Research Paper: NMR Spectra Nuclear Magnetic Resonance

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[. . .] Carbon 13 (13C) is not naturally abundant but provides a spin state of 1/2 or -1/2 and gives a NMR signal. NMR spectroscopy of 13C observes the skeleton of an organic molecule and allows determination of the chemical shift for every carbon atom whether a proton is attached or not. One useful property of 13C NMR is the broad chemical shift when compared to proton NMR chemical shifts. While a 1H NMR spectrum might have a shift of 10-20 ppm, a 13C spectrum can cover ranges of up to 200 ppm allowing far less overlapping of peaks for carbon atoms with different chemical shifts (Carey 2008). In addition, 13C has a low natural abundance and this low percentage results in few adjacent carbon atoms in a molecule that are 13C so there is no spin-spin coupling observed for 13C spectra. Coupling can occur between protons attached to 13C but the splitting can be minimized or negated by a magnetic field that is pulsed to align all of the protons, this type of spectrum is called a decoupled 13C NMR spectrum (Carey). The low abundance of 13C makes the estimation of relative peak intensity difficult; the primary information obtained from a 13C NMR spectrum is the number of carbon atoms in the structure and the relative chemical shifts which can allow assignment of structure.

2D NMR

The advent of pulsed spectrometers capable of analyzing signals using Fourier transforms allowed excitation and the understanding of resonance techniques in multiple dimensions (Skoog, D.A. Leary, J.L. 1992) A 2D spectrum is typically shown as a contour map of intensity rather than an actual spectrum. Correlated spectroscopy (COSY) allows a relative determination of couplings that occur between one type of atom. The spectrum is plotted with the X and Y axis showing the chemical shift for the molecule being analyzed and often the peaks from the 1D spectrum are overlaid on the axes for simplicity. Figure 2 below shows the COSY 2D 1 HNMR Spectrum for 2-hexanone (Carey p. 548). The diagonal down the center corresponds directly to the 1H peaks of the NMR spectrum. The important structural information is found in the off diagonal peaks which indicate a relationship between two protons that are correlated with coupling and are adjacent or nearby in the structure. The peak at 2.4 ppm consisting of a triplet has an off diagonal correlation contour at approximately 1.6 ppm. This correlation means that the protons at 1.6 ppm are adjacent to the protons at 2.4 ppm. Further, there is a correlation in the diagonal of the protons at 1.6 ppm with the protons at 1.3 ppm indicating that these two sets of protons are adjacent. Finally the protons at 0.9 ppm correlate to the 1.3 ppm completing the assignment of the structure. The final assignments determined are:

CH3CO-CH2-CH2-CH2-CH3

2.1-2.4-1.6-1.3-0.9

The power of the COSY and resulting assignments become more important with molecules where significant overlap and multiple structures are possible and the assignment of carbon atoms and protons to the structure is not possible with conventional 1D 1H and 13C NMR.

A second 2D NMR technique called heteronuclear chemical shift correlation (HETCOR) is actually a form of COSY where the two axes differ from each other. One axis represents the chemical shifts and spectrum for 1H NMR while the other represents the chemical shifts and spectrum for 13C NMR (Carey p. 549). The HETCOR spectrum lacks the diagonal correlation as there is no cross correlation of the spectra. All peaks in the spectrum correlate between 1H and 13C couplings and allow rapid assignment of the spectrum as shown in Figure 3:

In a HETCOR spectrum, each peak indicates correlation between a 13C atom and a 1H atom. The assignment continued from the COSY spectrum for 2-hexanone is:

CH3CO-CH2-CH2-CH2-CH3

2.1-2.4-1.6-1.3-0.9 1H

30 43-26 22-14 13C

Summary

The determination and validation of structure of organic molecules is dramatically simplified through the use of NMR techniques. Careful application of 1D 1H and 13C spectra can determine the structure of most simple organic molecules. Powerful techniques such as 2D NMR allow rapid correlation of atoms within the geometry of the molecules.

References

Carey, F.A., & Giuliano, R.M. (2008). Organic Chemistry. New York: McGraw Hill.

Pauli, W. (1940) . The connection between spin and statistics. Physical Review B. 15, 716-730.

Roberts, R.M.; Glibert, J.C.; Rodewald, L.B.; Wingrove, A.S.(1982) Modern Experimental Organic Chemistry (pp. 239). New York, NY: CBS College Publishing.

Silverstein, R.M.; Bassler, G.C.; Morrill, T.C. (1974) Spectrometric Identification of Organic Compounds, 3rd Ed. (p. 24). New York, NY: Wiley.

Skoog, D.A.; Leary, J.J.(1992) Principles of Instrumental… [END OF PREVIEW]

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