Showing papers in "Journal of the American Chemical Society in 1978"
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TL;DR: In this paper, the authors proposed a modified Wolfsberg-Helmholz formula to reduce the effect of counterintuitive orbital mixing in extended Hiickel and limited basis set ab initio SCF calculations.
Abstract: In both extended Hiickel and limited basis set ab initio SCF molecular orbital calculations an intriguing phenomenon occurs, especially in calculations on transition metal complexes. Some of the lower orbitals, which are made up primarily of ligand 2s combinations, mix into themselves the metal virtual orbitals, 4s and 4p, in an out-of-phase manner, opposite to what one would have predicted from a simple perturbation analysis. We call this counterintuitive orbital mixing (COM). The conse- quences of COM are lowered metal to ligand overlap populations and negative Mulliken gross atomic populations of the virtual orbitals. We analyze the prerequisites for occurrence of COM in both semiempirical and SCF methods and propose a modified Wolfsberg-Helmholz formula for reducing the magnitude of the effect in EH calculations.
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TL;DR: In this article, the authors investigated the photodecomposition of acetic acid on anatase powders and showed that the results were mainly methane and CO/sub 2, with small amounts of hydrogen and ethane.
Abstract: The heterogeneous photocatalytic decomposition of acetic acid/acetate mixtures was studied on n-type TiO/sub 2/ powder. The influence of solution composition, reaction temperature, light intensity, and semiconductor properties (crystallographic structure, doping) was investigated. For photodecarboxylation in the absence of molecular oxygen, anatase powders proved to be most efficient with increased reactivity due to doping and/or partial coverage of the powder particles with platinum. The products of the photodecomposition of acetic acid on platinized anatase powder were mainly methane and CO/sub 2/, with small amounts of hydrogen and ethane. Other saturated carboxylic acids (propionic, n-butyric, n-valeric, pivalic, adamantane-1-carboxylic acid) were also photocatalytically decarboxylated to the corresponding alkanes. An interpretation based on the photoelectrochemical properties of n-type TiO/sub 2/, with photooxidation and (dark) reduction reactions occurring on the particles on local cell processes, is proposed.
TL;DR: In this article, an approach to the construction of asymmetric hydrogenation catalysts based on embedding an (effectively) achiral diphosphinerhodium(l) moiety at a specific site in a protein was described.
Abstract: We wish to describe an approach to the construction of asymmetric hydrogenation catalysts based on embedding an (effectively) achiral diphosphinerhodium(l) moiety at a specific site in a protein: the protein tertiary structure provides thc chirality required for enantioselective hydrogenation. Although it is presently difficult to predict the enantioselectivity of any hydrogenation from knowledge of the structures of catalyst and substratc, phosphine-rhodium(l) complexes having rigid. conformationally homogeneous structures seem generally to be more effective catalysts than those which are conforma-tionally mobile .2 A globular protein modified by introduction of a catalytically active metal at an appropriate sitc could, in principle, provide an exceptionally wcll-defined steric environment around that mctal. and should do so for considerably smaller effort than would be required to construct a synthetic substancc of compa rable stereochemical complexi ty. Our initial efforts have focused on avidin. This well-characterized protein is composed of four ide ntical subunits, each of which binds biotin and many of its dcrivativcs sufficiently' tightly that association is effectively irreversible (K.r : l0-r2*10-rs M).1'a Biotin was converted to a chelating di-phosphine and complexed with rhodium(l) by the sequencc outlined in cq I (NBD = norbornadiene, Tf = triflate).5.6 The i n term ed ia te N,I/-bi s (2-d i ph en y I phos prh i noet h 1,'l) bi ot i n a m i dc (l) was fully characterizcd:s the rhodium complex l.
TL;DR: In this paper, the heterogeneous photocatalytic decomposition of acetic acid on n-type TiO/sub 2/ to yield almost exclusively methane and carbon dioxide is reported.
Abstract: The heterogeneous photocatalytic decomposition of acetic acid on n-type TiO/sub 2/ to yield almost exclusively methane and carbon dioxide is reported. Probable reaction mechanisms are discussed. It is assumed that the TiO/sub 2/ powder acts in a dual function causing the photooxidation of acetate and the reduction of intermediately formed methyl radicals. This property is not found at metal electrodes under usual Kolbe conditions. (JSR)
TL;DR: In this article, a tandem quadrupole mass spectrometer is used to create ion species from a sample, separate one individual ion species, fragment it, and obtain the mass spectrum of the fragments.
Abstract: : A tandem quadrupole mass spectrometer is used to create ion species from a sample, separate one individual ion species, fragment it, and obtain the mass spectrum of the fragments. Practicability of the Selected Ion Fragmentation Tandem Quadrupole Mass Spectrometer (SIFT QMS) is experimentally demonstrated. Normally large ion losses due to scattering in the collision-induced dissociation (CID) process are eliminated by performing CID in a 'RF-only' quadrupole. CID spectra with unit mass resolution are produced by the quadrupole mass filters. The CID spectrum of each component of a cyclohexane mixture containing three minor components is presented. Potential applications of SIFT QMS in mixture analysis and structure elucidation are discussed. (Author)
TL;DR: In this article, the carbenoid character of the carbonyl carbon resulting from an unusual ''side-on'' coordination of acyl and formyl groups was attributed to the coordination of alkyl and hydride derivatives of zirconium with CO.
