Showing papers in "ChemInform in 1988"

Flavans and proanthocyanidins

Abstract: Haslam’s previous accounts of this area of flavonoid chemistry (Haslam, 1975, 1982a) eloquently summarized the historical developments in the chemistry and biochemistry of this important and complex group of plant phenolics. Also covered were their relationship with condensed tannins and aspects of their characteristic chemical and spectroscopic properties and metabolism.

Ozone Destruction and Photochemical Reactions at Polar Sunrise in the Lower Arctic Atmosphere

Abstract: There is increasing evidence that at polar sunrise sunlight-induced changes in the composition of the lower Arctic atmosphere (0–2 km) are taking place that are important regarding the tropospheric cycles of ozone, bromine, sulphur oxides1, nitrogen oxides2 and possibly iodine3. Here we focus on recent ground-level observations from the Canadian baseline station at Alert (82.5° N, 62.3° W) and from aircraft that show that ozone destruction is occurring under the Arctic surface radiation inversion during March and April as the Sun rises. The destruction might be linked to catalytic reactions of BrOx radicals and the photochemistry of bromoform, which appears to have a biological origin in the Arctic Ocean. This may clarify previously unexplained regular springtime occurrences of ozone depletion at ground level in a 10-year data record at Barrow, Alaska4, as well as peaks in aerosol bromine observed throughout the Arctic in March and April3. Current information does not allow us to offer more than a speculative explanation for the chemical mechanisms leading to these phenomena.

Absolute Electronegativity and Hardness: Application to Inorganic Chemistry

Abstract: The recent concepts of absolute electronegativity, {chi}, and absolute hardness, {eta}, are briefly reviewed. Experimental values for a large number of molecules and radicals are presented. The values are shown to be in good agreement with the known chemical behavior, both as to nucleophilic-electrophilic properties and as to rates of reaction. Applications are also given for the use of empirical rank orders of the local hardness, {bar {eta}}. The uses of {chi} and {eta} are consistent with frontier orbital theory. To make meaningful comparisons in a series of molecules, it is necessary that these orbitals remain the same. Also it is necessary that the observed I and A values of the molecules relate to the appropriate frontier orbitals. 39 refs., 10 tabs.

Asymmetric Synthesis with Boronic Esters

Abstract: Reaction of (dichloromethyl)lithium with (+)—pinanediol alkylboronates at —100 °C folloed by treatment with anhydrous zinc chloride and warming to 0—25 C results in insertion of a chloromethyl group into the carbon—boron bond to increase the chain length by one carbon and form (l)—l—chloroalkylboronic esters in 85—99Z yields and usually 99Z or better diastereoselectivities. With (+)—pinanediol methylboronate the diastereoselectivity falls to 95%. The chloride can be replaced by Grignard reagents to form sec—alkylboronic esters, or by lithium benzyloxide to form l—benzyloxyboronic esters. A second chiral center can be introduced by another reaction with (dichloromethyl)lithium, and in principle there is no limit to the number of adjacent chiral centers that could be assembled in this manner. The process tolerates other functional groups as long as they are inert toward carbanions. It is possible to use (—)—pinanediol to obtain the corresponding (1R)—l—chloroboronic esters. The utility of this approach to chiral synthesis has been demonstrated with simple insect pheromones containing two chiral centers. Another useful chiral directing group is (R,)—2,3—butanediol, which yields 95—96 diastereoselectivities, even in the formation of the l—chloroethylboronic ester. An advantage of this directing group is that it yields the same results via reaction of (R,)—2,3—butanediol dichloromethylboronate with Grignard or lithium reagents, because the C.:, symmetry of the chiral group leads to the same borate complex intermeaiate regardless of the order of attachment of the dichloromethyl and alkyl groups. A second advantage is that the butanediol l—chloroalkylboronic esters hydrolyze readily on contact with water, and the resulting (l)—l—chloroalkylboronic acids are usually crystalline solids which can be enantiomerically purified by recrystallization, and which could easily be esterified with a chiral group which directs the next l—chloroboronic ester to be the (l)—isomer.

