- An in Situ Solid-State NMR Study of the Formation and Reactivity of Trialkylonium Ions in Zeolites
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In situ 13C and 77Se magic angle spinning (MAS) NMR was used to investigate the formation and reactivity of trialkyonium species on zeolite catalysts HZSM-5 and HY.Trimethyloxonium, trimethylsulfonium, and trimethylselenonium ions were all formed on HZSM-5 by adsorption of the corresponding dimethyl chalcogenide.The identity of these species was confirmed by comparison to solution-state chemical shifts as well as to an authentic sample of trimethylsulfonium-ZSM-5.Trimethyloxonium formed only on the strongly acidic zeolite HZSM-5 whereas trimethylsulfonium formed on both HZSM-5 and the less acidic HY.This result suggests that onium ions may be useful for measuring the strengths of Bronsted acid sites in catalysts.Unlike trialkyloxonium ions, trialkylsulfonium ions were stable at high temperatures and in the presence of alcohols.The adsorption of more than 1 equiv per acid site of dimethyl sulfide poisoned the catalyst by titration of the Bronsted acid sites.With less than 1 equiv per acid site of dimethyl sulfide, the remaining sites were active for MTG chemistry, but the trimethylsulfonium was not consumed in the formation of hydrocarbons.These results are interpreted in terms of onium-ylide mechanism proposed for methanol-to-gasoline chemistry on HZSM-5.In situ 13C MAS NMR studies of dimethyl ether at various loadings reconcile these studies with a previous in situ FTIR investigation by Forester and Howe (J.Am.Chem.Soc. 1987, 109, 5076).Trimethyloxonium ion formation on HZSM-5 is strongly loading dependent.At low loadings of dimethyl ether (e.g., 0.2 equiv per acid site), no trimethyloxonium formed, and protonation shifts were observed as the sample was heated.Trimethyloxonium formed readily at higher dimethyl ether loadings (e.g., 2 equiv per acid site).
- Munson, Eric J.,Kheir, Ali A.,Haw, James F.
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- Gas phase versus solution chemistry: On the reversal of regiochemistry of methylation of sp2- and sp3-nitrogens
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The nicotine analogue 3-(N,N-dimethylaminomethyl)pyridine 2, which reacts with methyl iodide in acetonitrile exclusively on the sp3-nitrogen, methylates exclusively on the pyridine nitrogen with trimethyloxonium in the gas phase. Calculations a
- Brodbelt, Jennifer S,Isbell, John,Goodman, Jonathan M,Secor, Henry V,Seeman, Jeffrey I
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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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- Unconventional Ionic Hydrogen Bonds. 1. Complexes of Quaternary Ions with n- and ?-Donors
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interaction energies are obtained from the clustering of quaternary onium ions with n-donor solvent molecules.The dissociation energies (ΔHoD) of Me4N(1+) clustered with the n-donor H2O, MeOH, MeNH2, and Me3N and with the ?-donors benzene and toluene range between 8 and 10 kcal mol-1.With the weak, bulky n-donor MeCl the interaction is weaker (6.5 kcal mol-1) while the more polar ligands Me2CO and MeCONMe2 attach strongly (14.6 and 18.0 kcal mol-1, respectively) to Me4N(1+).Strong interactions, 20-23 kcal mol-1, are also observed with polyethers and CH3CO-gly-OCH3, indicating polydentate complexing.The attachment energies of ligands to Et4N(1+) are smaller by 2 kcal mol-1 than those to Me4N(1+).Ab initio calculations show that in the Me4N(1+)*H2O, MeOH, MeNH2, and MeCl complexes the ligands attach electrostatically to a cavity created by protons of three CH3 gropus rather than hydrogen bonding to one proton or to one CH3 group.Both experiment and theory indicate that a second solvent molecule (H2O or CH3OH) attaches preferentially to the first solvent molecule rather than to Me4N(1+).
- Meot-Ner (Mautner), Michael,Deakyne, Carol A.
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p. 469 - 474
(2007/10/02)
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- Chloronium Ions as Alkylating Agents in the Gas-Phase Ion-Molecule Reactions with Negative Temperature Dependence
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The kinetics of the reactions Me2Cl+ + B = MeB+ + MeCl and MeEtCl+ + B = MeB+ or (EtB+) + EtCl (or MeCl) were studied with a pulsed-electron-beam, high-pressure mass spectrometer.At room temperature the rate constants were found to increase in the order B = benzene, toluene, isopropylbenzene, EtOH, Me2O, Et2O.At this point k become equal to the orbiting capture rate constant k1 ca. 10-9 molecule-1 cm3 s-.NH3 and Me3N were alkylated at orbiting capture rates.The temperature dependence of the rate constants for B = toluene, Me2O, and Et2O was examined.The rate constants were found to increase with decrease of temperature.This increase continued until the rate constants reached the magnitude of the orbiting rate constant kL.The rates remained approximately independent of temperature below this temperature.At low temperatures the collision-stabilized Me2Cl+B and MeEtCl+B could be observed.The temperature dependence of the equilibrium Me2Cl+ + toluene = (Me2Cl-toluene)+ was measured and led to the corresponding ΔHo and ΔSo.The reaction Me2Cl+ + benzene = Me-benzene+ + MeCl was found to have positive temperature dependence.On the basis of the above data it is suggested that the reactions Me2Cl+ + B = MeB+ + MeCl have an internal barrier in the potential energy of the reaction coordinate.This barrier protrudes above the energy level of the reactants (Me2Cl+ + B) for B = benzene.This leads to positive temperature dependence.For all other B, the top of the internal barrier lies below the level of the reactants and sinks lower, roughly in the order of increasing basicity of B.This lead to negative temperature dependence (toluene, isopropylbenzene, Me2O, Et2O).For B = NH3, MeNH2, Me3N, the barrier is so low that the reactions have orbiting capture rates equal to kL.Alkylation of bases B by chloronium ions like Me2Cl+ might have considerable utility in mass spectrometric analysis by chemical ionization.Ethers can be distinguished from alcohols and tertiary amines from primary and secondary amines.The alkylated ethers and the tertiary amines have no protic hydrogens and therefore do not form strongly hydrogen-bonded adducts.
- Sharma, D. K. Sen,Kebarle, P.
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