Abstract: Bis(pentamethylcyclopentadienyl)dihydridozirconium(IV), (eta/sup 5/-C/sub 5/Me/sup 5/)/sub 2/ZrH/sub 2/ (2) is prepared by the reaction of H/sub 2/ with ((eta/sup 5/-C/sub 5/Me/sub 5/)/sub 2/ZrN/sub 2/)/sub 2/N/sub 2/ (1). 2 forms unstable adducts with PF/sub 3/ and CO at -80/sup 0/C. The carbonyl adduct (eta/sup 5/-C/sub 5/Me/sub 5/)/sub 2/ZrH/sub 2/(CO) yields ((eta/sup 5/-C/sub 5/Me/sub 5/)/sub 2/ZrH)/sub 2/(OCH == CHO) and/or (eta/sup 5/-C/sub 5/Me/sub 5/)/sub 2/ZrH(OCH/sub 3/), depending on reaction conditions. Carbonylation of (eta/sup 5/-C/sub 5/Me/sub 5/)/sub 2/Zr(CH/sub 3/)/sub 2/, obtained from (eta/sup 5/-C/sub 5/Me/sub 5/)/sub 2/ZrCl/sub 2/ and methyllithium, yields successively (eta/sup 5/-C/sub 5/Me/sub 5/)/sub 2/Zr(CH/sub 3/)(CH/sub 3/CO) and (eta/sup 5/-C/sub 5/Me/sub 5/)/sub 2/Zr(O(CH/sub 3/)C == C(CH/sub 3/)O). The zirconacyclopentane ecomplex (eta/sup 5/-C/sub 5/Me/sub 5/)/sub 2/Zr(CH/sub 2/(CH/sub 2/)/sub 2/CH/sub 2/) (10) is prepared from 1 and ethylene. Carbonylation of 10 affords (eta/sup 5/-C/sub 5/Me/sub 5/)/sub 2/Zr(H)-(OC == OH(CH/sub 2/)/sub 2/CH/sub 2/). Treatment of 2 with isobutylene yields (eta/sup 5/-C/sub 5/Me/sub 5/)/sub 2/Zr(H)(Ch/sub 2/CHMe/sub 2/) (13), which undergoes a reaction with CO to form (eta/sup 5/-C/sub 5/Me/sub 5/)/sub 2/Zr(H)(OCH == CHCHMe/sub 2/) (15). The results of /sup 13/C and deuterium labeling studies indicate that the conversion of 13 to 15 is mediated by (eta/sup 5/-C/sub 5/Me/sub 5/)/sub 2/Zr(H)(Me/sub 2/CHCH/sub 2/CO). The observed patterns formore » these reactions of alkyl and hydride derivatives of zirconium with CO are attributed to carbenoid character of the carbonyl carbon resulting from an unusual ''side-on'' coordination of acyl and formyl groups.« less
TL;DR: In this paper, a molecular orbital study of the insertion of ethylene into a Pt-H bond was performed. But the insertion pathway was not easy, nor a facile reaction by a direct route from a four-coordinate complex with ethylene and hydride trans to each other.
Abstract: A molecular orbital study of the insertion of ethylene into a Pt-H bond begins with an analysis of the interaction of a hydride with an ethylene in the absence of the metal. Identification of the crucial orbitals along a simplified reaction coordinate allows one to focus on how the metal atom with one to three other ligands attached to it facilitates the insertion. Associative and dissociative processes from a four-coordinate reactant lead to five- and three-coordinate intermediates whose complex po- lytopal rearrangements are explored in detail. We do not find an easy insertion pathway from a five-coordinate intermediate, nor a facile reaction by a direct route from a four-coordinate complex with ethylene and hydride trans to each other. The neces- sary final waypoint of ethylene and hydride cis seems in the calculations to be best achieved by a sequence of associative and, preferably, dissociative steps.
TL;DR: In this paper, it was shown that di-and tripositive metal ions bind to o-semiquinone radical centers within melanin polymers to yield chelate complexes, and the binding is often accompanied by large increases in total radical con-centration, which are considered to arise via metal ion induced shifts in a comproportionation equilibrium.
Abstract: ESR experiments reveal, for the first time, that di- and tripositive metal ions bind to o-semiquinone radical centers within melanin polymers to yield chelate complexes. The binding is often accompanied by large increases in total radical con- centration, which are considered to arise via metal ion induced shifts in a comproportionation equilibrium. The effects observed are likely to be general for radicals with chelating ability, and may find use in the separation and identification of composite ESR signals in other biological systems.
TL;DR: In this article, a graph-theoretical theory of aromaticity for polyhedral boranes with a general formula of B/sub n/H/Sub n/sup 2 -/ has been proposed.
Abstract: Resonance energies of typical polyhedral boranes with a general formula of B/sub n/H/sub n//sup 2 -/ have been calculated by means of a graph-theoretical theory of aromaticity previously reported by Aihara. Huckel-type molecular orbitals employed are those of Kettle and Tomlinson with three-center BBB localized orbitals as basis functions. Most polyhedral boranes investigated are predicted to be aromatic with positive resonance energies, in general agreement with their chemistry. The present resonance energy is fairly proportional to the logarithm of the number of valence structures allowed for the polyhedral borane.