Variable temperature measurements

Variable-temperature measurements of the magnetic susceptibility at 0.1 and 1.0 T between 2 and 290 K indicated temperature-independent diamagnetism 44). That is, flat % versus T curves, for both 3 (Xdiayrip(3) =-10.2 X lO emumor ) and 4 (Xdia/Tip(4) =-9.8 x 10 emu mol were obtained. By comparison these numbers for the... 

Variable temperature measurements were carried out on a Varian A-60 spectrometer with a Varian-4060 probe heater attachment. Probe temperatures were calibrated with a standard ethylene glycol sample. Samples were prepared by dissolving up to 25 per cent by weight of polymer in tetrachloroethylene. Even with such concentrated solutions no viscosity problem was encountered above 60°. 

Variable temperature measurements which provide information about the state of aggregation of the chain ends will also be presented. 

Figure 9. Variable-temperature measurements on expanded aliphatic portion of...

Figure 12. Variable-temperature measurement of y-carbon resonance of sample from Figure 11a + 50 to — 50°C.

Variable temperature measurements of the enriched samples show that exchange processes are operating and different states of aggregation are observed. It is not clear at this point if inter- or intra-aggregate exchange is dominating. 

The methodologies for studying both the conformers at equilibrium and their rates of interconversion are mainly based on NMR spectroscopy. Variable temperature measurements are of great help to freeze the conformational equilibria. Also, quantitative determinations of ring interconversion and N-inversion barriers rely exclusively on dynamic NMR methods. In addition to the state of the art, already reviewed previously (vide supra), only a very few methods and applications were added. 

The temperature dependence of the spectral spin diffusion and crossrelaxation was examined by Mueller et a/.287,288 with spin- and spin-1 systems. They showed that the diffusion rate can be strongly temperature dependent if it is motionally driven. It is therefore, unreliable to discriminate spin diffusion and chemical exchange by variable-temperature measurement of 2D exchange spectra. Mueller et al. suggested that the dependence of the polarization transfer rate on the spectral difference of the relevant resonances should be measured in a single crystal to safely distinguish the two different polarization transfer processes (see also ref. 289). They also explained satisfactorily why the relaxation of the quadrupolar order is much faster than the Zeeman order. This... 

However, diffraction methods have severe drawbacks. Disordered crystals are often difficult to tackle. If the disorder is of dynamic nature, e.g. arising from small-or large-amplitude motions in the crystal, the use of devices for variable-temperature measurements is compulsory and can also yield very useful information (see below for some examples) on the existence of enantiotropic systems related by phase transitions. In some, not frequent, cases the crystals are sufficiently robust to be used for direct phase transition measurements on the diffractometer. Figure 3 shows an example of multiple diffraction data sets collected on the same specimen... 

As stated earlier, the amount of information available about a paramagnetic species from a powder EPR pattern is largely determined by the resolution of the spectra. If information on the gand A anisotropy can be extracted from the spectrum, then details on the electronic structure of the paramagnet can be derived. If variable temperature measurements are performed, then information on the dynamics of the species can also be derived. This can be illustrated through an example, based on the EPR spectrum of adsorbed NO2 on an oxide surface [19]. 

Anet and Ghiaci83 have studied how the nitrogen-15 chemical shift is related to the barrier of C—N internal rotation in urea and aniline derivatives. Based on H and 13C variable-temperature measurements, they observed a barrier to C—N internal rotation of 6.1 kcal mol 1 for A-methylaniline, which is in reasonable agreement with barriers predicted by 15N NMR chemical shifts and from the Hammett relationship. 

The variable temperature measurements of MAS NMR offer very variable information on the dynamic character of the protons of zeolites as solid-acid catalysts. The temperature-dependent lineshape of MAS NMR spectra of acidic protons (acidic OH groups) in zeolites shows the first and unambiguous evidence for proton migration in zeolite structure. There is a possibility that the mobility of protons influences a catalytic activity of solid-acid catalysts. 

The final electrical connections to the STM can be done with copper wires. A small amount of helium is used as an exchange gas to anchor the temperature of the whole assembly to the cryogenic fluid. The body of the STM can be made out of copper, which will respond quickly to temperature changes for variable temperature measurements and provide a uniform temperature environment for the tunnel junction. One has to estimate the differential thermal contraction of the component parts to make sure that a tunnel junction separation set at room temperature is sufficiently large to prevent tip crash on cooling. Other materials like Macor or Invar , which closely match the thermal expansion properties of the piezoelectric transducers, are used as well but take more time to thermally stabilize. Some references are given in [6.30-6.43]... 

Early measurements relied on direct H, C, and P variable-temperature measurements, where appropriate, to establish structures peak coalescence provided information on exchanging groups. More unambiguous assignments were later made possible by the construction of probes enabling H- Rh INDOR, C- X ... 

Variable temperature measurements were made using an Oxford Instruments DN 1704 liquid nitrogen cryostat. Clear glasses were obtained by the addition of 80% glycerol to the buffer. 

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