Magnetic field units,

A small amount of TMS is added to the sample, and the instrument measures the difference in magnetic field between where the protons in the sample absorb and where those in TMS absorb. For each type of proton, the distance downfield of TMS is the chemical shift of those protons. Newer spectrometers operate at a constant magnetic field, and they measure the chemical shift as a frequency difference between the resonances of the protons in the sample and those in TMS. Remember that frequency units (v) and magnetic field units (B0) are always proportional in NMR, with v = jBq/2tt. 

When you look at an NMR spectrum you will see that the scale does not appear to be in magnetic field units, nor in frequency units, but in parts per million (p.p.m.). There is an excellent reason for... 

Fig. 6. uS Repolarization. Longitudinal polarization of Mu as a function of the magnetic field (unit the internal mimetic field intensity, = 1585 G). Carve 1 corre nds to Eq. 23 for Mu, and curve 2 is that for a pseudo-Mu with a hyperfine constant smaller than that of Mu. Broken curves 3 and 4 correspond to Eq. 24 calculated for Mu and pseudo-Mu, respectively, and they indicate that the longitudinal polarization is enhanced at zero field due to chemical reactions. The shape of the repolarization curve thus contains information on the species and reactions that cause the loss of spin polarization... 

The measured separation between the two lines, directly gives the hyperfine splitting, 50.5 mT, in magnetic field units. The coupling when expressed in frequency units as is usual in ENDOR studies (Chapter 2) is converted to field units by ... 

Chemical shifts are measured in parts per million (ppm), a dimensionless fraction of the total applied field. By custom, the difference in field (the chemical shift) between the NMR signal of a proton and that of TMS is not measured in gauss, but in frequency units (hertz or Hz). Remember that frequency units and magnetic field units are always proportional in NMR, with v = The horizontal axis of the NMR spectrum is calibrated... 

Note that if the unit for y was given in frequency units per magnetic field units, for example, as MHz T instead of the 2n MHz T as we have had, then you would need to divide the y xB product by 27T, that is, vq = y C) x B/2it. So - like in all other areas of physical chemistry - watch for units. 

The units of M and AM can be mass units, voltage, or magnetic field units. If the desired distance between the peaks is taken as the base width, then the distance between the peaks becomes 1. However, a 10% peak overlap criterion is typically used. In a magnetic sector instrument, resolution is set by the beam-defining slits at the exit of the ionizing chamber (electrostatic accelerator) and entrance to the multiplier. These are vertical slits whose widths are adjustable from the outside of the instrument. These sUts define the beam and resolution [56] ... 

Fig. 26. Parameters describing diffraction from vortex lattice in superconducting UPtj as a function of field H applied along a, (a) Internal field deduced from the unit cell size. The line with slope unity is expected for singly quantized vortices, (b) Crystallographic angle a defined in the inset. For a conventional hexagonal lattice, a = 30°. (c) Scattering length, in magnetic field units, for the (1,0) Bragg reflection from the vortex lattice. (From Klerman ct al. 1992.)...

Figure 15. EPR spectra of the high-spin Fe-NO complex of soybean LOX-1 at two frequencies. The soybean lipoxygenase (LOX-l)-nitric oxide complex (3.2 mM in 0.1 M potassium phosphate, pH 7.0) was sealed under argon in a quartz EPR tube of 0.7 mm ID. Spectra were recorded 9.26 GHz and 94 GHz, and the temperature was 6 K. The original 94 GHz spectrum was scaled to the same g-value scale as the 9.26 GHz spectrum, and the magnetic field units are shown for the X-band spectrum. Because of the scaling, the high-fiequency spectrum is labeled in the figure as if it was recorded at 92.6 GHz, and the corresponding magnetic field scale would be ten times that shown.

The equation is usually written using Hertz (Hz), a unit of frequency, rather than Tesla, a magnetic field unit. The numerator gives the differences between the resonance position of hydrogen atoms in the sample and those in tetramethylsilane (TMS), whose chemical shift is set at zero. This value is about 10 that of the frequency of the NMR spectrometer. Hence, the quotient is multiplied by 10" to obtain delta units. 

Strength of magnetic field unit—ampere/meter (magnetic field induction, divided by Hq/j)... 

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