Reaction of Trimethyl-and Dimethyl-Silyl Radicals with Pentamethyldisilane

1996 ◽  
Vol 51 (1-2) ◽  
pp. 102-104
Author(s):  
C. Kerst ◽  
P. Potzinger ◽  
H. Gg. Wagner
Keyword(s):  
Rate Constant ◽  
Disproportionation Reaction ◽  
Constant Ratio ◽  
A Value ◽  
Rate Constant Ratio ◽  
Silyl Radicals

Abstract The rate constant for the H atom abstraction of trimethylsilyl radicals from pentamethyldisilane (k(4)) was measured relative to the trimethylsilyl combination reaction k(3). A value for =(8.53 ± 0.08) · 10-11cm3/2s–1/2 was obtained. For the dimethylsilyl radical, a smaller value for the corresponding rate constant ratio (5.9 ± 0.2) · 10-11 cm3/2 s–1/2 was measured, and this was attributed to a disproportionation reaction between the dimethylsilyl and the pentamethyldisilyl radical leading to dimethylsilylene.

1999 R.U. Lemieux Award Lecture Adventures with azo-, azoxy-, and hydrazoarenes: from the Wallach to the benzidine rearrangement. Molecular electronics

2000 ◽  
Vol 78 (10) ◽  
pp. 1251-1271 ◽  
Author(s):  
Erwin Buncel
Keyword(s):  
Molecular Electronics ◽  
Rate Constant ◽  
Reaction Pathway ◽  
Solvent Polarity ◽  
Molecular Switch ◽  
Reaction Pathways ◽  
Optical Data ◽  
Constant Ratio ◽  
Rate Constant Ratio ◽  
Acid Catalyzed

The author's studies with aromatic azo-, azoxy-, and hydrazo-dye molecules, comprising kinetic and equilibrium investigations, as well as synthesis of novel molecules having photogenic properties, are described under the following highlights: A. Wallach rearrangement and cognate studies with azoxyarenes — (1) Elucidation of the mechanism of the Wallach rearrangement of azoxybenzene through the kinetic observation of a two-proton process which, together with a pKa study, was interpreted on the basis of formation of a deoxygenated, dicationic, symmetrical species as a key, short-lived reaction intermediate. (2) The proposal of a general acid-catalyzed pathway in concentrated sulfuric acid (catalysis by H2SO4 and H3SO+4. (3) Elucidation of the consecutive sulfonations of reaction products of azoxybenzene in the 100% H2SO4 region, and the diprotonation equilibria for p-hydroxyazobenzene, thus shedding light on past reaction pathway and product studies. (4) The observation of a novel reaction pathway for 2,4,6,2',4',6'-hexamethy lazoxybenzene. (5) The observation of a dichotomy of reaction pathways for α- and β-2-phenylazoxynaphthalenes: reaction via the dicationic intermediate and via quinoid intermediate species; comprising two isomeric compounds reacting by different pathways to give the same product. (6) Identification and structure proof of α- and β-isomers observed for the first time in the peracid oxidation of phenylazopyridine. (7) Observation of a rate constant ratio of 22 000 in the rearrangement of these α- and β-isomers, and the proposal of differential barriers for transition states leading to a tricationic intermediate. B. Benzidine rearrangement and cognate studies — (8) Observation of the acid-catalyzed hydroxylation of phenylazopyridine to p-hydroxyphenylazopyridine and the proposal of an SNAr mechanism with formation of an intermediate hydrazo species in the reaction. (9) First study of benzidine type rearrangement-disproportionation of phenylhydrazopyridine in acid media. (10) Proposal of a A [Formula: see text] B [Formula: see text] C [Formula: see text] D type reaction profile for the consecutive hydroxylation[Formula: see text]disproportionation processes of phenylazopyridine in aq H2SO4. (11) Proposal of 10-π and 14π-electron electrocyclic processes in the benzidine type rearrangement-disproportionation of phenylhydrazopyridine. (12) Identification and structural elucidation of a dimer formed from phenylazopyridine as a minor product and proposal of a reaction mechanism. C. Facile acid-catalyzed demethylation via SNAr/A-SE2 mechanisms and studies of tautomerism — (13) Observation of an abnormally facile acid-catalyzed cleavage (demethylation) of 4-methoxyphenylazopyridine via an SNAr mechanism. (14) Observation of two reaction pathways, SNAr and A-SE2, for the consecutive demethylations of 3,4-dimethoxyphenylazopyridine, with rate constant ratio of 7 000:1 favoring the SNAr process. (15) Quantitation of the tautomeric and protonation equilibria of 4-hydroxyphenylazopyridine, produced in (13). D. A new solvent polarity scale, molecular switches, and molecular electronics — (16) Establishment of a π*azo solvent polarity scale based on solvatochromism of a series of azomerocyanine molecules ("Buncel's dye"). (17) Some glimpses are presented of current forays into molecular electronics, as emanating from the above studies: (a) spiropyran (SP) <—> merocyanine (MC) thermo- and photochromic "molecular switch" systems; (b) synthesis and characterization of azo-functionalized star-burst dendrimers with photoswitchable properties and potential applications in optical data storage systems, holographic gratings, and drug delivery systems as host molecules.Key words: Wallach rearrangement, benzidine disproportionation, azoarenes, azoxyarenes, dendrimers, hydrazoarenes, dendrimers, solvatochromism, photochromism, thermochromism, spiropyran-merocyanine molecular switch.

