CODEX exchange

Fig. 15. (a) Normalized pure-exchange CODEX intensities E(tm) as a function of tm for the aromatic ternary CH and the quaternary Cquat in Td-G2(-Me),6 dendrimer (T=363K). The fit curve for the ternary carbons is a stretched exponential cxp[—(rln/rcyi with /I = 0.51 and tc = 401 ms. The dotted line indicates the final CODEX exchange intensities, (b) Motional model of the localized, cooperative dynamics in polyphenylene dendrimers, including two-site jumps of all phenyl substituents of a pentaphenyl benzene building block. As indicated by X-ray analysis and computer simulations, the peripheral aromatic rings are inclined by 30° with respect to an axis normal to the face of the central benzene ring. For details, see ref. 44. 

Fig. 37. CODEX exchange intensities of the carbonyl 13C signal of / -poly(ethyl methacrylate) as a function of 13C chemical shift anisotropy recoupling time N Tr for various temperatures below the glass-transition temperature.64 Each curve can be simulated by assuming a 180° flips of the side group and small-angle liberations of the main chain. Simulations of the expected CODEX curves for these motions are shown on the figure.

CODEX experiments can be applied at very high spinning speeds, at which undesirable dipolar spin exchange may be suppressed, so that it does not interfere with the detection of motions on the second timescale. An up to 10-fold slow-down of the l3C spin diffusion has been demonstrated by CODEX at 28 kHz for unprotonated equivalent sites in several organic solids.27... 

Dipolar spin exchange in CODEX and its slow-down under fast MAS... 

A second dipolar effect in CODEX is the relaxation-induced dipolar exchange due to dipolar couplings to heteronuclei,82 83 in particular 14N and 15N, which undergo significant longitudinal relaxation during tm. The relaxation removes a heteronuclear coherence term from the density operator and thus reduces the stimulated echo. This effect is described in detail in ref. 83. 

Fig. 11. Temperature dependence of the fraction of flipping sidegroups in aPMMA (solid triangles), two PEMA samples (open circles, commercial PEMA with 7g = 66°C filled squares, nCOO-labeled PEMA with Tg = 81°C), and in PiBMA (open triangles). Adapted from T. J. Bonagamba, F. Becker-Guedes, E. R. deAzevedo and K. Schmidt-Rohr. Slow ester sidegroup flips in glassy poly(alkyl methacrylate)s characterized by centerband-only detection of exchange (CODEX) NMR, Journal of Polymer Science, Copyright 2001 John Wiley Sons. Reprinted by permission of John Wiley Sons, Inc.

Fig. 14. (a) Eull reference spectrum 5 0, spectrum S after CODEX dephasing and pure-exchange... 

CODEX spectrum /S.S = Sq — S of natural abundance PMMA at 300 K, for Tni = 500ms. The intensity of A S is low since only a minority of the side groups (35%) are undergoing flips, (b) Series of pure-exchange CODEX spectra of PMMA at 300 K, = 6.5 kHz, as a function of as indicated. Large-amplitude side group and smaller-amplitude backbone motions are observed. (Adapted with permission from Ref. 662.)... 

For an excellent review of all the one-dimensional exchange methods, their relative efficacy and the analysis of their results, the reader is referred to ref. 56. An experimental comparison of CODEX and trODESSA is undertaken in ref. 60. 

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