Proton chemical shifts and thermodynamics of the formation of hydrogen-bonded dimers and mixed 1:1 associates in the ternary system acetic acid/methanol/tetrahydrofuran-d8

Gerritzen, Detlef; Limbach, Hans−Heinrich

doi:10.1021/j100444a019

Cited by 12 publications

(2 citation statements)

References 7 publications

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“…In principle, c$ is a function of Ci and C2; k0bsd should, therefore, depend slightly on Ci and C2. However, one can show that these variations of k0bsd are smaller than the experimental error of &0bsd• From eq 13 one deduces that the energy of activation, £a(obsd), is the sum of the energy of activation of the proton transfer in the active complex, £a(ex), and the enthalpy, AH, of its formation: £a(obsd) -£a(ex) 4-AH (14) Similarly, the observed frequency factor, ^40bsd. depends on the frequency factor of the proton transfer, Acx, and the entropy, AS, of the formation of the active complex, and on ns and Cs:…”

Section: Resultsmentioning

confidence: 95%

Proton NMR spectroscopic study of cyclic proton exchange between acetic acid and methanol in tetrahydrofuran-d8

Limbach¹,

Seiffert²

1980

J. Am. Chem. Soc.

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Section: Resultsmentioning

confidence: 95%

Proton NMR spectroscopic study of cyclic proton exchange between acetic acid and methanol in tetrahydrofuran-d8

Limbach¹,

Seiffert²

1980

J. Am. Chem. Soc.

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“…Unauthenticated Download Date | 5/10/18 3: 29 PM still remains because the vibrational frequencies of transition states cannot be observed spectroscopically and because of proton tunneling, which also affects kinetic hydrogen/deuterium isotope effects [1], In order to check the validity of the RGM we have, for a number of years, been studying kinetic isotope effects of well characterized multiple symmetric pro ton transfer reactions by dynamic NMR spectroscopy in non aqueous liquids [17,[20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] and, recently, also in the solid state [35][36][37][38][39][40][41][42]. Large deviations from (2) were observed for the intramolecular HH migrations in meso-tetraphenylporphine [17] and azophenine [17,32,33] as well as for proton exchange between acetic acid and methanol in tetrahydrofuran (THF) [17,21], as replacement of the first H atom by D resulted in a larger decrease in the rate constants than replacement of the second H atom by D. These results have been useful in elucidating kinetic results of enzyme reaction mechanisms [ 10,11 ].…”

Section: Introductionmentioning

confidence: 99%

NMR Stratagems for the Study of Multiple Kinetic Hydrogen/Deuterium Isotope Effectsof Proton Exchange. Example: Di-p-fluorophenylformamidine/THF

Limbach

Meschede

Scherer

1989

Zeitschrift Für Naturforschung A

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Dedicated to Professor Jacob Bigeleisen on the occasion of his 70th birthday Stratagems are presented for the determination of kinetic isotope effects of proton exchange reactions by dynamic NMR spectroscopy. In such experiments, lineshape analyses and/or polariza tion transfer experiments are performed on the exchanging protons or deuterons as well as on remote spins, as a function of the deuterium fraction in the mobile proton sites. These methods are NMR analogs of previous proton inventory techniques involving classical kinetic methods. A theory is developed in order to derive the kinetic isotope effects as well as the number of transferred protons from the experimental NMR spectra. The technique is then applied to the problem of proton exchange in the system 15N,15N'-di-p-fluorophenylibrmamidine, a nitrogen analog of formic acid, dissolved in tetrahydrofuran-d8 (THF). DFFA forms two conformers in THF to which s-trans and s-cis structures have been assigned. Only the s-trans conformer is able to dimerize and exchange protons. Lineshape simulations and magnetization transfer experiments were carried out at 189,2 K, at a concentration of 0.02 mol l-1, as a function of the deuterium fraction D in the 'H-15N sites. Using 1H NM R spectroscopy, a linear dependence of the inverse proton lifetimes on D was observed. From this it was concluded that two protons are transported in the rate limiting step of the proton exchange. This result is expected for a double proton transfer in an s-trans dimer with a cyclic structure. The full kinetic HH/HD/DD isotope effects of 233:11:1 at 189 K were determined through 19F NMR experiments on the same samples. The deviation from the rule of geometric mean, although substantial, is much smaller than found in previous studies of intramolecular HH transfer reactions. Possible causes of this effect are discussed.

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Kinetic and Equilibrium Isotope Effects of Proton Exchange and Autoprotolysis of Pure Methanol Studied by Dynamic NMR Spectroscopy

Gerritzen

Limbach

1981

Ber Bunsenges Phys Chem

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The rate constants of the intermolecular proton exchange in pure methanol, i.e. the reciprocal proton lifetimes, τ0−1, have been determined as a function of the temperature by total lineshape analysis of the 1H‐NMR spectra. Since CH3OH is an AB3 spin system of high order the quantum mechanical density matrix formalism was employed for the simulation of the spectra. The neglect of high order effects as well as the presence of impurities had led to inconsistencies in previous studies. For the first time, the primary kinetic isotope effects were determined indirectly by simulation of the 1H‐NMR spectra of CH3OD samples containing 1 vol‐% CH3OH. The results are given by for CH3OH, and for CH3OD, with a kinetic isotope effect of 3.2 ± 0.4 at 298 K. ‐ The data cannot be explained by a cyclic exchange mechanism. However, they can be quantitatively related to the autoprotolysis constant of methanol and to Grunwald's kinetic data on proton transfer in buffered methanol solutions. It is concluded that the proton lifetimes in pure methanol are determined by the natural amount of free solvated CH3OH2+ and CH3O− ions generated by autoprotolysis. The observed energy of activation is then the sum of two terms, namely the energy of activation of the proton jumps between the ions and a methanol molecule, and half the enthalpy of methanol selfdissociation. In the presence of acid or basic impurities the second term becomes negligible. We propose a method for the quantitative determination of these impurities in the 10−9 to 10−7 mol 1−1 range. From our results we derive an equilibrium isotrope effect of (KH/KD)298K = 6 ± 3 for the autoprotolysis of pure methanol, which has not been reported before. It is shown that the literature data on proton exchange in pure water and other protoc systems can be explained in a similar way. The mechanism of the neutralisation in water and methanol is discussed. For the neutralisation of solutions containing 1 mol of each ion it is found that the mean number of proton transfer steps necessary is 5.3 for water and 2.5 for methanol. These data correlate well with the molar solvent density.

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Proton chemical shifts and thermodynamics of the formation of hydrogen-bonded dimers and mixed 1:1 associates in the ternary system acetic acid/methanol/tetrahydrofuran-d8

Cited by 12 publications

References 7 publications

Proton NMR spectroscopic study of cyclic proton exchange between acetic acid and methanol in tetrahydrofuran-d8

Proton NMR spectroscopic study of cyclic proton exchange between acetic acid and methanol in tetrahydrofuran-d8

NMR Stratagems for the Study of Multiple Kinetic Hydrogen/Deuterium Isotope Effectsof Proton Exchange. Example: Di-p-fluorophenylformamidine/THF

Kinetic and Equilibrium Isotope Effects of Proton Exchange and Autoprotolysis of Pure Methanol Studied by Dynamic NMR Spectroscopy

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