Relaxation source

Such rapid pulsing may seem excessively fast for the resonances one wishes to observe, especially for those protons with longer relaxation times. However, one should recall that the proton Tis generally measured relate to carbon-12 bound protons, whereas in HMQC one is interested in carbon-13 bound protons. This nucleus acts as an additional dipolar relaxation source for the directly attached proton leading to shortening of the proton Ti value, consistent with the rapid repetition. 

Figure 8.11. A three-spin system in which the I-spin neighbour, N, acts as an external relaxation source for spin-I through their mutual dipolar interaction.

We think that the H,D replacement method is the method of choice to get an accurate determination of the dipolar term in the hydride moiety, as the overall T, of the hydride resonances may receive relevant contributions also from other relaxation sources. For instance, Beringhelli et al. showed that in (p-H)2Re2(CO)g the dipolar H—H relaxation accounts for only the 9.5 % of the total relaxation rate.[21] In this derivative the importance of the dipolar interactions with the metal itself is remarkable. In the case of Re i = 5/2 natural abundance = 37.07 % Re I = 5/2, natural abundance = 62.93 %) the small Y of rhenium isotopes is partially counterbalanced by the factor 1(1+1) = 8.75 which makes the effective 7 ca. 18 10 rad T s- (to be compared with 26.7223 10 rad T s- for H). In the extreme narrowing condition, for a hydride bound to it (r Re rhenium atom is as effective as an hydrogen atom... 

Figure 8.10. Schematic illustration of the maximum homonuclear steady-state NOE in the presence (solid line) and absence (dotted Une) of external relaxation sources that compete with cross-relaxation.

The direct consequence of this is that to correctly interpret steady-state NOE data, it becomes essential to consider not only the I-S intemuclear separation, but also the proximity of all other nuclei (relaxation sources) surrounding I. 

Figure 4.9 Acrylic fiber structural model showing effect of stretching and relaxation. Source. Reprinted with permission from Capone GJ, Wet-Spinning Technology, Masson JC ed., Acrylic Fiber Technology and Applications, Marcel Dekker, New York, p. 96 and 99, 1995. Copyright 1995, CRC Press, Boca Raton, Florida.

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