Variable-temperature experiments

NMR spectra and Tj measurements at different temperatures. The local polymer chain motion varies over a frequency range of 104-106 Hz in the nematic phase. The activation energy of this motion is found to increase with decreasing number ( ) of methylene units in the spacer, and exhibits odd-even fluctuations. In a study of a homologous series of main-chain LC polyesters, 13C CP/MAS and variable-temperature experiments reveal a conformation-ally more homogeneous and a less dynamic nature for the even-chained than for the odd-chained polymer structures.300... 

Care should be taken during variable temperature experiments. When using liquid nitrogen for variable temperature work, skin contact should be avoided as severe frostbite or cryogenic burns can result. Also, certain parts of the equipment may become hot during the experiment. 

The linear dependence on ks is a strong advantage compared to the force-modulation SFM. Due to the inertia of the cantilever mass at high frequencies, the tip cannot follow completely the displacement of the sample. This results in smaller elastic deformations and low forces in the pN range, which are measured by accelerated mass md2zldt2. Recently, the SLAM technique has been advanced toward variable temperature experiments [138]. Before this development, temperature ramps have been used to perform local calorimetry [139]. 

The formation of photoproducts by the fine-tuning of wavelengths and variation of temperature of the sample being irradiation deserve attention. However, with variable-temperature experiments, the possibility of a phase transition taking place must not be overlooked. 

It seems that the question of dynamic buckling is still to some extent discussed in the literature, even though the variable temperature experiments seemed to have proven beyond doubt that the flat dimer structure in room temperature experiments is a dynamic effect. Given the large distance between tip and surface, the assumption of current induced buckling [55] or buckling due to tip-surface interactions [56] lack experimental and theoretical confirmation. 

Taking an experimental point of view variable temperature experiments used for dynamics studies are easiest to perform under static conditions as rotation of the sample cause frictional heating and at very high or very low temperatures it may be difficult to construct MAS probe that would allow sufficiently fast spinning when studying half-integer quadrupolar nuclei. This makes the... 

No variable temperature experiments have yet been performed on the platinum sample, and any explanation of the linewidth in this system would be premature. 

Another advantage of SS-NMR is that the observed chemical shift differences between polymorphs can be related to particular molecular sites based on known assignments (see Figure 6). Site-specific mobility can also be determined by probing the relaxation properties of the compound. This is an important application, since mobility is usually related to polymorphic interconversions and solid-state reactions.94 Solid-state reactions between active pharmaceutical ingredients and excipients can also be followed by SS-NMR. The study of polymorphic transitions can be performed by variable temperature experiments. 

Variable-temperature, EXSY and NOESY NMR spectra of (C-N)PdCl(X), where H(C-N)+ = (199) (Ar = 2,6-1Pr2C6H3) provided a detailed analysis of the molecular dynamics, e.g. for X = Cl, there was evidence for a flip of the boat conformation.959 The dynamic behaviour of [(ri3-allyl)Pd(L)]+, where L = (200) (R = Ph, Cy) was followed by variable-temperature experiments.960 Variable-temperature and 2-D 111 and 31P NMR spectra for PdC2(L), where L = (201), reveal restricted rotation about a number of P-QpSO aryl bonds.961... 

Additional assays may also be conducted on this material to more comprehensively characterize its thermal properties. The following may be considered variable temperature XRD (VT-XRD) studies may be conducted to observe changes in the powder diffraction patterns of the material as a function of temperature. Selection of appropriate temperatures depends on several factors the DSC transition temperatures, corroborating TGA and hot-stage evidence, and the accuracy of the heating controller for the XRD unit. It should also be noted that although XRD data may be collected at several different temperatures to simulate a variable temperature experiment, the follow-up VT-XRD assay is essentially a series of isothermal experiments, and the DSC is a dynamic assay. It is therefore reasonable to expect some variation in transition temperatures. 

Attempts to detect an equilibrium process through dilution and variable temperature experiments were uirfruitful. 

Variable temperature operation was achieved by placing the quartz sample tube within a standard quartz dewar insert through which nitrogen flowed at a controlled temperature in the range -130 to 30°C. Compared with the majority of previous cell designs, the Bond cell offers several advantages in that variable temperature experiments are possible, only small volumes of solution are used (helpful where materials are precious),... 

In specialized H NMR topics, variable-temperature experiments have been used to study the conformational behavior of oxazole-containing cyclophanes, contrasting them to the phenyl or thiazolyl analogs. ... 

CH2 carbons have shown the existence of two components with different Ti values for each carbon. This Ti result is in accord with the previous Ti results for poly[(R)-3-hydroxybutyric acid] [P(3HB)] homopolymer and other P(3HB)-type random copolyesters. In previous papers, however, the assignment of the two components with different Ti values have been remained unsolved. By performing variable-temperature experiments and by comparing with the results... 

The elucidation of a substitution reaction mechanism depends on reliable kinetic and thermodynamic data obtained by measuring changes in the reaction rate as a function of a chemical property (e.g., concentration, pH, ionic strength, solvent polarity) or physical quantity (e.g., temperature). The determination of an empirical rate law, the observation of steric or electronic effects induced by the entering, spectator, or leaving groups, and the estimation of activation parameters from variable-temperature experiments (i.e.. A// and Aj ) contribute to the adjudication of a plausible mechanism for a given reaction. 

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