The gas-phase acidity scale

Bartmess, John E.

doi:10.1016/b978-0-12-120802-8.50009-6

Cited by 162 publications

(105 citation statements)

References 68 publications

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“…42 The experimental result that methane is a poorer proton donor than dihydrogen by 17 kcal mol -1 supports the interpretation of the computational results that sigma-bond metathesis transition states resemble proton transfer transition states, i.e., they are proton transfer reactions. A corollary is the expectation that methane activation will always be more difficult than dihydrogen activation by about 15-18 kcal-mol -1 when oxidative addition does not occur.…”

Section: Chartsupporting

confidence: 68%

Reactions of Monomeric [1,2,4-(Me₃C)₃C₅H₂]₂CeH and CO with or without H₂: An Experimental and Computational Study

et al. 2007

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Abstract:Addition of CO to [1,2,4-(Me 3 C) 3 C 5 H 2 ] 2 CeH, Cp' 2 CeH, in toluene yields the cis-(Cp' 2 Ce) 2 (μ-OCHCHO), in which the cis-enediolate group bridges the two metallocene fragments. The cis-enediolate quantitatively isomerizes intramolecularly to the trans-enediolate in C 6 D 6 at 100°C over seven months. When the solvent is pentane, Cp' 2 Ce(OCH 2 )CeCp' 2 forms, in which the oxomethylene group or the formaldehyde dianion bridges the two metallocene fragments. The cis-enediolate is suggested to form by insertion of CO into the Ce-C bond of 1 Cp' 2 Ce(OCH 2 )CeCp' 2 generating Cp' 2 CeOCH 2 COCeCp' 2 . The stereochemistry of the cisenediolate is determined by a 1,2-hydrogen shift in the OCH 2 CO fragment that has the OC(H 2 ) bond anti-periplanar relative to the carbene lone pair. The bridging oxomethylene complex reacts with H 2 , but not with CH 4 , to give Cp' 2 CeOMe, which is also the product of the reaction between Cp' 2 CeH and a mixture of CO and H 2 . The oxomethylene complex reacts with CO to give the cis-enediolate complex. DFT calculations on C 5 H 5 model metallocenes show that the reaction of Cp 2 CeH with CO and H 2 to give Cp 2 CeOMe is exoergic by 50 kcal mol -1 . The net reaction proceeds by a series of elementary reactions that occur after the formyl complex, Cp 2 Ce(η 2 -CHO), is formed by further reaction with H 2 . The key point that emerges from the calculated potential energy surface is the bifunctional nature of the metal formyl in which the carbon atom behaves as a donor and acceptor. Replacing H 2 by CH 4 increases the activation energy barrier by 17 kcal mol -1 .

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Section: Chartsupporting

confidence: 68%

Reactions of Monomeric [1,2,4-(Me₃C)₃C₅H₂]₂CeH and CO with or without H₂: An Experimental and Computational Study

et al. 2007

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“…At present there are only very limited experimental data about the changes of DHO (A-H) due to substitution from gas phase experiments (13)(14)(15), and none of them deal with carboxylic acids. In a similar way there are only a few papers (16-18) which consider the FD effect in both the neutral acid and the anion.…”

Section: Introductionmentioning

confidence: 99%

Electrostatic effects on ionization equilibria: MNDO study of proton and hydrogen transfer reactions of 5-fluoropentanoic acid

Friedl¹

1985

Can. J. Chem.

