scholarly journals The Role of Carbonium Ions in Color Reception

1965 ◽  
Vol 48 (5) ◽  
pp. 753-760 ◽  
Author(s):  
Paul E. Blatz
Keyword(s):  
Lewis Acid ◽  
Lewis Acids ◽  
Benzene Solution ◽  
Fundamental Property ◽  
Conjugated Systems ◽  
Carbonium Ion ◽  
Carbonium Ions ◽  
Ion Treatment ◽  
Good Agreement

It is a fundamental property of conjugated systems to accept a proton or Lewis acid and form a stable carbonium ion. Polyenes that are protonated or add Lewis acids in this manner undergo substantial red shifts. For example, vitamin A1 acetate absorbs at 350 mµ in neutral and at 650 mµ in acidic benzene solution. The fundamental basis for absorption of polyene systems was described in detail in quantum mechanical terms. Applying the carbonium ion treatment to the visual chromophores retinal1 and retinal2 gives a very satisfactory explanation why these polyenes can be made to absorb in the visual region. Furthermore, by proper placement of the Lewis acid several absorption maxima can be gained from the carbonium ions which result. This treatment can be applied to explain experimental results. Individual cones from the frog are now known to absorb at 455, 537, and 625 mµ. If the value for the green cone (537 mµ) is used to calculate the Vo value in Kuhn's equation, the other two wave lengths may then be calculated. The calculated values are 460 and 600 mµ; this is in good agreement with the results from experiment.

scholarly journals Theoretical Insight into the Reversal of Chemoselectivity in Diels-Alder Reactions of α,β-Unsaturated Aldehydes and Ketones Catalyzed by Brønsted and Lewis Acids

Organics ◽  
2021 ◽  
Vol 2 (1) ◽  
pp. 38-49
Author(s):  
Lakhdar Benhamed ◽  
Sidi Mohamed Mekelleche ◽  
Wafaa Benchouk
Keyword(s):  
Lewis Acids ◽  
Major Product ◽  
Diels Alder ◽  
Unsaturated Ketone ◽  
Unsaturated Aldehydes ◽  
Aldehydes And Ketones ◽  
Theoretical Insight ◽  
Insight Into ◽  
Good Agreement

Experimentally, a reversal of chemoselectivity has been observed in catalyzed Diels–Alder reactions of α,β-unsaturated aldehydes (e.g., (2E)-but-2-enal) and ketones (e.g., 2-hexen-4-one) with cyclopentadiene. Indeed, using the triflimidic Brønsted acid Tf2NH as catalyst, the reaction gave a Diels–Alder adduct derived from α,β-unsaturated ketone as a major product. On the other hand, the use of tris(pentafluorophenyl)borane B(C6F5)3 bulky Lewis acid as catalyst gave mainly the cycloadduct of α,β-unsaturated aldehyde as a major product. Our aim in the present work is to put in evidence the role of the catalyst in the reversal of the chemoselectivity of the catalyzed Diels–Alder reactions of (2E)-but-2-enal and 2-Hexen-4-one with cyclopentadiene. The calculations were performed at the ωB97XD/6-311G(d,p) level of theory and the solvent effects of dichloromethane were taken into account using the PCM solvation model. The obtained results are in good agreement with experimental outcomes.

The stabilities of carbonium ions

1973 ◽  
Vol 26 (2) ◽  
pp. 301
Author(s):  
IJ Miller
Keyword(s):  
Gas Phase ◽  
Bond Energy ◽  
Positive Charge ◽  
Classical Electrodynamics ◽  
Chemical Bonds ◽  
Carbonium Ion ◽  
Carbonium Ions ◽  
Solvation Energies ◽  
Experimental Values ◽  
Good Agreement

An empirical bond energy scheme for carbonium ions is derived which gives good agreement with experimental values for some primary, secondary, and tertiary ions. If the chemical bonds are considered as polarizable, the bond energy scheme, and some reactions of carbonium ions, can be rationalized solely from the application of classical electrodynamics. As example it is deduced that dienylic ions are less stable than cyclopentenylic ions and cyclohexenylic ions. It is also shown that the activation energy for the cyclization of dienylic ions is largely composed of electrostatic repulsion between the equally charged ends. While the results are mainly applicable for the gas phase an approximate discussion of solvation energies shows that, while all C-C bonds stabilize a carbonium ion centre to a greater extent in the gas phase, a methyl will stabilize positive charge to a greater extent than, say, an ethyl in aqueous solution if the positive charge is less than 0.6e. That is, the Baker-Nathan order is predicted in solution if the charge is delocalized. It is concluded that the concept of hyperconjugation is unnecessary for a description of carbonium ions.

