Cheat codes for 13C qNMR

Proton (¹H) remains the most widely used nuclide for nuclear magnetic resonance (NMR), however, heteronuclear NMR certainly has its niche and uses. In today’s blog, I will be talking about ¹³C quantitative NMR (qNMR). It is not as common of a nuclide to use in qNMR studies due to the inherent difficulties as compared to ¹H qNMR. When conducting a qNMR experiment, the T₁ relaxation times need to be considered in order to obtain accurate integrations. Typically, an interscan delay of ~5 times the longest T₁ would be chosen to allow the full relaxation (or a very close approximation!) of the signals of interest. However, unlike ¹H resonances, ¹³C resonances typically experience much longer T₁ times. Additionally, due to the natural abundance of ¹³C (1.1%) compared to ¹H (99.98%), these experiments often take much longer to perform, as more scans are required to build a sufficient signal-to-noise ratio (SNR) for accurate integrations. Another consideration for ¹³C qNMR is the nuclear Overhauser effect (NOE), which can lead to confusion when choosing a decoupling mode.¹ If you would like to read more on which decoupling mode to choose, please read our previous post here and expect a follow-up describing the NOE in the near future!

In this study, analogous to a previous blog post by Glenn Facey, I analyzed acetone neat using a 0 second scan delay, 10 second scan delay, and lastly, after adding 5 mg of chromium(III) acetylacetonate, Cr(acac)₃. This paramagnetic metal complex is used in NMR studies as a relaxation agent.¹ Acetone has the molecular formula (CH₃)₂CO, which contains 2 types of carbon environments that we would expect to integrate to a 1:2 ratio. Shown in Figure 1a and Figure 1b are ¹³C{¹H} NMR spectra of acetone using a 0 second delay and 10 second delay, respectively. Evidently, we observe inaccurate integration ratios for both spectra with ratios of 0.6:2 in Figure 1a and 0.7:2 in Figure 1b, indicating that a 10 second scan delay is still insufficient.

Figure 1. ¹³C{¹H} (15.1 MHz) NMR spectra of neat acetone acquired using a 0 second scan delay (a) and 10 second scan delay (b). An inverse gated decoupling sequence was used for these experiments.

Figure 2 shows the ¹³C{¹H} spectrum after adding 5 mg of Cr(acac)₃ and using a 0 second scan delay, where the regions now integrate to 1:2, as expected.

Figure 2. ¹³C{¹H} (15.1 MHz) spectrum of neat acetone with 5 mg of Cr(acac)₃ added and acquired using a 0 second scan delay. An inverse gated decoupling sequence was used for this experiment.

As shown in this blog post, when conducting ¹³C qNMR studies, one can bypass the stupendously long T₁ relaxation times prevalent in ¹³C NMR just by adding a simple relaxation agent. As a note, if you use this method, please be wary that adding too much relaxation agent can lead to broader signals and a subsequence loss of SNR. Thanks for reading and if you have any questions, please do not hesitate to contact us!

[1] Findeisen, M.; Berger, S. in 50 and More Essential NMR Experiments, 4^th ed., Wiley-VCH, 2014.