VII. 13C-{1H} NMR Spectra - II

The ¹³C{¹H} spectrum obtained using a standard 1PULSE experiment is not quantitative, i.e., the integration of the peaks will not give a true indication of relative ratios because of the nuclear Overhauser enhancement (nOe) of the ¹³C nuclei due to their attached ¹H nuclei. For the case of ¹³C spectra acquired with proton decoupling, an enhancement of up to 1.98 (i.e., 198%), or an almost threefold improvement in signal-to-noise is expected for those carbon nuclei that are attached to protons.

The ¹H digital resolution given by the default parameters on both the Unity+300 and VXR-S 400 differ from the ¹³C digital resolution by a factor of five. This is mainly due to the need for a larger spectral width used to accomodate the wider chemical shift range for ¹³C and the need to use a fewer points to conserve space on the hard disk. Due to the large spectral width typical of ¹³C{¹H} spectra, it is important that the number of points not be too small, or distortions of the peaks can occur. Figure VII-4 shows a spectrum of 30% menthol in CDCl₃ collected with only 8192 points at 75.432 MHz. Compare it to Figure VII-2 and note the anamolous peak heights in Figure VII-4, as well as the phase distortion in all the peaks. These effects are due to an insufficient number of data points and is not an instrument problem. The distortion can be eliminated by increasing the number of data points as is the case in the standard default parameters for ¹³C NMR spectroscopy on the Unity+300 and VXR-S 400 NMR spectrometers.