- Nucleophilic Reactions of Anions with Trimethyl Phosphate in the Gas Phase by Ion Cyclotron Resonance Spectroscopy
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The gas-phase ion-molecule reactions of several negative ions (SF6-, SF5-, SO2F-, F2-, F-, CF3Cl-, Cl-, CD3O-, DNO-, OH-, and NH2-) with trimethyl phosphate are investigated using ion cyclotron resonance techniques.Nucleophilic attack on OP(OCH3)3 occurs chiefly at carbon, resulting in displacement of O2P(OCH3)2-.This behavior contrast with that observed in solution, where attack at phosphorus is favored for hard nucleophiles.This difference is ascribed to solvation energetics for the intermediates involved in the two reactions.The failure of SF6- to transfer F- to OP(OCH3)3 places an upper limit of 11 +/- 8 kcal/mol on the fluoride affinity of OP(OCH3)3.The significance of the results for the negative chemical ionization mass spectrometry of phosphorus esters is briefly discussed.
- Hodges, Ronald V.,Sullivan, S. A.,Beauchamp, J. L.
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- Degradation of Organic Cations under Alkaline Conditions
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Understanding the degradation mechanisms of organic cations under basic conditions is extremely important for the development of durable alkaline energy conversion devices. Cations are key functional groups in alkaline anion exchange membranes (AAEMs), and AAEMs are critical components to conduct hydroxide anions in alkaline fuel cells. Previously, we have established a standard protocol to evaluate cation alkaline stability within KOH/CD3OH solution at 80 °C. Herein, we are using the protocol to compare 26 model compounds, including benzylammonium, tetraalkylammonium, spirocyclicammonium, imidazolium, benzimidazolium, triazolium, pyridinium, guanidinium, and phosphonium cations. The goal is not only to evaluate their degradation rate, but also to identify their degradation pathways and lead to the advancement of cations with improved alkaline stabilities.
- You, Wei,Hugar, Kristina M.,Selhorst, Ryan C.,Treichel, Megan,Peltier, Cheyenne R.,Noonan, Kevin J. T.,Coates, Geoffrey W.
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supporting information
p. 254 - 263
(2020/12/23)
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- Catalytic etherification of alcohols in Shilov system: C[sbnd]O versus C[sbnd]H bond activation
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A novel catalytic reaction of alcohol etherification in the system ROH ? PtCl42? ‐ PtCl62? was found. Methanol easily transforms into dimethyl ether in the presence of catalytic amounts of PtII chloro complexes at 70?°C. Under the same conditions reaction of ethanol affords diethyl ether (catalytic) and π-ethylene PtII complex (stoichiometric). The reactions are accompanied by multiple H/D exchange, which is indicative of intermediacy of corresponding alkyl platinum derivatives. The plausible reaction mechanism involves oxidative addition of alcohol forming intermediate alkyl platinum(IV) derivative followed by decomposition of it via reductive elimination step under the action of alcohol giving the ether and regenerating catalyst. In the case of ethyl alcohol reaction, β-hydrogen abstraction from the intermediate Pt-ethyl species yields π-ethylene platinum(II) complex. Although it seems that the reaction does not involve the initial breaking of C[sbnd]H bonds of an alcohol, this system can be regarded as a model for studying of some peculiarities of Shilov chemistry, in particular, of isotope scrambling mechanisms in Shilov alkane activation. In contrast to reactions of dimethyl and diethyl ethers formation, tert-butyl ethers formation in CD3OH/t-BuOH medium is catalyzed by PtIV chloro complexes also and is not accompanied by isotope scrambling. These observations argue against intermediacy of alkyl platinum derivatives suggesting that acid-catalyzed mechanism operates in tert-butyl alcohol etherification.
- Khazipov, Oleg V.,Nykytenko, Denys V.,Krasnyakova, Tatyana V.,Vdovichenko, Alexander N.,Fuentes Frias, Dario A.,Mitchenko, Serge A.
