13C chemical shifts are also reported relative to the standard, tetramethylsilane (TMS). Hence, the chemical shift of the four equivalent carbons of TMS appear at =0 ppm. The range of chemical shifts in which most carbon resonances appear is 0 to 200 ppm, which is about 10-20 times the range of proton chemical shifts. As a consequence, a peak is usually observed for each carbon or sets of equivalent carbons; whereas, resonances for 1H nuclei often overlap in proton NMR spectra. Carbon chemical shifts, as discussed earlier for 1H chemical shifts, are influenced by electronegative substituents and -bonds, but also more strongly by the shielding from an abundance of electrons in non-spherical p-orbitals. A large effect from electrons in p-orbitals is the down field shift of carbon resonances with branching. For example compare the 13C chemical shifts for the carbons of 2-methylhexane and hexane shown in Figure 9. The methyls at the branched end of 2-methylhexane (22 ppm) appear approximately 8 ppm down field from the methyl at the unbranched end or both methyls of hexane (14). Further, the methylene at the 3-position of 2-methylhexane (39) appears 7 ppm downfield from the methylene at the 3-position of hexane (32).
A number of empirical rules have been developed for predicting chemical shifts. A very rough rule is that 13C chemical shifts are approximately 10 to 20 times the corresponding 1H chemical shifts. For example the protons of the methyl groups of hexane appear at about 1 ppm in the 1H NMR spectrum and the methyl carbons of hexane appear at about 14 ppm in the 13C NMR spectrum. An example of how 1H NMR and 13C NMR spectra appear as printed out is given in Figure 10, the spectra of 2-bromobutane.
A broader comparison of 13C and 1H chemical shifts appears in the summary at the end of this section.
A gross exception to the rule that carbon chemical shifts are 10-20 times the corresponding proton chemical shifts appears with the substituent iodine, and to a lesser extent with the substituent bromine. This is often referred to as the heavy atom effect and results from the abundance of electrons on the heavy atom in p and d orbitals. As shown in Figure 9, the chemical shift of the carbon at the 1-position of 1,1-diiodopropane appears about 40 ppm higher field than the carbons of TMS. As a consequence the chemical shift value is a negative number.
Unique to 13C NMR spectra (versus 1H NMR spectra) is direct information about the molecular environment of carbons with no directly bonded hydrogens, commonly called unprotonated carbons. Examples are the two unprotonated carbons of t-butylbenzene shown below (Figure 11). Also of importance are peaks for carbonyl and carboxyl carbons which appear very far down field in the region from 160 to 210 ppm as shown in the summary at the bottom of this page.
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Copyright information: Original content © University of Colorado, Boulder, Chemistry and Biochemistry Department, 2011. The information on these pages is available for academic use without restriction.