Showing papers in "Journal of the American Chemical Society in 1983"

Absolute hardness: companion parameter to absolute electronegativity

Abstract: For neutral and charged species, atomic and molecular, a property called absolute hardness eta is defined. Let E(N) be a ground-state electronic energy as a function of the number of electrons N. As is well-known, the derivative of E(N) with respect to N, keeping nuclear charges Z fixed, is the chemical potential ..mu.. or the negative of the absolute electronegativity chi: ..mu.. = (deltaE/deltaN)/sub Z/ = /sup -/chi. The corresponding second derivative is hardness: 2eta = (delta..mu../deltaN)/sub Z/ = (deltachi/deltaN)/sub Z/ = (delta/sup 2/E/deltaN/sup 2/)/sub Z/. Operational definitions of chi and eta are provided by the finite difference formulas (the first due to Mulliken) chi = 1/2(I+A), eta = 1/2(I-A), where I and A are the ionization potential and electron affinity of the species in question. Softness is the opposite of hardness: a low value of eta means high softness. The principle of hard and soft acids and bases is derived theoretically by making use of the hypothesis that extra stability attends bonding of A to B when the ionization potentials of A and B in the molecule (after charge transfer) are the same. For bases B, hardness is identified as the hardness of the species B/sup +/. Tables ofmore » absolute hardness are given for a number of free atoms, Lewis acids, and Lewis bases, and the values are found to agree well with chemical facts. 1 figure, 3 tables.« less

Combination of theoretical ab initio and experimental information to obtain reliable harmonic force constants. Scaled quantum mechanical (QM) force fields for glyoxal, acrolein, butadiene, formaldehyde, and ethylene

Abstract: Fully optimized geometries, complete in- and out-of-plane force fields, and dipole moment derivatives have been calculated for the title compounds at the ab initio HartreeFock level using the 4-21 Gaussian basis set. The theoretical information is combined with experimental data by fitting the calculated force constants through a few parameters to the observed frequencies to obtain the final, scaled quantum mechanical (SQM) force fields. Recommendations for a standard procedure of this type are given. The SQM force fields give excellent reproduction of the fundamental frequencies and are considered as approaching the best accuracy which can be achieved in a harmonic treatment. The infrared intensities obtained at this level of theory are only qualitative estimates, but they are still useful for making assignments more reliable.

Chain-length dependence of electronic and electrochemical properties of conjugated systems: polyacetylene, polyphenylene, polythiophene, and polypyrrole

Abstract: The valence effective Hamiltonian (VEH) technique is used to compute ionization potentials, optical transition energies, and electron affinities of oligomers and polymers in four conjugated systems: polyacetylene, poly@-phenylene), polythiophene, and polypyrrole. The theoretical results compare very favorably with experimental data on gas-phase ionization potentials, optical absorption, and electrochemical redox potentials. The latter case is especially important, and the calculated oxidation and reduction potentials are in remarkably good agreement with experiment. For polyacetylene the predicted oxidation potential is 0.4 V vs. SCE, and the predicted reduction potential is -1.1 V, both of which are in good agreement with experimentally observed oxidation and reduction onsets. In these systems, the electronic and electrochemical properties predicted by VEH theory for the oligomers extrapolate to those of the polymer with an inverse chain-length dependence.

