- The Ionic Hydrogen Bond and Ion Solvation. 2. Solvation of Onium Ions by One to Seven H2O Molecules. Relations between Monomolecular, Specific, and Bulk Hydration
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The relation between enthalpies of solvation of onium ions BH+ by one water molecule, -ΔHo0.1, and by four water molecules, -ΔHo0.4, is constant for most onium ions: ΔHo0.4/ΔHo0.1 is 2.8 +/- 0.1 for all oxonium ions and monoprotonic ammonium and pyridinium ions, and 3.1 +/- 0.1 for polyprotonic ammonium ions.These relations, in conjunction with the correlation between ΔHo0.1 and the proton affinity difference ΔPA = PA(B) - PA(H2O), allow the prediction of the total four-molecule specific hydration energy -ΔHo0.4 for all onium ions within the experimental accuracy of +/-3 kcal mol-1.The observed (or predicted) fourfold specific relative hydration energies simulate closely the relative bulk hydration enthalpies for most ions.In other words, for most onium ions differential hydration effects are determined by the specific hydrogen-bonding interactions.Deviations are useful to identify bulk solvation effects.For example, such deviations indicate attenuated bulk solvation of ions with phenyl substituents.
- Meot-Ner (Mautner), Michael
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- Kinetic and Theoretical Study on the Ion/Molecule Reactions of Methoxymethyl Cation with Ammonia
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The reaction between C2H5O+ and NH3 is investigated by using the ion-trapping technique.Rate constants of reactions 3, 4, and 5 are determined.Their potential energy profiles are sought by use of the ab initio MO calculation.Reaction 3 gives the products methanol and protonated methylenimine.It is composed of the nucleophilic addition of NH3 to the carbonyl carbon and the subsequent 1,3-proton shift.Reaction 4 gives formaldehyde and protonated methylamine.It is of the SN2 type.Reaction 5 gives ammonium ion and ethylene oxide.The last reaction is of the specific pattern in the gas phase, although the reverse reaction involves the popular mechanism of the electrophilic ring cleavage.
- Okada, Satoshi,Abe, Yasuo,Taniguchi, Setsuo,Yamabe, Shinichi
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- Photochemistry of 2-azido-1-methylimidazole in aqueous solutions. Observation of the 1-methyl-2-imidazolylnitrenium ion
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Irradiation of 2-azido-1-methylimidazole (12) in aqueous solution gives products from two reaction channels. One pathway involves a ring opening typical of azidoheterocycles. The observed products are glyoxal bis-hydrate, the methylammonium ion, and cyanamide; a glyoxal bis-oxime is presumed to be the intermediate initially formed in the ring opening. The other pathway leads to products that retain the five-membered ring, the 2-amino-4,5-dihydro-4,5-dihydroxy-1-methylimidazolium ion 3, its monophosphate ester 6 when the irradiation is carried out in phosphate buffer, and glutathione adducts 7 and 8 when glutathione (GSH) is present. These products have been previously observed in the reactions of 2-hydroxylamino-1-methylimidazole in aqueous solution, and arise from reaction of the 1-methyl-2-imidazolylnitrenium ion (2+) with water, phosphate, and GSH. This pathway is therefore proposed to involve formation of the cation 2+ via protonation of the singlet 1-methyl-2-imidazolylnitrene 13 formed upon irradiation of the azide. A single transient species undergoing exponential decay with λmax at 230-235 nm is observed with flash photolysis. This transient is assigned to 2+ on the basis of the pH dependence of the yields of products, and especially because of the correspondence of k2(GS-):ks ratios measured directly with flash photolysis and by competition kinetics starting from the hydroxylaminoimidazole. The cation 2+ has a lifetime in water of 100 ms, and shows a high selectivity for GSH with k2(GS-) = 3 × 107 M-1 s-1. There is evidence that this class of nitrenium ion is formed upon reductive metabolism of 2-nitroimidazoles. Thus this class of drugs is capable of producing a relatively long-lived electrophile in biological systems.
- Gadosy, Timothy A.,McClelland, Robert A.
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- Trends in alkyl substituent effects on nucleophilic reactions of carbonyl compounds: Gas phase reactions between ammonia and R1R2COCH3+ oxonium ions
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The reactivity of carbonyl substituted methyl oxonium ions (R1R2COCH3-) towards ammonia has been investigated using an FT-ICR mass spectrometer and ab initio calculations. The monosubstituted ions (R1=H: R2 = H, CH3, C2H5 and i-C3H7) show different reaction patterns with variable degree of: (1) nucleophilic substitution, (2) addition elimination and (3) proton transfer, when reacted with ammonia. In all cases addition-elimination dominates over nucleophilic substitution, and the observed reactions are slow. The trends in reactivity are consistent with the alkyl group's electronic properties, as expressed by a single parameter linear or slightly non-linear model.