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p. 490 - 498
(2016/12/16)
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- Imidazolium Cations with Exceptional Alkaline Stability: A Systematic Study of Structure-Stability Relationships
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Highly base-stable cationic moieties are a critical component of anion exchange membranes (AEMs) in alkaline fuel cells (AFCs); however, the commonly employed organic cations have limited alkaline stability. To address this problem, we synthesized and characterized the stability of a series of imidazolium cations in 1, 2, or 5 M KOH/CD3OH at 80 °C, systematically evaluating the impact of substitution on chemical stability. The substituent identity at each position of the imidazolium ring has a dramatic effect on the overall cation stability. We report imidazolium cations that have the highest alkaline stabilities reported to date, >99% cation remaining after 30 days in 5 M KOH/CD3OH at 80 °C.
- Hugar, Kristina M.,Kostalik, Henry A.,Coates, Geoffrey W.
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supporting information
p. 8730 - 8737
(2015/07/27)
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- Gas-Phase Nucleophilic Displacement Reactions
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Displacement reactions of each of a variety of anionic nucleophiles reacting with each of a variety of neutrals have been studied by pulsed ion cyclotron resonance (ICR) spectroscopy.Rate constants for these reactions are interpreted in terms of a three-step reaction sequence.RRKM calculations are used to obtain information about the energy of transition states.The origin of the barrier to reaction in solution is discussed.
- Olmstead, William N.,Brauman, John I.
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p. 1653 - 1662
(2007/10/03)
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- Carbon versus phosphorus site selectivity in the gas-phase anion-molecule reactions of dimethyl methylphosphonate
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The reactions of dimethyl methylphosphonate and its conjugate base with a variety of anions and neutral substrates, respectively, have been examined with use of the thermally equilibrated conditions (298 K) of the flowing afterglow. The conjugate base of dimethyl methylphosphonate reacts readily with alcohols and carbonyl compounds; its reaction with alcohols yields products from proton transfer, proton transfer followed by substitution at carbon, and proton transfer followed by substitution at phosphorus, while its reaction with carbonyl compounds generates products from proton transfer, Horner-Emmons-Wadsworth reaction, addition/elimination, and adduct formation. Dimethyl methylphosphonate undergoes facile reaction with a diverse set of anions ranging in base strength from amide to hydrogen sulfide and in structure from localized heteroatomic bases and localized carbon bases to delocalized carbanions. Four reaction pathways account for the interaction of anions with dimethyl methylphosphonate: proton transfer, nucleophilic substitution at carbon, reductive elimination, and nucleophilic substitution at phosphorus. Proton transfer and nucleophilic substitution at carbon dominate all reactions, while reductive elimination is observed only for the strongest base examined, amide. Methoxide and fluoride are the only anions that react at phosphorus. A reaction coordinate diagram is used to interpret the reactions of dimethyl methylphosphonate and its conjugate base. The acidity of dimethyl methylphosphonate was bracketed to be ΔHoacid[(CH3O)2(CH 3)PO] = 373 ± 3 kcal mol-1.
- Lum, Rachel C.,Grabowski, Joseph J.
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p. 7823 - 7832
(2007/10/02)
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- ALKYL TRANSFER REACTIONS BETWEEN PROTONATED ALCOHOLS AND ETHERS. GAS-PHASE ALKYLATION OF FORMALDEHYDE
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Alkyl-transfer reactions involving protonated alcohols and ethers, of the general type R2OR'+ + R''2O -> R""OR'+ + R2O, may be classified according to the degree of alkylation, i.e., the total number n of alkyl groups in the system.For systems containing two alkyl groups, the reaction is alkyl transfer between protonated and neutral alcohols, ROH2+ + ROH -> R2OH+ + H2O, and we measured rate constants for R=Me, Et, i-Pr, and t-Bu.The rate constants are (0.6-1.5) x 1E-10 cm3s-1 when R is a normal alkane and (6 +/- 1) x 1E-10 when R=i-Pr and t-Bu.The larger rate constants in the latter may be due to a lowered barrier for the initial partial R+-OH2 bond dissociation.For reaction systems containing three alkyl groups, i.e., the reactions of protonated ethers R2OH+ with alcohols R'OH, the possible channels are alkyl transfer from the alcohol, yielding R2OR'+, or alkyl transfer from the ether, yielding ROR'H+.Both processes are observed in (CH3)2OH+ + C2D5OH which yields both (CH3)2OC2D5H+ and CH3OC2D5H+.For systems containing four alkyl groups an example is the reaction(CH3)2OH+ + (CH3)2O -> (CH3)3O+ + CH3OH, which is a slow reaction with k3s-1.Finally, for the highest possible degree of alkylation, n=5, an example is methyl transfer in (CH3)OCD3+ + (CH3)2O -> (CH3)2OCH3+ + CH3OCD3 which is a very slow reaction, observed only above 500K.The rate constants for alcohols show negligible temperature dependence between 300 and 670 K, but in the most highly alkylated system the rate increases strongly with temperature, and an activation energy of 15 kcal-1 is observed.The results show that alkyl transfer occurs in systems with all possible degrees of alkylation, but the rates tend to decrease with increasing alkylation.In addition to saturated systems, alkyl transfer is also observed with unsaturated ions or neutrals.Examples are alkyl transfer between unsaturated oxocarbonium ions C2H5O+ and methanol and ethanol and between protonated alcohols and CH2O.These reactions have rate constants of (1-4) x 1E-11 cm3s-1.Depending on the temperature coefficients, the alkylation of formaldehyde may be important in astrochemical synthesis.
