Wyniki wyszukiwania - BazTech

1

Wewnętrzna solwatacja a reakcje przeniesienia protonu w fazie gazowej dla związków azotowych

Raczyńska E.D.

Wiadomości Chemiczne

|

2000

|

[Z] 54, 1-2

67-86

EN

Intramolecular hydrogen bonding which can be formed in the neutral or ionic forms of polyfunctional compounds strongly influences the simple proton-transfer reactions and tautomeric equilibria in the gas phase. This effect, called internal solvation increases the proton affinities of nitrogen ligands, such as proton sponges with a rigid structure, and diamines, amidinamines, bipyridines and amidinazines with a flexible conformation by 20-100 kJ/mol in comparison to monofunctional or model compounds. In consequence diamines and amidinamines belong to superbases (PA>1000 kJ/mol). In aqueous solutions, only proton sponges are strong bases. Due to strong steric effects, water molecules cannot open the ionic NźźźH-N+ bridge in the proton sponges. Bidentate nitrogen ligands with a flexible conformation behave in solution similarly as monofunctional derivatives. External solvation by water molecules eliminates the NźźźH-N+ bridge, changes conformation of the monocation (from cyclic to linear), reduces polarizability of the chain, and thus decreases the basicity. For tautomerizing azoles, the internal solvation changes the pK(T) values by more than 2 pK(T) units in comparison to derivatives without this interaction. Application of the Taft and Topsom equation to series of 4(5)-substituted imidazoles indicates that the internal solvation in the gas phase has similar influence on the transmission of substituent effects as the external solvation in water.

2

Reakcje przeniesienia protonu od C-kwasów do zasad organicznych w rozpuszczalnikach aprotonowych

Jarczewski A.

Wiadomości Chemiczne

|

2000

|

[Z] 54, 3-4

203-226

EN

The proton transfer reactions in solutions seem to be simple, yet a number of consecutive steps can be distinguished. The useful tool in determining the mechanisms of these reactions was, and still is the primary deuterium kinetic isotope effect]. Therefore the factors influencing the rate constants and then KIE of these reactions as: steric hindrance of reactants, polarity of solvents and symmetry of transition state were widely discussed. The commonly acepted view on the deuterium kinetic isotope effects is that they are large when bulky substituents are crowded around the reaction site], and also the reactions are carried out in low polarity solvent]. However, there was found that even in polar aprotic solvents as acetonitrile large KIE values take place]. The KIE values are not very sensitive on the steric hindrance, showing in some extreme cases reduced KIE values, what is incompatible with previous papers. Likewise some other effects as "scrambling effect" simulating very large KIE are considered . The products of proton transfer reactions between C-acids and strong organic bases in polar aprotic solvents in contrary to previous findings appeared to be highly dissociated into free ions. The reason and consequences of erroneous fulfillment of Benesi-Hildebrand equation are discussed. It was essential to carry out the kinetic study of the proton transfer reactions, in particular those of low equilibrium constants in a BH+/B buffer. Then the association effect should be considered. Also a number of homoconjugation constants for selected organic bases and equilibrium acidities of some C-acids in acetonitrile solvent are given. The study of proton transfer reactions in mixed H2O-Me2SO solution prevented the homoconjugation effects and gave the thermodynamic and kinetic acidity, what enabled to evaluate the intrinsic barriers or intrinsic reactivity. To assess the aci-nitro behavior of C-acids the protonation reaction of 4-nitrophenylnitromethane carbanion at different pH values were performed. The influence of residual water which cannot be removed completely from the reaction systems of C-acids and strong organic bases in aprotic solvent was a subject of interest for many years. After careful examination it appeared that the traces of water in reaction medium, in aprotic solvents, caused a marked decrease of reaction rate instead of its acceleration [97]. It was clamed that the reactivity of ion pairs is negligible compared to free ions. However, in some cases the reactions are going via ion pair reagents with distinct differentiation of reactivity between loose and tight ion pairs . Using literature values of the fractionation of cesium n-propoxide for ions and ion pairs, 2.5 times larger reactivity has been found for ion pairs than free ions in proton abstraction from 1-(4-nitrophenyl)-1-nitroethane. The recent progress in the study of proton transfer reactions indicates new aspects in understanding the mechanism and theory of these processes.

3

Zastosowanie spektrometrii masowej cyklotronowego rezonansu jonowego do badania reakcji przeniesienia protonu w fazie gazowej

Raczyńska E.D.

Wiadomości Chemiczne

|

1998

|

[Z] 52, 5-6

385-403

EN

The principle of ion cyclotron resonance (ICR) reported in 1930 by Lawrence et al. [9] was firstly applied in mass spectrometry by Hipple, Sommer and Thomas in 1949 [26]. Their instrument has never been commercilized due to several problems with electronics and vacuum. Next instrument constructed by Wobschall et al. In the 1960s [27] had more chamce. It was modified by Llewellyn [28] in cooperation with Baldeschwieler et al. [29] and became a commercial Varian mass spectrometer in the late 1960s. Its drift cell contained three separated regions: ion source, analyser and ion collector. Important modification in ICRMS was proposed by McIver in 1970 [10a, b]. He introduced a one region trapped-ion analyzer cell and pulsed mode of operation. First FT-ICR experiments were carried out by Comisarow and Marshall in 1974 [11]. They also applied a one region trapped-ion analyzer cell. The pulsed mode of operation in the one region cell was combined with Fourier transform techniques in first commercial Nicolet FT-ICR mass spectrometer. From this moment the FT-ICR mass spectrometry has become more attractive and more frequently used by chemists. Since ions may be trapped for extended periods prior to detection in an ICR cell, both, ICR and FT-ICR mass spectrometry have been used to the study of gas-phase ion-molecule reactions [1-6]. First experimental gas-phase data of sufficient precision obtained by ICRMS became available from the 1970s for the proton-transfer reactions [2, 7, 8, 10, 41-43]. Relative acidity (DGA) or basicity (DGB) determinations were based on measurements of the equilibrium constants of proton exchange between two anions (A-1 and A-2) or two bases (B1 and B2) [7, 8, 13a]. These reactions have been studied in order to determine intrinsic gas-phase acidities or basicities of compounds and to establish general gas-phase acidity/basicity scale. Presently there are few thousands gas-phase data compiled in this scale by Lias et al. [50]. The upper limit of the GB scale is separated from the lower limit of the GA scale by ca 150 kJ/mol (Fig. 6), and thus spontaneous neutralization reactions between neutral acids and bases can not yet be observed in the gas phase.