Interrupting decoupling

Fig. 47. 15 MHz 13C MAS NMR spectrum of cured furfuryl alcohol resin a. The spectrum of the same resin obtained with dipolar dephasing (100 ps interrupted decoupling) b (reprinted from Ref.2341 with permission)...

The pulse sequences used for this purpose are known as interrupted decoupling (Gerasimowicz et al. 1984, Haw et al. 1984, Barron et al. 1985, Maciel et al. 1985), delayed decoupling (Manders 1987) or dipolar dephasing (Hatcher 1987). All involve an interval, t inserted in the pulse sequence between the end of the contact time and the beginning of data acquisition as shown in Fig. 4.5.2. Both nC and proton RF transmitters are switched off for the duration of t and strong static dipolar interactions between, 3C... 

Some of the pulse sequences involve additional refocusing pulses in the l3C channel, the proton channel, or both channels. This discussion and examples presented here are confined to the simplest of interrupted-decoupling sequences, as illustrated in Fig. 4.5.2, since the reader may not have access to the computer code for the more sophisticated versions. 

The operating parameters listed in Section 4.5.3.3 are appropriate for interrupted decoupling also, except that ... 

Interrupted-decoupling experiments provide the spectra required for the analytical procedures described in the following two sections. 

Fig. 4.5.8. Interrupted-decoupling CP/MAS NMR spectra of wood from Eucalyptus obligua (solid line), Pseudowintera axillaris (broken line) and Picea abies (dotted line)...

Interrupted-decoupling CP/MAS NMR spectra of softwoods show a band of signals from about 140 to 156 ppm, with some evidence of partly resolved structure. Variations in the shape of this band are attributed to variations in the abundance of fi-O-4 ether linkages. The degree of etherification can be estimated by curve fitting. The procedure described here follows Leary et al. (1987). 

Fig. 4.5.9. Interrupted-decoupling CP/MAS NMR spectrum of wood from Picea abies. The dotted lines show components of a simulated spectrum based on contributions from etherified left) and nonetherified right) lignin units...

There are significant differences between the CP-MAS spectra of the cured resins with a formaldehyde/phenol (FP) ratio = 1.2 and FP = 1.8. There is no peak at 110 ppm in the FP = 1.8 resin, indicating complete substitution at the ortho position , unlike in the FP = 1.2 resin. The cured FP = 1.8 resin contains methyl (10 ppm), phenoxy (150 ppm) and carbonyl groups (190 ppm) while the cured FP = 1.2 resin does not. Interrupted decoupling identifies the carbonyl in this material as an aldehyde. 

Molecular mobility can be studied by solid-state NMR and X-ray crystallography. Solid-state NMR offers several approaches to studying the molecular mobility of solids. These include (a) the study of processes which result in peak coalescence of solid-state NMR resonances using variable temperature solid-state NMR, (b) determination of the 7 , relaxation of individual carbon atoms using variable temperature solid-state NMR, (c) use of interrupted decoupling to detect methylene and possible methine groups with unusual mobility, and (d) comparison of solid-state MAS spectra measured with and without cross-polarization. 

Fig. 17. 15N interrupted decoupling spectra of a cured 15N enriched melamine-formaldehyde resin with the delays (t) indicated (adapted from [53])... 

Figure 17 Selected C MAS NMR results supporting the assignment of the methoxy resonance at 58 ppm (a) Bloch decay spectrum (b) cross-polarization spectrum (c) spectrum with cross-polarization and interrupted decoupling (d) slow-spced spectrum obtained at 830 Hz. All spectra were acquired at 298 K.

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