Synthesis of the New 2-Alkyl-nido-2,7,10-C3B8H11 Tricarbaborane by Protonation of [7-Alkyl-nido-7,8,10-C3B8H10]-: A Reversible Cage-Carbon Rearrangement

1999 ◽  
Vol 64 (5) ◽  
pp. 865-882 ◽  
Author(s):  
Alexandra M. Shedlow ◽  
Larry G. Sneddon
Keyword(s):  
Transition State ◽  
Activation Barrier ◽  
Original Position ◽  
Skeletal Rearrangement ◽  
Nmr Studies ◽  
Two Phase ◽  
Protonation Reaction ◽  
Pathway Activation ◽  
Incoming Proton ◽  
Open Face

Protonation of the [7-R-nido-7,8,10-C3B8H10]- (where R = PhCH2 (1a) or R = Me (1b)) tricarbollide anion, with concentrated H2SO4 in a two-phase aqueous/CH2Cl2 system, yields the new neutral tricarbaborane: 2-R-nido-2,7,10-C3B8H11 (where R = PhCH2 (2a) or R = Me (2b)). The three cage-carbons of the [7-R-nido-7,8,10-C3B8H10]- anion are located on the open face, but spectroscopic and DFT/GIAO/NMR studies of 2a and 2b show that during the protonation reaction, isomerization of the cage framework occurs to produce the neutral tricarbaborane having a 2,7,10-structure in which only two of the carbons remain on the open face. The third (R-substituted) carbon adopts a five-coordinate vertex off of the open face, thus enabling the incoming proton to adopt a bridging position on the B-B edge of the new C2B3-open face. The skeletal rearrangement is reversible, since deprotonation of 2a or 2b regenerates the anions 1a and 1b, respectively, having the 7,8,10-configuration. In agreement with the experimentally observed structures of the anionic (7,8,10-structure) and neutral (2,7,10-structure) species, DFT calculations at the B3LYP/6-311G*-level show that the [7-Me-nido-7,8,10-C3B8H10]- anion (1b, structure 16) is 28.9 kcal/mol more stable than the [2-Me-nido-2,7,10-C3B8H10]- isomer (3b, structure 18), while for the neutral tricarbaborane, the 2-R-nido-2,7,10-C3B8H11 (2b, structure 14) structure is more stable than any 7,8,10-structure (structures 7-11) which has the added proton in an endo position on the open face. Transition state calculations at the HF/6-31G*-level yielded a simple, low-energy pathway (activation barrier of only 6.5 kcal/mol for the transition state TS18/16) for the rearrangement of [2-Me-nido-2,7,10-C3B8H10]- (3b, structure 18) to [7-Me-nido-7,8,10-C3B8H10]- (1b, structure 16) requiring the movement of only one cage atom, B11, from its original position in the C7-B8-B9-C10-B11 plane of 3b, to the C7-C8-B9-C10-B11 plane of 1b.

scholarly journals Asymmetric Cyanation of Activated Olefins with Ethyl Cyanoformate Catalyzed by Ti(IV)-Catalyst: A Theoretical Study

Catalysts ◽  
2020 ◽  
Vol 10 (9) ◽  
pp. 1079
Author(s):  
Zhishan Su ◽  
Changwei Hu ◽  
Nasir Shahzad ◽  
Chan Kyung Kim
Keyword(s):  
Transition State ◽  
Self Assembly ◽  
Theoretical Study ◽  
Activation Barrier ◽  
Cinchona Alkaloid ◽  
Activation Process ◽  
Construction Step ◽  
Dual Activation ◽  
Membered Transition State ◽  
Step Mechanism

