (AFP) pulses into the field of protein dynamics. AFP pulses employ a frequency sweep, and have not only superior inversion properties with respect to offset effects, but they are also easily implemented into a pulse sequence. As magnetization is dragged from the +z to...
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09-30-2011 08:01 PM
[U. of Ottawa NMR Facility Blog] Probe Tuning and 90 Degree Pulses
Probe Tuning and 90 Degree Pulses
In order to get meaningful results from multiple-pulse NMR pulse sequences, it is essential that the 90° and 180° pulses are calibrated at the power levels used in the sequences (see this post for example). The calibrations are usually done on a standard sample in a well tuned and matched probe. The calibrations are typically stored in a file which is called up when setting up particular NMR experiments. It is important to know that these calibrations are correct for the particular sample of interest only when the probe is well tuned and matched. For...
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09-16-2011 10:02 PM
[U. of Ottawa NMR Facility Blog] Shaped Pulses
Shaped Pulses
Shaped rf pulses are used frequently in modern NMR experiments for selective excitation and more efficient inversion. The figure below shows some of the pulse shapes in the Bruker shape library measured with an oscilloscope on an AVANCE III console. Each 50.3 MHz rf pulse was 1 msec in duration and was measured at the output of the signal generation unit.http://2.bp.blogspot.com/--GjbnC0soco/TeU9TDXS7nI/AAAAAAAAA4c/Kt3DwPKLjx8/s400/shaped_pulses_scope.jpghttps://blogger.googleusercontent.com/tracker/3300702123878659843-4230712340261495616?l=u-of-o-nmr-facility.blogspot.com...
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05-31-2011 11:41 PM
[U. of Ottawa NMR Facility Blog] Excitation Profiles for Shaped Pulses
Excitation Profiles for Shaped Pulses
Shaped pulses are very commonly used for selective excitation and nonselective inversion in a large number of NMR pulse sequences. The frequency domain excitation profile of a radio frequency pulse is the Fourier transform of the time dependent pulse shape and determines the width, uniformity and phase of the frequency spectrum excited. Since time and frequency are reciprocals of one another, short rf pulses have very wide excitation profiles and long rf pulses have very narrow selective excitation profiles. In a previous BLOG post the excitation...
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01-21-2011 03:31 AM
[NMR paper] Improved excitation pulse bandwidths using shaped pulses, with application to heteron
Improved excitation pulse bandwidths using shaped pulses, with application to heteronuclear half filters in macromolecular NMR.
Related Articles Improved excitation pulse bandwidths using shaped pulses, with application to heteronuclear half filters in macromolecular NMR.
J Magn Reson B. 1995 Jul;108(1):12-21
Authors: Hyre DE, Spicer LD
The advantageous use of sinc-shaped pulses in heteronuclear half filters is explored for studying biological macromolecules. The typical square, or hard, pulse used in half-filter pulse sequences for...
FT NMR Spectra Without Pulses
Linear Mode
