Application of Chemical Shifts to Structure Determination

To see how nmr and infrared spectra can be used together for structure determination we shall work through a representative example.

The objective is to assign a structure to the compound C4H8O3 whose nmr spectrum is shown in Figure 9-30 and whose infrared spectrum shows prominent bands at 2900cm−1, 1750cm−1, 1000cm−1, and 1100cm−1.

Figure 9-30: Proton nmr spectrum of a compound, C4H8O3, at 60MHz relative to TMS =0.00ppm.

The infrared spectrum indicates  (1750cm−1), C−H(2900cm−1), and C−O(1000cm−1,1100cm−1). The position of the carbonyl band suggests that it is probably an ester, . A carboxylic acid is ruled out because there is no sign of an O−H stretch.

The nmr spectrum shows three kinds of signals corresponding to three kinds of protons. The integral shows these are in the ratio of 2:3:3. From this, we can conclude that they are two different kinds of CH3− groups and a −CH2− group.

The chemical shifts of the presumed CH3 groups are at 3.70ppm and 3.35ppm. Because the compound contains only C, H, and O, the data of Table 9-4 suggest that these resonances arise from OCH3 groups. The low-field resonance is likely to be  (we know from the infrared that there probably is an ester function), while the higher-field resonance is possibly an ether function, −OCH3. If you put all of this information together, you find that CH3OCH2CO2CH3 is the only possible structure. To check whether the CH2CH2 resonance at 3.9ppm3.9ppm is consistent with the assigned structure we can calculate a shift value from Equation 9-4:

The agreement between the calculated and observed shifts is not perfect, but is within the usual range of variation for Equation 9-4. We can be satisfied that the assigned structure is correct.

Figure 9-31: Proton nmr spectrum of a compound, C4H8O3, at 60MHz relative to TMS at 0.00ppm The stepped line is the integral running from left to right.