 小中大资料：核磁共振 NMR Basics FAQ 3 核磁共振 NMR Basics FAQ 3
I can't phase correct my spectrum.
The aph (automatic phase correction) command usually does a good job of correcting the phase, and should be the first thing you try. Sometimes (for example in noisy spectra) the aph command is unable to correct the phase, and in these situations it often leaves lp at a high value (say one or two thousand). In these situations you will have to correct the phase manually. First a couple of obvious things: if you ran a DEPT or APT experiment or something similar, there will be some positive and some negative peaks, so don't try and phase them all positive! Similarly in a 1:1 binomial solvent suppression sequence, half the spectrum will be positive and half negative.
Having established that you are not running an exotic pulse sequence that produces strange phases, the next thing to consider is foldback. Are you sure that you used a large enough spectral width when acquiring the spectrum? If one or more resonances occurred outside the observe region, the method used to digitise the signal results in these resonances appearing within the observed spectral width, but with a phase error. If in doubt, double or triple the spectral width, run the spectrum again, and see if the resonance that could not be phase corrected remains at the same chemical shift as before.
To perform manual phase correction, proceed as follows:
Type lp=0 rp=0. This sets the left phase and right phase to zero. On Varian spectrometers, "right phase" is the zero-order phase adjustment and "left phase" is the first-order phase adjustment. The zero-order phase affects the entire spectrum equally, while the first-order phase is frequency dependent. The zero-order phase should always be in the range -360° to +360° and the first order phase should also usually be in this range. If you have a first order phase correction of more than a thousand degrees, not only is it probably incorrect, but you will also probably be generating baseline roll. On Gemini or XL spectrometers, type QP to get into quick phase mode. On Sun based spectrometers or data stations, click the "phase" button with the mouse. Perform a zero-order phase correction on the largest peak as described in the manual for the spectrometer you are using. Now choose another peak some distance from the largest peak, and adjust the first-order phase. On Sun based systems you only get one shot at adjusting the zero-order phase - all subsequent corrections are made to the first-order phase, so there is no point clicking on the largest peak again. If you want to readjust the zero-order phase, get out of the phase-correction routine (by for example typing ds) then click on the phase button again. If for some reason a large first-order phase correction is required, it may be easier to choose a peak for the first-order adjustment that is close to the peak you used for the zero-order adjustment. On Sun based systems, set the phasing parameter to 100. This causes the effect of the phase values to be shown for the entire spectrum as you are making the adjustments, thus making it easier to see what you are doing.
I need to run my spectrum at a higher field to get better resolution.
No you don't! The resolution of a high field spectrometer may even be worse than a low field spectrometer. What a high field instrument has more of is dispersion. This means that resonances with different chemical shifts are further apart. Multiplets due to coupling will not show any improvement unless the higher field instrument separates overlapping multiplets with different chemical shifts, or the multiplet showed strong coupling effects at lower field. Some nuclei such as 31P may have worse resolution because of a property called chemical shift anisotropy which increases with field strength.
There are no parameters for the solvent I want to use.
If you're running a proton spectrum, set up for 1H / CDCl3, double the spectral width, run a quick spectrum, and put the two cursors around the spectrum. Then do a movesw and acquire the final spectrum. If you're running a carbon spectrum, set up for 13C / CDCl3, increase the spectral width by 20 percent, and run as normal. If your solvent has carbon nuclei which show up quickly, reference the solvent and check that the observed spectral range is correct.
If you are running a phosphorus spectrum, set up for 31P / CDCl3, increase the spectral width by 20 percent, and run as normal. Then supply an NMR tube containing the solvent to the NMR staff so that they can set up H3PO4 referencing parameters for you.
How can I suppress a strong solvent resonance in a proton spectrum?
If the solvent signal is less than two or three times the size of the largest signal from your compound, it may not be worth bothering. On the Geminis, XL200 and VXR300, the usual method is to presaturate the solvent signal using the decoupler (instructions for doing this are in the folders near the spectrometers). Although it is simple, this method has the disadvantage that NH or OH protons that are exchanging with water also have their signals reduced or eliminated. Another method is the 1:1 binomial pulse sequence. The signals on one side of the solvent resonance are of opposite phase to the other side when this method is used. On the Inova spectrometers, the method of choice is watergate solvent suppression. Simply set the observe transmitter on the solvent position, type wgate and acquire a spectrum. The watergate sequence set up by the wgate macro uses hard pulses and therefore does not require pulse phases etc. to be optimised. The other watergate technique available uses shaped pulses. Simply type autowatergate, and wait while it automatically optimises the parameters and runs a final spectrum. Watergate is only available on the Inovas because it uses pulsed field gradients. If chemical exchange is very rapid, watergate may not be suitable, in which case a binomial pulse sequence is the best choice.
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