Carbon and Proton NMR How to Solve a Structure Problem

FIGURE 6.2 4 The C proton-decoupled spectrum of CH3PO(OCH3)2 (75 MHz). 

8 to 1.8 ppm Protons in this region are generally associated with sp carbon atoms such as CH, CH2, and CH3 groups at some distance from electronegative atoms. (Groups with more attached protons are more shielded and will appear upfield (closer to TMS). Thus, CH3 is more shielded than a CH2 group and will appear at a lower parts-per-million (ppm) value. 

8 to 3.0 ppm This region is generally associated with protons on a sp carbon atom next to C=0, C=C, and aromatic groups. Examples include CH2—C=0, C=C—CH2—, and CH2—Ar. One exception to this is a proton directly attached to a triple bond, C C—H, that also appears in this range. 

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5 to 7.0 ppm This region is generally associated with protons directly attached to C=C sp carbon atoms in alkenes (vinyl protons). Example C=C—H. However, it should be remember that multiple electronegative atoms attached to a carbon can shift protons downfield into this region. Examples include —0—CH2—O— and Cl—CH2—Cl. 

5 to 8.5 ppm This region is generally associated with protons directly attached to C=C sp carbon atoms in a benzene ring or other aromatic compounds. 

0 to 10 ppm This region is always associated with aldehyde protons, protons directly attached to a C=0 group. 

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You can see from this example how NMR and NMR spectroscopy complement each other. NMR spectra provide an estimate of the electronic environment (i.e., electron rich versus electron poor) of a hydrogen nucleus under observation (S), a measure of its relative abundance (integration), and an indication of how many neighbors (and their number of types) it has (spin-spin splitting). Proton-decoupled NMR provides the total number of chemically distinct carbons, their electronic environment (5), and, in the DEPT mode, even the quantity of their attached hydrogens. Application of both techniques to the solution of a structural problem is not unlike the methods used to solve a crossword puzzle. The horizontal entries (such as the data provided by NMR spectroscopy) have to fit the vertical ones (i.e., the corresponding NMR information) to provide the correct answer. 

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