Carbon-13 spin echo pulse sequences

A few relatively recent published examples of the use of NMR spectroscopy for studying polymer degradation/oxidation processes will now be discussed briefly. At the early stages of degradation, the technique can be used to provide chemical identification and quantification of oxidised species for polyolefins, oxidation sites can be identified by the chemical shifts of -CH2- groups a and ji to carbons bonded to oxygen [85]. Spin-spin relaxation times may be determined by a pulse sequence known as the Hahn spin-echo pulse sequence. 

The delay is generally kept at Vi x> The coupling constant Jcc for direcdy attached carbons is usually between 30 and 70 Hz. The first two pulses and delays (90J -t-180 2-t) create a spin echo, which is subjected to a second 90J pulse (i.e., the second pulse in the pulse sequence), which then creates a double-quantum coherence for all directly attached C nuclei. Following this is an incremented evolution period tu during which the double quantum-coherence evolves. The double-quantum coherence is then converted to detectable magnetization by a third pulse 0,, 2, and the resulting FID is collected. The most efficient conversion of double-quantum coherence can... 

The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment [6, 7] was the first broadband pulsed experiment for polarization transfer between heteronuclei, and has been extensively used for sensitivity enhancement and for spectral editing. For spectral editing purposes in carbon-13 NMR, more recent experiments such as DEPT, SEMUT [8] and their various enhancements [9] are usually preferable, but because of its brevity and simplicity INEPT remains the method of choice for many applications in sensitivity enhancement, and as a building block in complex pulse sequences with multiple polarization transfer steps. The potential utility of INEPT in inverse mode experiments, in which polarization is transferred from a low magnetogyric ratio nucleus to protons, was recognized quite early [10]. The principal advantage of polarization transfer over methods such as heteronuclear spin echo difference spectroscopy is the scope it offers for presaturation of the unwanted proton signals, which allows clean spec-... 

In the case of a heteronuclear AX system, e.g. a CH bond with A = 1H and X = 13C, the transverse magnetization dephases to the doublet components with v0 + J(H and v0 — 7CI, because of CH coupling (Fig. 2.38(a)). A 13C- H pair thus responds to the echo sequence shown in (Fig. 2.37 (a-c), provided the 180" pulse covers the range of carbon-13 shifts At time 2z after the initial pulse, a spin-echo builds up along the negative y axis. Subsequent Fourier transformation computes a negative signal (Fig. 2.38(a)). 

A constant evolution period t, is the new feature of. /-modulated spin-echo, DEPT, and INADEQUATE sequences. During this time period, a 7-modulation or a polarization transfer may evolve. Such pulse sequences provide FID signals S(t2) which are still functions of one variable time t2. The Fourier transforms, however, are NMR spectra with specific information, depending on the constant evolution period ti. One simple example is the generation of the quaternary carbon-13 subspectrum by means of a. /-modulated spin-echo experiment with an evolution time of tj2 = x = as in... 

Subsequently, Uhrinova et al.29 reconsidered the problem using both proton-and carbon-detected experiments. For example, couplings of anomeric carbons were measured from the 13C satellites in proton NMR spectra. The critical factor in these methods is the suppression of signals from protons bound to, 2C atoms. In the pulse-sequence proposed, these protons were selectively inverted by a BIRD (Bilinear Rotation Decoupling) pulse,30 and the spin-echo method introduced by Bendall et al.31 was used. 

The spin-echo experiment is particularly simple to set up as it does not require proton pulses or their calibration, a desirable property when the experiment was first introduced but of little consequence nowadays. The same results can, in fact, be obtained by the use of proton 180° pulses rather than by gating of the decoupler [23] (Fig. 4.15b). In this case the A period is broken in two periods of 1/27 separated by the simultaneous application of proton and carbon 180° pulses. These serve to refocus carbon chemical shifts but at the same time allow couplings to continue to evolve during the second A/2 period (Section 2.2). Hence, the total evolution period in which coupling is active is 1/7, as in the decoupler-gating experiment above, and identical modulation patterns are produced. It is this shorter pulsed form of the heteronuclear spin-echo that is widely used in numerous pulse sequences to refocus shift evolution yet leave couplings to evolve. 

The poor editing accuracy of spin-echoes in the presence of a wide range of 7 values and the inability to fuUy characterise all carbon multiplicities are the major limitations of these techniques. More complex variations on the pulsed 7-modulated spin-echo are to be found that do allow a complete decomposition of the carbon spectra into C, CH, CH2 and CH3 sub-spectra [25] and that also show greater tolerances to variations in 7ch [26]. Likewise, 7-compensated APT sequences have been developed for greater tolerance to a spread of 7 values [27], and for the direct generation of complete sub-spectra [28], Invariably, the simplest sequences stiU find widest use. 

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