DEPT: A tool for 13C peak assignments

Distortionless Enhancement by Polarization Transfer (DEPT) is a double resonance pulse program that transfers polarization from an excited nucleus to another – most commonly 1H → 13C. This results in a sensitivity enhancement relative to the standard decoupled 1D carbon spectra (13C{1H}), which benefits only from the small Nuclear Overhauser Effect (NOE) enhancements.

The DEPT sequence is a clever combination of the coherence transfer techniques of the Insensitive Nuclei Enhanced by Polarization Transfer (INEPT) experiment and the spin-echo protocol of the attached proton test (APT). This astute manipulation of nuclear spins affords spectra that has BOTH signal enhancement AND phase separated methine (CH), methylene (CH2) and methyl (CH3) resonances.[1] Awesome – I know!!

There are three DEPT experiments (when run together we refer to them as the “DEPT-trio”). These experiments have the same “INEPT sequence” and differ only in the “APT portion” of the pulse program – that is, they differ only in magnitude of the final 1H tip angle (X = 45, 90 or 130°). Although there is a significant increase in the SNR, the limitation of this method is that quaternary carbons are not typically observed.

The resultant phase of each resonance then depends on:
1) this tip angle
2) the number of protons attached to the carbon that gives rise to a specific resonance.

So what is it exactly that we observe? Well, DEPT-45 leaves all resonances with a positive phase (so pretty similar to a basic 1D except that due to polarization transfer, you can get the spectrum much faster – with the exception of quaternary carbons). DEPT-90 only shows CH, and DEPT-135 shows CH/CH3 with a positive phase and CH2 with a negative one (Caution – be careful with DEPT-135 phasing!!)

Because we only see resonances that have attached protons, ideally, a 13C{1H} should be paired with DEPT-90 and DEPT-135 for full peak assignment. See, for example, the 30v/v% (~1.5 M) sample of diethyl phthalate in d6-DMSO. The standard 13C{1H} spectrum was acquired with about 3x the number of scans as were the DEPT spectra.

In the 13C{1H} spectrum there are 6 carbon peaks (δ 167.35, 132.41, 131.61, 129.03, 61.63 and 14.14 ppm) as we would expect given the top/bottom symmetry in diethyl phthalate. In the DEPT-45 there are only 4 peaks (suggesting two quaternary and 4 with attached protons). There are only 2 peaks in the DEPT-90 spectrum – so we can easily assign the quaternary aromatic-C (132.41 ppm)) and the primary aromatic-C’s (131.61 & 129.03 ppm). Finally, the DEPT-135 obviously distinguishes between the –CH2– (61.63 ppm) and –CH3 (14.14 ppm) of the ethyl chain.

So Cliffnotes, DEPT really simplifies peak assignments for 13C spectra:
1) CH – positive in DEPT-45, DEPT-90 and DEPT-135
2) CH2 – positive in DEPT-45, negative in DEPT-135, not observed in DEPT-90
3) CH3 – positive in DEPT-45 and 135, and are silent in DEPT-90.

DEPT is a great tool to add to your structural elucidation/peak assignment tool box! Happy structure elucidating!

[1] Jacobsen, N. E. “NMR Spectroscopy Explained: Simplified Theory, Applications and Examples for Organic Chemistry and Structural Biology” 2007, John Wiley & Sons, Inc.: Hoboken, Chapter 7.