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ZDENEK FRIEDL. Can. J. Chem. 63, 1068Chem. 63, (1985. Relative proton transfer enthalpies &AH" and homolytic bond strengths 6AH" (A-H) of sp and np conformers of 5-fluoropentanoic acid, as well as some related aliphatic and 4-X-bicyclo[2.2.2]octane-I-carboxylic halogenoacids, have been calculated by the MNDO method. The results, together with literature data concerning the gas phase acidities, were compared with the prediction of the electrostatic theory. It is shown that the substituent effects on acidities of carboxylic acids are largely due to substituent interactions in the anions and only to a smaller extent to interactions in the neutral acids. A small contribution (about one-eighth to one-fourth as large in magnitude but of opposite sign) is possibly also made by substituent effects on homolytic bond strengths DH" (A-H).ZDENEK FRIEDL. Can. J . Chem. 63, 1068Chem. 63, (1985. Utilisant la mtthode MNDO. on a calculC les enthalpies relatives de transferts de protons (&AH") et les forces relatives des liaisons homolytiqucs (6AH0 (A-H)) des conformeres sp et np de I'acide fluoro-5 pentano'ique, de quelques halogtnoacides aliphatiques apparentis et de quelques acides X-4 bicyclo[2.2.2] octanecarboxyliques-1. On a cornpart les rtsultats obtenus ainsi quc des donntes tirees de la litterature relatives aux aciditts en phase gazeuse avec des predictions obtenues I'aide de la theorie tlectrostatique. On dtmontre que les effets des substituants sur les aciditts des acides carboxyliques sont dus en grande partie a des interactions des substituants dans les anions alors que les interactions des substituants des acides neutres n'ont qu'une faible influence. II cst possible que les effets des substituants puissent faire une Itgkre contribution (de I'ordre du huititme au quart de I'amplitude mais de signe inverse) sur les forces des liaisons homolytiques DH" (A-H).[Traduit par le journal] Introduction Substituent effects on proton transfer reactions may be theoretically predicted within the framework of classical electrostatics. Considering ionization equilibria in the gas phase (1, 2), the electrostatic theory developed by Smallwood (3) seems to be more appropriate than the generally used theory of Kirkwood and Westheimer (4,5). The second theory was created especially for the description of solvent effects on ionization equilibria in aqueous solutions. These effects are expressed by means of the effective relative permittivity E,,, which must be calculated from the solvent bulk permittivity E,,, and the internal relative permittivity attributed to the solute molecule (4, 5). On the other hand, Smallwood (3) had cancelled all the solvation terms and considered the effect of the dipolar substituent X on the isodesmic reaction [l]

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“…Recent measurements of the equilibrium constant for reaction [2] by high-pressure mass spectrometry (2), flowing afterglow (3), and ion cyclotron resonance spectrometry (4) have provided reliable gas-phase data on the acidities of a variety of carbon acids and a few carboxylic acids ( I , 5). The gas-phase studies have yielded valuable information on the effect of substituents on the intrinsic properties of carbanions, whereas until recently our knowledge has been based on thermodynamic and kinetic studies in solution.…”

Section: Introductionmentioning

confidence: 99%

Molecular orbital treatment of substituent effects. III. Proton affinities of some methyl and carboxylate anions

Edgecombe¹,

Boyd²

1984

Can. J. Chem.

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. Can. J. Chcm. 62, 2887Chcm. 62, (1984. Extensive ab initio molecular orbital calculations for the proton affinitics of CH,,IX4-rr-r where t~ = 1 , 2, 3, or 4 and X = F, CN or NOz, formate, fluoroformate, and a numbcr of acetates, CH,,-IX,+,,C02-, where n = 1 , 2, 3, or 4 and X = F or C1 are presented. Comparison of the theoretical and experimental gas-phase data indicates that the gcneral chemical trends are well described by small basis sets of the split-valence typc, but that bettcr quantitative agreement is obtained by use of the diffuse function-augmented basis sct (3-21+G) recommended by von R. Schleyer and co-workers (J. Comput. Chem. 4, 294 (1983)) for anion calculations. Large basis scts and post Hartree-Fock calculations are included for CH, -, CHzF-, and CHzCN-. [Traduit par le journal]

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The gas-phase acidity scale

Cited by 162 publications

References 68 publications

Reactions of Monomeric [1,2,4-(Me₃C)₃C₅H₂]₂CeH and CO with or without H₂: An Experimental and Computational Study

Reactions of Monomeric [1,2,4-(Me₃C)₃C₅H₂]₂CeH and CO with or without H₂: An Experimental and Computational Study

Electrostatic effects on ionization equilibria: MNDO study of proton and hydrogen transfer reactions of 5-fluoropentanoic acid

Molecular orbital treatment of substituent effects. III. Proton affinities of some methyl and carboxylate anions

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The gas-phase acidity scale

Cited by 162 publications

References 68 publications

Reactions of Monomeric [1,2,4-(Me3C)3C5H2]2CeH and CO with or without H2: An Experimental and Computational Study

Reactions of Monomeric [1,2,4-(Me3C)3C5H2]2CeH and CO with or without H2: An Experimental and Computational Study

Electrostatic effects on ionization equilibria: MNDO study of proton and hydrogen transfer reactions of 5-fluoropentanoic acid

Molecular orbital treatment of substituent effects. III. Proton affinities of some methyl and carboxylate anions

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Reactions of Monomeric [1,2,4-(Me₃C)₃C₅H₂]₂CeH and CO with or without H₂: An Experimental and Computational Study

Reactions of Monomeric [1,2,4-(Me₃C)₃C₅H₂]₂CeH and CO with or without H₂: An Experimental and Computational Study