Interpretation of a Simple 13C Spectrum Diethyl Phthalate

The proton peak is split by two deuterium nuclei with a spin number (7) of one. The multiplicity can be calculated by the familiar formula 2nl + 1 thus 2 X 2X1 + 1 = 5. The —CD3 group does not interfere. 

Substitution of D for H on a carbon results in a dramatic diminution of the height of the 13C signal in a broadband-decoupled spectrum for the following reasons. Since deuterium has a spin number of 1 and a magnetic moment 30% that of H, it will split the 13C absorption into three lines (ratio 1 1 1) with a J value equal to 0.30 X 7CH. Furthermore, 7, for 13C—D is longer than that for 13C—3H because of decreased dipole-dipole relaxation. Finally, the NOE is lost, since there is no irradiation of deuterium. A separate peak may also be seen for any residual 13C— H since the isotope effect usually results in a slight shift to lower frequency of the l3C—D absorption (—0.2 ppm per D atom). The isotope 

FIGURE 4.6 (a) Diethyl phthalate, decoupled 13C spectrum at 150.9 MHz in CDC13, (b) coupled 13C spectrum (c—f) expansions of coupled 13 C spectrum. 

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