Nuclear Magnetic Resonance

DEPT Spectroscopy

Introduction

While modern NMR spectrometers make recording ¹³C-NMR spectra a matter of routine, the fact remains that it is more difficult to obtain a ¹³C-NMR spectrum than a ¹H-NMR spectrum. The difficulty stems from two sources. First, the natural abundance of ¹³C is low, so there are fewer NMR-active nuclei per mole of compound to absorb energy. Second, the inherent signal intensity per nucleus is less for ¹³C than for ¹H. For equal numbers of ¹H and ¹³C nuclei, the signal intensity for ¹³C is roughly 1/4 that of ¹H. When combined with the fact that the natural abundance of ¹³C is roughly 1% of that of ¹H, this means that the signal intensity of ¹H is over 400 times greater than that of ¹³C. Consequently NMR spectroscopists have sought ways to increase the signal intensity of carbon. All of the methods they have developed involve a phenomenon known as polarization transfer. In this topic we will look briefly at one polarization transfer technique called Distortionless Enhancement by Polarization Transfer, DEPT.

DEPT Spectroscopy

In the context of this discussion, the word polarization is used to describe the differences in populations of various spin states that are produced when a sample is subjected to an external magnetic field. Recall that in the absence of B_o the magnetic moments of the individual nuclei are randomly oriented and that they all have essentially the same energy. Application of a strong external magnetic field removes the randomness, forcing the nuclei to align with or against the direction of B_o. This change from a random state to an ordered state is called polarization. The diagram at the top of the left hand panel of Figure 1 shows the polarization of the ¹³C and ¹H nuclei in a sample of chloroform, CHCl₃. The spin states labeled 1-4 are produced by spin-spin coupling of the C and the H atoms. The red dots represent the population differences between the various spin states. The population difference between state 1 and state 2 is 4 units, the same as that between state 3 and state 4. By the same token, the population difference between states 2 and 4 is 16 units, which is the same as the difference between states 1 and 3. The transitions associated with ¹³C nuclei are colored magenta, while those of the ¹H nuclei are shown in cyan. The diagram at the bottom of the left hand panel represents the corresponding spectrum. Note that the ¹H signal is approximately 4 times the intensity of the ¹³C signal.

Figure 1

Polarization Transfer

Now imagine irradiating this system with a radio frequency that just matches the value of DE between spin states 1 and 3. Some of the nuclei will change spin states. The diagram at the top of the right hand panel of Figure 1 shows the spin state that results when the populations of states 1 and 3 become equal. Furthermore, the population difference between states 1 and 2 has inverted. There are 4 more units in state 2 than in state 1. If you were to record a spectrum of the sample at this point, it would look like that shown at the bottom right of the figure. The 2,4 transition remains unchanged. The intensity of the 1,3 transition has dropped to zero. The 1,2 transition now produces a negative peak! The signal for the 3,4 transition has become stronger, i.e. enhanced by polarization transfer. Manipulating the populations of spin states by the selective irradiation of specific transitions is the basis for DEPT spectroscopy.

The utility of DEPT spectroscopy stems from the fact that the number of hydrogen atoms attached to a carbon determines whether the ¹³C resonance will appear as a positive or a negative peak. In other words, DEPT spectroscopy differentiates CH₃ groups from CH₂ groups from CH groups from carbons that have no hydrogens attached. In order to make the distinction, it is necessary to perform three measurements. First you record a normal broad-band decoupled spectrum. The second spectrum is referred to as a DEPT 90 spectrum, and the last as a DEPT 135 spectrum, where the numbers indicate different durations of irradiation. Figure 2 presents a logic table for these spectra. A similar table was first published by O.L. Chapman and A.A. Russell in the Journal of Chemical Education 1992, 69, 779-782.

Figure 2

Interpreting DEPT Spectra

Notice the following points:

1. Carbon atoms without attached hydrogens do not appear in DEPT spectra. (Why not?)
2. Methine carbons (CH) appear as positive peaks in both DEPT 90 and DEPT 135 spectra.
3. Methylene carbons (CH₂) don't appear in DEPT 90 and appear as negative peaks in DEPT 135 spectra.
4. Methyl carbons (CH₃) don't appear in DEPT 90 and appear as positive peaks in DEPT 135 spectra

	A	B	C	D	E
	A	B	C	D	E

Topics