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), which benefits only from a 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 should be paired with DEPT-90 and DEPT-135 for full peak assignment. See, for example, 4M diethyl phthalate in DMSO-d₆.

In the ¹³C 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!

If you would like a concise reminder of the various experiments discussed in this post, please check out our infographic!

References

^[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.