The NMR Spectrometer

Simplified block diagram of a nuclear magnetic resonance spectrometer. 

A stable magnet, with a sensitive controller to produce a precise magnetic field 

A radio-frequency (RF) transmitter, emitting a precise frequency 

A recorder to plot the output from the detector versus the applied magnetic field 

Proton NMR spectrum of methanol. The more shielded methyl protons appear toward the right of the spectrum (higher field) the less shielded hydroxyl proton appears toward the left (lower field) 

Details of the FT process are given elsewhere [1], but briefly, an NMR spectrometer generates pulsed RF radiation (typically of microsecond duration) which excites nuclei in the sample to non-Boltzmann energy levels the resulting decay to the ground state generates RF radiation 

At the other extreme, there are only five 600 MHz NMR spectrometers in the UK they are at the Universities of Edinburgh, Oxford and Leicester, with the remaining two at the laboratories of Glaxo and 

Wellcome. Both Oxford and Wellcome have also recently bought 750 MHz NMR spectrometers. 

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Replacing one of these protons by chlorine as a test group gives (/ )-2-chloro-l-propanol replacing the other gives (.S)-2-chloro-l-propanol. Enantiotopic protons have the sane chemical shift, regardless of the field strength of the NMR spectrometer. 

The Fourier transform of a pure Lorentzian line shape, such as the function equation (4-60b), is a simple exponential function of time, the rate constant being l/Tj. This is the basis of relaxation time measurements by pulse NMR. There is one more critical piece of information, which is that in the NMR spectrometer only magnetization in the xy plane is detected. Experimental design for both Ti and T2 measurements must accommodate to this requirement. 

Fig. 28. Room temperature 2H NMR spectra of the smectic liquid crystalline polymer (m = 6), oriented in its nematic phase by the magnetic field (8.5 T) of the NMR spectrometer with director ii parallel (left) and perpendicular (right) to the magnetic field...

The next step is to set the same conditions for the HPLC system which is coupled with the NMR spectrometer. The field homogeneity of the probehead is first optimized (shimmed) using the same separation column and solvent mixture. 

Moving on to some wider stereochemical considerations, just as enantiomers are indistinguishable as far as their physical and chemical properties are concerned (except, of course, as regards their reactions with other optically active reagents) so their spectra, acquired under normal conditions, are identical. The NMR spectrometer does not differentiate between optically pure samples and racemic ones. Note there is a way of differentiating between enantiomers by NMR but it involves using certain chiral reagents which we ll discuss in detail later. 

We have seen that the spectra of enantiomers, acquired under normal conditions, are identical. The NMR spectrometer does not differentiate between optically pure samples, and racemic ones. The wording is carefully chosen, particularly normal conditions , because it is often possible to distinguish enantiomers, by running their spectra in abnormal conditions - in the presence of a chiral resolving agent. Perhaps the best known of these is (-)2,2,2,trifluoro-l-(9-anthryl) ethanol, abbreviated understandably to TFAE. (W.H. Pirkle and D.J. Hoover, Top. Stereochem., 1982,13, 263). Structure 7.4 shows its structure. 

The OPENCORE NMR project is not going to follow up what the modern commercial systems offer. Since the developer has been trying to keep the system at the root of evolution of the NMR spectrometer, efforts have been paid to keep the system simple, or even to make the system simpler. In this sense, it would be fair to mention that the commercial spectrometers would be much more convenient for many practical circumstances. Nevertheless, the author believes that there is room for making contributions in scientific researches through the OPENCORE NMR project, which is looking at the different direction of development from that of the other spectrometers. 

The Fourier transform (FT) relates the function of time to one of frequency—that is, the time and frequency domains. The output of the NMR spectrometer is a sinusoidal wave that decays with time, varies as a function of time and is therefore in the time domain. Its initial intensity is proportional to Mz and therefore to the number of nuclei giving the signal. Its frequency is a measure of the chemical shift and its rate of decay is related to T2. Fourier transformation of the FID gives a function whose intensity varies as a function of frequency and is therefore in the frequency domain. 

The output of the NMR spectrometer must be transformed from an analog electrical signal into digital information that can be stored in the computer s dedicated computer. The minicomputers used in NMR spectroscopy have memory used for data accumulation, programs for manipulating the data, and storage devices to store large collections of data for future or additional manipulation into useful spectral results. 

When the tube has been placed inside the NMR spectrometer, the procedure used is exactly as for a standard NMR tube. 

The polarization patterns are dependent upon the strength of the magnetic field, in which the reactions are carried out. If the reactions are carried out at high fields (i.e., notably within the NMR spectrometer), the resonances appear in antiphase - that is, there is an equal number of absorption and emission lines and no net polarization. At low field however (i.e., when the reaction is carried out at zero or a very low field and then transferred into the high field of the NMR spectrometer for subsequent investigation), the resonances display net polarization, as has been outlined by Pravica and Weitekamp [9]. 

