Dennis D. Davis

Photoredox reactions of metal ions for photochemical solar energy conversion

Abstract Solar energy conversion to chemical potential energy is thermodynamically feasible by many routes. One possible route is the photochemical reaction of metal ions in water to produce hydrogen and an oxidizer. The photooxidation of several low-valent transition metal ions, including europium(II), vanadium(II), and copper(I) complexes, proceeds in aqueous acidic media according to: Quantum yields in 1.0 M hydrochloric or perchloric acid at 313 nm are: Φ Eu(III) = 0.16, Φ V(III) = 0.15, and Φ Cu(II) = 0.34. This reaction proceeds in visible light with a minimum of photochemical complications for Eu(II) and Cu(I) salts, and since the oxidation of copper(I) halo-complexes is endergic and hence potentially useful for energy storage, the mechanism of photooxidation has been studied. The product quantum yield is strongly affected by the acidity, irradiation wavelength and H-atom scavengers. Photoredox reactions of a number of metal ions and the requirements for using such in a solar energy scheme are discussed.

Isolation and identification of 5,7-dimethoxyisoflavone, an inhibitor of Aspergillus flavus from peanuts

An inhibitor of Aspergillus flavus has been isolated from peanuts and identified as 5,7-dimethoxyisoflavone. Authentic 5,7-dimethoxyisoflavone, synthesized from benzyl-2-hydroxy-4,6 dimethoxyphenyl (3), and the isolated compound display identical ultraviolet spectra in methanol, in methanol+sodium methoxide, and in methanol+AlCl3; similar behavior in solvent extraction, column chromatography, TLC, and fungal inhibition.

Deoxymetalation reactions.the mechanisms of deoxysilylation of mono-trimethylsilyl-and bis-trimethylsilyl-substituted alcohols and a comparison to the mechanism of deoxystannylation and deoxyplumbylation

Abstract The relative rates of acid-catalyzed deoxysilylation of 2-trimethylsilyl-1- hydroxyethane, 4, 1-trimethysilyl-2-hydroxypropane, 5, 1,3-bis(trimethysilyl)- 2-hydroxypropane, 6, and 1-trimethylsilyl-2-methyl-2-hydroxypropane, 7, were found to be 1 : 10 3.30 : 10 5.92 : 10 6.77 , respectively, in 9 vol % aqueous-methanol at 40°C. These rates are directly proportional to the sum of the δ + constants of the substituents on the carbon bearing the nucleofuge (ϱ + = − 11). The additive rate-accelerating effect of two trimethylsilyl groups requires equal conjugative stabilization by each trimethylsilyl group, and a mechanism involving a hyper- conjugatively-stabilized carbocation intermediate is proposed. In contrast, the deoxymetalation reactions of the triphenyltin-, triphenyllead-, and iodomercury-analogs exhibits very different structure-reactivity relationships and have been described as proceeding through concerted E 2 -like or bridge-ion mechanisms. These mechanistic regimes are reconciled by considering the conjugative interactions, electrofugalities and nucleophilic solvent assistance at the organometal-leaving groups in terms of Thorntons Reacting Bond Rules. This analysis suggests a spectrum of merging mechanisms, the acid-catalyzed deoxysilylation representing one extreme, the E 1 M (carbocation intermediate) mechanism and the other Group IV deoxymetalation reactions more nearly concerted E 2 M-paths.

Deoxymetalation reactions. The concerted nature of 1,3-Deoxystannylation

Abstract Kinetic and product studies of the solvolysis of γ-R 3 M-substituted mesyloxypropanes allow a distinction to be made between a concerted 1,3-elimination and reaction pathways involving metalated cyclopropanes or percaudally-homoconjugated ions.

Non-isothermal kinetics of hydrazine decomposition

Abstract Accelerating rate calorimetry has been used to study the kinetics of the titanium thermal decomposition of liquid and vapor hydrazine over the temperature range 475–590 K. In a titanium reaction vessel, hydrazine decomposes exothermically to ammonia, nitrogen and trace amounts of hydrogen. While both liquid and vapor hydrazine are present the reaction has an apparent zero-order activation energy of 98.3±1.7 kJ mol −1 . Cessation of the hydrazine liquid-vapor equilibrium is noted as a dislocation of the log (temperature rate) -inverse temperature relationship and an inflection point on the pressure rate profile. Procedures for the analysis of chemical reaction rate data from the accelerating rate calorimeter and for modeling adiabatic reaction kinetics are presented. Comparison with the experimental results for hydrazine decomposition is made.