Photochemistry of Ru( bpy)32+. Solvent Effects

Abstract: The excited-state lifetime of the metal-to-ligand charge-transfer (MLCT) excited state or states of Ru(bpy)3Z+ has been measured in a series of solvents at a series of temperatures. The data can be fit to the equation r(T)-' = k + k"' exp (-(AE'/k,T)) where k is the sum of the radiative (k,) and nonradiative (k,,) rate constants for decay of the MLCT state(s) and the temperature-dependent term involves a thermally activated transition from the MLCT state to a low-lying state or states presumably d-d in character. From a combination of lifetime and emission quantum yield measurements, values for k, and k,, have been obtained in the series of solvents. From the variations of the various kinetic parameters with solvent the following conclusions can be reached: (1) k, is only slightly solvent dependent; (2) the variations in k,, and emission energy with solvent are in quantitative agreement with the predictions of the energy gap law for radiationless transitions; and (3) the solvent dependence of the kinetic parameters k'O and AE', which characterize the MLCT - dd transition, can be considered in the context of electron-transfer theory including the observation of a linear relationship between In k'O and AE'(Barc1ay-Butter plot). In a final section the implications of solvent effects on the use of Ru(bpy)32+* as a sensitizer are discussed. We have shown that the energy gap law can be applied to nonradiative decay in a series of polypyridyl complexes of OS(II).'~~ The studies were based on the metal-to-ligand charge-transfer (MLCT) excited states of the two series of complexes (phen)- OSI*L,~+ and (bpy)OsL42+ (bpy is 2,2'-bipyridine, phen is 1,lO- phenanthroline; L = '/2bpy, 1/2phen, pyridine, PR,, Me2S0, CH,CN, ...). The nature of the experiment was to show from excited-state lifetime and emission measurements that plots of In k,, vs. E,, are linear where k,, is the nonradiative decay rate constant and E,, the emission energy. In addition, it was possible to account for the origin of the solvent dependence of k,, on the basis of the energy gap law using the series of Os-phen based MLCT excited states., In these studies, radiative rate constants, k,, for excited-state decay, were shown to be relatively insensitive to variations either in complex or in solvent. The earlier studies based on the Os(I1) complexes are part of a larger effort to explore in detail the photochemical and pho- tophysical properties of MLCT excited states. In many ways the "parent" compound associated with MLCT excited states is Ru- (b~y),~+. The excited-state electronic structures of R~(bpy),~+ and related complexes have been investigated by spectroscopic studie~,~ low-temperature emission and lifetime measurement^,^

Group-transfer polymerization. 1. A new concept for addition polymerization with organosilicon initiators

Abstract: Polymerisation du methacrylate de methyle avec l'acetal de dimethylcetone methyl trimethylsilyle. Reaction rapide a temperature ambiante conduisant a un polymere vivant de distribution etroite de la masse moleculaire, avec rendement quantitatif

The calculation of molecular volumes and the use of volume analysis in the investigation of structured media and of solid-state organic reactivity

Abstract: Proposition d'une methode de calcul, rapide et precise, des volumes moleculaires a partir des geometries moleculaires et des rayons atomiques. Application au calcul du volume de la cavite dans des composes de type cage, a la prevision de la stœchiometrie et de la stabilite de composes d'inclusion dans des matrices solides, a l'analyse des facteurs steriques agissant sur la reactivite de composes organiques a l'etat solide

Description of conjugation and hyperconjugation in terms of electron distributions

Abstract: Definition d'un indice de liaison et d'une ellipticite de liaison pour les liaisons C-C a partir de la distribution de charge p et de ses courbures principales du point critique de liaison, l'ellipticite caracterisant le caractere π d'une liaison et la direction de son axe principal. Application a des hydrocarbures cycliques et acycliques, satures ou non, et a quelques anions et cations organiques

Poly[1-(trimethylsilyl)-1-propyne]: a new high polymer synthesized with transition-metal catalysts and characterized by extremely high gas permeability

Abstract: Polymerisation du (trimethylsilyl)-1 propyne-1 par les pentachlorures et pentabromures de niobium et de tantale pour donner un nouveau polymere de masse moleculaire elevee. Permeabilite tres elevee a l'oxygene

Rate constants for the reactions of free radicals with oxygen in solution

Abstract: The kinetics of the rections of several free radicals with oxygen have been examined in solution at 300 K using laser flash photolysis techniques. The reactions of resonance-stabilized radicals are only slightly slower than those of nonstabilized radicals: for example, for tert-butyl (in cyclohexane), 4.93 x 10/sup 9/; benzyl, 2.36 x 10/sup 9/ (in cyclohexane); cyclohexadienyl (in benzene), 1.64 x 10/sup 9/ M/sup -1/ s/sup -1/. The reaction of butyl-tin (n-Bu/sub 3/Sn.) radicals is unusually fast (7.5 x 10/sup 9/ M/sup -1/ s/sup -1/), a fact that has been tentatively attributed to a relaxation of spin selection rules due to heavy atom effects. 1 table.

Catalytic asymmetric epoxidations with chiral iron porphyrins

Abstract: Epoxydation asymetrique d'olefines prochirales par des porphyrines de fer chirales et par des composes iodosyles

Silicon-29 NMR study of dehydrated/rehydrated silica gel using cross polarization and magic-angle spinning

Abstract: A /sup 29/Si NMR study was carried out on silica gel, using cross polarization and magic-angle spinning (CP/MAS). Spectra were examined on silica samples prepared at various stages of dehydration. It was found that the spectral changes observed in these experiments could not be accounted for by a single structural model of the types that have been advanced previously for silica surfaces. However, these /sup 29/Si spectral features are consistent with a heterogeneous silica surface consisting of separate regions resembling the 100 face and 111 face of ..beta..-cristobalite.