The reaction mechanism and origin of asymmetric induction for conjugate addition of cyanide to the C=C bond of olefin were investigated at the B3LYP-D3(BJ)/6-31+G**//B3LYP-D3(BJ)/6-31G**(SMD, toluene) theoretical level. The release of HCN from the reaction of ethyl cyanoformate (CNCOOEt) and isopropanol (HOiPr) was catalyzed by cinchona alkaloid catalyst. The cyanation reaction of olefin proceeded through a two-step mechanism, in which the C-C bond construction was followed by H-transfer to generate a cyanide adduct. For non-catalytic reaction, the activation barrier for the rate-determining C-H bond construction step was 34.2 kcal mol−1, via a four-membered transition state. The self-assembly Ti(IV)-catalyst from tetraisopropyl titanate, (R)-3,3′-disubstituted biphenol, and cinchonidine accelerated the addition of cyanide to the C=C double bond by a dual activation process, in which titanium cation acted as a Lewis acid to activate the olefin and HNC was orientated by hydrogen bonding. The steric repulsion between the 9-phenanthryl at the 3,3′-position in the biphenol ligand and the Ph group in olefin raised the Pauli energy (ΔE≠Pauli) of reacting fragments at the re-face attack transition state, leading to the predominant R-product.

Elucidation of the mechanism and energy barrier for anesthetic triggered membrane fusion in model membranes

2019 ◽  
Vol 97 (6) ◽  
pp. 474-482
Author(s):  
Trinh T. Nguyen ◽  
David T. Cramb
Keyword(s):  
Transition State ◽  
Membrane Fusion ◽  
Energy Barrier ◽  
Activation Barrier ◽  
Correlation Spectroscopy ◽  
Model Membranes ◽  
General Anesthetics ◽  
New Class ◽  
Two Photon ◽  
Photon Excitation

Membrane fusion is vital for cellular function and is generally mediated via fusogenic proteins and peptides. The mechanistic details and subsequently the transition state dynamics of membrane fusion will be dependent on the type of the fusogenic agent. We have previously established the potential of general anesthetics as a new class of fusion triggering agents in model membranes. We employed two-photon excitation fluorescence cross-correlation spectroscopy (TPE-FCCS) to report on vesicle association kinetics and steady-state fluorescence dequenching assays to monitor lipid mixing kinetics. Using halothane to trigger fusion in 110 nm diameter dioleoylphosphatidylcholine (DOPC) liposomes, we found that lipid rearrangement towards the formation of the fusion stalk was rate limiting. The activation barrier for halothane induced membrane fusion in 110 nm vesicles was found to be ∼40 kJ mol−1. We calculated the enthalpy and entropy of the transition state to be ∼40 kJ mol−1 and ∼180 J mol−1 K−1, respectively. We have found that the addition of halothane effectively lowers the energy barrier for membrane fusion in less curved vesicles largely due to entropic advantages.

Ion Mixing During Film Deposition: Growth of Metastable Semiconducting and Metallic Alloys

1981 ◽  
Vol 7 ◽  
Author(s):  
K.C. Cadien ◽  
M.A. Ray ◽  
S.M. Shin ◽  
J.M. Rigsbee ◽  
S.A. Barnett ◽  
...  
Keyword(s):  
Activation Barrier ◽  
Essential Feature ◽  
Rf Sputtering ◽  
Ion Bombardment ◽  
Film Deposition ◽  
Low Energy ◽  
Diffusion Annealing ◽  
Transformation Rate ◽  
High Temperature Stability ◽  
Two Phase

ABSTRACTEpitaxial metastable (GaSb)1−Gex alloys with compositions across the pseudobinary phase diagram have been grown on (100) GaAs substrates by multitarget rf sputtering. An essential feature allowing the growth of these unique materials was low energy ion bombardment of the growing film during deposition to promote collisional mixing and enhance adatom diffusion. Annealing experiments indicated that the metastable films exhibit good high temperature stability. While the free energy difference between the single phase metastable and two phase equilibrium states is only ∼ 20 meV/atom, the activation barrier for the transformation is ∼ 3 eV. All films were ptype with room temperature carrier concentrations and mobilities between 1016 and 1019 cm−3 and 10 and 720 cm2/V∼sec, respectively, depending on film composition. Collisional mixing due to low energy ion bombardment of the growing film was also found to affect both the transformation rate kinetics as well as the reaction path during subsequent annealing of amorphous GaSb/Ge mixtures deposited on glass at 60°C. Finally, some initial results on the deposition of metastable fcc Cu/Cr alloys are presented.