δ+-substituent constants for (organometal)methyl groups: a bond polarizability model for hyperconjunction

Abstract Internally consistent and properly scaled δ+-substituent constants for (organometal)—methyl groups are derived from several photophysical data sets. These δ+-values are well correlated by a model which considers that stabilization of an adjacent electron-deficient center is proportional to the polarizability of the carbonmetal bond as measured by bond refractions and expressed in a group polarizability function. The model also accommodates the hyperconjugative effects shown by β-deuterium substitution in solvolysis reactions and points to nonhyperconjugative effects as the basis of the Baker-Nathan order. A “best set” of δ+-values for the —CH2MRn substituents (M = C, Si, Ge, Sn, Pb, Hg; R = Me, Ph or X) is suggested. Seventeen other δ+ values for organometallic substituents are presented.

Synthesis of some trimethyltinnorbornanols

The stereospecific syntheses of four trimethyltinnorbornanols are reported. These include syn-7-trimethyltin-exo-2-norbornanol, anti-7-trimethyltin-exo- 2-norbornanol, syn-7-trimethyltin-endo-2-norbornanol, and anti-7-trimethyltin- endo-2-norbornanol. The syn-trimethyltin group is found to have little steric interference towards exo attack at the C-2 and C-3 positions of the norbornyl skeleton in both hydroboration and metal hydride reduction.

An estimate of the steric bulk of the trimethyltin group.

Abstract Epoxidation and hydroboration reactions of syn -7 substituted norbomenes have been used as probes for effective steric bulk of several substituents including the Me 3 Sn group. Steric hindrance in these reactions decreases in the following order: t-Bu > Me > Br > Me 3 Sn > Cl > H.

Transition metal catalysis of the heterogeneous decomposition of hydrazine: adiabatic kinetics by accelerating rate calorimetry

Abstract The thermochemical kinetics of the transition-metal-catalyzed decomposition reaction of hydrazine have been studied using accelerating rate calorimetry. The reaction stoichiometry was determined by both product analysis and thermochemical balance. In the range 350–515 K when both liquid and vapor hydrazine are present, the decomposition proceeds according to: N 2 H 4 (1)→ 1 3 N 2 (g)+ 4 3 NH 3 (g) with Δ H rxn = −123.3 kJ mol −1 (515 K). In the range 515–590 K when only vapor is present in the reaction system, Δ H rxn = −154.8 kJ mol −1 (515 K). The reaction shows apparent zero-order kinetics and the rates were found to be proportional to the surface area of added metals in the form of powders or foils. Rates and activation parameters are reported for ten transition metals and the Fe : Ni : Cr alloy, 304L stainless steel. The activation energies and relative specific activities at 353 K are: Co (43 kJ mol −1 , 3900); Ni (93 kJ mol −1 , 310); Mo (76 kJ mol −1 , 94); V (85 kJ mol −1 , 66); Fe (119 kJ mol −1 , 33); W (68 kJ mol −1 , 11); Au (74 kJ mol −1 , 7.8); Ti (95 kJ mol −1 , 1.0); 304L (100 kJ mol −1 , 0.43); Cr (105 kJ mol −1 , 0.099); Ta (106 kJ mol −1 , 0.06). The Arrhenius activation parameters for the various metals (and different samples of the same metal) show a linear compensation effect: In A = (1/θ) E a / R + 1n k o with a compensation temperature, θ, of 445 K. Different samples of molybdenum powders showed θ = 432 K and an isokinetic temperature of 430 K, at which the rates for all molybdenum samples were equal. The relative activities are explained in terms of the binding energies of N-adatoms to the metal surface and an empirical thermochemical relationship is proposed. Additionally it is noted that the least catalytically reactive metals are those whose ambient-atmosphere oxide films are not reduced by hydrazine.

Kinetics and mechanism of the thermal decomposition of monomethylhydrazine by accelerating rate calorimetry

Abstract The thermal decomposition reaction of monomethylhydrazine was studied using an accelerating rate calorimeter. Nitrogen, ammonia, methylamine, methane and ethane were the major products. Dimethylamine, 1,2-dimethyldiazene and traces of nine other nitrogenous compounds or hydrocarbons were observed in the product mixture. The Arrhenius parameters for the decomposition reaction are compensated and linearly related by the relationship In A = E a /Rθ + ln k o , with a compensation temperature θ of 478 K. The isokinetic temperature is 475 K. Activation energies were found to be in the range 71.8–114.8 kJ mol −1 . A mechanism involving surface-bound amido, imido, nitrido, hydrido, methylene and methyl species is proposed to account for the major and minor products.