Temperature, NMR measurements

A Relaxation time measurement in the solid (Al) in solution (A2). B Mechanical spectroscopy. C Variable-temperature NMR spectroscopy (coalescence temperature measurement). D Variable-temperature EPR spectroscopy... 

The synthetic method described in this section can be successfully applied not only to the benzo-annelated thiepins but also to the various kinds of annelated and non-annelated thiepin derivatives. The annelated thiepins thus obtained, though some of them were not isolated, are shown in Scheme 1 491. The thermal stability of these thiepins can be evaluated by their half-lives at an appropriate temperature measured by H-NMR spectroscopy and will be discussed in Section 5.1. 

Figure 10. Line-width of the H-NMR signal measured on solid P NMe) as a function of temperature.

A NMR probe for very high temperature and high pressure was published by Jonas in 1979 [5]. The probe had a relatively low spectral resolution but allowed NMR relaxation measurements at temperatures up to 700 °C. The pressure vessel was made from the IMI-680 titanium alloy and could withstand pressures up to 200 MPa. Because of the high temperatures attained with the probe, gaseous argon was used as the pressure transmitting medium. A first NMR relaxation study of supercritical water performed with this probe was published in 1981 [25]. 

Little difference was found between the NMR results on the LP and HP samples at either room temperature or as the temperature was raised, although the T1 for the HP sample was 10% shorter than that for the LP sample. After maintaining the samples at temperatures near 200°C for 1 hr., a major difference was noted upon cooling, with the HP sample exhibiting a marked time-temperature hysteresis of all of the measured NMR properties while the LP sample exhibited no time-temperature hysteresis in any of the NMR parameters measured. A more dramatic manifestation of this is that after the sample was cooled to room temperature, the proton motion took weeks to return to the original state... 

C- NMR was measured in the mode of 1H noise decoupling without NOE, using D2O sealed up in a capillary tube. All UV and NMR measurements were performed under precise temperature control. 

The Evans method gives excellent results provided adequate care is taken. A most important requirement is that the solution temperature is measured reliably. One effective means of accomplishing this for H NMR is to insert into the NMR tube a capillary or additional coaxial sample of an NMR temperature calibrant solvent, usually methanol (158) or ethylene glycol (88). In this way the temperature measurement is made simultaneously with the susceptibility measurement. A second important factor is the variation of the solvent density with temperature (126). Because the density difference between the solvent and solution depends linearly on the concentration of the solute, it is only... 

Evidently, we should be wary of drawing conclusions from what may be a deceptively-simple NMR spectrum measured at room temperature wherever fluxionality seems plausible, it must be suspected. Organometallic compounds involving the cyclopentadienide ligand... 

There has been a controversy 54 89) regarding the accuracy of equations (iii) and (iv), i.e. the validity of inferring rate constants from coalescence temperatures. It now appears B9) that, provided no additional coupling is present, and provided Av > 3 Hz (eqn. (iii)) and Av > J (eqn. (iv)), the simple coalescence method gives rate constants within 25 % of those obtained by more sophisticated line shape analysis (vide infra). This leads to an error in AG° no greater than that produced by the uncertainty of temperature measurement ( 2 °C m) in the NMR probe. 

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