Spectral width, irradiated

The details of how multidimensional spectra are obtained is beyond the scope of this chapter, but it suffices to say that, like most other modem NMR experiments, they involve irradiation of the sample with a set of rf pulses of defined length, frequency, and phase, with specific interpulse delays. The pulse programs for such experiments are commonly provided with the spectrometer as part of a standard library of experiments and may easily be run by novice users after input of an appropriate set of parameters to define the relevant spectral widths and type of experiment required. 

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). 

NMl Spectroscopy. CPMAS C NMR spectra were obtained with a JEOL FX60QS NMR spectrometer operating at 15.04 MHz. The H decoupling rf irradiation field strength was 11 G the contact time was 0.5 sec and the recycle time was 1.5 sec. A spectral width of 8000 Hz and a sampling rate of 2k data points, zero filled to 4k, were used. Chemical shifts were assigned relative to the methyl resonances, 17.36 ppm, of hexamethylbenzene. 

NMR experiments were carried out at 15 C on a Bruker AMX-500 spectrometer equipped with a 5 mm inverse detection probe and an X-32 computer. All ID spectra were recorded with a 5500 Hz spectral width and 8k data points. The water resonance was suppressed either by a pre-saturation irradiation or by using a tailored jump-return excitation pulse l. Phase-sensitive detection in the tl dimension of 2D experiments was achieved using the time-proportional phase incremental scheme. ... 

For proton NMR the main consideration is ensuring complete relaxation between successive pulses for all the different types of hydrogen atoms present. This requires the interpulse delay to be at least 5 times the longest Tj. In addition, for nuclei such as C. where broadband decoupling is usually required, the inverse gated technique (Section 18.3.4.2) should be used to prevent the occurrence of NOE effects. A further consideration in the case of spectra from nuclei such as C and F, which may have very wide spectral widths, is whether the RF pulse has sufficient power to irradiate all the nuclei equally effectively. The digital resolution and data processing requirements for a particular application also require careful selection. 

Utilizing NMR prediction ahead of data acquisition is clearly valuable when the class of compound is known and when it is necessary to set a specific spectral window. While there are a number of cases where the defect between the experimental and predicted shifts differs by considerably more than the 20 ppm window, we have found that in our hands this value is appropriate in ah cases we have faced in our own laboratory. Clearly the ideal however is improved spectrometer and probe efficiency to irradiate the entire spectral width in which case such calculations would become... 

The sinc fiinction describes the best possible case, with often a much stronger frequency dependence of power output delivered at the probe-head. (It should be noted here that other excitation schemes are possible such as adiabatic passage [9] and stochastic excitation [fO] but these are only infrequently applied.) The excitation/recording of the NMR signal is further complicated as the pulse is then fed into the probe circuit which itself has a frequency response. As a result, a broad line will not only experience non-unifonn irradiation but also the intensity detected per spin at different frequency offsets will depend on this probe response, which depends on the quality factor (0. The quality factor is a measure of the sharpness of the resonance of the probe circuit and one definition is the resonance frequency/haltwidth of the resonance response of the circuit (also = a L/R where L is the inductance and R is the probe resistance). Flence, the width of the frequency response decreases as Q increases so that, typically, for a 2 of 100, the haltwidth of the frequency response at 100 MFIz is about 1 MFIz. Flence, direct FT-piilse observation of broad spectral lines becomes impractical with pulse teclmiques for linewidths greater than 200 kFIz. For a great majority of... 

Precisely controllable rf pulse generation is another essential component of the spectrometer. A short, high power radio frequency pulse, referred to as the B field, is used to simultaneously excite all nuclei at the T,arm or frequencies. The B field should ideally be uniform throughout the sample region and be on the order of 10 ]ls or less for the 90° pulse. The width, in Hertz, of the irradiated spectral window is equal to the reciprocal of the 360° pulse duration. This can be used to determine the limitations of the sweep width (SW) irradiated. For example, with a 90° hard pulse of 5 ]ls, one can observe a 50-kHz window a soft pulse of 50 ms irradiates a 5-Hz window. The primary requirements for rf transmitters are high power, fast switching, sharp pulses, variable power output, and accurate control of the phase. 

Another consequence of the large sweep width needed for 19F acquisition is that the electronics of the instrument are pushed to the limit it is difficult to generate uniform r.f. irradiation over such a large frequency range and for this reason it may be necessary to acquire spectra in different spectral ranges, depending on the expected fluorine environment. This is particularly so in the case of high-field (>400 MHz) spectrometers. 

Figure 10. Selective irradiation of linear PE (2 X 10 mol wt, 1 — K 0.5). Spectral details are 35°C 67.9 MHz sweep width 5 KHz (quadrature detection) line broadening 9.7 Hz pulse width 35 jisec (90°C = 48 /jsec) delay = 1.0 sec, 4K data points 1024 scans accumulated 10-mm sample tube. Decoupling 7W (forward), 0.4W (reflected), broad band noise modulated decoupling.

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