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)