Low-temperature solutions

From X-ray diffraction experiments28 it is known that in the crystalline phase the erythrodiisotactic poly(l,2-dimethyltetramethylene) has a (g+aaa g aaa)n structure as shown in Fig. 13. The bold printed letters in the denotation give the conformation of the CH—CH bond. In agreement with this structure and low temperature solution state spectra of 2,3-dimethylbutane, 3,4-dimethylhexane, and 4,5-dimethyloctane 29 30) in which the CHCH bond rotation is frozen the crystalline signals can be assigned conclusively. Like for the crystalline state of poly(l,2-... 

Morgan and Kwolek33 have described a large number of PA derived from phenylenediamines and aliphatic diacids by low-temperature solution polymerization starting with aliphatic diacid chlorides. 

Mixing of high temperature hydrothermal solution with high salinity and low temperature solution with low salinity of meteoric water origin seems the most likely mechanism for the base-metal vein-type deposition. 

The investigations described in this short communication were aimed at reactivity studies of equivalent silaethene derivatives in high temperature gas phase and low temperature solution reactions. 

Of course, even in low temperature solutions, unstable compounds may not be very long-lived. Modern fast-scanning FT-IR interferometers can produce high signal-to-noise spectra in a single scan. This means that metal carbonyl compounds with half-lives as short as 2 seconds can be easily detected using an unmodified interferometer (28,29). With improved interferometers, we anticipate that such studies will soon be extended to compounds with lifetimes —100 mseconds. However, detection of shorter lived species, such as reaction intermediates, requires much faster and more sensitive techniques. 

The matrix experiments thus reveal some complex photochemistry of relevance to solution chemistry but the experiments do not provide information about kinetics. For this we need a fluid medium e.g. gas or liquid, and we consider such experiments in the next two sections. Flash photolysis suggests itself as the technique for detecting a species as reactive as Cr(C0)5 but before describing these experiments we show what can be achieved from low-temperature solutions. 

Low-temperature Solutions. The matrix spectroscopic data for Cr(C0)5...X (Figure 2) suggest that the interaction between Cr(C0)5 and saturated hydrocarbons may be quite substantial,... 

Films at NASA GRC were deposited using homemade spray or aerosol-assisted chemical vapor deposition (AACVD) reactors to exploit the lower deposition temperature enabled by the simpler decomposition chemistry for the SSPs.6 9 AACVD is a simple and inexpensive process that offers the advantage of a uniform, large-area deposition, just like metal organic CVD (MOCVD), while also offering the low-temperature solution reservoir typical of spray pyrolysis methods. 

Of course carbene C-H insertion reactions are well known absolute kinetics have been reported for the insertions of ArCCl into isooctane, cyclohexane, and n-hexane,67 and of PhCCl into Si-H, Sn-H, and C-H bonds.68 More recently, detailed studies have appeared of PhCCl insertions into a variety of substrates bearing tertiary C-H bonds, especially adamantane derivatives.69 Nevertheless, because QMT is considered important in the low temperature solution reactions of MeCCl,60,63 and is almost certainly involved in the cryogenic matrix reactions of benzylchlorocarbene,59 its possible intervention in the low temperature solution reactions of the latter is a real possibility. We are therefore faced with two alternative explanations for the Arrhenius curvature exhibited by benzylchlorocarbene in solution at temperatures < 0°C either other classical reactions (besides 1,2-H shift) become competitive (e.g., solvent insertion, azine formation), or QMT becomes significant.7,59,66... 

The possible intervention of classical, competitive reactions in the low temperature solution chemistry of benzylchlorocarbene (10a) requires careful investigation. There are reasons to suspect azine (48) formation Goodman reported minor yields of azine in analogous MeCCl experiments,60 and Liu et al. found 40% of 48 in the photolysis of neat diazirine 9a.65 Perhaps azine formation is also significant at low temperature in hydrocarbon solvents. If so, the intervention of bimolecular azine formation, in competition with the unimolecular carbene 1,2-H shift, could lead to a nonlinear temperature dependence for the disappearance of 10a. Arrhenius curvature could then be explained without invoking QMT. 

The solution experiments may be made in aqueous media at around ambient temperatures, or in metallic or inorganic melts at high temperatures. Two main types of ambient temperature solution calorimeter are used adiabatic and isoperibol. While the adiabatic ones tend to be more accurate, they are quite complex instruments. Thus most solution calorimeters are of the isoperibol type [33]. The choice of solvent is obviously crucial and aqueous hydrofluoric acid or mixtures of HF and HC1 are often-used solvents in materials applications. Very precise enthalpies of solution, with uncertainties approaching 0.1% are obtained. The effect of dilution and of changes in solvent composition must be considered. Whereas low temperature solution calorimetry is well suited for hydrous phases, its ability to handle refractory oxides like A1203 and MgO is limited. 

High-molecular-weight fluorine-containing aromatic poly(benzoxazole)s have not been obtained either by the direct solution polycondensation in PPA at 200°C or by the low-temperature solution polycondensation in DMAc at 0 to 5°C from 2,2-bis(3-amino-4-hydroxyphenyl)-l,l,l,3,3,3-hexafluoropropane and aromatic diacid derivatives because the fluorine-containing monomer has low nucleophilicity owing to the presence of the electron-withdrawing hexafluoroiso-propylidene unit. 

Mamyama et al.25 have obtained high-molecular-weight poly(benzoxazole)s by the low-temperature solution polycondensation of A,A 0,0 -tetrais(trimethyl-silyl)-substituted 2,2-bis(3-amino-4-hydroxyphenyl)-l,l,l,3,3,3-hexafluoro-propane (25) with aromatic diacids and subsequent thermal cyclodehydration of the resulting poly(o-hydroxy amide)s in vacuo. In this method, aromatic diamines with low nucleophilicity are activated more positively through the conversion to the /V-silylated diamines, and the nucleophilicity of the fluorine-containing bis(o-aminophenol) can be improved by silylation. 

FLF)- (L = H or D) anion in low temperature solutions of (C4H9)4N+ (FL)nF . The authors were able to determine zero-, one-, and two-bond, H/D isotope effects on hydrogen and fluorine NMR chemical shifts for the series n = 1 to n = 3, and to relate the observed spectra to H/D isotope effects on the hydrogen bond geometries. Isotope effects on spin-spin L-F and F-F coupling 13C constants were reported. 

Tung VC, Chen L, Allen MJ et al (2009) Low-temperature solution processing of graphene-carbon nanotube hybrid materials for high-performance transparent conductors. Nano Lett 9 1949-1955... 

Synthetic fluor-containing apatites are prepared and investigated for biomedical applications and serve also as models to understand the formation of biological fluorapatites and some of their properties. The synthesis of fluoridated apatites has been accomplished in various ways from simple ion exchange in solution to more elaborate techniques involving sol-gel routes or thermal processes. Two main classes of synthesis routes are presented in this chapter high-temperature routes and low-temperature solution routes. 

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