Low temperature studies

Much of our knowledge of the frequency dependence of VER rates in polyatomic molecules stems from low-temperature studies of molecular crystals [2] such as pentacene (PTC 221 4) guest molecules in a crystalline naphthalene (N C,., H ) host. In naphthalene, the phonon cut-off frequency is -180 cm [97]. At low temperature,... 

The first crystal structure investigation was reported by Romming (1959, 1963) for benzenediazonium chloride. An unpublished low temperature study ( —160 °C) by Romming is mentioned by Sorriso (1978). Romming and Tjornhom (1968) investigated the structure in the presence of one equivalent of CH3COOH. More... 

The mechanism of reduction of dioxygen by fully reduced cytochrome oxidase. Correlation of room and low temperature studies. G. M. Clore, Rev. Inorg. Chem., 1980,2, 343-360 (52). 

The components of the low lying Raman doublet (h2g> b ) at about 30 cm exhibit a different temperature dependence which gives rise to a reverse assignment at 300 K [103, 104] in comparison to the low temperature studies (< 20 K) see Figs. 2 and 3. In fact, the bands overlap at about 50 K [114]. 

In 1999, we reported21 low-temperature studies of oxygen states at Cu(110). At 110K the oxygen state present at high coverage is largely disordered but... 

Applications The general applications of XRD comprise routine phase identification, quantitative analysis, compositional studies of crystalline solid compounds, texture and residual stress analysis, high-and low-temperature studies, low-angle analysis, films, etc. Single-crystal X-ray diffraction has been used for detailed structural analysis of many pure polymer additives (antioxidants, flame retardants, plasticisers, fillers, pigments and dyes, etc.) and for conformational analysis. A variety of analytical techniques are used to identify and classify different crystal polymorphs, notably XRD, microscopy, DSC, FTIR and NIRS. A comprehensive review of the analytical techniques employed for the analysis of polymorphs has been compiled [324]. The Rietveld method has been used to model a mineral-filled PPS compound [325]. 

When low-temperature studies are performed, the maximum resolution is imposed by data collection geometry and fall-off of the scattered intensities below the noise level, rather than by negligible high-resolution structure factor amplitudes. Use of Ag Ka radiation would for example allow measurement of diffracted intensities up to 0.35 A for amino-acid crystals below 30 K [40]. Similarly, model calculations show that noise-free structure factors computed from atomic core electrons would be still non-zero up to O.lA. 

M. G. H. Wallbridge The Crystal Structure of Aluminium Borohydride Trimethylamine Low-temperature Study. Chem. Comm. 1966, 286. 

For the low temperature study in solution the silaethene Vi(Me)Si=CHCH2rBu (9) was chosen as equivalent to 2a. It can be generated from methyldivinylchlorosilane Vi2Si(Me)Cl (8) by the fBuLi route according to Scheme 3 [3],... 

The mechanism for the thermal decomposition of 03 is well established309-312. Both low-temperature studies (in clean glass vessels) and high-temperature shock-tube studies suggest that a simple mechanism, involving ground-state O atoms,... 

Most chemical reactions can be slowed down by lowering the temperature. With low-temperature studies it is possible to prolong the lifetimes of the reactive intermediates so that they can be characterised by normal techniques. Matrix isolation allows experiments to be carried out at temperatures as low as 4K, in order to study species, such as radicals, that are produced photochemically at very low temperatures. The initial photoproduct is trapped within a rigid matrix that inhibits diffusion of the reactive species. The matrix material must be ... 

Dating from 1990, electrochemical studies have gradually revealed the capacity of C60 to receive reversibly from 2 to 5 electrons. In 1992 low temperature studies clearly proved the ability of C60 to receive 6 electrons.10 Figure 14 shows the low temperature cyclic voltammogram of C60. 

There are substantial difficulties in the interpretation of temperature-dependent shifts of protein spectra because of the thermal lability of proteins and the possibility of temperature-dependent conformational transitions. Low-temperature studies in aqueous solutions revealed that for many of the proteins investigated the observed shifts of the fluorescence spectra within narrow temperature ranges were probably the result of cooperative conformational transitions, and not of relaxational shifts/100 1 Spectral shifts have also been observed for proteins in glass-forming solvents, 01) but here there arise difficulties associated with the possible effects of viscous solvents on the protein dynamics. 

The reduction of the rate of hydrolysis due to lowering the temperature 45 C in the solution containing 7 M NH4NO3 is Ah2o/ nh4N03,7m = 4.78, with an activation energy of 5 kcal mol . This solvent is not suitable for low-temperature studies of the lysozyme reaction. 

Stewart s conclusion underscores the need for short-wavelength, low-temperature studies, if very high accuracy electrostatic properties are to be evaluated by Fourier summation. But, as pointed out by Hansen (1993), the convergence can be improved if the spherical atoms subtracted out are modified by the k values obtained with the multipole model. Failure to do this causes pronounced oscillations in the deformation density near the nuclei. For the binuclear manganese complex ( -dioxo)Mn(III)Mn(IV)(2,2 -bipyridyl)4, convergence of the electrostatic potential at the Mn nucleus is reached at 0.7 A" as checked by the inclusion of higher-order data (Frost-Jensen et al. 1995). 

Transferability of atomic densities was tested by Brock et al. (1991), who applied atomic charge density parameters from an accurate low-temperature study of perylene (I) to data collected at five and six different temperatures on naphthalene (II) and anthracene (III), respectively. The molecules are all aromatic hydrocarbons. To reduce the number of variables, all H atoms were assigned... 

Interestingly, during a low-temperature study of the conformational equilibria, in addition to the generally observed chair conformation of the 1,3-oxathiane ring, a significant contribution of the 2,5-twist conformer (8-9% at 180 K) was also observed (Equation 10) <2005STC369>. 

Standard optical metal or metal-glass cryostats are commonly used in low-temperature studies of TSL and TSC. Less expensive facilities for liquid N2 temperatures have been designed, and closed cycle cryotips may be employed as well. TSR requires dynamic temperature cycles. Therefore, it is generally advantageous to design the sample holder-dewar arrangement as small as possible to enable quick turnaround times. 

Abstract—A brief review is given of earlier low temperature studies (4° 20°K of the hydrogen stretching mode of H bonded species. Evidence for sharpening of band structure is quoted (e.g., acetyl glycine, hydrate salts). In contrast are studies for which the band breadth has been found to be virtually independent of tsmoerature (H80 CH3OH). 

Low temperatures studies, for instance in THF at 175K, can elucidate annular tautomerism (92JCS(P2)1737) a large collection of 15N chemical shifts of NH-pyrazoles with tautomerism frozen has been published (94MRC699). The tautomerism is also frozen in the solid state. Thus two 15N shifts at 150 ppm (NH) and 222 ppm ( = N-) are observed for solid imidazole the average is close to the single signal at 186 ppm found in solution. 

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