Search results for "Isomerization"
showing 10 items of 281 documents
Mononuclear heterocyclic rearrangements 5. Kinetic Investigation of the behaviour of (e)- and (z)-phenylhydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadia…
1980
The kinetic behaviour of the geometrical isomers I-E and I-Z of the title compound in the presence of piperidine in benzene has been investigated. The kinetic results suggest that I-Z rearranges directly into 2,5-diphenyl-4-benzoylamino-1,2,3-triazole (II), whereas I-E probably rearranges only through the intermediate I-Z formed by isomerization. All the reactions studied are piperidine-catalyzed.
On the low-lying singlet excited states of styrene: a theoretical contribution
2000
The present contribution analyses the trans–cis photoisomerization mechanism of ethene and styrene on the singlet manifold. Within the framework of multiconfigurational second-order perturbation theory (CASPT2), the extended multistate approach (MS-CASPT2) is found to be flexible enough to describe energy hypersurfaces adiabatically. For ethene, torsion about the CC bond towards a perpendicular structure leads to a situation where the energy difference between the ground and the lowest excited state is still too large (2.5 eV) for efficient radiationless decay. However, the energy gap decreases to 0.4 eV when one of the methylene moieties is, in addition, pyramidalized from the twisted stru…
Photoactive Yellow Protein Chromophore Photoisomerizes around a Single Bond if the Double Bond Is Locked
2020
Photoactivation in the Photoactive Yellow Protein, a bacterial blue light photoreceptor, proceeds via photo-isomerization of the double C=C bond in the covalently attached chromophore. Quantum chemistry calculations, however, have suggested that in addition to double bond photo-isomerization, the isolated chromophore and many of its analogues, can isomerize around a single C-C bond as well. Whereas double bond photo-isomerization has been observed with x-ray crystallography, experimental evidence for single bond photo-isomerization is currently lacking. Therefore, we have synthesized a chromophore analogue, in which the formal double bond is covalently locked in a cyclopentenone ring and ca…
On the role of the triplet state in the cis/trans isomerization of rhodopsin: A CASPT2//CASSCF study of a model chromophore
2011
The possibility of population of the lowest-lying triplet state (T1) in the early events of the photochemical isomerization process of a model chromophore of Rhodopsin (Rh) has been analyzed using multireference perturbation theory (CASPT2//CASSCF) methods. It is shown that the characteristics of the isomerization process namely small S1−T1 gap, presence of hydrogen out of plane active vibrational modes, and existence of a dense manifold of vibrational states, render possible the fulfilment of the conditions needed for the population of T1. The possible consequences for the photochemistry and photophysics of Rh are also discussed. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem 111:3431–3…
Synthesis of a labile sulfur-centred ligand, [S(H)C(PPh2S)2]-: structural diversity in lithium(i), zinc(ii) and nickel(ii) complexes
2016
A high-yield synthesis of [Li{S(H)C(PPh2S)2}]2 [Li2·(3)2] was developed and this reagent was used in metathesis with ZnCl2 and NiCl2 to produce homoleptic complexes 4 and 5b in 85 and 93% yields, respectively. The solid-state structure of the octahedral complex [Zn{S(H)C(PPh2S)2}2] (4) reveals notable inequivalence between the Zn–S(C) and Zn–S(P) contacts (2.274(1) Å vs. 2.842(1) and 2.884(1) Å, respectively). Two structural isomers of the homoleptic complex [Ni{S(H)C(PPh2S)2}2] were isolated after prolonged crystallization processes. The octahedral green Ni(II) isomer 5a exhibits the two monoprotonated ligands bonded in a tridentate (S,S′,S′′) mode to the Ni(II) centre with three distinctl…
Efficient Conversion of Light to Chemical Energy : Directional, Chiral Photoswitches with Very High Quantum Yields
2020
Abstract Photochromic systems have been used to achieve a number of engineering functions such as light energy conversion, molecular motors, pumps, actuators, and sensors. Key to practical applications is a high efficiency in the conversion of light to chemical energy, a rigid structure for the transmission of force to the environment, and directed motion during isomerization. We present a novel type of photochromic system (diindane diazocines) that converts visible light with an efficiency of 18 % to chemical energy. Quantum yields are exceptionally high with >70 % for the cis–trans isomerization and 90 % for the back‐reaction and thus higher than the biochemical system rhodopsin (64 %). T…
The Boulton-Katritzky Reaction: A Kinetic Study of the Effect of 5-Nitrogen Substituents on the Rearrangement of Some (Z)-phenylhydrazones of 3-Benzo…
2014
The kinetics of the ring-into-ring conversion of some new (Z)-phenylhydrazones of 3-benzoyl-1,2,4-oxadiazole containing different nitrogen-substituents at C-5 (3b-d; X = NHMe, NMe2, and NHCOMe) into the relevant triazoles 4b-d have been examined in a wide range of pS+ (0.1-11.9) in 1:1 (v/v) dioxane/water solution. The obtained results have been compared with previous data concerning the (Z)-phenyl-hydrazone of 5-amino-3-benzoyl-1,2,4-oxadiazole (3a; X = NH2). All of the studied (Z)-phenylhydrazones rearrange through three different pathways (specific acid-catalysed, uncatalysed and general base-catalysed). The different effects of the substituents on the course of the rearrangement in the …
Light-induced, site-selective isomerization of glyoxylic acid in solid xenon
2014
Abstract The isomerization of glyoxylic acid (GA) and its water complex was studied in a low temperature xenon matrix. The aim of these studies was to understand how xenon environment affects the cis-trans GA interconversion upon near infrared irradiation. In solid xenon, the GA conformers are embedded in two different matrix sites. These show up as different vibrational bands of GA that exhibit different kinetic rates of isomerization. Upon complexation with water, the isomerization process slows down. Xenon matrix appears not to affect energy relaxation process via intramolecular or intermolecular hydrogen bond as compared with previous experiments in an argon.
Impact of α,β-dehydroamino acid residues on the binding abilities of di-, tri- and tetra-peptides
2000
Insertion of a dehydroamino acid residue into a sequence of di-, tri- or tetra-peptide changed considerably the binding abilities of peptide ligands towards copper(II) ions. Potentiometric and spectroscopic (EPR, UV-VIS and CD) data have shown that the amide nitrogen of the dehydroamino acid residue is more effective in co-ordination than its parent analogue. In the case of the bulky ΔPhe residue also the (Z–E) isomerisation has a critical impact on the co-ordination equilibria in the system studied.
ChemInform Abstract: Base-Catalyzed Isomerization of 2-Isoxazolines Enables a Two-Step Enantioselective Synthesis of β-Hydroxynitriles from Enals.
2011
Treatment of chiral isoxazoles with catalytic amounts of DBU results in efficient formation of optically active β-hydroxynitriles.