Figure 12.3 outlines the essential features of the PASADENA/PHIP concept for a two-spin system. If the symmetry of the p-H2 protons is broken, the reaction product exhibits a PHIP spectrum (Fig. 12.3, lower). If the reaction is carried out within the high magnetic field of the NMR spectrometer, the PHIP spectrum of the product consists of an alternating sequence of enhanced absorption and emission lines of equal intensity. This is also true for an AB spin system due to a compensating balance between the individual transition probabilities and the population rates of the corresponding energy levels under PHIP conditions. The NMR spectrum after the product has achieved thermal equilibrium exhibits intensities much lower than that of the intermediate PHIP spectrum. 

The formation and decay of these product-catalyst-7i-complexes are expected to occur according to the sequence of reactions as outlined in Scheme 12.4. The kinetic constants associated with the occurrence of kHYD and the decay of k0FF> respectively, can both be determined by PHIP-NMR using a process termed dynamic PASADENA (DYPAS) spectroscopy, as has been outlined previously [37]. For this purpose the addition of parahydrogen to the reaction is synchronized with the pulse sequences of the NMR spectrometer, whereby the time for acquiring the NMR spectra is delayed by variable amounts. The results thereof are listed in Table 12.3. A variety of kinetic constants can be determined, and the method is reasonably accurate the margins of error are also indicated in Table 12.3 [37]. 

It is worthwhile pointing out that it is desirable to acquire all spectra under identical conditions, such as hydrogen pressure, elapsed time prior to the acquisition of the spectrum, temperature, and amount of catalyst used. For this reason it is desirable to use a set-up which permits the spectra to be recorded in totally standardized fashion, which does not depend on any individual human factor . Such a system would allow the reaction to be conducted at a low magnetic field and would thereafter transfer the solution automatically (notably quickly and adiabatically ) into the NMR spectrometer for subsequent analysis. 

The best results are obtained when using substrates associated with high hydrogenation rates and long spin-lattice relaxation time for all nuclei of interest, and if the reactions are carried out in the absence of the strong field of the NMR spectrometer. Therefore, in order to study the consequences of polarization transfer to 19F, the hydrogenations of 19F-containing ethynylbenzenes and... 

Watanabe et al. published the first paper to appear in the literature dealing with the coupling of LC and NMR in 1978 [82], This early exploration of LC-NMR led to the modification of a standard NMR probe to include a flow cell comprised of a thin-wall Teflon tube with an inner diameter of 1.4 mm. The dimensions of this flow-cell were 1 cm in length and a total volume of 15 pi. This modification not only made the NMR spectrometer amenable to a flow system, but also overcame some of the inherent sensitivity issues associated with NMR as an LC... 

It is appropriate at this time to discuss some of the limitations associated with LC-NMR. It is more accurate to say the limitations of the NMR spectrometer in an LC-NMR instrument. As compared to MS, NMR is an extremely insensitive technique in terms of mass sensitivity. This is the key feature that limits NMR in its ability to analyze very small quantities of material. The key limiting factor in obtaining NMR data is the amount of material that one is able to elute into an active volume of an NMR flow-probe. The quantity of material transferred from the LC to the NMR flow-cell is dependant on several features. The first being the amount of material one is able to load on an LC column and retain the resolution needed to achieve the desired separation. The second is the volume of the peak of interest. The peak volume of your analyte must be reasonably matched to the volume of the flow-cell. An example would be a separation flowing at lml/min with the peak of interest that elutes for 30 s. This corresponds to a peak volume of 500 pi, which clearly exceeds the volume of the typical flow-cell. This is the crux of the problem in LC-NMR. There is a balance that must be struck between the amount of compound needed to detect a signal in an... 

While the early days of LC-NMR and LC-NMR-MS were plagued by the poor sensitivity of the NMR spectrometer, the recent probe design advances have provided a means to potentially overcome this hurdle. As reported in the literature, it is possible to get both ID and 2D homo-nuclear and heteronuclear correlation data on sub micrograms of materials in quite complex mixtures utilizing cryogenic flow-probes in tandem with SPE peak trappings [98]. While these technologies are still in their infancy, they have the potential to revolutionize LC-NMR as a structure elucidation technique. 

Artifacts may be roughly categorized into those due to inherent limitations (e.g. pulses cannot excite unlimited bandwidths even if all hardware components work perfectly) and those that result from improper set-up of the experiment or nonideal functioning of the NMR spectrometer system. In this chapter we will mainly focus on the latter two. These artifacts are more likely to appear in multiple-pulse experiments. Quite often, they are avoided by clever programming of the experiments (e.g. interleaved acquisition of data for NOE spectra, use of pulsed-field gradients instead of phase-cycling). 

As described in Section 10.2, the final output from the NMR spectrometer to the computer is an FID. Typically 2048 096 digital points are accumulated in the FID and the next step is to improve the potential resolution of the FID by zero-filling the FID to 16384 digital points by adding zeroes to the end of the FID. Upon Fourier transformation (FT) the resultant spectrum contains 16384 points describing a spectral width of 2000-4000 Hz depending on the settings in the ADC. 

Automation of LC-NMR is now at a stage where the operator can inject a sample and leave the HPLC interface to detect and store peaks and the NMR spectrometer to collect one- and two-dimensional data with signal-to-noise-dependent collection. An example of automated loop collection and transfer of closely eluting peaks is shown in Figure 6.38. Structures were deduced from the aromatic peak patterns and LC-MS information. Peaks 1-5 all elute within 5 min with no carry-over present in any of the H spectra. 

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