Elimination of solvent signals in carbon magnetic resonance spectroscopy

Abstract

The problem of a strong or obscuring solvent signal in proton resonance spectroscopy was alleviated by the use of perdeuterated solvents. The practice of stabilizing the magnetic field-frequency ratio for the spectrometer via the deuterium resonance of the solvent followed from the almost routine use of deuterated solvents and was continued for carbon magnetic resonance. Perdeuterated solvents depleted in 13C were developed to remove interfering solvent signals in those cases where the reactivity of the reagents restricted the choice of solvent. However, these are expensive and only available for a few solvents. It has recently become possible to distinguish the number of hydrogen atoms directly bonded to a carbon atom by the modulation of the signal of that carbon atom in response solely to an appropriately gated proton decoupling sequence (I4). The inclusion of 180 and 90” proton pulses in the overall pulse sequence, although more complicated and demanding upon instrumentation, has been familiar longer as INEPT type experiments (5,6). With both types of pulsing sequences the spectrum can be displayed with lines arising from carbon atoms with an even number of attached hydrogens pointing down and those with an odd number pointing up. If such a spectrum is added with one obtained in the normal manner there should be complete cancellation of signals from quaternary and secondary carbons and reinforcement of signals for tertiary and primary carbon atoms. For most organic molecules the cancellation of signals from secondary and quaternary carbons is not quite complete because of long-range carbon-hydrogen coupling and because of the range of values found for one-bond carbon-hydrogen coupling constants. Therefore very weak signals are often found for them. In perdeuterated molecules the cancellation of the carbon resonance signals should be complete since there is no carbon-hydrogen spin coupling. This is illustrated in Fig. 1 for spectra acquired by the common one-pulse sequence and by the solvent signal removing sequence of Fig. 2. It can be seen that the signal of the quatemary carbon in 2-methylbutene-2 (indicated by an arrow) is not completely canceled. The deuterated tetrahydrofuran (Merck) was claimed to contain 99% deuterium. Therefore 8% of the molecules will contain one hydrogen. These are responsible for the residual solvent signals. NMR studies of model anion-pairs valuable in the investigation of