S. F. Bureiko

Mechanism of non-catalytic proton exchange in aprotic low-polar solvents

The kinetics of proton exchange for the CD3OH−(CH3)3COH and CD3OH−CH3COOH systems in various solvents have been studied by dynamic1H and2H NMR. The mechanism of the process is discussed.AbstractМетодом динамического1H и2H ЯМР изучена кинетика протонного обмена в системах CD3OH−(CH3)3COH и CD3OH−CH3COOH в различных растворителях. Обсуждён механизм исследованного процесса.

STRUCTURE OF MOLECULAR AND IONIC H-BONDED COMPLEXES OF HEXAMETHYLPHOSPHORTRIAMIDE BY LOW TEMPERATURE NMR IN FREON SOLUTION

Abstract The 1 H NMR spectra have been obtained for a number of H-bonded complexes formed by the interaction of OH-acids of increasing strength and [(CH 3 ) 2 N] 3 PO (HMPT) in a freon mixture (CDF 2 Cl + CDFCl 2 ) at 100 K.The signals of the complexes with moderately strong acids (pK a > −2) are singlets,which proves that they have a molecular structure,AH…B.With stronger acids,a doublet spin-spin coupling of the bridge proton with 31 P nucleus occurs (J PH =cca 20 Cps) indicating a complete proton transfer,A − …HB + .The conclusion has been drawn that a sharp change in structure is caused by gradual change in the interaction energy as measured by ΔpK a .The spectra of a number of conjugated complexes,[BHB]A − ,have been taken.If B=B,the homoconjugated cation with a centrosymmetric H-bond is formed,followed by a strong decrease in J PH .

Effect of intramolecular hydrogen bonding on the kinetics of fast hydrogen exchange

The kinetics of hydrogen exchange between phenol derivates and methanol have been followed in CCl4 solution by IR spectroscopy, using a multicapillary mixer with a time resolution of 3 msec. The influence on the rate of the electronic structure and the ability to form intra- and intermolecular hydrogen bonds is discussed.AbstractМетодом кинетической ИК-спектроскопии с использованием струевого жидкостного смесителя с временным в мсек изучена кинетика процессов водородного обмена замещенных фенолов со спиртом в CCl4. Обсуждено влияние злектронного строения молекул и их способности к образованию внутрии межмолекулярных водородных связею на скорость реакции.

Correlation relationships between the energy and spectroscopic parameters of complexes with F···HF hydrogen bond

Twenty six complexes with a F···HF hydrogen bond have been theoretically considered. Correlation relationships between the bond energy and IR and NMR parameters—the frequency shift Δν of the ν(HF) stretching vibration and the change in the proton chemical shift Δδ1Н caused by complexation—have been constructed. In the entire range of existence (energy 1–200 kJ/mol), the plots obey the power law for the optical spectrum and the linear law for the NMR spectrum.

Kinetic isotope effect in proton exchange processes between methanol and water in inert solvents

The kinetic isotope effect in proton exchange between methanol and water in CCl4 solution was studied by1H and2H NMR and kinetic IR spectroscopy. The exchange rate increases in the series CD3OH+H2O<CD3OD+H2O<CD3OD+D2O. It has been concluded that the limiting step of the reaction involves the formation of an H-bonded cyclic intermediate.AbstractМетодами1H и2H ЯМР и кинетической ИК-спектроскопии исследован кинетический изотопний эффект в протонном обмене между метанолом и водой в растворах в CCl4. Показано, что скорость реакции увеличивается в ряду CD3OH+H2O<CD3OD+H2O<CD3OD+D2O.Сделан вывод, что лимитирующей стадией процесса является образование цикличес-кого промежуточного комплекса с Н-связями.

