ChemInform Abstract: Fitness of Driving Force and Catalytic Space in Chiral Catalyst Design. Application of Axial Biscarboline N-O Chiral Catalyst for Enantioselective Allylation of Allyltrichlorosilane to Bulky Substituted Aldehydes.

ChemInform ◽  
2015 ◽  
Vol 46 (37) ◽  
pp. no-no
Author(s):  
Li Liu ◽  
Qin Yang ◽  
He Yu ◽  
Jing-Liang Li ◽  
Yu-Ning Pei ◽  
...  
Keyword(s):  
Driving Force ◽  
Catalyst Design ◽  
Chiral Catalyst

scholarly journals Interrogation of O-ATRP Activation Conducted by Singlet and Triplet Excited States of Phenoxazine Photocatalysts

2021 ◽  
Author(s):  
Yisrael M. Lattke ◽  
Daniel Corbin ◽  
Steven M. Sartor ◽  
Blaine G. McCarthy ◽  
Garret Miyake ◽  
...  
Keyword(s):  
Electron Transfer ◽  
Driving Force ◽  
Catalyst Design ◽  
Rate Coefficients ◽  
Quantum Yields ◽  
Collisional Quenching ◽  
Initiator Concentration ◽  
Data Set ◽  
Back Electron Transfer ◽  
Electron Transfer Theory

Organocatalyzed ATRP (O-ATRP) is a growing field exploiting organic chromophores as photoredox catalysts (PCs) that engage in dissociative electron transfer (DET) activation of alkyl halide initiators following absorption of light. Characterizing DET rate coefficients (<i>k<sub>act</sub></i>) and photochemical yields across various reaction conditions and PC photophysical properties will inform catalyst design and efficient use during polymerization. The studies described herein consider a class of phenoxazine PCs where synthetic handles of core-substitution and <i>N</i>-aryl substitution enable tunability of the electronic and spin character of the catalyst excited state as well as DET reaction driving force ( ). Using Stern-Volmer quenching experiments through variation of diethyl 2-bromo-2-methylmalonate (DBMM) initiator concentration, collisional quenching is observed. Eight independent measurements of <i>k<sub>act </sub></i>are reported as a function of for four PCs: four triplet reactants and four singlets with <i>k<sub>act</sub></i> values ranging from 1.1´10<sup>8</sup> M<sup>-1</sup>s<sup>-1</sup> where DET itself controls the rate to 4.8´10<sup>9</sup> M<sup>-1</sup>s<sup>-1</sup> where diffusion is rate limiting. This overall data set, as well as a second one inclusive of five literature values from related systems, is readily modeled with only a single parameter of reorganization energy under the frameworks of adiabatic Marcus electron transfer theory and Marcus-Savéant theory of DET. The results provide a predictive map where <i>k<sub>act</sub></i> can be estimated if is known and highlight that DET in these systems appears insensitive to PC reactant electronic and spin properties outside of their impact on driving force. Next, on the basis of measured <i>k<sub>act</sub></i> values in selected PC systems and knowledge of their photophysics, we also consider activation yields specific to the reactant spin states as the DBMM initiator concentration is varied. In <i>N</i>-naphthyl-containing PCs characterized by near-unity intersystem crossing, the T<sub>1</sub> is certainly an important driver for efficient DET. However, at DBMM concentrations common to polymer synthesis, the S<sub>1</sub> is also active and drives 33% of DET reaction events. Even in systems with low yields of ISC, such as in <i>N</i>-phenyl-containing PCs, reaction yields can be driven to useful values by exploiting the S<sub>1</sub> under high DBMM concentration conditions. Finally, we have quantified photochemical reaction quantum yields, which take into account potential product loss processes after electron-transfer quenching events. Both S<sub>1</sub> and T<sub>1</sub> reactant states produce the PC<sup>·+</sup> radical cation with a common yield of 71%, thus offering no evidence for spin selectivity in deleterious back electron transfer. The sub-unity PC<sup>·+</sup> yields suggest that some combination of solvent (DMAc) oxidation and energy-wasting back electron transfer is likely at play and these pathways should be factored in subsequent mechanistic considerations.

scholarly journals Interrogation of O-ATRP Activation Conducted by Singlet and Triplet Excited States of Phenoxazine Photocatalysts

2021 ◽  
Author(s):  
Yisrael M. Lattke ◽  
Daniel Corbin ◽  
Steven M. Sartor ◽  
Blaine G. McCarthy ◽  
Garret Miyake ◽  
...  
Keyword(s):  
Electron Transfer ◽  
Driving Force ◽  
Catalyst Design ◽  
Rate Coefficients ◽  
Quantum Yields ◽  
Collisional Quenching ◽  
Initiator Concentration ◽  
Data Set ◽  
Back Electron Transfer ◽  
Electron Transfer Theory