scholarly journals Bypassing the Inertness of Aziridine/CO2 Systems to Access 5-Aryl-2-Oxazolidinones: Catalyst-Free Synthesis Under Ambient Conditions

2020 ◽  
Author(s):  
Giulio Bresciani ◽  
Emanuele Antico ◽  
Gianluca Ciancaleoni ◽  
Stefano Zacchini ◽  
Guido Pampaloni ◽  
...  
Keyword(s):  
Activation Barrier ◽  
Research Area ◽  
Ambient Conditions ◽  
Nmr Studies ◽  
Co Catalyst ◽  
Critical Issues ◽  
High Activation Barrier ◽  
Sequential Addition ◽  
Synthetic Routes ◽  
Active Research

The development of sustainable synthetic routes to access valuable oxazolidinones via CO<sub>2</sub> fixation is an active research area, and the aziridine/carbon dioxide coupling has aroused a considerable interest. This reaction is featured by a high activation barrier, so to require a catalytic system, and may present some other critical issues. Here, we describe the straightforward gram-scale synthesis of a series of 5-​aryl-​2-oxazolidinones at ambient temperature and atmospheric CO<sub>2</sub> pressure, in the absence of any catalyst/co-catalyst. The key to this innovative procedure consists in the direct transfer of the pre-formed amine/CO<sub>2</sub> adduct (carbamate) to common aziridine precursors (dimethylsulfonium salts), replacing the classical sequential addition of amine (intermediate isolation of aziridine) and then CO<sub>2</sub>. The reaction mechanism has been investigated by NMR studies and DFT calculations applied to model cases.<br>

scholarly journals Substrate binding in the processive cellulase Cel7A: Transition state of complexation and roles of conserved tryptophan residues

2019 ◽  
Vol 295 (6) ◽  
pp. 1454-1463 ◽  
Author(s):  
Nanna Røjel ◽  
Jeppe Kari ◽  
Trine Holst Sørensen ◽  
Silke F. Badino ◽  
J. Preben Morth ◽  
...  
Keyword(s):  
Transition State ◽  
Turnover Rate ◽  
Activation Barrier ◽  
Free Energy Barrier ◽  
Binding Rate ◽  
Tryptophan Residues ◽  
High Conservation ◽  
Polymeric Ligand ◽  
Important Enzyme ◽  
Tunnel Entrance

Cellobiohydrolases effectively degrade cellulose and are of biotechnological interest because they can convert lignocellulosic biomass to fermentable sugars. Here, we implemented a fluorescence-based method for real-time measurements of complexation and decomplexation of the processive cellulase Cel7A and its insoluble substrate, cellulose. The method enabled detailed kinetic and thermodynamic analyses of ligand binding in a heterogeneous system. We studied WT Cel7A and several variants in which one or two of four highly conserved Trp residues in the binding tunnel had been replaced with Ala. WT Cel7A had on/off-rate constants of 1 × 105m−1 s−1 and 5 × 10−3 s−1, respectively, reflecting the slow dynamics of a solid, polymeric ligand. Especially the off-rate constant was many orders of magnitude lower than typical values for small, soluble ligands. Binding rate and strength both were typically lower for the Trp variants, but effects of the substitutions were moderate and sometimes negligible. Hence, we propose that lowering the activation barrier for complexation is not a major driving force for the high conservation of the Trp residues. Using so-called Φ-factor analysis, we analyzed the kinetic and thermodynamic results for the variants. The results of this analysis suggested a transition state for complexation and decomplexation in which the reducing end of the ligand is close to the tunnel entrance (near Trp-40), whereas the rest of the binding tunnel is empty. We propose that this structure defines the highest free-energy barrier of the overall catalytic cycle and hence governs the turnover rate of this industrially important enzyme.

