- Rotational isomerism of acetic acid isolated in rare-gas matrices: Effect of medium and isotopic substitution on IR-induced isomerization quantum yield and cis→ trans tunneling rate
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The rotational isomerization of acetic acid was studied in Ar, Kr, and Xe matrices. Using resonant excitation of a number of modes in the 3500-7000 cm-1 region, the light induced trans→cis reaction was promoted. The quantum yields for this process were also measured for various acetic acid isotopologues and matrix materials. For excitation of acetic acid at energies above the predicted isomerization energy barrier, it was found that the measured quantum yields were in average 2%-3%, one order of magnitude smaller that the corresponding values known for formic acid.
- Macoas,Khriachtchev,Pettersson,Fausto,Raesaenen
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- New mechanistic insight into intramolecular arene hydroxylation initiated by (μ-1,2-peroxo)diiron(III) complexes with dinucleating ligands
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(μ-1,2-Peroxo)diiron(iii) complexes (2-R) with dinucleating ligands (R-L) generated from the reaction of bis(μ-hydroxo)diiron(ii) complexes [Fe2(R-L)(OH)2]2+ (1-R) with dioxygen in acetone at -20°C provide a diiron-centred electrophilic oxidant, presumably diiron(iv)-oxo species, which is involved in aromatic ligand hydroxylation.
- Sekino, Mio,Furutachi, Hideki,Tasaki, Kyosuke,Ishikawa, Takanao,Mori, Shigeki,Fujinami, Shuhei,Akine, Shigehisa,Sakata, Yoko,Nomura, Takashi,Ogura, Takashi,Kitagawa, Teizo,Suzuki, Masatatsu
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- Catalytic Oxidative Cracking of Benzene Rings in Water
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Efficient degradation of harmful benzene rings in water is indispensable for achieving a clean water environment. We report herein unprecedented catalytic oxidative benzene cracking (OBC) in water using a ruthenium(II)-aqua complex having an N-heterocyclic carbene ligand as a catalyst and a cerium(IV) salt as a sacrificial oxidant under mild conditions. The OBC reactions produced carboxylic acids such as formic acid, which can be converted to dihydrogen directly from the OBC solution using a rhodium(III) catalyst with adjustment of the solution pH to 3.3. The OBC reactions can be applied to monosubstituted benzene derivatives such as ethylbenzene, chlorobenzene, and benzoic acid. Initial rates of the OBC reactions showed a linear relationship in the Hammett plot with a negative slope, indicating the electrophilicity of a Ru(III)-oxyl complex as the reactive species in the catalytic OBC reaction. Also, we discuss a plausible mechanism of the catalytic OBC reactions based on the kinetic analysis and the product stoichiometry for the OBC reaction of nonvolatile sodium m-xylene sulfonate. The addition of an electrophilic radical to the aromatic ring to form arene oxide/oxepin is proposed as the initial step of the OBC reaction.
- Shimoyama, Yoshihiro,Ishizuka, Tomoya,Kotani, Hiroaki,Kojima, Takahiko
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p. 671 - 678
(2019/01/08)
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- Secondary deuterium isotope effects on the acidity of carboxylic acids and phenols
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Secondary deuterium isotope effects (IEs) on acidities have been accurately measured by an NMR titration method applicable to a mixture of isotopologues. Deuteration definitely decreases the acidity of carboxylic acids and phenols, by up to 0.031 in the ΔpK per D. For aliphatic acids, the IEs decrease as the site of deuteration becomes more distant from the OH, as expected, but a surprising result is that IEs in both phenol and benzoic acid do not decrease as the site of deuteration moves from ortho to meta to para. The experimental data are supported by ab initio computations, which, however, substantially overestimate the IEs. The discrepancy does not seem to be due to solvation. The IEs originate in isotope-sensitive vibrations whose frequencies and zero-point energies are lowered upon deprotonation. In the simplest case, formate, the key vibration can be recognized as the C-H stretch, which is weakened by delocalization of the oxygen lone pairs. For the aromatic acids, delocalization cannot account for the near constancy of IEs from ortho, meta, and para deuteriums, but the observed IEs are consistent with calculated vibrational frequencies and electron densities. Moreover, the ability of the frequency analysis to account for the IEs is evidence against an inductive origin.
- Perrin, Charles L.,Dong, Yanmei
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p. 4490 - 4497
(2008/02/04)
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- Concerted or stepwise mechanisms for acyl transfer reactions of p-nitrophenyl acetate? Transition state structures from isotope effects
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Isotope effects have been measured for the acyl transfer reactions of p-nitrophenyl acetate (PNPA) with the oxyanion nucleophiles hydroxide, phenolate, and the anion of hexafluoroisopropyl alcohol; with the sulfur anions of mercaptoethanol and methyl 3-me
- Hengge, Alvan C.,Hess, Robert A.
