Interrupted 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)...

Thus, what is observed is a decoupling between the motions of the transition and those involved in the a transition. Indeed, in PMMA, along the high-temperature side of the j3 peak, more and more cooperativity between ester flips and chain motions is involved, in such a way that the chain motions in this temperature region could be considered as the beginning of cooperative chain motions characteristic of the a processes. The presence of rigid CMI units within the chain backbone interrupts this cooperativity and, consequently, the motions involved in the j3 transition are confined to small MMA sequences and cannot reach the extent required by the chain motions occurring at the onset of the a transition. 

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...

Interrupted-Proton-Decoupling Experiments. Interrupted-pro-ton-decoupling experiments were carried out with a 50- xs interruption. This technique selects quaternary carbons (those lacking an attached proton) and rapidly moving methyl carbons. Resonances from methylene and methine carbons are suppressed unless, for some reason, their motion within the solid occurs at a rate similar to that of methyl groups. In essence, the interrupted-proton-decoupling experiment, which has not been previously reported for amber samples, provides an alternative fingerprint for the samples. 

Figure 3 gives the results of interrupted proton decoupling for the Simojovel sample with the 13C NMR spectrum shown in Figure IB and for the Baltic sample with the 13C NMR spectrum shown in Figure 2A. 

Figure 3. The 13C NMR spectra of Mexican amber from Simojovel and Baltic amber taken with interrupted proton decoupling.

Stage 1. Raw material storage Unless the raw materials (also called feed stocks or feeds) are supplied as intermediate products (intermediates) from a neighboring plant, some provision will have to be made to hold several days or weeks worth of storage to smooth out fluctuations and interruptions in supply. Even when the materials come from an adjacent plant, some provision is usually made to hold a few hours or even days worth of inventory to decouple the processes. The storage required depends on the nature of the raw materials, the method of delivery, and what assurance can be placed on the continuity of supply. If materials are delivered by ship (tanker or bulk carrier), several weeks stocks may be necessary, whereas if they are received by road or rail, in smaller lots, less storage will be needed. 

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