NMR Active Nuclei & Benchtop NMR

One of the common questions that we get asked here at Nanalysis is: “what nuclei does the NMReady run?”
Currently that answer is: “proton and fluorine.”
The typical response: “Oh really……not carbon??”
Our classic answer: “Fact is, this new technology is not yet wired to measure carbon-13 NMR – but it’s not impossible!”

Why not?

1) Carbon is one of the proverbial dogs of NMR nuclei (along with 15N and 17O, of course).  This NMR is plagued with two standard plights:  Low natural abundance.  Poor receptivity (see table below).   The NMR active carbon isotope marks only 1.07 % of the carbon present (cf., 99.99 or 100% for 1H and 19F, respectively) and it is intrinsically less receptive (by 4 orders of magnitude vs. both 1H and 19F!!).

Some Properties of Selected Magnetically Active Nuclei[i]

[i]Data taken from: Silverstein, Robert M.; Webster, Francis, X.; Kiemle, David J.. “Spectrometer Identification of Organic Compounds” 7th Ed. John Wiley & Sons, Inc.: USA

All chemists struggle with these less than desirable NMR characteristics – regardless at what field the data is acquired. Even at high field, carbon data collections usually require an overnight time slot and samples are generally prepared at higher concentration.  Unfortunately, given the lower resolution and sensitivity of a 60 MHz instrument, these realities are even more challenging.  Samples would need to be very concentrated and would likely require a 24 hour acquisition to get suitable data.

 

2) On a 60 MHz instrument, such as the NMReady, the resonance Larmor frequency of carbon is ~15 MHz (rough rule of thumb is carbon is ¼ the field strength of proton).  Despite the large spectral window of carbon NMR (~0 – 250 ppm), this could be problematic given that carbon NMR provides two pieces of vital information:
(i) the types of carbon present; and (perhaps more importantly)
(ii) the number of carbon present.  At such a low frequency this valuable information may be lost.  If this is the case, it could be confusing and potentially misleading for structural elucidation.

 

Conclusion:

Subsequently, carbon NMR was not an initial priority for Nanalysis.  This does not mean that it is impossible to tune for carbon NMR; just that proton and fluorine were identified as more suitable test nuclei for for the benchtop machine as they were predicted to be more useful for rapid, routine NMR analysis.