Carbon‐13 Nuclear Magnetic Resonance Spectroscopy

Abstract

AbstractCarbon‐13 (13C) nuclear magnetic resonance spectroscopy (NMR) is the measurement of the precession or resonance frequencies of the net magnetization for13C nuclei whose individual magnetic moments have been oriented in a strong magnetic field. Nuclei differing in their electronic shielding precess about the magnetic field at different Larmor or resonance frequencies. A high‐power radiofrequency (rf) pulse is used to perturb the magnetization vectors from their equilibrium distribution, generating an observable transverse magnetization. Precession of this net magnetization vector about the static magnetic field induces a voltage into the NMR probe coil. Relaxation pathways promote the repartitioning of the individual magnetic moments to their equilibrium Boltzmann distributions and a dephasing of the individual magnetization vectors in the transverse plane. This signal is detected as a function of time through a phase‐sensitive receiver, digitized and Fourier transformed (FT) into a frequency domain spectrum. The NMRs, referenced relative to the resonance frequency of a standard are shifted in a manner characteristic of hybridization of the atom, electronegativity of the substituents attached and the steric environment of the nucleus. These shifts typically follow a standard set of rules and thus spectra can be simulated either empirically through measurement of the additive effects of substituents or through ab initio and semi‐empirical computational methods. Scalar couplings to directly attached hydrogen‐1 (1H) nuclei split resonance lines into (nH + 1) lines and require the use of broadband1H decoupling to remove this splitting for sensitivity enhancement. The presence of attached or nearest neighbor1H atoms also provides a means of enhancing the signal intensity of13C spectra and for selective observation of13C signals through polarization transfer. Multidimensional NMR experiments are available which permit the13C13C or13C1H correlations as well as a means of measuringnJCHcouplings.Analysis of samples can be done without the need for internal standards or calibration standards since NMR signals are directly proportional to the moles of analyte present and there are no response factors or absorptivities to be determined for quantitative analysis. Although13C NMR spectroscopy is widely used and is a very powerful structural technique in organic and polymer chemistry, the technique suffers from an inherent lack of sensitivity due to a natural abundance of only 1.1 percent and a small magnetogyric ratio (γC). Long relaxation times present in small molecules undermine the quantitative accuracy of this method or require lengthy amounts of instrument time to acquire an NMR spectrum. Quantitative results can be obtained with the selection of appropriate experimental parameters often combined with the use of relaxation agents.