Theory of INEPT and DEPT Spectroscopy

Bendall, Pegg, and Doddrell (20) showed that the results of multipulse sequences, such as INEPT and DEPT, can be analyzed using the Heisenberg vector approach. The INEPT sequence is amenable to pictorial representation using familiar vector diagrams. However, the mechanism of the DEPT sequence is not well understood, and a more mathematical approach will be employed to describe it. 

By employing a doubly rotating frame of reference, polarization of I and H in an IH system can be followed simultaneously (Fig. 11). It is important 

If a decoupled spectrum is desired an additional delay time, A = 1/2J, is employed to allow the two halves of the doublet to come into phase (Fig. llg). As before, a refocusing pulse is applied halfway through this delay period. 

The interpretation of pulse sequences is somewhat more complex in IH systems where n 1. For n = 2 the nuclei IA and IB receive polarization from the surplus population of protons in the state over the [[ state. There is, however, no net proton magnetization for I nuclei coupled to protons in opposite eigenstates (1, ). Hence the expected triplet has lines of intensities +1 0 — 1. Such a zero-intensity center line is found in all cases where n is even and allows easy distinction between even and odd multiline patterns. 

In the case of n = 3, all four lines of the quartet are observed, however the intensities are distorted. As stated previously I nuclei receive polarization from the excess of protons aligned with the field over those opposed to the field. In this case two such proton conditions exist, 11J. versus [ and 111 versus [[[. The first can be formed by three different combinations of spins while the second has only one form. This provides the usual 1 3 3 1 pattern. The second however, has three times as great an energy gap as the first and hence a threefold greater Boltzmann population difference. When these two effects are combined the result is a 1 1 — 1 — 1 spectrum. 

---