Transmitter offset

The transmitter offset describes the location of the observation frequency and is closely related to the spectral width. With quadrature phase detection of sample signals (Section 5-8), the frequency of the transmitter is positioned in the middle of the spectral width. In so doing, the operator has the best chance of irradiating, with equal intensity, those nuclei whose resonances are both close to and far from the transmitter frequency. Irradiation is not a problem for protons, with their small chemical shift range, but it can be for nuclei with large chemical shift ranges (Chapter 3). 

Standard sets of acquisition parameters include typical transmitter offset values. If wider spectral widths are required, simple trial and error with concentrated samples or standards permits the operator to widen the spectral window in a selective manner. For example, the operator may suspect the presence of highly deshielded H signals and wish to open the H spectral width from the usual 10 ppm (4,000 Hz at 400 MHz) to 15 ppm (6,000 Hz). In order to add all of the additional 5 ppm (2,000 Hz) worth of sw capacity to the downfield (high-frequency) end of the spectral range (i.e., from 10-15 ppm), the transmitter offset value is increased by approximately 2.5 ppm (1,000 Hz at 400 MHz). This procedure also keeps the transmitter offset positioned in the middle of the widened spectral window. 

---

In quadrature detection, the transmitter offset frequency is posidoned at the center of the F domain (i.e., at F2 = 0 in single-channel detection it is positioned at the left edge). Frequencies to the left (or downfield) of the transmitter offset frequency are positive those to the right (or upheld) of it are negative. 

SWi, which in turn is related to the homonuclear or heteronuclear coupling constants. In homonuclear 2D spectra, the transmitter offset frequency is kept at the center of (i.e., at = 0) and F domains. In heteronuclear-shift-correlated spectra, the decoupler offset frequency is kept at the center (Fi = 0) of thei i domain, with the domain corresponding to the invisible or decoupled nucleus. 

A comparison between the one- and two-dimensional data shown for this compound is interesting. As we have said, the 2-D ROESY does offer the advantage of displaying all enhancements occurring in the molecule simultaneously but against that, the data is probably more prone to artifacts than the corresponding 1-D technique. This can be particularly apparent in cases where the transmitter offset... 

The idea of frequency selectivity is certainly not restricted to J-coupling mediated polarization transfer. Furthermore, frequency selective polarization transfer can also be realized by DQ recoupling techniques. For the technique of shift-evolution-assisted selective homonuclear recoupling (SEASHORE), which employs POST-C7 to construct an effective DQ coupling [82, 83], frequency selective polarization can be achieved when the transmitter offset is set to the... 

Use of isotopic enrichment. Largest Cq 50 ever reported for Mg. Spectra (very broad) recorded with changing RL transmitter offset. Experimental data supported by first-principles calculations. See Ref. 40 for comparison of these experimental data with other results of first-principles calculations. 

In some respects, the H-decoupling operation can be viewed as a simultaneous H NMR experiment. Just as there is a transmitter offset that positions the X-nucleus observation frequency, so is there a corresponding decoupler offset for the H-decoupling fiequency. Many spectrometers have both transmitter and decoupler power levels. Three parameters, however, are specific to decoupling and have no counterparts among the spectral observation parameters that have been discussed. 

With the test sample in the magnet, the probe is tuned to H and the magnet homogeneity maximized. Next, a test spectrum is determined in which the tp used is unimportant. This spectrum then serves as a starting point for another spectrum, which has a reduced spectral width. The original sw is now reduced to about 500 Hz, and the transmitter offset is adjusted so that it is in the middle of the reduced sw. Many spectrometers have programs that do both operations with one command. Since sw has been considerably reduced, the number of data points should also be decreased, to approximately 4,000, so as to maintain an acquisition time of around 4 s. 

When sw is reduced, modem spectrometer programs have a series of commands, called macros, which automatically move the transmitter offset so that it is in the middle of the reduced sw2. The operator can verify that the transmitter has moved by noting the offset values for the original and reduced sw s. 

Fig. 3. Two-dimensional MQNMR spectrum of 3-chloroiodobenzene (2) oriented in liquid crystalline solution with non-selective excitation and detection (Fig. 2). 1,024 increments in fu 12 scans per increment, 48 kHz in f2, 160 kHz in j, 400 MHz at 295 K, transmitter offset 18 kHz from the centre of the 1H spectrum. The transmitter offset was set to provide dispersion between the various MQ spectra.

Whilst maximum excitation occurs at l/2t Hz from the transmitter offset, further nulls occur at offsets of n/x (n = 1, 2, 3,. .. corresponding to complete revolutions during each x) so a judicious choice of x is required to provide excitation over the desired bandwidth. The excitation profiles of the 1-1 and 1-3-1 sequences are shown in Fig. 9.25b and c. Clearly the excitation is non-uniform, so places limits on quantitative measurements, and once again there exists a phase inversion either side of the solvent. Both provide an effective null at the transmitter offset and suppression ratios in excess of 1000-fold can be achieved. 

Parameter set mode acquisition mode spectrometer proton frequency transmitter offset receiver offset sweep vidth... 

We can control both the width and the location of the SW. The width is controlled by varying the sampling rate of the analog-to-digital converter (A/D). We can translate (move side to side) the center of the spectral window by varying the transmitter frequency. The transmitter frequency is also known as the transmitter offset or the carrier frequency, or sometimes simply the transmitter (this is an imprecise term and should be avoided unless the context is well understood). On a Bruker instrument, the transmitter frequency is determined by a coarse value listed in MHz (sfrq, for spectrometer frequency)... 

The NOE difference experiment is carried out as follows. First, we collect a regular 1-D spectrum. Next, we record the transmitter offset required to make each resonance to be irradiated on resonance. Third, we find a suitable location in or near the spectral rvindow for dummy (or control) irradiation (perhaps —5 ppm or +15 ppm) where no resonances are observed. Fourth, we prepare both FlDs using a long period of single-frequency low power RF irradiation followed by a hard 90° RF pulse, except that in the preparation of the... 

---