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p. 11256 - 11263
(2007/10/02)
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- Incorporation of Label from 18O2 into Acetate during Side-chain Cleavage Catalysed by Cytochrome P-45017α (17α-hydroxylase-17,20-lyase)
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Samples of pregnenolone and 17α-hydroxypregnenolone deuteriated at the C-21 methyl group have been prepared and subjected to side-chain cleavage with a pig tested microsomal preparation under 18O2. 17α-Hydroxypregnenolone was exclusively cleaved to dehydroisoandrosterone and the acetic acid released during the process was found to incorporate 0.90 atom of 18O.When a similar incubation was performed with pregnenolone two steroidal products, dehydroisoandrosterone and androsta-5,16-dien-3β-ol, were formed in an approximate ratio of 1:2-3 and the acetic acid formed in the process was again shown to contain 0.85 - 0.90 atom of 18O.Complementary experiments in which the two substrates labelled with 18O at the C-20 carbonyl group were incubated under 16O2 gave acetic acid retaining between 65 - 85percent of the original carbonyl oxygen.The experiments strengthen the hypothesis that the two C(17) - C(20) bond cleavages described above correspond to the acyl-carbon fission process of the equation below showing the indicated fate of the various oxygen atoms:
- Akhtar, Muhammad,Corina, David L.,Miller, Sharon L.,Shyadehi, Akbar Z.,Wright, J. Neville
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p. 263 - 268
(2007/10/02)
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- Effect of Reactivity on Virtual Transition-State Structure for the Acylation Stage of Acetylcholinesterase-Catalyzed Hydrolysis of Aryl Esters and Anilides
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The acylation stage of acetylcholinesterase-catalyzed hydrolysis of p-methoxyphenyl formate and of three anilides (o-nitrochloroacetanilide, o-nitroacetanilide, and o-nitroformanilide) has been studied by measuring substrate secondary and solvent isotope effects and by determining pL (L = H, D)-rate profiles and Eyring plots.The results of each of these probes support a model for acylation rate determination that involves a virtual transition state that contains contributions from the transition states of sequential physical and chemical steps.Eyring plots for all substrates are nonlinear and are interpreted in terms of temperature-dependent changes in fractional rate determination of sequential microscopic steps.For all substrates acylation reactivity increases sigmoidally with pH and depends on pKaH2O = 5.6-5.8, which is well below the intrinsic pKa = 6.3 of the active site histidine.Solvent isotope effects for the anilide substrates are in the range 1.3-1.6.Proton inventory experiments indicate that intrinsic solvent isotope effects of ca 2 that arise from general acid-base stabilization of the chemical transition state partially masked by a solvent isotope-insensitive transition state that contributes 58-67percent to acylation rate determination.For the most reactive substrate, p-methoxyphenyl formate, the solvent isotope effect is 1.09, which indicates that the solvent isotope-insensitive transition state is almost entirely rate determining.Substrate secondary deuterium kinetic isotope effects are consistent with decreasing nucleophilic interaction at the carbonyl carbon of the scissile bond of the substrate in the virtual acylation transition state with increasing kcat/Km.Hence, both solvent and substrate isotope effects indicate a general trend toward less acylation rate determination by chemical transition states as reactivity increases.The virtual transition-state model delineated herein lends quantitative support to Rosenberry's notion that the acylation stage of acetylcholinesterase-catalyzed hydrolysis of neutral substrates is prominently rate limited by an induced fit conformation change that precedes chemical catalysis.
- Acheson, Scott A.,Barlow, Paul N.,Lee, Gerald C.,Swanson, Michael L.,Quinn, Daniel M.
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p. 246 - 252
(2007/10/02)
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- Parallel Behavior in Kinetic and NMR Effects: Secondary Deuterium Isotope Effects on the Alkaline Hydrolysis of Esters
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β-Deuterium secondary kinetic isotope effects (β-D KIEs) on the alkaline hydrolysis of the p-nitrophenyl esters of acetic, propanoic, butanoic, and pentanoic acids in pH 10.70, 0.20 M carbonate buffer at 25 deg C tend to increase with increasing chain length of the esters up to the pentanoate.The β-D KIEs are respectively 0.975 +/- 0.004, 0.960 +/- 0.002, 0.940 +/- 0.001, and 0.948 +/- 0.004.The activation energies of the esterolyses of the isotopically light esters follow a similar pattern, as do the 13C NMR nuclear shieldings in CDCl3 of the isotopically light parent carboxylic acids (20.9, 27.4, 35.9, and 33.8 (ppm)) and 13C NMR one-bond isotope shifts produced by disubstitution of deuterium for hydrogen at the α-carbons of the acids (0.45, 0.55, 0.60, and 0.59 (ppm)).Correlation of nuclear shieldings and isotope shifts is known from previous work.The possibility is considered that all of the kinetics-based and NMR relationships are linked through the operation of a common ground-state feature of the ester and acid alkyl chains.