Chiral Phosphine Ligands in the Structural Chemistry of Gold

1997 ◽  
Vol 52 (12) ◽  
pp. 1477-1483 ◽  
Author(s):  
Angela Bayler ◽  
Andreas Bauer ◽  
Hubert Schmidbaur
Keyword(s):  
Silver Chloride ◽  
Variable Temperature ◽  
Phosphine Ligands ◽  
Hydroxyl Group ◽  
Molar Ratio ◽  
Racemic Mixture ◽  
Nmr Studies ◽  
Two Phase ◽  
Ambient Temperatures ◽  
High Yields

Abstract The reaction of chloro(dimethylsulfide)gold(I) with (2-hydroxybutyl)diphenylphosphine (L) (racemic mixture) in dichloromethane affords high yields of the racemic chiral complex (L)AuCl. Metathesis reactions with KBr or KI, respectively, in a two-phase solvent mix­ ture (CH2CI2/H2O) lead to a conversion into the corresponding bromide and iodide complexes (L)AuBr and (L)AuI. Treatment of silver chloride with the ligand L in acetonitrile yields the analogous 1:1 adduct (L)AgCl. The compounds have been characterized by their analytical and spectroscopic data, and the structures of (L)AuCl and (L)AuBr have been determined by single-crystal X-ray diffraction. The compounds build isomorphous crystal lattices (monoclinic, P21/c) in which pairs of enantiomers (R-and S-configuration of L) are aggregated by two long hydrogen bonds between the hydroxyl group of the ligand and the halogen substituent of the neighbouring molecule. Cationic, linear two-coordinate 1:2 complexes (L)2Au+ X-(X = BF4, CIO4, SO3CF3) and (L)2Ag+ X-(X = BF4) are obtained in good yields by reaction of L with Me2SAuCl and AgX (molar ratio 2:1:1) or with AgBF4 (molar ratio 2:1), respectively. 31P NMR studies at variable temperature show an equilibrium between all possible stereoisomers (RR, SS, RS, SR) in solution with rapid ligand exchange at ambient temperatures.

Product Energy Distributions

1996 ◽  
Author(s):  
Tomas Baer ◽  
William L. Hase
Keyword(s):  
Potential Energy ◽  
Transition State ◽  
Potential Energy Surface ◽  
Energy Surface ◽  
Vibrational Energy ◽  
Activation Barrier ◽  
Dissociation Rate ◽  
Energy Partitioning ◽  
The Other ◽  
Energy Distributions

The measurement of product translational and rotational energies, and in some cases vibrational energy, is often more readily accomplished than the measurement of the dissociation rate. As a result there exists a considerable body of experimental information about product energy distributions (FED) for many classes of reactions. The only simple model for treating these FED is the statistical one; however, there is a considerable diversity in its application. In the dissociation of large molecules at moderate to large excess energies, the translational, rotational, and vibrational energy distributions can be treated as continuous functions. On the other hand, in the dissociation of triatomic molecules, it is often possible to measure the quantized rotational energy distribution for specific vibrational energy levels of the diatomic product. Just as in the determination of the dissociation rates, product energy partitioning is highly sensitive to the potential energy surface. If there is no reverse activation barrier, the product energies are often distributed statistically. That is, the distributions depend only upon the product phase space and are independent of the detailed shape of the potential energy surface. On the other hand, for reactions with a “tight” transition state located at the top of a reverse activation barrier, statistical redistribution of the product energies is often not possible. After passing through the transition-state region, the products move down the repulsive wall and rapidly dissociate with little chance to exchange and equilibrate the available energy. Often, such products are ejected with considerable translational energy. This happens in large as well as small molecules or ions. The resulting product energy partitioning is then highly nonstatistical, even though the dissociation rate is perfectly predicted by RRKM theory. That is, the dissociation rate and product energy partitioning are separate and uncoupled events. The rate is governed early in the reaction history by the structure of the transition state, while product energy partitioning is determined late in the reaction and is governed by the shape of the potential energy surface at large internuclear distances. The most effective model for treating product energy distributions (PEDs) of reactions with no reverse activation barriers is the statistical theory.