- Karoas, Zeev,Meot-Ner (Mautner), Michael
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p. 1859 - 1863
(2007/10/02)
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- Bis(dimethoxymethyl) peroxide and bis(1,1-dimethoxyethyl) peroxide
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The title compounds 1 and 2, the first examples of peroxides polysubstituted at the α and α' positions by alkoxy groups, are formed by benzophenone sensitized photooxygenation of trimethoxymethane and 1,1,1-trimethoxyethane, respectively.No peroxide was formed from tetramethoxymethane.Allowing 98percent hydrogen peroxide and trimethylmethane to stand results in an 80percent yield of 1, so that 1 and 2 are probably formed by such a disproportionation reaction during photooxygenation.Compound 1 is converted quantitatively to methanol, methyl formate, and dimethyl carbonate in pyridine solution at 60 deg C.In acidic methanol both 1 and 2 undergo solvolysis rapidly with exclusive cleavage of the carbon - peroxy oxygen bond.Signals for the ether and peroxy oxygens of 1 appear at 34 and 263 ppm and those of 2 appear at 40 and 264 ppm in the 17O nuclear magnetic resonance spectrum.Luminescence results when 1 and 2 are heated to 150 deg C.
- Kopeczky, Karl R.,Molina, Jose
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p. 2350 - 2355
(2007/10/02)
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- Phosphoric Amides. Part 8. The Effect of the Ethyleneimine Substituent on the Solvolytic Reactivity of Phosphate and Phosphoramidate Bonds
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Rates and products of the base-catalysed hydrolysis of some amidoesters of phosphoric acid have been determined in the N,N-dimethyl derivative, the P-N bond is resistant, and the P-O bond deactivated towards hydrolysis, while in the N-methyl substrate, the reactivity of the ester link is similar to that in trimethyl phosphate.In the N-ethylene compound, both P-O and P-N bonds are strongly activated.The N-(β-chloroethyl) substrate reacts via fast, base-catalysed cyclization to the N-ethylene amidate.
- Davidowitz, Bette,Modro, Tomasz A.
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p. 303 - 306
(2007/10/02)
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- Simultaneous Occurrence of Solvent Reorganization and Heavy-Atom Reorganization in a Transmethylation Transition State
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The methylation of methoxide anion by S-methyldibenzothiophenium cation in methanol solution from 10 to 25 deg C (ionic strength 0.100), and in binary mixtures of methanol-h and methanol-d at 25 deg C, exhibits (a) a carbon isotope effect at transferring methyl, k12/k13 = 1.072 +/- 0.006 (10 deg C), 1.072 +/- 0.007 (20 deg C), 1.080 +/- 0.011 (25 deg C). (b) an α-deuterium secondary isotope effect at transferring methyl, k3H/k3D = 0.996 +/- 0.006 (10 deg C), 0.994 +/- 0.008 (20 deg C), 0.972 +/- 0.012 (25 deg C), and (c) a solvent isotope effect, kCH3OD/kCH3OH = 2.04, with the data in mixtures being consistent with transition-state solvation models involving any number of solvent hydrogen bonds to transition-state sites.The results require a transition state with substantial amounts of both C-S bond fission and C-O bond formation and with reorganisation of the initial-state solvation structure being considerably advanced but still incomplete.Specific salt effects are observable and are treated by a modified Setschenow formulation.
- Wong, Osborne S.-L.,Schowen, Richard L.
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p. 1951 - 1954
(2007/10/02)
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