13C NMR of "Perdeuterated" Solvents

When one acquires a ¹³C NMR spectrum of a sample, the deuterated solvent is observed with resonances characterized by the J coupling pattern of the deuterons attached to the carbon atoms. (CD = 1:1:1 triplet, CD₂ = 1:2:3:2:1 quintet, CD₃ = 1:3:6:7:6:3:1 septet). Often, however; small peaks are observed near the main solvent resonances. These are due to other isotopomers of the solvent. An example of this is shown in the bottom spectrum of the figure below for the high frequency resonance of "THF-d₈". The spectrum consists mainly of the expected 1:2:3:2:1 quintet however, there is a small peak present on the high frequency side of the quintet marked by the arrow. It is due to one of the components of the C1 resonance from THF-1,2,2,3,3,4,4-d₇. The supplier of the solvent claims that the isotopic purity is 99.5 atom % D. If the deuteration is uniform, approximately 0.5 % of the molecules will contain a single proton. Half of these molecules will be mono-protonated on the high frequency carbons. If a DEPT spectrum is acquired on the same sample, the non-protonated carbons are suppressed leaving only the mono-protonated carbons. This is shown in the top spectrum of the figure, which shows a 1:1:1 triplet from CHD. The isotope shift between CHD and CD₂ is 0.354 ppm. Other examples can be found here and here.

Effect of 1H Tuning on the Signal-to-Noise Ratio in 13C NMR Spectra

Most liquid state ¹³C NMR data is collected with ¹H decoupling so that all of the ¹³C resonances (not coupled to nuclei other than protons) will appear as singlets. Since ¹³C is only 1.1% naturally abundant and often the chemist is limited by the quantity of sample, acceptable signal-to-noise ratios in ¹³C NMR spectra are often difficult to obtain in a reasonable period of time. Any hints to improve the signal-to-noise ratio are welcome. One way to ensure that the signal-to-noise ratio is what it should be, is to make sure that the proton channel of the probe is well tuned. A poorly tuned proton channel will lead to incomplete decoupling resulting in broad lines or residual splittings and hence a low signal-to-noise ratio. The reason for this is that when the proton channel is poorly tuned the sample receives less power from the proton channel of the spectrometer and the decoupler pulses in the decoupling scheme (usually WALTZ 16) are no longer calibrated for proper decoupling. This is illustrated in the figure below.For the spectrum in the left-hand panel, the ¹H channel was well tuned. In the right-hand panel, the ¹H channel was poorly tuned. One can see that the spectrum acquired with the poorly tuned ¹H channel has a lower signal-to-noise ratio. There are even more problems with pulse sequences (such as DEPT experiments) which require hard pulses from the ¹H channel. This sort of problem can also be seen in proton detected experiments with poorly tuned ¹³C channels when ¹³C decoupling is required (eg. HMQC/HSQC experiments)

Collecting 13C NMR Spectra of Highly Paramagnetic Compounds

I am sometimes asked to acquire ¹³C NMR spectra of highly paramagnetic compounds. Those of you attempting to acquire such spectra for the first time should be aware of the following:
1. You should use a large spectra width as the ¹³C chemical shifts can be very extreme. In order to get a wide uniform excitation, use very short high power pulses.
2. Both the T₁'s and the T₂'s are short, so you can use short acquisition times and short recycle delays.
3. The ¹H chemical shift range for paramagnetic compounds can span hundreds of ppm. The ¹H decoupling schemes used to collect ¹³C data do not permit such a broad decoupling bandwidth and you may have to collect several ¹³C spectra with different ¹H offsets. It is always best to collect a proton spectrum first to evaluate the ¹H decoupling needs in the ¹³C spectrum.
4. The lines are often very broad and therefore many scans are needed to build up the signal-to-noise ratio.
5. The ²H lock signal may be shifted by the paramagnetic compound, so don't be surprised if your automatic locking routines do not work.