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资料：核磁共振 NMR Basics FAQ 4

　　核磁共振 NMR Basics FAQ 4

　　What is nuclear spin?

　　All nuclei carry a charge. In some nuclei this charge "spins", causing the nucleus to behave like a tiny bar magnet. This is why it aligns with or against the magnetic field of an NMR spectrometer. However unlike a bar magnet, the low energy state is aligned with the field and the high energy state is aligned against the field. Up to now we have been talking about nuclei with a uniform spherical charge distribution. These nuclei are said to have a spin of ½. Protons, 13C and 31P are all spin half nuclei. Note that the most common isotope of carbon, 12C, has no spin and can therefore not be observed using NMR. Nuclei with a non-spherical charge distribution have a spin number I of 1, 3/2 or higher (in steps of ½ ), and are referred to as quadrupolar nuclei. Spin ½ nuclei have two orientations (with or against the field). Spin 1 nuclei have three orientations, spin 3/2 nuclei have 4 orientations, etc. Deuterium is an example of a spin 1 nucleus. Although deuterium is chemically the same as hydrogen, for the purposes of NMR it is completely different. For example a carbon spectrum of CDCl3 is a 1:1:1 triplet regardless of whether you turn on the proton decoupler. This is because the deuterium attached to the carbon can have three orientations, and occurs at a different frequency to protons.

　　What is a double quantum coherence?

　　When you put your sample in the magnet, all the spin half nuclei align either with or against the magnetic field. The population difference between these two orientations (known as the Boltzman distribution) is field dependent, and is determined by their energy difference. An NMR signal is observed when nuclei flip from one orientation to the other. This is a single quantum coherence. When two nuclei are coupled, they can flip together as though they were a single unit. If they flip in opposite directions, the flips "cancel each other out" (sort of) resulting in a zero quantum coherence. If they both flip the same way, you get a double quantum coherence. The frequency of a zero quantum coherence is between zero and a few kilohertz, so it is not directly observed. Similarly the frequency of a double quantum coherence is roughly twice the normal observe frequency, so that is not observed directly either. You can also have triple quantum coherences from groups of three coupled nuclei. The effect of double and triple quantum coherences can only be observed by inserting pulses or delays into a pulse sequence to convert them to single quantum coherences before acquisition of the NMR signal. Do not confuse double quantum coherences with coupling in a normal spectrum. A doublet for example, arises when there are two coupled spins, but only one of these spins flips.

　　What are pulsed field gradients?

　　Imagine if you could really mess up the Z1 resolution for a few milliseconds then restore it to its proper value during the course of the pulse sequence. This is an oversimplification, since pulsed field gradients do not use the normal shim circuits. A special PFG probe, and a PFG amplifier are necessary. By applying a gradient to the magnetic field, the top of the sample experiences a slightly different magnetic field to the bottom of the sample. Since magnetisation precesses at different rates in different fields, it is possible after a 90 degree pulse and a PFG of a few milliseconds to have the magnetisation vectors along the length of the tube pointing in all directions instead of nicely aligned along one axis of the rotating frame. Obviously if the magnetisation vectors are pointing in all directions, there is no net signal. The vectors are said to be dephased. If you now apply a PFG of opposite sign for the same time, you will rephase the magnetisation, and get your signal back. You could achieve the same thing by giving the dephased vectors a 180 degree pulse, then applying a PFG of the same sign. The other thing to be aware of is that double quantum coherences dephase at twice the rate of normal single quantum coherences, so by adjusting the strength or duration of pulsed field gradients, you can select single, double or triple quantum coherences. The "old fashioned" way of selecting certain types of coherences is to use elaborate phase cycles which cause the unwanted signals to cancel out on successive scans. The PFG method acquires only the desired signal on each scan, resulting in fewer artifacts and allowing fewer scans. The old method can be thought of as "cancellation of unwanted signals over time" whereas the PFG method can be thought of as "cancellation of unwanted signals over space" where "time" refers to successive scans, and "space" refers to the physical length of the sample in an NMR tube.

　　What is the Nuclear Overhauser Effect?

　　Glad you asked. Have a look at our NOE guide.

　　How do I run a quantitative spectrum?

　　A quantitative spectrum is simply a spectrum where you can trust the integral ratios. In other words, if the integral of resonance A is twice the height of the integral of resonance B, you can say with certainty that resonance A is due to twice the number of nuclei as resonance B. Why do we use integrals? Because it is the area of the resonances that is proportional to the number nuclei. The height of a broad line may be less than that of a sharp line, but its area may be greater. How do we get accurate integrals? By ensuring that all resonances are equally excited, well digitised, and properly relaxed.

　　Equally excited : if the pulse power is not high enough, some resonances far from the observe frequency may experience a reduced flip angle, resulting in a smaller observed signal.

　　Well digitised : if the number of data points in the spectrum is too low, there will not be enough points to accurately define each resonance, resulting in inaccurate integrals (and peak heights).

　　Properly relaxed : resonances that are not fully relaxed give a weaker signal than fully relaxed resonances. The nuclei in your compound will not all relax at the same rate, so if you pulse too rapidly the quickly relaxing resonances will appear stronger than the slowly relaxing ones. To be sure of obtaining accurate integrals, you need to measure the relaxation times of your compound, and set a delay equal to 5 times the longest relaxation time. Fortunately it is easy to run an inversion - recovery experiment to measure relaxation times.

　　It is harder to obtain quantitative carbon spectra, because carbon relaxes more slowly than protons, is less intense, and steps have to be taken to eliminate the Nuclear Overhauser Effect which builds up when protons are decoupled.