NMR基础知识简介摘要.ppt

* For a single carbon spin or a few spins, we can still unravel the coupling patterns, however when a lot of chemically different carbon atoms are present in the sample, the scalar coupling of the carbon with the proton spins give rise to a very complicated spectrum. * When the proton spin is up, the carbon resonates at w+J/2. After the proton p-pulse, the carbon resonates at w-J/2. If we apply a series of p-pulses at the proton resonance, and if we do this fast compared to the size of the J-coupling, the carbon will resonate at the average of the frequencies (w-J/2) and (w+J/2), which is w. The carbon thus no longer sees the effect of the proton spin, hence it is decoupled. * intensity is distributed over two or more resonances. * Preparing for a carbon experiment with proton decoupling: 1. Measure a proton spectrum; 2. Set O1 to the center of the proton resonances; 3. Use the proton O1 value as the O2 value in the carbon experiment; 4. Measure the carbon spectrum with low decoupling power (pulse program zgcw); 5. Increase the proton power until sufficient decoupling is achieved, while monitoring the lock signal (it will drop when the sample heats up) and the FID (which will show spikes if the probe starts arcing). * * * * * * * * * * * * * * * Detector si sigledirection.it can only detect the magnitization along detecting direction. * * * * * * * * * * * * * * * * * * * * In the example below the proton chemical shifts are referenced to the proton resonance frequency of TMS. (Set frequency scale to ppm (HZ/PPM); press calibrate;middle mouse button on TMS peak; type 0 ppm). * The presence of a nuclear spin will cause a small perturbation of its surrounding electron cloud. In the case of a chemical bond the electron clouds overlap, and as a result of this perturbation the neighboring spin feels a slightly different magnetic field, dependent on whether the spin that it is coupled to is up or down. Note that the spins must be from nuclei that are part of the same