(distinction between CH, CH2, and CH3 carbons by phase labeling). The three "flavors" of the DEPT experiment are called "DEPT-45", "DEPT-90", and "DEPT-135" depending on the pulse width (45°, 90o, and 135°, respectively) of the final 1H pulse in the sequence. We need only to perform a normal13C spectrum, a DEPT-90, and a DEPT-135 spectrum in order to assign each carbon in the molecule to quaternary (Cq), methine (CH), methylene (CH2), or methyl (CH3) based on its exact number of attached protons. The expected behavior is as follows: quaternary carbons (Cq) will be present in the 13C spectrum but absent in all DEPT spectra; methine carbons (CH) will be positive in the DEPT-90 and in the DEPT-135; methylene carbons (CH2) will be negative in the DEPT-135 and absent in the DEPT-90; and methyl carbons (CH3) will be positive in the DEPT-135 and absent in the DEPT-90. Because the 1H frequency is being used for "hard" pulses as well as for decoupling in this sequence, we need either a 1H decoupler separate from the 1H and 13C pulse transmitters, or a means of rapidly switching the power level from high power (for 1 H pulses) to low power (for 1H decoupling) and back again. Modern spectrometers use the latter approach, with power level changing in a matter of microseconds without relays.

The expected results for the 13C and DEPT experiments are diagramed in Figure 7.35 for 4-hydroxy-3-methyl-2-butanone, which has one quaternary carbon, one methine, one methylene, and two methyl groups. Note that the solvent resonance (CDCl3 in this case) is absent in the DEPT spectra because solvent carbon is effectively quaternary (no attached hydrogens) and cannot undergo INEPT transfer from 1H to 13C. This presents a problem for referencing the DEPT spectra; usually the exact chemical shift of an easily identified peak in the referenced 13 C spectrum is used to reference the DEPT spectra. The ketone carbonyl peak, which appears at 213 ppm in the 13 C and APT spectra, is missing in the DEPT spectra because it has no attached proton. This identifies it as a quaternary carbon, Cq. The next peak, at 64 ppm, is negative in the DEPT-135, so it must be the


0 0

Post a comment