New insights on the theoretical study of hydrolysis mechanism of carbon disulfide (CS2) at low temperature: A density functional theory study

Density functional theory (DFT) is used to investigate the two-step hydrolysis mechanism of CS2. By optimizing the structure of reactants, intermediates, transition states, and products, the conclusion shows that the first step of CS2 (CS2 reacts with H2O first to form COS intermediate); The second step (COS intermediate reacts with H2O to form H2S and CO2). Therefore, hydrogen migration is crucial to the mechanism of CS2 hydrolysis. In the first step of the reaction, the rate-determining step in both the single C=S path and the double C=S path has a higher barrier of 199.9 kJ/mol, but the 127.9 kJ/mol barrier in the double C=S path has a lower barrier of 142.8 kJ/mol in the single C=S path. So the double C=S path is better. Similarly, the order of the barriers for the three paths in the second reaction is C=S path < C=S path and C=O path < C=O path. So the C=S path is better. Also, to further explore the reaction of CS2 hydrolysis, the natural bond orbital (NBO) analysis of the transition states was carried out. Besides, to further explain which reaction path is better, the hydrolysis kinetics of CS2 was analyzed. It was found that the hydrolysis of CS2 was an exothermic reaction, and the increase in temperature was unfavorable to the reaction. During the hydrolysis of CS2, the six reaction paths are parallel and competitive. The results will provide a new way to study the catalytic hydrolysis of CS2.

The Role of Homoaromaticity in the Tropylium-Catalyzed Carboxylic Acid O-H Insertion with Diazoesters

The tropylium catalyzed carboxylic acid O-H insertion with diazoesters providing α-hydroxy esters was reported recently through an activated carbene as the key intermediate. We report a revised mechanism involving a unique homoaromatic intermediate with the tropylium ion and the diazoester based on the DFT calculations. Our computational model provides a clear insight into the binding of the tropylium ion with the diazoester providing the homoaromatic intermediate. The reaction profiles of four different pathways were compared. The energies of the intermediates and the transition states are reported at B97-D3(SMD)/def2TZVP//B97-D3/def2TZVP (in dichloromethane). The energy profiles were compared across a few computational methods to study the sensitivity of our model across methods.

Constitutionalism within Times of Change: Authority, Society and Democracy

This article, following classical methodological patterns, as well as their evolution framework, identifies key features of the two most predominant constitutionalism traditions — political and legal, simultaneously drawing indispensable red lines with regard to correlation of the doctrine and a Fundamental Law itself. Respectively, the features have been rendered as the very elements of constitutionalism’s role within times of change — i. e., over the aforementioned time frames and transition states in between — whereas the doctrine’s capacity to answer so-called “questions of constitutionalism” constitutes its underlying response mechanism. The article addresses the phenomena of authority, society and democracy in their modern perception, and makes crucial points upon the constitutionalism’s effect on their sheer structures.

Ab initio study of the mechanism of the reaction ClO + O --> Cl + O2

Ab initio investigation on the reaction mechanism of ClO + O --> Cl + O2 reaction has been performed using correlation consistent triple zeta basis set. The geometry and frequency of the reactants, products, minimum energy geometries and transition states are obtained using MP2 method and energetics are obtained at the QCISD(T)//MP2 level of theory. Primarily, a possible reaction mechanism is obtained on the basis on IRC calculations using MP2 level of theory. To obtain true picture of the reaction path, we performed IRC calculations using CASSCF method with a minimal basis set 6-31G**. Some new equilibrium geometries and transition states have been identified at the CASSCF level. Energetics are also obtained at the QCISD(T)//CASSCF method. Possible reaction paths have been discussed, which are new in literature. Heat of reaction is found to be consistent with the experimental data. Bond dissociation energies to various dissociation paths are also reported.

Observing the base-by-base search for native structure along transition paths during the folding of single nucleic acid hairpins

Biomolecular folding involves searching among myriad possibilities for the native conformation, but the elementary steps expected from theory for this search have never been detected directly. We probed the dynamics of folding at high resolution using optical tweezers, measuring individual trajectories as nucleic acid hairpins passed through the high-energy transition states that dominate kinetics and define folding mechanisms. We observed brief but ubiquitous pauses in the transition states, with a dwell time distribution that matched microscopic theories of folding quantitatively. The sequence dependence suggested that pauses were dominated by microbarriers from nonnative conformations during the search by each nucleotide residue for the native base-pairing conformation. Furthermore, the pauses were position dependent, revealing subtle local variations in energy–landscape roughness and allowing the diffusion coefficient describing the microscopic dynamics within the barrier to be found without reconstructing the shape of the energy landscape. These results show how high-resolution measurements can elucidate key microscopic events during folding to test fundamental theories of folding.

Tracking the Source of Enantioselectivity

<p>Enantioselectivity remains one of synthetic chemistry’s most formidable problems. It arises due to the formation of diastereomeric species in a reaction, either in the form of diastereomeric intermediates or a set of diastereomeric transition states. Without control a racemic mixture is formed. A resolution method is then required to separate the enantiomers. Any given resolution method will rely on the differing energies of diastereomers to allow for their separation. Experimentally there are a myriad of different options that may be used to induce separation; for example chromatography and/or crystallisation. The actual process that occurs through- out this separation has not, however, been fully investigated in all cases. A better understanding of the process is able to provide an understanding of how resolution methods work i.e. when diastereomers occur and how great their energy differences are. This is vital in increasing the efficiency and effectiveness of any given resolution method. This theoretical study completed an investigation of the reaction pathway between the enantiomers of 2-formyl-3-hydroxyl[2.2]paracyclophane (FHPC) with (S)-valyl-(S)-valine. A subsequent investigation of an alternative resolution method, involving (R)-α-PEAM, was also conducted. This latter resolution method was proposed experimentally as a simpler method that could aid in improving the separation of the enantiomers. This investigation was carried out using Density Functional Theory (DFT) with the PBE0 functional and the triple-ζ TZVP basis set. The complete reaction profile was determined and diastereomeric intermediates and transition states for both resolution methods along two different pathways were determined; the ‘N-deprotonation Pathway’ and the ‘O-deprotonation Pathway’. The inadequacy of the first resolution method was found to be due to the presence of copper(II). Furthermore it was discovered that the re- action for both pathways would most likely proceed through the ‘O-deprotonation Pathway’ due to the barriers being lower in energy.</p>