Showing papers in "Chemical Reviews in 1989"

Magnetic field effects in chemical kinetics and related phenomena

Abstract: ions was studied by Margulis et al.156 These authors used solvent mixtures of water or ethanol with glycerol to obtain very highly viscous solvents. Varying the temperature and the solvent composition, they showed that the MFEs are determined by the value of T/q. MFEs on the free radical yield (determined by flash spectroscopy) or on the permanent bleaching reaction (determined by continuous photolysis) became detectable only at T/q values smaller than 10 K/cP. At room temperature this corresponds to viscosities larger than 20 cP. In the systems investigated by Margulis et (cf. Table 8). The magnetic field causes an increase of free radical yield, which results from a suppression of tripletsinglet transitions by a magnetic field. The MFD curves are of the case 2 type, which is taken as evidence for a contribution of the relaxation mechanism. Remarkably, Margulis et al. also found MFEs on the second-order recombination rate constant in the case of riboflavin semiquinone radicals and benzophenone ketyl radicals. The second-order bulk recombination rate is slowed down by the magnetic field, demonstrating that F pairs behave rather like geminate triplet pairs, which has also been confirmed in many CIDNP investigations. MFEs in the photochemistry of quinoline and isoquinoline derivatives have been reported by Hata et al.318-320 Photochemical hydrogen abstractions by the ring nitrogen from the solvent ethanol are believed to be the primary reactions in the photoreactions of 1isoquinolinecarbonitrile (7) and 4-methylquinoline-2carbonitrile (9). For the reaction of 7 (eq 46) the

Allylic 1,3-strain as a controlling factor in stereoselective transformations

Abstract: 5. Stereoselective Intermolecular Addhion Reactions to Double Bonds Controlled by Allylic 1,bStrain 5.1. Hydrogenation 5.2. Hydroboration 5.3. Epoxldation and Cycbpropanation 8. Diastereoselecthre Enolate Alkylations Controlled by Allyllc 1.3-Strain 7. Chirality Transfer in Reactions of Allylsilanes and Allylboronates 8. Nucleophilic AddRion to Double Bonds Controlled by Allylic 1.3-Strain 9. Reactions of Benzylic Systems Controlled by Allylic 1,3-Strain 10. Allylic 1 ,bStrain Control of Dlastereoselecthre Reactions InvoMng Heteroallyl Systems 10.1. 2-Azaallyl Systems 10.2. 2-Oxonlaallyl Systems 10.3. Ntrones 10.4. Other Azaallyllc Systems 10.5. Vinyl Sulfoxides 11. Conformational Control around Bonds to Three-Membered Rings Related to Allylic 1.3-Strain 12. Epilogue

Aerogels and related porous materials

Abstract: Aerogels are extremely porous, low-density materials, consisting of inorganic oxides such as silica, alumina, zirconia, stannic or tungsten oxide, or a mixture of these oxides. They have large surface areas and can be translucent as well as transparent, have extremely low thermal conductivities, and have fascinating acoustic properties (sound velocities as low as 100 m/s). Th is review surveys the literature and summarizes the historical background of aerogel development, their production by the sol-gel process, possible drying methods, structural investigations, and various novel applications in catalysis, high-energy physics, passive solar energy uses and energy conversions, and low-temperature glass formation.

The molecular mechanism of retention in reversed-phase liquid chromatography

Abstract: The term “reversed-phase” chromatography seems at first inappropriate for what is by far the most popular mode of modern liquid chromatography. Estimates of the popularity of the technique range from 57% of all analytical chromatography1 to as high as 80-90% .* The term itself can be traced to Howard and Martin in 1950.3 In attempting the separation of long-chain fatty acids they realized that the “normal” mode of chromatography, using a polar stationary phase and nonpolar mobile phase, would not work, as the hydrophobic compounds had too little retention to effect a separation. They were able to treat Kieselguhr with dimethyldichlorosilane vapor and then coat this hydrophobic support with a nonpolar liquid stationary phase. Both the polarity of the phases and the respective elution order of solutes were reversed from traditional chromatographic systems, and they christened the technique “reversed-phase” partition chromatography. The popularity of reversed-phase liquid chromatography (RPLC), as practiced today, can be attributed to the development of chemically stable, microparticulate bonded phases that provide rapid mass transfer and a high degree of reproducibility. Attempts to utilize liquid-liquid chromatography, with a liquid stationary phase physically coated on an inert support, were rapidly abandoned with the introduction of commercially available bonded phases. Interesting perspectives on the early development of bonded phases for modern liquid chromatography can be found in a book devoted to the history of liquid chromatography.* H. A. Laitinen, in an editorial in Analytical Chemistry, described the seven ages of an analytical method from the birth of an idea to the ultimate replacement of the method by newer technique^.^ The fourth phase he described as “...detailed studies of principle and mechanisms are pursued with the aid of improved instrumentation. This represents the stage at which the method matures as an accepted procedure in competition and cooperation with other approaches. This stage represents the crest of analytical research as distinguished from instrumentation research.” This most clearly describes the present status of reversedphase liquid chromatography. Many methods of investigation are being brought to bear on the problem of understanding the molecular mechanism of retention of RPLC. These range from spectroscopic studies, including UV-visible, IR, NMR, fluorescence, and others, to thermal methods, to neutron scattering, to chromatographic methods themselves. Experimental studies alone are not enough. The cooperation of theorists and experimentalists is leading to dramatic advances in the understanding of the retention process. The goal of this review is to critically assess the current understanding of retention of small molecules in reversed-phase chromatography from both a theoretical and experimental perspective. We first discuss the synthetic methodologies for the preparation of reversed-phase stationary phases and deal next with the partitioning processes and stationary-phase structural details.