Question

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 isomerNMR spectrum, including the DEPT technique, would allow you to distinguish.

Short answer

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 finaltip 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.

Step-by-step solution

Explanation of part (a):

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 finaltip 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.

Explanation of part (b):

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 finaltip 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 (b), there are five 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. Two peaks are observed in DEPT-90 as two methine carbons are present. Structure has two methylene groups; thus, two peaks appear downward in DEPT-135 and one peak of methyl carbon appears upwards in DEPT-135.

Explanation of part (c):

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 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 (c), there are five 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 two methylene groups; thus, two peaks appear downward in DEPT-135 and one peak each of methine and methyl carbon appears upwards in DEPT-135.

Explanation of part (d):

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 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 (d), there are five 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. Two peaks are observed in DEPT-90 as two methine carbons are present. Structure has one methylene group; thus, one peak appears downward in DEPT-135 and two peaks each of methine and methyl carbon appears upwards in DEPT-135.