Catalytic Reduction of Nitrogen Oxides. A Review on the Fundamentals and Technology

Abstract: A survey is presented of recent developments in NO{sub x} removal processes. Particular attention is paid to selective catalytic reduction, with emphasis being placed on catalysts for the reduction of NO{sub x} from stationary sources. Chapters are included on: sources of nitrogen oxides and emission standards, control technology, catalyst testing, effect studies, kinetics and mechanism, and process development (including a patent index). 303 refs., 64 figs., 15 tabs.

The Interaction of Water with Solid Surfaces: Fundamental Aspects

Abstract: The purpose of this review is to compare and discuss recent experimental and theoretical results in the field of H2O-solid interactions. We emphasize studies of low (submonolayer) coverages of water on well-characterized, single-crystal surfaces of metals, semiconductors and oxides. We discuss the factors which influence dissociative versus associative adsorption pathways. When H2O adsorbs molecularly, it tends to form three-dimensional hydrogen-bonded clusters, even at fractional monolayer coverages, because the strength of the attractive interaction between two molecules is comparable to that of the substrate-H2O bond. The template effect of the substrate is important in determining both the local orientation and long-range order of H2O molecules in these clusters. The influence of surface additive atoms (e.g., O, Br, Na, K) and also surface imperfections (e.g. steps and defects) on the surface structure and chemistry of H2O is examined in detail. Some results on single-crystal substrates are compared with earlier measurements of H2O adsorption on high-area materials.

Recent Advances in the Concept of Hard and Soft Acids and Bases

Abstract: The hard / soft acids / bases principle has been justifiably criticized because of the lack of a precise definition of hardness and the inability to quantify this property; recent developments have overcome these objections, however.

X-Ray Absorption Edge Determination of the Oxidation State and Coordination Number of Copper: Application to the Type 3 Site in Rhus vernicifera Laccase and Its Reaction with Oxygen

Abstract: Cu X-ray absorption edge features of 19 Cu(I) and 40 Cu(II) model complexes have been systematically studied and correlated with oxidation state and geometry. Studies of Cu(I) model complexes with different coordination number reveal that an 8983-8984-eV peak (assigned as the Cu 1s ..-->.. 4p transition) can be correlated in energy, shape, and intensity with ligation and site geometry of the cuprous ion. These Cu(I) edge features have been qualitatively interpreted with ligand field concepts. Alternatively, no Cu(II) complex exhibits a peak below 8985.0 eV. The limited intensity observed in the 8983-8985-eV region for some Cu(II) complexes is associated with the tail of an absorption peak at approx. 8986 eV which is affected by the covalency of the equatorial ligands. These models studies allow accurate calibration of a normalized difference edge procedure which is used for the quantitative determination of Cu(I) content in copper complexes of mixed oxidation state composition. This normalized difference edge analysis is then used to quantitatively determine the oxidation states of the copper sites in type 2 copper-depleted (T2D) and native forms of the multicopper oxidase, Rhus vernicifera laccase. The type 3 site of the T2D laccase is found to be fully reduced and stable tomore » oxidation by O/sub 2/ or by 25-fold protein equivalents of ferricyanide, but it can be oxidized by reaction with peroxide. The increase in intensity of the 330-nm absorption feature which results from peroxide titration of T2D laccase is found to correlate linearly with the percent of oxidation of the binuclear copper site.« less

The Design of Molecular Hosts, Guests, and Their Complexes

Abstract: The origins, definitions, tools, and guiding principles of host-guest chemistry are developed. Perching, nesting, and capsular complexes are exemplified through molecular model and crystal structure comparisons. The degree of preorganization of a host for binding is a central determinant of its binding power. Complementarity of binding site placement in host and guest is a central determinant of structural recognition in complexation. Examples are given of chiral recognition in complexation, of partial transacylase mimics, of caviplexes, and of a synthetic molecular cell.