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Cross-polarization is usually used to assist in observing the dilute spins, such as C13, though it can also be used to preform some spectral editing, and to obtain information on which spins are close in space. Because of the low abundance of some elements, the signal-to-noise ratio of such nuclei could be very poor, in addition, the relexation time of low abundance nuclei tend to be very long, which means that you have to wait for one day or longer time to get the spectral. The CP tech can solve this problem well. In a doubly rotating frame, taking H1 and species X for example, the CP transfer is mediated by the dipolar interaction between H1 and X spins. At first you will give a 90 degree pulse to the H1, and then adjust the magnetic field strengh of those two to match the Hartman-Hahn condition, and the effect of the sequence is to transfer magnetization from the abundant H1 spins in the sample to the X spin via the agency of the dipolar coulpling between H1 and X spins. * r3 n+ W! }% N" \' s
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Nuclear spins exhibit a dipole moment, which interacts with the dipole moment of other nuclei. The magnitude of the interaction is dependant on the spin species, the internuclear distance, and the orientation of the vector connecting the two nuclear spins with respect to the external magnetic field. The dipolar coupling interaction depends on the orientation of the internuclear vector with the external magnetic field by D~3cos2θ－1. Consequently, two nuclei with a dipolar coupling vector at an angle of 54.7° to a strong external magnetic field, which is the angle where D becomes zero, have zero dipolar couping. θ is called the magic angle. $ Y, s5 q; D, m& M A
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Hope you can find what you want to know in my explanation. | 
发表于 2008-12-28 21:38:31