- Bache-Andreassen, Lihn,Uggerud, Einar
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p. 705 - 713
(2007/10/03)
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- Site of Gas-phase Methylation of 1-Phenyl-2-aminopropane
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The regioselectivity of methyl cation transfer from (CH3)2F(1+), (CH3)2Cl(1+) and (CH3)3O(1+) to 1-phenyl-2-aminopropane was studied by Fourier transform ion cyclotron resonance in combination with collision-induced dissociation and neutralization-reionization mass spectrometry of the stable (1+) ions formed in a chemical ionization source.The (CH3)2F(1+) ion transfers a methyl cation to the NH2 group and the phenyl ring with almost equal probability.Predominant CH3(1+) transfer to the NH2 group is observed for the (CH3)2Cl(1+) ion whereas the (CH3)3O(1+) ion reacts almost exclusively at the amino group.The preference for m ethylation at NH2 is discussed in terms of a lower methyl cation affinity of the phenyl ring than of the amino group and the existence of an energy barrier for methylation of the phenyl moiety.
- Zappey, Herman,Fokkens, Roel H.,Ingemann, Steen,Nibbering, Nico M. M.,Florencio, Helena
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p. 587 - 594
(2007/10/02)
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- An ICR mass spectrometry study of ion/molecule reactions in mixtures of methylamine with ethylamine, propylamine, or butylamine
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Ion/molecule reactions of protonated alkylamine ion species, RNH3+, were studied in mixtures of methylamine with ethylamine, propylamine, or butylamine by ICR mass spectrometry at 1E-5 Torr.The occurrence of methyl group transfer from methylamine to higher protonated alkylamine species was observed.The rate constants of some bimolecular reactions occurring in these systems were estimated by numerical simulation.
- Xu, Guoying,Herman, Jan A.,Wojcik, Leszek
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p. 570 - 574
(2007/10/02)
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- Unconventional Ionic Hydrogen Bonds. 2. NH(1+)...? Complexes of Onium Ions with Olefins and Benzene Derivatives
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Unconventional strong ionic hydrogen bonds of the -XH(1+)...? type, where the electron donor is a ?-bond or an aromatic ?-system, are formed in the clustering reactions of NH4(1+) and MeNH3(1+) with C2H4 and benzene derivatives.The interaction energies range from 10 to 22 kcal mol-1.The experimental results and ab initio calculations on C2H4*NH4(1+), C6H6*NH4(1+), and C6H5F*NH4(1+) indicate that the interaction is primarily electrostatic in nature with little ?-donation into the bond.The most stable structure of C2H4*NH4(1+) is the conformer where one N-H(1+) bond points at the center of the double bond.For C6H6*NH4(1+) and C6H5F*NH4(1+), the lowest energy ?-dimers have two NH4(1+) hydrogens directed towards the ring.The F...H-NH3(1+) ?-complex was studied also for C6H5F*NH4(1+).The latter complex is the more stable of the two at this level of calculation.
- Deakyne, Carol A.,Meot-Ner (Mautner), Michael
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p. 474 - 479
(2007/10/02)
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- The Ionic Hydrogen Bond. 2. Multiple NH+...O and CH?+...O Bonds. Complexes of Ammonium Ions with Polyethers and Crown Ethers
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Complexes of ammonium ions RNH3+ (R = CH3, c-C6H11), (CH3)3NH+, and pyridineH+ with polyethers and crown ethers are observed in the gas phase in the abscence of the solvent effects.The dissociation energies, ΔH0D, of the RNH3+ polyether complexes range from 29.4 kcal mol-1 (for RNH3+*CH3OCH2CH2OCH3) to 46 kcal mol-1 (RNH3+*18-crown-6).The large ΔH0D values for complexes of polydentate ligands indicate multiple -NH+...O-hydrogen bonding.Such mutiple bonding can contribute up to 18 kcal mol-1 to the bonding in RNH3+*CH3(OCH2CH2)3OCH3 and 21 kcal mol-1 in RNH3+*18-crown-6.Multiple interactions are also evident in the (CH3)3NH+*polyether complexes where -CH?+...O-hydrogen bonding seems to occur; and consecutive -CH?+...O-bonds contribute approximately 6, 4, and 2 kcal/mol-1 respectively for up to three such bonds.Total ΔH0D values in the (CH3)3NH+*polyether complexes thus range from 26.7 kcal mol-1 in (CH3)3NH+*CH3O(CH2)2OCH3 to 41 kcal mol-1 in (CH3)3NH+*18-crown-6.Multiple interaction effects, possibly including van der Waals dispersion forces, are observed also in pyridineH+*polyether complexes.Large negative entropies in RNH3+*acyclic polyether complexes vs.RNH3+*cyclic crown ethers make the acyclic polyethers less efficient ligands.
- Meot-Ner (Mautner), Michael
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p. 4912 - 4915
(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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- Kinetics and Mechanism of the Oxidation of N-Methylformamide by Aquothallium(III) in Perchloric Acid Media
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Kinetics of oxidation of N-methylformamide (NMF) by thallium(III) is studied in perchloric acid medium and the results are compared with the oxidation of formamide and dimethylformamide.When initial > , actual reducing agent is formic acid whereas if initial > , both HCOOH and unhydrolysed NMF act as reducing agents.The reactivity sequence, formamide > NMF > dimethylformamide fits the Taft structure-reactivity correlation which yields a value of -1.2 +/- 0.05 for reaction constant, ρ, showing the amide oxidation to be strongly influenced by polar effects.
- Gupta, K. S.,Saxena, S. D.
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p. 818 - 820
(2007/10/02)
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