Kinetics and Mechanism of Hydrogen Exchange in the 2,4,6-Trihydroxybenzenonium Ion

1971 ◽  
Vol 49 (17) ◽  
pp. 2777-2784 ◽  
Author(s):  
A. J. Kresge ◽  
Y. Chiang ◽  
S. A. Shapiro
Keyword(s):  
Reaction Mechanism ◽  
Hydrogen Exchange ◽  
Intermediate Formation ◽  
Line Broadening ◽  
Tracer Method ◽  
Concerted Mechanism ◽  
Kinetic Acidity ◽  
Unique Relationship ◽  
Good Agreement ◽  
Aromatic Hydrogen

The n.m.r. line-broadening method was used to measure rates of hydrogen exchange in the 2,4,6-trihydroxybenzenonium ion (carbon-protonated phloroglucinol) in a series of concentrated sulfuric and perchloric acid solutions. Comparison of separately determined rates of exchange of protons in the methylene and vinyl groups of this ion allows a concerted mechanism for the exchange reaction to be excluded and shows that exchange must occur through intermediate formation of completely deprotonated 1,3,5-trihydroxybenzene. This assignment of reaction mechanism allows transformation of the data into deprotonation rate constants, which, together with previously measured positions of equilibrium, permit estimation of specific rates of protonation of 1,3,5-trihydroxybenzene, k+; the latter are in good agreement with the rate of aromatic hydrogen exchange of 1,3,5-trihydroxybenzene measured by an isotopic tracer method at lower acidities. The kinetic acidity dependence of k+ is unusually shallow; this, coupled with the much steeper acidity dependences of other aromatic protonation reactions, serves to reinforce the growing realization that there is no unique relationship between kinetic acidity dependence and reaction mechanism.

Investigation of Residual Strains of the Second Kind in Plastically Deformed Metallic Materials

1994 ◽  
Vol 376 ◽  
Author(s):  
M. Vrána ◽  
P. Klimanek ◽  
T. Kschidock ◽  
P. Lukáš ◽  
P. Mikula
Keyword(s):  
High Resolution ◽  
Neutron Diffraction ◽  
Stacking Faults ◽  
Metallic Materials ◽  
Line Broadening ◽  
X Ray Diffraction ◽  
X Ray ◽  
Neutron Diffractometer ◽  
Residual Strains ◽  
Good Agreement

ABSTRACTInvestigation of strongly distorted crystal structures caused by dislocations, stacking-faults etc. in both plastically deformed f.c.c. and b.c.c. metallic materials was performed by the analysis of the neutron diffraction line broadening. Measurements were realized by means of the high resolution triple-axis neutron diffractometer equipped by bent Si perfect crystals as monochromator and analyzer at the NPI Řež. The substructure parameters obtained in this manner are in good agreement with the results of X-ray diffraction analysis.

Reaction mechanism studies. 5. The mechanism of the diaxial → diequatorial rearrangement of β-chlorothioethers

1968 ◽  
Vol 46 (1) ◽  
pp. 9-13 ◽  
Author(s):  
J. F. King ◽  
K. Abikar
Keyword(s):  
Reaction Mechanism ◽  
Transition State ◽  
Nitro Group ◽  
Ion Pair ◽  
Concerted Mechanism

p-Methoxy- and p-nitro substituted analogues (1b and 1c) of the diaxial β-chlorothioether 2β-chloro-3α-(phenylthio)-5α-cholestane (1a), have been prepared and found to undergo the diaxial → diequatorial rearrangement. The rates of rearrangement of these compounds show the sequence p-methoxy > H > p-nitro. It is concluded that the transition state for the rearrangement is polarized in the sense of a sulfonium chloride (3). The rearrangement of 1a is 1600 times faster in butanol than in decalin (at 110°). There is thus no inherent insensitivity to solvent change in a rearrangement in which there may be a "four-atom arrangement" in the transition state, a conclusion relevant to previous work on the diaxial → diequatorial rearrangement of 1,2-dibromides (1). It was further found that the nitro group slowed the rearrangement (at 110°) more in butanol than in decalin, an observation regarded as consistent with, but not requiring, the incursion of a merged ion-pair, cyclic concerted mechanism.

From the Understanding of the Reaction Mechanism Towards Optimizing the Deposition Rate and Optoelectronic Properties of a-Si:H

1989 ◽  
Vol 149 ◽  
Author(s):  
S. Veprek ◽  
M. Heintze ◽  
R. Bayer ◽  
N. Jurčik-Rajman
Keyword(s):  
Reaction Mechanism ◽  
Surface Coverage ◽  
Kinetic Studies ◽  
Reactive Intermediates ◽  
Sticking Coefficient ◽  
High Quality ◽  
Rate Determining Step ◽  
Homogeneous Good ◽  
Good Agreement

ABSTRACTWe present new results of kinetic studies of the deposition of high quality a-Si:H which strongly support the reaction mechanism suggested in our earlier papers: 1. SiH4 → SiH2; 2. SiH2 + SiS4 → Si2H6 (SiH2 + Si2H6 → Si3H6); 3. Si2H6 → 2a-Si:H (Si3H8 → 3a-Si:H). The “SiH3 mechanism”, as promoted by several workers, is in contradiction with these experimental facts.The di- and trisilane, which have a much higher reactive sticking coefficient than monosilane, play the role of reactive intermediates which facilitate the heterogeneous decomposition of silicon carrying species at the surface of the growing film. The values of the reactive sticking coefficient of Si2H6 and Si3H8 depend on the surface coverage by chemisorbed hydrogen; they increase with decreasing surface coverage. Under the conditions of the growth of high quality a-Si:H films the reactive sticking coefficient of disilane amounts to 10−4 to 10−2 which is in a good agreement with recent data of other authors.The rate determining step of the growth of high quality a-Si:H films is the desorption of hydrogen from the surface of the growing film. This can be strongly enhanced by ion bombardment at impact energy of <100 eV. In this way, homogeneous, good quality films were deposited at rates up to 1800 Angströms/min, and there is a well justified hope that this rate can be further increased.

