Decoupling heating effects

Steady-state scans (Section 2-4i) are used before the start of essentially all 2D experiments. They are particularly important in a number of pulse sequences in order to compensate for spin-lock (Sections 7-7b and 7-10b) and decoupler (Section 7-8) heating effects. Larger numbers of steady-state scans are employed in experiments that have either particularly long spin-lock times or X-nucleus decoupling over especially wide spectral widths. 

These problems have been overcome for dilute spins, that is, those of low natural abundance such as C, by high-power decoupling to eliminate hetero-nuclear dipolar broadening. The technical difficulties due to the heating effects of the high-power input to the sample were overcome by pulsed decoupling... 

Thus, efficient decoupler operation is crucial especially at the higher frequencies where certain samples may absorb more power than at the lower frequencies. Sample heating effects for ionic solutions have been discussed by Led and Petersen (1978) and by Bock, et al. (1980) and a more efficient coil design by Alderman and Grant (1979). A key fact to remember is that ionic heating is an electric field effect like the piezoelectric resonance discussed in VI.B.5. so that some sort of an electrostatic shielding as described there should work here, too. 

J. J. Led and S. B. Petersen, "Heating effects in carbon-13 NMR spectroscopy on aqueous solutions caused by proton noise decoupling at high frequencies," J. Magn. Resonance 32, 1-17 (1978). 

In most cases, however, heat transfer and mass transfer occur simultaneously, and the coupled equation (230) thus takes into account the most general case of the coupling effects between the various fluxes involved. To solve Eq (230) with the appropriate initial and boundary conditions one can decouple the equation by making the transformation (G3)... 

Shorten or even eliminate the relaxation delay is only effective for relatively fast-relaxing nuclei, and can lead, if decoupling is turned on, to serious radio-frequency heating problems if the relaxation delay is very short. Recently, Kupce and Freeman have introduced an alternative... 

The state of polarization, and hence the electrical properties, responds to changes in temperature in several ways. Within the Bom-Oppenheimer approximation, the motion of electrons and atoms can be decoupled, and the atomic motions in the crystalline solid treated as thermally activated vibrations. These atomic vibrations give rise to the thermal expansion of the lattice itself, which can be measured independendy. The electronic motions are assumed to be rapidly equilibrated in the state defined by the temperature and electric field. At lower temperatures, the quantization of vibrational states can be significant, as manifested in such properties as thermal expansion and heat capacity. In polymer crystals quantum mechanical effects can be important even at room temperature. For example, the magnitude of the negative axial thermal expansion coefficient in polyethylene is a direct result of the quantum mechanical nature of the heat capacity at room temperature." At still higher temperatures, near a phase transition, e.g., the assumption of stricdy vibrational dynamics of atoms is no... 

Separating antiaromaticity effects for the (CH)2EH species from the aromaticity effects in (CH)4EH species is as difficult as it was to decouple strain and antiaromaticity in the analysis above. Again the analysis is complicated by the absence of thermochemical data for the group 15 metalloles. Indeed, the only case known to the authors of experimental heat of formation data for an entire series of such compounds is for the group 16 species furan, thiophene, and selenophene, (CH)40, (CH)4S, and (CH)4Se . 

---