Why are my 13C peaks split?
This is a good question and there are a number of reasons why your carbon peaks may be split into mutiplets. Incredibly enough some of these may even fall into the "Something is wrong with the spectrometer. I know it's not my chemistry." category. The following list is by no means comprehensive but rather a listing of some of the more common reasons you may see "carbon splitting". Some of these are specific to the MIT facility but some are not. These include:
Probe tuning (most common)...
A poorly tuned probe is the most common reason we see for split 13C peaks. In order for the spectrometer to most effectively and efficiently decouple you must tune both the proton ( 1H ) and carbon ( 13C ) channels. When decoupling, as with most all NMR experiments, it is important to remember that the pulse width is related to how well the probe is tuned. A poorly tuned probe will have a correspondingly 'longer' pulse width. As a result, the 'pulse timing' will be wrong for a given pulse sequence. When tuning a probe it is best to iterate between the channels since the tuning of one channel can (and most likely will) influence the other.
Incomplete decoupling (can be related to probe tuning)...
Incomplete decoupling can result from a poorly tuned or a poorly calibrated probe. If you have determined that the probe is well tuned it's possible that the probe may need to be recalibrated. Here the decoupler field strength is determined by measuring the residual couplings in a 60% C6D6/ 40% dioxane sample. The strength of the decoupler field is know as gH2 (gamma-H-two). The field strength is important to know since numerous pulse sequences require decoupler pulses which require a knowledge of the decoupler field strength. For example, the decouple field strength is required to properly set up Waltz-16 decoupling since the decoupler modulation frequency (dmf) is set equal to 4*gH2. If you feel the decoupler field strength needs to be examined please notify a staff member.
Low decoupler power output (can be hardware or software related)...
Incorrect probe file loaded (software)...
Some of the facility spectrometers (the Varian Unity 300, the Varian Inova 500 and 501 in particular) have several probe options available, and the probe used depends upon the desired experiment. Each of the probes has a probe file which contains the necessary calibration data (i.e., pulse width, transmitter power, decoupler power, etc.) required for each probe. If the wrong probe file is loaded it can not only have deleterious effects on your experiment it may actually damage the probe. Verify that the correct probe file is loaded by typing probe? in the input window. The currently selected probe will be displayed in the status window. The current software selected probe should correspond to the currently installed probe. The currently installed probe is clearly marked on the probe body.
Decouple mode (software)...
There are numerous decoupler modulation modes, some more or less efficient than others. The DCIF standard 13C parameter default mode is Waltz-16 (dmm='w'). Some of the other modulation modes along with abbreviations are:
- Continuous wave (dmm='c')
- fm-fm modulation (dmm='f')
- GARP (dmm='g')
- Noise modulation (dmm='n')
- Square-wave modulation (dmm='r')
- XY32 modulation (dmm='x')
Poor shimming can manifest itself in split 13C peaks. Actually, if the shims are bad enough you can split almost anything. Our advice for bad shims is to keep shimming. If you are having trouble shimming see our "My sample won't shim." help page.
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