IR spectra of cyclic hydrogen-bonded complexes of bifunctional nitrogen compounds in solution

We measured the IR absorption spectra of self-associates and complexes with carboxylic acids of 3,5-dimethylpyrazole (DMP), diphenylformamidine (DPFA), diphenyltriazene (DPT), and diphenylguanidine (DPG) in solutions in a wide range of concentrations and temperatures and calculated spectroscopic, geometric, and energy characteristics of complexes in the quantum-mechanical harmonic and anharmonic 1D and 2D approximations. Spectroscopic data show that, in the case of DMP, cyclic trimers are predominantly formed; DPFA and DPG form cyclic dimers with two NH...N bonds in inert solvents, whereas, upon the complexation of DPT, cyclic structures do not occur, and only open dimers are formed. Upon the interaction of DMP, DPFA, and DPT with weak carboxylic acids (HCOOH, CH3COOH, CH2ClCOOH) in CCl4 or in CH2Cl2, molecular cyclic structures with NH...O=C and OH...N H-bonds are formed, whereas cyclic dimer complexes with stronger acids (CHCl2COOH, CCl3COOH, CF3COOH) predominantly have the structure of hydrogen-bonded ion pairs with proton transfer from the hydroxyl group to the proton-acceptor nitrogen atom. The calculations of the structure and vibrational frequencies using various basis sets of atomic functions confirm the formation of cyclic complexes in accordance with experimental results and, in the case of interaction with strong carboxylic acids, the proton transfer along the OH...N hydrogen bridge.

Structure of adducts of the intermolecular interaction of dimethylpyrazole and diphenylformamidine with hydrogen halides in the solution

The structure of adducts forming in the solution due to the interaction of bifunctional azo compounds (dimethylpyrazole (DMP) and diphenylformamidine (DPFA)) with hydrogen halides (HF, HCl, and HBr) is found from the data of the IR absorption spectra and quantum chemical calculations. It is shown that in the interaction with HCl or HBr proton donors, proton transfer via the hydrogen bond to the basic N atom of the azo compound occurs with the formation of an NH+…Hal− ionic pair. Strong evidences of proton transfer and the anion-cation pair formation are not found for the DMP-_F structure, and complexes with the molecular N…HF hydrogen bond are the dominant structures. Geometric parameters of the formed structures are calculated. The formation of trimers, containing two molecules of the azo compound and one HHal molecule, with an increase in the nitrogenous base concentration is experimentally proved, and the trimer structure is determined.

STUDIES OF HYDROGEN-BONDED COMPLEXES OF 4-METHOXYBENZONITRILE IN GROUND AND LOWEST TRIPLET STATES BY IR, PHOSPHORESCENCE, AND 15N NMR SPECTRA

Abstract The formation of hydrogen-bonded complexes between 4-methoxybenzonitrile and various OH proton donors was studied in solution in the ground electronic state by means of IR absorption spectra, and the ΔH values were measured. The upfield shift of purely electronic 0–0 transition is observed in low temperature phosphorescence spectra under the interaction, the values of this shift for different proton donor molecules being correlated with the chemical shifts of 15 N NMR signals under the complex formation. The nature of observed anomalous N signal shift in NMR spectra was explained by a decrease of paramagnetic deshielding because of the mixing of ground and lowest triplet electronic states of the nitrile studied.

Influence of external electric field on the rate of hydrogen isotope exchange between hydroxyl-containing molecules in CCl4

Rates of the fast proton-deuteron exchange between CH3OD and C6H5OH (1), and CH3OD and CH3COOH (2) in dilute CCl4 solutions have been measured by a kinetic IR spectroscopy technique. Under an imposed external electric field (10 V/m), the exchange rate in system (2) increases, which is not observed in (1). The effect is interpreted assuming that, in the second case, a cyclic ion pair with a high dipole moment is formed as a reaction intermediate.AbstractМетодом кинетической ИК-спектроскопии измерены скорости быстрого дейтерообмена между молекулами CH3OD и C6H5OH (1), CH3OD и CH3COOH (2) в разбавленных растворах в CCl4. При наложении внешнего электрического роля (IO7 B/м) скорость обмена в системе (2) возрастает, чего не наблюдается для процесса (1). Эффект интерпретирован в предположении промежуточного образования в случае (2) циклической ионной пары, обладающей высоким дипольным моментом.