Alkylation of amino acids without loss of the optical activity: preparation of .alpha.-substituted proline derivatives. A case of self-reproduction of chirality

Abstract: Preparation d'un enolate par condensation de proline avec le pivalaldehyde suivie de deprotonation. Etude des reactions de cet enolate avec divers electrophiles. Condensation du pivalaldehyde avec d'autres aminoacides

Activation of carbon-hydrogen bonds in saturated hydrocarbons on photolysis of (.eta.5-C5Me5)(PMe3)IrH2. Relative rates of reaction of the intermediate with different types of carbon-hydrogen bonds and functionalization of the metal-bound alkyl groups

Abstract: Irradiation of (eta/sup 5/-pentamethylcyclopenta-dienyl)(trimethyl phosphine) dehydroiridium (eta/sup 5/-C/sub 5/Me/sub 5/)(PMe/sub 3/)IrH/sub 2/ in saturated hydrocarbons using a 500 W Oriel focused-beam mercury lamp leads to extrusion of hydrogen and production of the hydrido alkyl iridium complexes (----HRIr). Competition experiments were used to measure relative rates at which the intermediate formed on hydrogen loss reacts with different types of C-H bonds. Relative to cyclohexane (1), these rates are: benzene (4), cyclopropane (2.65), cyclopentane (1.6), neopentane (1.14), cyclodecane (.23), and cyclooctane (.09). Reductive elimination of hydrocarbon occurs at elevated temperature, regenerating the intermediate, which may then react with another hydrocarbon acting as solvent. The factors which presumed to influence the rates of reaction of transition-metal complexes with saturated C-H bonds are discussed.

Methane exchange reactions of lanthanide and early-transition-metal methyl complexes

Abstract: Les complexes M(η 5 -C 5 Me 5 ) 2 CH 3 reagissent avec les liaisons sp 3 completement non activees du methane, comme cela a ete detecte avec la reaction d'echange avec 13 CH 4

Laser and pulse radiolytically induced colloidal gold formation in water and in water-in-oil microemulsions

Abstract: Reduction of HAuCl/sub 4/ has been investigated pulse radiolytically in water and in water-in-oil microemulsions. Rate constants have been determined for Au/sup 3 +/ + e/sub aq//sup -/ ..-->.. Au/sup 2 +/, 2Au/sup 2 +/ ..-->.. Au/sup 3 +/ + Au/sup +/, and Au/sup +/ + R/sup -/. ..-->.. Au/sup 0/ + R (where R/sup -/. is an unidentified radical). On the longer time scale formation of colloidal gold, nAu/sup 0/ ..-->.. (Au/sup 0/)n, has been observed. Rate of colloidal gold formation has also been studied in the bombardment of HAuCl/sub 4/ solutions by 353-nm 60-mJ 3-5-ns laser pulses. Hydrodynamic diameters and polydispersities of empty and colloidal-gold-containing microemulsions have been determined by electron micrography. There are a number of advantages of forming colloidal particles in microemulsions. Under identical conditions a greater amount of colloidal particles is formed than that in water. Colloidal gold particles formed in microemulsions are smaller and more uniform than those obtained in homogeneous solutions.

Aliphatic hydroxylation catalyzed by iron porphyrin complexes

Abstract: Hydroxylation et oxydation de cyclohexane, cycloheptane et cis-decaline. Selectivite et stereospecificite

On the origin of nonexponential fluorescence decay in tryptophan and its derivatives

Abstract: The nonexponential fluorescence decay of tryptophan and its derivatives is discussed in terms of a simple model based on conformers about the Ca-CB bond and the relative rates of charge transfer from indole to various electrophiles. Accurate predictions concerning the relative fluorescence lifetimes and the form of the fluorescence decay law are made for tryptophan and 17 of its derivatives, including three new derivatives synthesized specifically to test the model.

Intramolecular nitrene carbon-hydrogen insertions mediated by transition-metal complexes as nitrogen analogs of cytochrome P-450 reactions

Abstract: La reaction est realisee a partir d'un derive d'imidoiodobenzene obtenu par reaction du diisopropyl-2,5 benzenesulfonamide avec le diacetoxyiodobenzene