Organocatalyzed ATRP (O-ATRP) is a growing field exploiting organic chromophores as photoredox catalysts (PCs) that engage in dissociative electron transfer (DET) activation of alkyl halide initiators following absorption of light. Characterizing DET rate coefficients (<i>k<sub>act</sub></i>) and photochemical yields across various reaction conditions and PC photophysical properties will inform catalyst design and efficient use during polymerization. The studies described herein consider a class of phenoxazine PCs where synthetic handles of core-substitution and <i>N</i>-aryl substitution enable tunability of the electronic and spin character of the catalyst excited state as well as DET reaction driving force ( ). Using Stern-Volmer quenching experiments through variation of diethyl 2-bromo-2-methylmalonate (DBMM) initiator concentration, collisional quenching is observed. Eight independent measurements of <i>k<sub>act </sub></i>are reported as a function of for four PCs: four triplet reactants and four singlets with <i>k<sub>act</sub></i> values ranging from 1.1´10<sup>8</sup> M<sup>-1</sup>s<sup>-1</sup> where DET itself controls the rate to 4.8´10<sup>9</sup> M<sup>-1</sup>s<sup>-1</sup> where diffusion is rate limiting. This overall data set, as well as a second one inclusive of five literature values from related systems, is readily modeled with only a single parameter of reorganization energy under the frameworks of adiabatic Marcus electron transfer theory and Marcus-Savéant theory of DET. The results provide a predictive map where <i>k<sub>act</sub></i> can be estimated if is known and highlight that DET in these systems appears insensitive to PC reactant electronic and spin properties outside of their impact on driving force. Next, on the basis of measured <i>k<sub>act</sub></i> values in selected PC systems and knowledge of their photophysics, we also consider activation yields specific to the reactant spin states as the DBMM initiator concentration is varied. In <i>N</i>-naphthyl-containing PCs characterized by near-unity intersystem crossing, the T<sub>1</sub> is certainly an important driver for efficient DET. However, at DBMM concentrations common to polymer synthesis, the S<sub>1</sub> is also active and drives 33% of DET reaction events. Even in systems with low yields of ISC, such as in <i>N</i>-phenyl-containing PCs, reaction yields can be driven to useful values by exploiting the S<sub>1</sub> under high DBMM concentration conditions. Finally, we have quantified photochemical reaction quantum yields, which take into account potential product loss processes after electron-transfer quenching events. Both S<sub>1</sub> and T<sub>1</sub> reactant states produce the PC<sup>·+</sup> radical cation with a common yield of 71%, thus offering no evidence for spin selectivity in deleterious back electron transfer. The sub-unity PC<sup>·+</sup> yields suggest that some combination of solvent (DMAc) oxidation and energy-wasting back electron transfer is likely at play and these pathways should be factored in subsequent mechanistic considerations.

In situ TEM Studies of agglomeration of sub-nanometer Ru layers in Ru/C multilayers

1992 ◽  
Vol 50 (1) ◽  
pp. 256-257
Author(s):  
Tai D. Nguyen ◽  
Ronald Gronsky ◽  
Jeffrey B. Kortright
Keyword(s):  
Layered Structure ◽  
Driving Force ◽  
High Energy ◽  
Superior Performance ◽  
In Situ Tem ◽  
Rayleigh Instability ◽  
Practical Applications ◽  
Transmission Electron ◽  
Diffraction Patterns

Nanometer period Ru/C multilayers are one of the prime candidates for normal incident reflecting mirrors at wavelengths < 10 nm. Superior performance, which requires uniform layers and smooth interfaces, and high stability of the layered structure under thermal loadings are some of the demands in practical applications. Previous studies however show that the Ru layers in the 2 nm period Ru/C multilayer agglomerate upon moderate annealing, and the layered structure is no longer retained. This agglomeration and crystallization of the Ru layers upon annealing to form almost spherical crystallites is a result of the reduction of surface or interfacial energy from die amorphous high energy non-equilibrium state of the as-prepared sample dirough diffusive arrangements of the atoms. Proposed models for mechanism of thin film agglomeration include one analogous to Rayleigh instability, and grain boundary grooving in polycrystalline films. These models however are not necessarily appropriate to explain for the agglomeration in the sub-nanometer amorphous Ru layers in Ru/C multilayers. The Ru-C phase diagram shows a wide miscible gap, which indicates the preference of phase separation between these two materials and provides an additional driving force for agglomeration. In this paper, we study the evolution of the microstructures and layered structure via in-situ Transmission Electron Microscopy (TEM), and attempt to determine the order of occurence of agglomeration and crystallization in the Ru layers by observing the diffraction patterns.

The nucleation of cavities at grain boundaries

1987 ◽  
Vol 45 ◽  
pp. 288-291
Author(s):  
P. J. Goodhew
Keyword(s):  
Surface Tension ◽  
Surface Energy ◽  
Grain Boundaries ◽  
Radiation Damage ◽  
Impurity Concentration ◽  
Driving Force ◽  
Nucleation And Growth ◽  
Phase Boundaries ◽  
Cavity Nucleation ◽  
Spherical Bubble

Cavity nucleation and growth at grain and phase boundaries is of concern because it can lead to failure during creep and can lead to embrittlement as a result of radiation damage. Two major types of cavity are usually distinguished: The term bubble is applied to a cavity which contains gas at a pressure which is at least sufficient to support the surface tension (2g/r for a spherical bubble of radius r and surface energy g). The term void is generally applied to any cavity which contains less gas than this, but is not necessarily empty of gas. A void would therefore tend to shrink in the absence of any imposed driving force for growth, whereas a bubble would be stable or would tend to grow. It is widely considered that cavity nucleation always requires the presence of one or more gas atoms. However since it is extremely difficult to prepare experimental materials with a gas impurity concentration lower than their eventual cavity concentration there is little to be gained by debating this point.

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