Different types of protons and carbons in alkanes tend to absorb at similar chemical shifts, making structure determination difficult. Explain how the among the following four isomer 13C NMR spectrum, including the DEPT technique, would allow you to distinguish.

Carbon-NMR, DEPT-90 and DEPT-135 peaks

Distortionless Enhancement by Polarization Transfer (DEPT) is a double resonance pulse program that transfers polarization from an excited nucleus to another. There are three DEPT experiments and they differ only in the “APT portion” of the pulse program, meaning, that they differ only in magnitude of the final 1H tip angle (X = 45, 90, 130). DEPT-45 leaves all resonances with a positive phase, DEPT-90 only shows methine carbon and DEPT-135 shows methine/methyl with a positive phase and methylene with a negative phase.

In part (a), there are six peaks observed in carbon-NMR and carbons have been labelled in alphabets in structure to depict different types of carbons which gives rise to peaks. One peak is observed in DEPT-90 as only one methine carbon is present. Structure has four methylene groups; thus, four peaks appear downward in DEPT-135 and one peak each of methine and methyl carbon appears upwards in DEPT-135

Carbon-NMR, DEPT-90 and DEPT-135 peaks

Carbon NMR, DEPT-90, and DEPT-135 peaks

Carbon NMR peaks, DEPT-90, and DEPT-135 peaks