Spectrometer frequency

Observed chemical shift (number of Hz away from TMS) Spectrometer frequency in MHz... 

By using a system of measurement in which NMR absorptions are expressed in relative terms (parts per million relative to spectrometer frequency) rather than absolute terms (Hz), it s possible to compare spectra obtained on different instruments. The chemical shift of an NMR absorption in 8 units is constant, regardless of the operating frequency of the spectrometer. A H nucleus that absorbs at 2.0 8 on a 200 MHz instrument also absorbs at 2.0 8 on a 500 MHz instrument. 

The NMR chart is calibrated in delta units (5), where 15=1 ppm of spectrometer frequency. Tetramethylsilane (TMS) is used as a reference point because it shows both 1H and 13C absorptions at unusually high values of the applied magnetic field. The TMS absorption occurs at the right-hand (upfield) side of the chart and is arbitrarily assigned a value of 0 5. 

Laboratory frame The Cartesian coordinates (x, y, and z) are stationary with respect to the observer, in contrast to the rotating frame, in which they rotate at the spectrometer frequency. 

Quadrature images Any imbalances between the two channels of a quadrature detection system cause ghost peaks, which appear as symmetrically located artifact peaks on opposite sides of the spectrometer frequency. They can be eliminated by an appropriate phase-cycling procedure, e.g., CYCLOPS. 

Fig. 9.12 (A) Cross-relaxation rates with respect to the correlation time and the spectrometer frequency cOq. (B) While only positive NOE (resulting in cross-peaks with inverted sign compared to the diagonal peaks, left) are present in a mixture of free ligands, the addition of the receptor leads to the...

Fig. 1.50. Relaxation time as a function of the molecular correlation time for two spectrometer frequencies 60 MHz and 220 MHz. rSGR, spin-lattice relaxation time rSSR, spin-spin relaxation time (Fig. 2.24 from [ 1.105]).

Fig. 7. Block diagram of the CP-ENDOR spectrometer. Frequency range of automatic phase and level control 2-50 MHz, rf fields 0.08 mT, . (Adapted from Ref. 43)...

In this simulation, a ligand/protein ratio of 10 1 was assumed. The kon rate was assumed to be 10 s The spectrometer frequency was set at 600 MHz. A somewhat longer correlation time of 10 s was assumed for the protein and the complex, primarily to ensure efficient spin diffusion. The ligand correlation time was fixed at 0.2966 ns, corresponding to null NOE at 600 MHz. A leakage factor of 0.3 s was assumed for all the protons in their free and bound states. This term was added to all the diagonal elements of the R matrix. 

Fig. 3 a STD curves left panel) for the symmetric ligand-protein complex in Fig. 2a. b STD curves right panel) for the asymmetric ligand-protein complex in Fig. 2b. The P3 and P3 protons are saturated. A spectrometer frequency of 600 MHz and a free ligand correlation time of 2.966 x 10 ° s corresponding to null NOE at 600 MHz were assumed. The protein correlation time was 10 s, Lt/Et =10 1, leakage rate = 0.3 s ...

When gf3eHhyperfine spectra of (27+ 1) equally spaced lines is to be expected only when the spectrometer frequency is large enough to require gfieH >A. For large hyperfine interactions or low spectrometer frequencies, the spectrum can deviate greatly from the simple pattern observed at high frequencies. 

Fig. 8.9 NMR spectrum of CH3CHO. The total area of the doublet peaks is three times the total area of the quartet peaks. The coupling constant 7=2.9 Hz, which is substantially less than the usual value 5 to 8 Hz for vicinal coupling constants in compounds with essentially free rotation. The splittings are drawn as equal to about one-tenth part-per-million, which corresponds to a spectrometer frequency of 29 MHz.

Ten years ago, most nmr spectrometers operated for protons with radiofrequency (rf) transmitters set at 60 MHz (6 x 107 cycles per sec) but there has been a proliferation of different proton-operating frequencies and now 30, 60, 90, 100, 220, 270,300, and 360 MHz machines are commercially available. The cost of these machines is roughly proportional to the square of the frequency, and one well may wonder why there is such an exotic variety available and what this has to do with the chemical shift. High operating frequencies are desirable because chemical shifts increase with spectrometer frequency,... 

To reiterate, chemical shifts are strictly proportional to spectrometer frequency, thus lines 100 Hz apart at 60 MHz will be 167 Hz apart at 100 MHz. This might seem to make comparisons of nmr spectra on different spectrometers hopelessly complex but, because of the proportionality of shifts to frequency (or field), if we divide the measured shifts in Hz, (relative to the same standard) for any spectrometer by the transmitter frequency in MHz, we get a set of frequency-independent shifts in parts per million (ppm), which are useful for all nmr spectrometers. Nmr shifts reported in ppm relative to TMS as zero, as shown in Figure 9-23, are called 8 (delta) values ... 

A steady-state diffuse reflectance spectroscopy instrument typically includes a broadband fight source, intermediate optics, spatially separated delivery-collection optical fiber probes,77 and a CCD-based grating spectrometer. Frequency-based approaches have also been pursued.78 Correlations between the glucose concentration and the tissue transport scattering coefficient have been observed.77,78... 

An added benefit of using thin mica is that the mica is optically transparent down to 1200 cm-l and at least partially transparent down to the spectrometer frequency limit at 200 cm-l. This allows spectral subtraction routines to be employed below 1200 cm-l for extraction of spectral information of the adsorbed species from the mica background (11). Secondly, the thin films of mica can be placed in series and this increases the amount of sample probed by the beam which has led to a direct improvement in the detection limit of monolayer and submonolayer coverages on mica surfaces. 

H NMR frequency in MHz the spectrometer frequency is usually a part of the spectrometer model name (e.g. UNITY 400 or AVANCE 400). c29Si NMR frequency in MHz. 

Measuring frequency seems to be a problem when concise data presentation in the format 29Si-NMR (XXX MHz, CDCI3, TMS int.) is required by the journal. Some authors correctly interpret XXX as the 29Si NMR measuring frequency, i.e. the resonance frequency of 29Si nuclei, (39.73, 59.63 and 99.32 MHz) others assume it to be the spectrometer frequency, i.e. the 1II NMR measuring frequency (200, 300 and 500 MHz, respectively). The use of the spectrometer frequency then leads to the occurrence of... 

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