Lewis Acid – Base Interactions Between Bis-π-cyclopentadienyl Tungsten and Molybdenum Dihydrides, Cp2MH2, and some Main Group III Moieties

1971 ◽  
Vol 49 (15) ◽  
pp. 2504-2507 ◽  
Author(s):  
A. Storr ◽  
B. S. Thomas
Keyword(s):  
Lewis Acid ◽  
Lewis Acids ◽  
Benzene Solution ◽  
Main Group ◽  
Acid Base ◽  
Group Iii ◽  
Lewis Bases

Combination of the Lewis bases Cp2MH2 (where M = W or Mo) with the Group III Lewis acids AlR3 (where R = Me, Et, or Ph) and AlMe2H has yielded a series of 1:1 complexes of the type Cp2MH2•AlR3. A number of the complexes undergoes a slow elimination of hydrogen or alkane in benzene solution. Attempts to isolate mixed hydride adducts of the type, Cp2MH2•M′H3 (where M′ = Al or Ga), were unsuccessful.

Lewis Acid Catalyzed Rearrangements of Nitroxide Radicals

1974 ◽  
Vol 52 (13) ◽  
pp. 2463-2470 ◽  
Author(s):  
C. T. Cazianis ◽  
D. R. Eaton
Keyword(s):  
Lewis Acid ◽  
Lewis Acids ◽  
Equilibrium Constants ◽  
Relative Equilibrium ◽  
Paramagnetic Complex ◽  
Nitroxide Group ◽  
Nitroxide Radicals ◽  
Lewis Acid Catalysts ◽  
Carbonium Ion ◽  
Acid Catalyzed

The interaction of nitroxide radicals with the Lewis acids, aluminum chloride and boron trifluoride has been investigated using e.s.r. and n.m.r. techniques. Three types of reaction have been demonstrated. Initially a paramagnetic complex with the nitroxide group is formed. This can then dimerize or polymerize to a diamagnetic complex. Concurrently rearrangement of t-butyl groups to s-butyl groups can occur giving a further series of radicals and complexes. The rearrangements are analogous to the well known carbonium ion rearrangements induced by Lewis acid catalysts. Relative equilibrium constants and rates for the various reactions are discussed in a qualitative manner.

REACTIONS OF AMIDES AND RELATED COMPOUNDS: I. N.M.R. (PROTON) INVESTIGATION OF N,N-DIMETHYLFORMAMIDE – LEWIS ACID ADDUCTS AND REMARKS ON THE RELATIVE STRENGTH OF LEWIS ACIDS

1965 ◽  
Vol 43 (2) ◽  
pp. 375-380 ◽  
Author(s):  
Stephen J. Kuhn ◽  
John S. McIntyre
Keyword(s):  
Chemical Shift ◽  
Lewis Acid ◽  
Lewis Acids ◽  
Methylene Chloride ◽  
Carbonyl Oxygen ◽  
Relative Strength ◽  
Proton Resonance ◽  
Lewis Acid Adducts ◽  
Related Compounds ◽  
Good Agreement

The proton resonance spectra of a number of Lewis acid – N,N-dimethylformamide adducts in 2-nitropropane and methylene chloride solutions have been investigated and an attempt has been made to correlate the relative strength of Lewis acids with the chemical shift of the methyl protons. The order obtained for the relative strength of Lewis acids shows good agreement with the results obtained by other methods. The complex formation has taken place on the carbonyl oxygen of the N,N-dimethylformamide in all cases. The DMF – Lewis acid mole ratios were 1:1 in ZnCl2, AlCl3, PF5, BiCl3, InCl3, AsF5, SbCl5, SbF5, BF3, BCl3, BBr3. The 2:1 DMF – Lewis acid complexes of ZnCl2, SnCl4, SnBr4, Snl4, TiCl4, ZrBr4 have also been investigated.

scholarly journals Different reactivity of phosphorylallenes under the action of Brønsted or Lewis acids: a crucial role of involvement of the P=O group in intra- or intermolecular interactions at the formation of cationic intermediates

2019 ◽  
Vol 15 ◽  
pp. 1491-1504
Author(s):  
Stanislav V Lozovskiy ◽  
Alexander Yu Ivanov ◽  
Aleksander V Vasilyev
Keyword(s):  
Nmr Spectroscopy ◽  
Lewis Acid ◽  
Crucial Role ◽  
Lewis Acids ◽  
Phosphonic Acids ◽  
Phosphorus Containing ◽  
Allyl Alcohols ◽  
Hydrolysis Of ◽  
Allene System

3-Methylbuta-1,2-dien-1-ylphosphonic acid derivatives (phosphorylallenes) [X2(O=)P–CR=C=CMe2, X = Cl, OMe, NR2, or SAr] undergo intramolecular cyclization into the corresponding 1,2-oxaphospholium ions in the Brønsted superacid TfOH. These cations have been thoroughly studied by means of NMR spectroscopy. The hydrolysis of superacidic solutions of these species afforded cyclic phosphonic acids and other phosphorus-containing compounds. Contrary to Brønsted acids, 3-methylbuta-1,2-dien-1-ylphosphonic dichloride [Cl2(O=)P–HC=C=CMe2] reacted with the Lewis acid AlCl3 in an intermolecular way forming noncyclic intermediates, which were investigated by NMR spectroscopy and DFT calculations. Hydrolysis of these species resulted in the formation of phosphoryl-substituted allyl alcohols and 1,3-butadienes. A strong coordination of the oxygen of the P=O group with AlCl3 prevented the formation of cyclic 1,2-oxaphospholium ions and played a crucial role in the different reactivity of such phosphorylallenes under the action of Brønsted or Lewis acids. Apart from that, the reaction of dichlorophosphorylallenes with arenes and AlCl3 led to products of hydroarylation of the allene system, phosphoryl-substituted alkenes and/or indanes. This is the first example of a Lewis acid-promoted intermolecular hydroarylation of allenes bearing electron-withdrawing substituents. Plausible reaction mechanisms have been proposed on the basis of the investigated reactions, and NMR analysis and DFT studies of the intermediate cationic species.