DFT study of phenol alkylation with propylene on H-BEA in the absence and presence of water

2021 ◽  
Author(s):  
Sajal Kanti Dutta ◽  
Vishal Agarwal
Keyword(s):  
Transition State ◽  
Transition State Theory ◽  
Rate Coefficient ◽  
Activation Barrier ◽  
State Theory ◽  
Dft Study ◽  
Phenol Alkylation ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Rate Limiting

Water reduces the activation barrier of the rate-limiting step of phenol alkylation with propylene in H-BEA. This, in turn, increases the transition-state theory rate coefficient by two orders-of-magnitude, suggesting much faster alkylation.

scholarly journals The Optical Properties of InGaAs(P)/InP Under Group V Sublattice Two-phase Interdiffusion

1995 ◽  
Vol 417 ◽  
Author(s):  
E. Herbert Li ◽  
Joseph Micallef ◽  
W. C. Shui
Keyword(s):  
Experimental Data ◽  
Optical Properties ◽  
Transition State ◽  
Diffusion Coefficients ◽  
Concentration Ratio ◽  
Diffusion Mechanism ◽  
Time Dependent ◽  
Two Phase ◽  
Group V ◽  
Lattice Matched

AbstractUsing the fundamental transition state, we will investigate the two phase interdiffusion of group V sublattice in a strained lattice matched InGaAs/InP quantum well (QW) structures. The model employs three parameters namely the diffusion coefficients in the barrier (Db)and in the well (Dw)and the concentration ratio (k) of the diffused species at the heterstructure. The QW model includes the effects of strain and the exciton. A pseudo time dependent calculation is also considered and results are fitted to the reported experimental data. These parameters which characterize the diffusion mechanism can be measure to form a better understanding of the interdiffsion process for group V sublattice.

scholarly journals Bypassing the Inertness of Aziridine/CO2 Systems to Access 5-Aryl-2 Oxazolidinones: Catalyst-Free Synthesis in Water Under Ambient Conditions

2020 ◽  
Author(s):  
Giulio Bresciani ◽  
Emanuele Antico ◽  
Gianluca Ciancaleoni ◽  
Stefano Zacchini ◽  
Guido Pampaloni ◽  
...  
Keyword(s):  
Co2 Fixation ◽  
Activation Barrier ◽  
Ambient Conditions ◽  
Nmr Studies ◽  
Co Catalyst ◽  
Water As Solvent ◽  
Critical Issues ◽  
High Activation Barrier ◽  
Sequential Addition ◽  
Synthetic Routes

<div>The development of sustainable synthetic routes to access valuable oxazolidinones via CO2 fixation is </div><div>currently a hot topic of research, and the aziridine/carbon dioxide coupling has aroused a particular </div><div>interest. This reaction is featured by a high activation barrier, thus it requires a catalytic system and </div><div>often presents some other critical issues. Here, we describe the first gram-scale synthesis of a large </div><div>number of 5-aryl-2-oxazolidinones at ambient temperature and atmospheric CO2 pressure, in the </div><div>absence of any catalyst/co-catalyst and using water as solvent. The key to this innovative procedure </div><div>consists in the direct transfer of the CO2/amine adduct (carbamate) to common aziridine precursors </div><div>(dimethylsulfonium salts), replacing the classical sequential addition of amine (intermediate isolation </div><div>of aziridine) and then CO2. The reaction mechanism has been elucidated by NMR studies and DFT </div><div>calculations applied to model cases. </div>

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