- Matta, Michael S.,Broadway, Dale E.,Stroot, Michele K.
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p. 4916 - 4918
(2007/10/02)
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- Oxidation of (CD3)2CX Radicals. III. Reaction of 2-Propanol and (CD3)2C(OH) Radicals with Atomic Oxygen
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The reaction of oxygen atoms with 2-propanol was studied in a fast flow system coupled with a photoionization mass spectrometer.Radicals formed in the primary attack of O(3P) atoms on 2-propanol were photoionized by the Xe lamp and identified as the (CH3)2C(OH) radical from (CH3)2CH(OH) and (CH3)2CD(OH) and the (CD3)2C(OH) radical from (CD3)2CH(OH).The major primary product in the subsequent reaction of O+(CD3)2C(OH) was found to be acetone, (CD3)2CO, (62+/-4percent).Formation of 1-propen-2-ol, CD3C(OH)=CD2, by deuterium abstraction and acetic acid, CD3COOH, by oxygen addition were observed in the reaction of O+(CD3)2C(OH).In the reaction of 18O+(CD3)2C(OH), the acetone produced was mostly (CD3)2CO.This fact shows that acetone is produced predominantly by the hydrogen abstraction by O(3P) from the O-H in the (CD3)2C(OH) radical.A part of the acetone is also produced also by the addition of O(3P) to the (CD3)2C(OH) radical; i.e., a small but significant signal of (CD3)2C18O was observed in the reaction 18O+(CD3)2C(OH).The competition experiment between O(3P) and O2 for the (CD3)2C(OH) radical shows that (CD3)2C(OH) react (0.16+/-0.02) times as fast with O2 as with O(3P).This result suggests that the reaction of (CD3)2C(OH) with O2 is very rapid.It is proposed that the rapid reaction is hydrogen abstraction by O2 from the O-H in the (CD3)2C(OH) radical to form acetone and HO2.
- Washida, Nobuaki
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p. 3757 - 3764
(2007/10/02)
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- Transition-State Properties and Rate-Limiting Processes in the Acetylation of Acetylcholinesterase by Natural and Unnatural Substrates
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The acetylation of acetylcholinesterase at pH 7.5 and 25.00 deg C proceeds at the same rate to within a few percent for acetylcholine, CD3CO2CH2CH2N(CH3)3+, CH3CO2CD2CH2N(CH3)3+, and CH3CO2CH2CD2N(CH3)3+ and possibly sligh
- Hogg, John L.,Elrod, James P.,Schowen, Richard L.
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p. 2082 - 2086
(2007/10/02)
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- Reaction Progress at the Transition State for Nucleophilic Attack on Esters
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β-Deuterium isotope effects (kCH3/kCD3) at 25 deg C in aqueous solution for reaction of the following nucleophiles and substrate acetate esters are as follows: CH3CO2(-), 2,4-(NO2)2C6H3O2CCL3 (L = H,D), 0.950 +/-0.008; C6H5O(-), 4-NO2C6H4O2CCL3, 0.971 +/-0.020; C6H5O(-), 2,4-(NO2)2C6H4O2CCL3, 0.968 +/-0.009; HO(-), 4-NO2C6H4O2CCL3, 0.970 +/-0.009; HO(-), C6H5O2CCL3, 0.980 +/-0.009.These are consistent with fractions of reactant progress at the transition state of 0.37, 0.21, 0.22, 0.22, and 0.15, respectively.Agreement is reasonably good with fractions of reaction progress estimated from α-deuterium isotope effects in formate ester reactions, measured by Cordes et al.For the nucleophilic reaction of water with C6H5O2CCL3, protolytically catalyzed by CH3CO2(-), the β-D effect is 0.914 +/-0.023 (fraction of reaction progress 0.65).This corresponds well with fractions of reaction progress of 0.58-0.66 from α-D effects for carboxylate general catalysis of three formate ester hydrolyses, studied by Cordes et al.For very strong bases (pK > 12) reacting as nucleophiles, the value of βnuc(ca. 0.2) is similar to, or smaller than, the fraction of reaction progress estimated from the isotope effects (ca. 0.2-0.4).The value of βnuc for nucleophilic reactions of weaker bases (pK = ca. 8-11) is larger than the fractions of reaction progress calculated from isotope effects (0.2-0.4 while βnuc = ca. 0.7).This evidence is consistent with negative deviations both for aryloxide ions and for strongly basic nucleophiles such as alkoxide and hydroxide ions from a hypothetical line of βnuc greater than 0.2 but smaller than 0.7.The major reason for negative deviation of aryloxide ions is probably disproportionate loss of resonance energy and of alkoxide ions is disproportionate loss of solvation energy, upon nucleophilic binding at the transition state.These two factors may be linked and may not be entirely separable.
- Kovach, Ildiko M.,Elrod, James P.,Schowen, Richard L.
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p. 7530 - 7534
(2007/10/02)
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