scholarly journals Cavitation Bubble Cloud Break-Off Mechanisms at Micro-Channels

Fluids ◽  
2021 ◽  
Vol 6 (6) ◽  
pp. 215
Author(s):  
Paul McGinn ◽  
Daniel Pearce ◽  
Yannis Hardalupas ◽  
Alex Taylor ◽  
Konstantina Vogiatzaki
Keyword(s):  
Cavitation Bubble ◽  
Cavitation Inception ◽  
Wave Dynamics ◽  
Bubble Cloud ◽  
Cloud Dynamics ◽  
Micro Channels ◽  
Physical Insight ◽  
Cloud Process ◽  
Good Agreement

This paper provides new physical insight into the coupling between flow dynamics and cavitation bubble cloud behaviour at conditions relevant to both cavitation inception and the more complex phenomenon of flow “choking” using a multiphase compressible framework. Understanding the cavitation bubble cloud process and the parameters that determine its break-off frequency is important for control of phenomena such as structure vibration and erosion. Initially, the role of the pressure waves in the flow development is investigated. We highlight the differences between “physical” and “artificial” numerical waves by comparing cases with different boundary and differencing schemes. We analyse in detail the prediction of the coupling of flow and cavitation dynamics in a micro-channel 20 m high containing Diesel at pressure differences 7 MPa and 8.5 MPa, corresponding to cavitation inception and "choking" conditions respectively. The results have a very good agreement with experimental data and demonstrate that pressure wave dynamics, rather than the “re-entrant jet dynamics” suggested by previous studies, determine the characteristics of the bubble cloud dynamics under “choking” conditions.

scholarly journals Seasonal predictability of Mediterranean weather regimes in the Copernicus C3S systems

2021 ◽  
Author(s):  
Ignazio Giuntoli ◽  
Federico Fabiano ◽  
Susanna Corti
Keyword(s):  
Mediterranean Region ◽  
Climate Models ◽  
Reanalysis Data ◽  
Seasonal Predictability ◽  
Economic Implications ◽  
Weather Regimes ◽  
Weather Patterns ◽  
The Mediterranean ◽  
Good Agreement

AbstractSeasonal predictions in the Mediterranean region have relevant socio-economic implications, especially in the context of a changing climate. To date, sources of predictability have not been sufficiently investigated at the seasonal scale in this region. To fill this gap, we explore sources of predictability using a weather regimes (WRs) framework. The role of WRs in influencing regional weather patterns in the climate state has generated interest in assessing the ability of climate models to reproduce them. We identify four Mediterranean WRs for the winter (DJF) season and explore their sources of predictability looking at teleconnections with sea surface temperature (SST). In particular, we assess how SST anomalies affect the WRs frequencies during winter focussing on the two WRs that are associated with the teleconnections in which the signal is more intense: the Meridional and the Anticyclonic regimes. These sources of predictability are sought in five state-of-the-art seasonal forecasting systems included in the Copernicus Climate Change Services (C3S) suite finding a weaker signal but an overall good agreement with reanalysis data. Finally, we assess the ability of the C3S models in reproducing the reanalysis data WRs frequencies finding that their moderate skill increases during ENSO intense years, indicating that this teleconnection is well reproduced by the models and yields improved predictability in the Mediterranean region.

Lewis Acid-Promoted Suzuki Reaction using Palladium Chloride Anchored on a Polymer as a Catalyst

2008 ◽  
Vol 61 (8) ◽  
pp. 610 ◽  
Author(s):  
Guozhi Fan ◽  
Hanjun Zhang ◽  
Siqing Cheng ◽  
Zhandong Ren ◽  
Zhijun Hu ◽  
...  
Keyword(s):  
Catalytic Activity ◽  
Lewis Acid ◽  
Lewis Acids ◽  
Heterogeneous Catalysts ◽  
Suzuki Reaction ◽  
Cross Coupling ◽  
Palladium Chloride ◽  
Coupling Reactions ◽  
Cross Coupling Reactions ◽  
Recoverable Catalyst

Palladium chloride anchored on polystyrene modified by 5-amino-1,10-phenanthroline was prepared and used as an efficient recoverable catalyst for Suzuki cross-coupling reactions. The heterogeneous catalysts can be easily separated from the reaction mixture and reused for five cycles without significant Pd leaching and loss of catalytic activity. Rate enhancement in the Suzuki reaction by Lewis acids was also studied.

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