Pulsed-mode NMR

In a pulsed-mode NMR experiment, which is performed at both constant magnetic field and constant rf frequency, rf radiation is supplied by a brief but powerful computer-controlled pulse of rf current through the transmitter coil. This monochromatic (single-frequency) pulse, centered at the operating frequency v0, is characterized by a power (measured in watts and controlling the magnitude of B,) and a pulse width (tp), the duration of the pulse measured in microseconds. However, as a direct consequence of the uncertainty principle [Eq. (1.6)], this brief pulse acts as if it covers a range... 

As mentioned in Section 3.4, the success of the pulsed-mode NMR technique depends on the careful selection of data acquisition parameters. These data for the six spectra discussed in this chapter appear in Table 5.2. The same sample was used for all the spectra. 

Modern pulse-mode NMR spectrometers allow the details of the Vj and v2 pulse sequence to be altered in an infinite variety of ways to yield useful information about molecular structure. Such pulse sequence variations include the following ... 

The original method employed was to scan eitiier the frequency of the exciting oscillator or to scan the applied magnetic field until resonant absorption occiined. Flowever, compared to simultaneous excitation of a wide range of frequencies by a short RF pulse, the scanned approach is a very time-inefficient way of recording the spectrum. Flence, with the advent of computers that could be dedicated to spectrometers and efficient Fourier transfomi (FT) algoritluns, pulsed FT NMR became the nomial mode of operation. 

The pulsed-mode technique for acquiring an NMR spectrum involves the following steps once the sample is immersed in the magnetic field ... 

Because of the low natural abundance and low sensitivity of l3C, the acquisition of carbon NMR spectra requires the use of signal-averaged pulsed-mode meth-... 

The design of EPR spectrometers resembles that of a field-sweep NMR instrument (Section 3.3.2), though pulsed-mode (Fourier transform Section 3.4) EPR spectrometers are now available. Many of the considerations (such as field stability, lineshape, saturation, relaxation, etc.) that were discussed in Chapters 2 and 3 for NMR are also important in EPR,1 but there are some significant differences. 

Without doubt the biggest advances in NMR technology over the past decade have involved the advent of higher field pulse-mode spectrometers together and the realization that, by careful manipulation of the Vj and v2 pulse sequences, the net magnetization vector M can be redirected at will to... 

Figure 24.5a Change in single-pulse static NMR spectra with temperature for tetramethylammonium dicyanamide. The spectra show line narrowing as the temperature increases, indicating the onset of various rotational modes within the material. The presence of multiple peaks within each spectrum indicates the existence of multiple modes of...

As mentioned above, ENDOR is the detection of NMR resonances by observing their effect on an EPR signal, whether in continuous wave (CW) or pulsed mode. The NMR... 

FIG. 5. Temperature-dependent Na NMR spectra recorded in the pulsed mode at 15-9 MHz for a sample consisting of 0-3 M cryptand C222 and 0-6M NaBr in ethylenediamine. (64)... 

FIG. 12. Temperature-dependent Be NMR spectra in Be(TMPA)J, recorded in the pulsed mode at 12-2 MHz in trimethyl phosphate (TMPA) (a) simulated, (b) experimental spectrum. (106)... 

Pulse NMR is not just confined to C spectroscopy, but is now used for the study of a wide range of nuclei, although this review concentrates on carbon and proton nuclei. All modern high-field spectrometers operate in the FT mode, and the success of pulse FT NMR is due to its versatility. Quantum... 

When the resonant condition is met, the NMR signal is collected at the RF receivers. NMR signals are generally weak and need to be amplified and processed prior to further analysis. Using the pulsed mode, the free induction decay (FID) spectrum in the time domain is recorded and while it contains all the information on frequencies, splitting and integrals, it must be converted into the frequency domain by Fourier Transformation (FT). This FT step enhances the S/N ratio of the signal. 

Very different intensities of the P NMR signals from (CH3)3P-derived chemical species on surfaces can be observed by using different NMR pulse modes for generating transverse P magnetization [90j. Figure 45 shows the CP and single-pulse (FT) MAS spectra for two surface concentrations of (CH3)3P adsorbed on y-alumina. The CP-MAS spectrum reveals the presence of Bronsted (-4 ppm) and Lewis ( — 48 ppm) complexes in addition to physisorbed phosphine (-58 ppm). At small (<1 ms) CP contact time the cross-polarization... 

Physical Measurements. For the electrolyses, a Wenking potentiostat model 70TS1 and a Koslow Scientific coulometer model 541 were used. Voltammetry with wax-impregnated graphite and rotating platinum electrodes was performed as described elsewhere (7, 8). IR and electronic spectra were measured on Perkin-Elmer 225 and Cary 14 instruments. X-band ESR spectra were recorded at room temperature on a JEOL MES-3X spectrometer. Phosphorus-31 NMR spectra were recorded in the pulse mode on a Varian XL-100 instrument at 40.5 MHz using a deuterium lock, or on a Bruker HFX-90 instrument at 36.43 MHz using a fluorine lock. 

---