Photolysis of, and the reactions of H-atoms with, isopropanethiol

1988 ◽  
Vol 66 (8) ◽  
pp. 2025-2033 ◽  
Author(s):  
W. W. Lam ◽  
T. Yokota ◽  
I. Safarik ◽  
O. P. Strausz
Keyword(s):  
Rate Constant ◽  
Thermal Reaction ◽  
Inert Gas ◽  
Constant Ratio ◽  
Temperature Range ◽  
Rate Constant Ratio ◽  
Photolytic Decomposition

Detailed investigation of the photolysis of i-C3H7SH has been carried out in the absence and presence of the inert gas, n-C4H10. A mechanism consisting of three primary photochemical steps: [Formula: see text], [Formula: see text]; [Formula: see text], [Formula: see text]; [Formula: see text],[Formula: see text]; six hot reaction steps and seven thermal reaction steps adequately explains all the experimental observations. As in the case of hot H* atoms both the H-atom abstraction, [Formula: see text], and the SH-displacement, [Formula: see text], reactions occur with thermalized H-atoms, with the rate constant ratio k7/k8 = 19.2 ± 4.3 at 25 °C. The Arrhenius expressions have been determined over the temperature range 25–145 °C, to be: k7 = (4.0 ± 0.7) × 1012 exp [(−1066 ± 43)/RT] and k8 = (2.8 ± 0.6) × 1012 exp[(−2597 ± 114)/RT] cm3 mol−1 s−1. It was found that the overall mechanism for the photolytic decomposition of i-C3H7SH is analogous to that established for C2H5SH in previous investigations.

Fluorescence différée et émissions photostimulées du carbazole et de la diphénylamine dans une matrice rigide d'éther à 77 °K

1974 ◽  
Vol 52 (22) ◽  
pp. 3707-3715 ◽  
Author(s):  
D. Muller ◽  
M. Ewald ◽  
G. Durocher
Keyword(s):  
Singlet State ◽  
Delayed Fluorescence ◽  
Quantum Yields ◽  
Constant Ratio ◽  
Light Stimulation ◽  
Excited Singlet ◽  
A Value ◽  
Rate Constant Ratio ◽  
Experimental Values ◽  
Triplet Quantum Yields

The isothermal delayed fluorescence along with the visible (6000 Å) photostimulated fluorescence and phosphorescence emissions have been recorded and analyzed in the following systems: diphénylamine (DPA) – ether rigid glass and carbazole (C) – ether rigid glass at 77°K. The electron–cation recombination after visible light stimulation has been analyzed in terms of a kinetic model already described (10). The triplet quantum yields of the DPA and C molecules are calculated and compared to the existing experimental values. The rate constant ratio (β) of populating the first triplet (T1) relative to populating the first excited singlet (S1) after the recombination event has taken place, is equal to about 2 in both systems investigated here. A value of 4.4 was obtained for indole and tryptophan in the same matrix. This difference can only be explained in terms of various singlet state energies for these molecules and also in terms of the stimulating energies used in these experiments.

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