Pulsed FT-NMR

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. 

Figure Bl.12.3. Schematic representation showing the components of a pulse FT NMR spectrometer.

Not only is pulsed FT NMR the best method for obtaining proton spectra it is the only practical method for many other nuclei including It also makes possible a large number of sophisticated techniques that have revolutionized NMR spectroscopy... 

However unlike H which is the most abundant of the hydrogen isotopes (99 985%) only 1 1% of the carbon atoms m a sample are Moreover the intensity of the signal produced by nuclei is far weaker than the signal produced by the same number of H nuclei In order for NMR to be a useful technique in structure deter mination a vast increase in the signal to noise ratio is required Pulsed FT NMR pro vides for this and its development was the critical breakthrough that led to NMR becoming the routine tool that it is today... 

In general a simple pulse FT NMR experiment involves the following stages... 

The key dimension m NMR is the frequency axis All of the spectra we have seen so far are ID spectra because they have only one frequency axis In 2D NMR a stan dard pulse sequence adds a second frequency axis Only pulsed FT NMR spectrometers are capable of carrying out 2D experiments... 

Carbon-13 nmr. Carbon-13 [14762-74-4] nmr (1,2,11) has been available routinely since the invention of the pulsed ft/nmr spectrometer in the early 1970s. The difficulties of studying carbon by nmr methods is that the most abundant isotope, has a spin, /, of 0, and thus cannot be observed by nmr. However, has 7 = 1/2 and spin properties similar to H. The natural abundance of is only 1.1% of the total carbon the magnetogyric ratio of is 0.25 that of H. Together, these effects make the nucleus ca 1/5700 times as sensitive as H. The interpretation of experiments involves measurements of chemical shifts, integrations, andy-coupling information however, these last two are harder to determine accurately and are less important to identification of connectivity than in H nmr. 

Pulsed FT-NMR has facilitated the study of nuclei other than H where the sensitivity obtainable from a CW instrument is totally inadequate. In particular, 13C NMR, the sensitivity of which is nearly 10-4 less than that of the proton (Table 9.9), is now a well-established technique that yields information on the skeletal structure of complex molecules. The pulsed technique also enables proton spectra to be obtained from samples as small as a few micrograms. 

The most abundant isotope of earbon ( C) cannot be observed by NMR. is a rare nucleus (1.1% natural abundance) and its low concentration coupled with the fact that has a relatively low resonance frequency, leads to its relative insensitivity as an NMR-active nucleus (about 1/6000 as sensitive as iff). However, with the increasing availability of routine pulsed FT NMR spectrometers, it is now common to acquire many spectra and add them together (Section 5.3), so C NMR spectra of good quality can be obtained readily. 

T. T. Nakashima, K. J. Harris and R. E. Wasylishen, Pulse FT NMR of non-equilibrium states of half-integer spin quadrupolar nuclei in single crystals. /. Magn. Reson., 2010, 202,162-172. 

New users of Pulse FT-NMR will find it veiy valuable." Choice... 

Pulse FT NMR has been used to study the spin-lattice relaxation times, Tv of U9Sn in a number of organic (37, 38, 40, 42, 43) and inorganic (44) tin compounds. The most important relaxation mechanism for this nucleus in a series of tetraorganotins appears to be spin-rotation (SR) (38, 43) although for larger molecules, such as hexabutylditin, dipole-dipole (DD) relaxation is important, even at room temperature. (37)... 

The sensitivity of pulsed FT NMR experiments is given by the signal to noise ratio ... 

The signal intensities, S, are theoretically proportional to the number of spins promoted from the lower to the upper spin state at resonance, but in pulsed FT NMR these intensities are perturbed in a nonuniform way by several factors that need to be suppressed for quantitative analysis ... 

Many workers have in fact used density matrix methods for the calculation of line shapes and intensities in multiple resonance experiments, and two excellent reviews of the background theory are available. (49, 50) In addition there is also a simple guide (51) to the actual use of the method which is capable of predicting the results of quite elaborate experiments. Major applications have included the calculation of the complete double resonance spectrum from an AX spin system which gives 12 transitions in all (52) an extremely detailed study of the relaxation behaviour of the AX2 systems provided by 1,1,2-trichloroethane and 2,2-dichloroethanol (53) the effects of gating and of selective and non-selective pulses on AB and AX spin systems and the importance of the time evolution of the off-diagonal elements of the density matrix in repetitively pulsed FT NMR and spin-echo work (54) the use of double resonance to sort out relaxation mechanisms and transient responses (55) the calculation of general multiple resonance spectra (56) and triple resonance studies of relaxation in AB and AX spin systems. (57)... 

---