The chemistry of <i>N,N</i>′-dimethylformamidine. III. Tautomerism and hindered rotation

Halliday, James D.; Symons, E. A.; Bindner, P.E.

doi:10.1139/v78-240

Cited by 33 publications

(14 citation statements)

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“…The first studies of proton exchange of amidines were per formed by Halliday et al [47] and Borisov et al [48], but they did neither report rate laws nor kinetic iso tope effects. Borisov et al also reported that N,N'-diarylformamidines exist in tetrahydrofuran (THF) in an s-trans form A and an s-cis form B, which show different exchange characteristics.…”

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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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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“…The associated mechanism may involve amidine ± amidine hydrogen bonding [21] or bridging of the amidine head groups Figure 2. Film thickness versus time during adsorption of octamidine on different SAMs on gold.…”

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confidence: 99%

Switchable Assembly of Stable, Ordered Molecular Layers

et al. 1999

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Bipolar amphiphiles belonging to a,w-bis(4-amidinophenoxy)alkanes self-assemble in neutral or alkaline aqueous solution on acid-functionalised surfaces to form pH-switchable layers, with the amphiphiles oriented nearly perpendicular to the substrate. Layer thicknesses estimated by in situ ellipsometry, surface plasmon spectroscopy (SPS) or neutron reflection measurements correlated, within a certain amphiphile size range, with the molecular length of the amphiphile. Above a certain chain length, the characterisation by infrared reflection absorption spectroscopy (IRAS) indicated a crystallinelike order of the layered amphiphiles and a tendancy for bilayer formation was observed. These layers were stable and charge selective towards plasma proteins and charged surfactants.The layer stability was strongly pH dependant and no self-assembly was observed in acidic solutions. By cycling the pH between 9 and 2 rapid disassembly and reassembly of the layers was thus observed. The pH-switchable assembly can be used to restore a chemically selective interface after binding of a target molecule, for enhancement of sensor responses and potentially for optoelectronic or molecular electronic applications.

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“…Hydrolysis of the NMF, eq. [2], was found to occur at a rate at least a factor of 10 slower than for the DMFA over the p H range 11.5-14; however, a t p H 1 the N M F reacted many orders of magnitude faster than the DMFA.…”

Section: Hydrolysis Prodirctsmentioning

confidence: 82%

“…[l] and [2], was confirmed by comparing the nmr and infrared spectra of reaction samples with those of aqueous solutions of authentic samples of N-methylformamide, methylamine, formic acid, and sodium formate. Hydrolysis of the NMF, eq.…”

Section: Hydrolysis Prodirctsmentioning

confidence: 93%

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The chemistry of N,N′-dimethylformamidine. II. Hydrolysis. Kinetically controlled formation of cis-N-methylformamide

Halliday¹,

Symons²

1978

Can. J. Chem.

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The hydrolysis of N,N′-dimethylformamidine (DMFA) has been investigated in acid and alkaline aqueous media by 1H nmr; only a narrow basic pH range could be extensively studied kinetically. The pseudo-first-order kobs rose steadily from pH 11.5 to 13.0 (reaction approximately first order in OH−), then became independent of pH above 13.5 (9.3 × 10−4 s−1 at 10 °C). In contrast to many amidines, DMFA is quite stable in acid solution (estimated value of the pseudo-first-order hydrolysis rate constant is 1.4 × 10−1 s−1 at 10 °C, pH 0.05, from measurements at 100 and 140 °C). This stability is ascribed to the difficulty of eliminating the fairly strong base methylamine from the tetrahedral intermediate in acid solution.N-Methylformamide (NMF), one of the products, is formed initially as the cis isomer. A somewhat slower conversion then occurs to the thermodynamically more stable trans isomer. This unusual result is explained in terms of Deslongchamps and co-workers' theory of stereoelectronic control for the orbital-assisted breakdown of tetrahedral intermediates.

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The chemistry of N,N′-dimethylformamidine. III. Tautomerism and hindered rotation

Cited by 33 publications

References 0 publications

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

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

Switchable Assembly of Stable, Ordered Molecular Layers

The chemistry of N,N′-dimethylformamidine. II. Hydrolysis. Kinetically controlled formation of cis-N-methylformamide

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