Electron microscope studies of minerals: phase boundaries in an extremely slowly cooled clinopyroxene (augite)

1983 ◽  
Vol 387 (1792) ◽  
pp. 21-30 ◽  
Keyword(s):  
Phase Interface ◽  
Intermediate Formation ◽  
High Resolution Electron Microscopy ◽  
Cooling History ◽  
The North ◽  
North Side ◽  
Resolution Electron ◽  
History Of ◽  
Ambient Surface ◽  
Good Agreement

High resolution electron microscopy has been used to study the nature of exsolution lamellae that developed during extremely slow and prolonged cooling and depressurization of an aluminium rich augite (high Ca clinopyroxene) taken from a layer of garnet-augite rich gneiss that outcrops on the north side of Scourie Bay, Sutherland, northwest Scotland. The parent clinopyroxene structure evolved with an average cooling rate of ca . 6 x 10 -6 K per year and an average depressurization rate of ca . 0.75 Pa per year over a span of ca . 2 x 10 9 years between 3.0 x 10 9 and 1.0 x 10 9 years ago, and was subsequently stored at close to ambient surface conditions. Three sets of lamellae, which probably formed mainly during the middle of this evolution, were identified as amphibole, pigeonite and hypersthene. Coherent amphibole lamellae, ca . 10 nm thick, exsolved parallel to (010) of the host augite whereas hypersthene formed thicker lamellae 150-250 nm wide, parallel to (100) augite. Again the phase interface is coherent but contains ledges, a few lattice spacings wide, of a pigeonite structure suggesting that the growth of the hypersthene lamellae proceeded via the intermediate formation of pigeonite. Pigeonite forms ca . 90 nm thick lamellae, which extend parallel to an irrational plane (7.96, 0, 1) of host augite at 12° to (100) augite, yet at the same time maintains a coherent interphase boundary. The angle between lamellae of this type and (100) of the host augite is known to be dependent upon composition of the host augite. The quoted value indicates a (Fe+Mn)/(Fe+Mn+Mg) ratio of 0.30 ± 0.05, in good agreement with microprobe data for the host augite, which fall in the range 0.372–0.382. The micrographs also indicate that hypersthene lamellae precede amphibole. The above observations have enabled the relation between the phases and the cooling history of the rock to be established.

Clay catalysis of aromatic hydrogen-exchange reactions

1982 ◽  
Vol 46 (2) ◽  
pp. 219-222 ◽  
Author(s):  
Robert Alexander ◽  
Robert Ian Kagi ◽  
Alfons Valentino Larcher
Keyword(s):  
Hydrogen Exchange ◽  
Exchange Reactions ◽  
Clay Catalysis ◽  
Aromatic Hydrogen

HYDROGEN EXCHANGE BETWEEN CELLULOSE AND WATER: I. MEASUREMENT OF ACCESSIBILITY

1961 ◽  
Vol 39 (10) ◽  
pp. 1934-1943 ◽  
Author(s):  
O. Sepall ◽  
S. G. Mason
Keyword(s):  
Surface Roughness ◽  
Water Vapor ◽  
Relative Humidity ◽  
Heavy Water ◽  
Hydrogen Exchange ◽  
Amorphous Material ◽  
Tritiated Water ◽  
Improved Method ◽  
Amorphous Fraction ◽  
Good Agreement

Estimates of the extent of the hydrogen exchange reaction between cellulose and water vapor determined by an improved method using tritiated water and by the use of heavy water were usually in good agreement. With certain samples, surface roughness had a small effect upon the measurement of tritium radioactivity in cellulose.The accessibility or limiting extent of the exchange increased slightly with relative humidity in the range 20 to 100% and was attributed to the reduction of crystalline order reported recently in similar experiments.In contrast to cellulose whose accessibility corresponds approximately to the amorphous fraction, a partially crystalline amylose was 100% accessible while an amorphous xylan was 52% accessible, indicating that in these substances the accessibility and the fraction of amorphous material are not identical.

scholarly journals The influence of active site conformations on the hydride transfer step of the thymidylate synthase reaction mechanism

2015 ◽  
Vol 17 (46) ◽  
pp. 30793-30804 ◽  
Author(s):  
Katarzyna Świderek ◽  
Amnon Kohen ◽  
Vicent Moliner
Keyword(s):  
Reaction Mechanism ◽  
Thymidylate Synthase ◽  
Active Site ◽  
Hydride Transfer ◽  
Md Simulations ◽  
Concerted Mechanism ◽  
X Ray ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Rate Limiting

QM/MM MD simulations from different X-ray structures support the concerted mechanism character in the rate limiting step of thymidylate synthase catalysis.

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