The NMR Magnet

From the foregoing discussion we can list the basic components of an NMR spectrometer. There will be a magnet to generate B0, an rf oscillator to generate B, in the transmitter coil, a receiver coil to pick up the signal, the electronics (including a computer and plotter) to turn the signal into a 

The three most important characteristics of the magnet in any NMR spectrometer are the strength, stability, and homogeneity of its magnetic field B0. Not only is the precessional (resonance) frequency of identical nuclei directly proportional to the strength of B0 (Section 2.2), but so is the difference in precessional frequencies (Av) of nonidentical nuclei  

The most common lock systems monitor the signal from deuterium (2H, 1= 1), so it is common in NMR to use deute-riated solvents such as DzO or CDC12 (deuteriated chloroform). Many such deuteriated solvents are readily available. 

As we will see later, this 2 ppm shift would be a very large shift indeed  

Once a stable field is established, the question remains as to whether that field is completely homogeneous (uniform) throughout the region of the sample. The level of homogeneity required for a given NMR experiment depends on the desired level of resolution, which in turn controls the precision of the measurement. In the case of H nuclei at 5.87 T, for example, Example 3.1 suggests that to achieve a precision of 1 Hz at a frequency of 250 MHz (four parts per billion ), the field must be homogeneous to the extent of 2.35 x 1 ()-S T  

This spinning helps to average out any slight inhoinogenei-ties of the field in the sample region. 

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The NMR magnetic shielding for atoms like carbon is affected greatly by what it is bonded to and the type of bond to its neighbor. Use the inner carbon atoms of normal butane as the reference atom and calculate the shift in C isotropic shielding for 2-butene and 2-butyne. Can you explain these shifts as a function of the changing molecular environments ... 

The strength of the NMR magnet is normally described by the frequency at which protons resonate in it - the more powerful the magnet, the higher the frequency. The earliest commercial NMR instruments operated at 40 megacycles (in those days, now MHz) whereas modem NMR magnets are typically ten times as powerful and the most potent (and expensive ) machines available can operate at fields of 1 GHz. 

Samples must be physically brought to the NMR magnet and probe via a sample line and sampling system. This limits the number of streams that a single NMR unit can effectively moiutor with a useful duty cycle. Heating of samples entering the NMR analyzer is required for several reasons ... 

NMR observes the chemistry of only the proton nucleus (though it can observe many other nuclei independently). This means that hetero and metallic chemistry cannot be observed directly. Thus, sulfur, nitrogen, oxygen, and metals cannot be directly analyzed by NMR, though secondary correlations can be obtained from the proton chemistry of the sample. In combination with electron spin resonance (ESR) analyzers that can operate in the fringe fields of the NMR magnet the presence of paramagnetic metals and free radicals can be quantified. 

Open Ixjtlom for the introduction of the tube into the NMR magnet... 

According to an X-ray structure determination (62) can best be described as a square pyramid having two P atoms and two CO ligands at the base and a third carbonyl at the apex, with the alkyl C atom below the basal plane pointing towards the Co centre, and the NMR magnetic equivalence... 

So the net magnetization at equilibrium is proportional to the number of identical spins in the sample (i.e., the concentration of molecules), the square of the nuclear magnet strength, and the strength of the NMR magnet, and inversely proportional to the absolute temperature. For example, M0 for H is 16 times larger than M0 for 13C because yn/yc = 4. This net magnetization vector is the material that we mold, transform and measure in all NMR experiments. 

A generalized CP pulse sequence is shown in Fig. 4.5.2, with vertical displacements indicating transmitter power and the horizontal axis indicating elapsed time. A CP pulse sequence begins with a pulse delay, td, to allow recovery of proton polarization along the static field of the NMR magnet. This can be achieved if td greatly exceeds T,(H), the time constant for recovery of equilibrium polarization. 

The first example (see Pig. 21.9) shows a H-PHIP spectrum ofthe hydration of perdeuterated styrene obtained under ALTADENA conditions, i.e. with hydration outside the NMR magnet taken from Ref [94]. The spectrum of the hydration product ethyl-benzene exhibits the typical strong spin polarized signals at 1 ppm and 3 ppm. Between 5 ppm and 6 ppm there are additional spin polarized signals, which stem from a side reaction. In Ref [94] it was shown that the ratio of hydration versus geminal exchange is controllable by addition of CO. 

Connect the upper part of the NMR assembly (Fig. 4.24(a)) to the lower part, and check the tuning and machining of the RF circuits (outside the NMR magnet). 

Shim the NMR magnet using a deuterium lock or free induction decay, (FID), signal. 

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