After 1H, 13C is easily the next most important nuclide in the NMR periodic table; 13C measurements can provide a wealth of valuable structural info. Unfortunately, with a receptivity that is around 5,500 smaller than that of 1H, 13C is a much less sensitive nuclide. This lower sensitivity demands the maximum performance from the NMR spectrometer to keep the measurement times and sample concentration within practical limits. Since 13C NMR has the reputation to be challenging even for high field spectrometers, people tend to think that only overnight experiments can be performed on bench top systems. In the first example below we want to show you that even at frequencies like 43, 60 or 80 MHz high quality 13C spectra can be acquired in a single scan. If your goal is to teach the principles of 13C NMR to students, it is worth knowing that good 13C NMR spectra can be acquired on concentrated organic liquid samples in just under a minute. Moreover students can collect a whole set of powerful multidimensional heteronuclear experiments in well under an hour. The spectrum below of neat propylbenzoate could serve as a useful example for teaching 13C NMR in an educational environment.
Figure1: 1D 13C NMR spectra of neat Propylbenzoate acquired with a single scan (blue), 4 scans (green) and 16 (red) scans totalling 5, 20 and 80 seconds of acquisition time respectively.
As the concentration of the sample decreases the experiment will take more time depending on the concentration and 13C sensitivity of your instrument. As described in the previous post, the effect of lower sensitivity can cause the experiment time to increase dramatically because of the square root relationship with the number of scans. Because 13C sensitivity is such a critical parameter we want to provide some standards that can be used as reference to understand and evaluate 13C measurements when considering a benchtop NMR instrument.