Search results for "effect"

Long term stability testing of oxide unicouple thermoelectric modules

2019

Thermoelectric devices based on oxides are good candidates for energy harvesting technologies for use in aggressive conditions where the materials should withstand high temperatures and corrosive environments over prolonged time. This leads to a natural concern for the stability of the electrical contacts, especially on the hot side of the module. In this work, we have assembled several prototype unicouple thermoelectric modules made by pyrolyzed and spark plasma sintered n-type CaMnO3 and p-type Ca3Co4O9 and then tested under different conditions mimicking end-user applications. For baseline experiments we have chosen to use nickel as the contact material in order to show the effect of its…

Fourier-transform spectroscopy and deperturbation analysis of the spin-orbit coupled A(1)Σ(+) and b(3)Π states of KRb.

2016

Fourier-transform A(1)Σ(+) - b(3)Π → X(1)Σ(+) laser-induced fluorescence spectra were recorded for the natural mixture of (39,41)K(85,87)Rb isotopologues produced in a heatpipe oven. Overall 4200 rovibronic term values of the spin-orbit coupled A(1)Σ(+) and b(3)Π states were determined with an uncertainty of about 0.01 cm(-1) in the energy range [10 850, 14 200] cm(-1) covering rotational quantum numbers J' ∈ [3, 280]. Direct deperturbation analysis of the A ∼ b complex performed within the framework of the A(1)Σ(+) ∼ b(3)ΠΩ=0,1,2 coupled-channel approach reproduced experimental data with a standard deviation of 0.004 cm(-1). Initial parameters of the internuclear potentials and spin-orbit …

Unexpected Substituent Effects in the Iso-Heterocyclic Boulton-Katritzky Rearrangement of 3-Aroylamino-5-methyl-1,2,4-oxadiazoles: A Mechanistic Stud…

2019

The kinetics of the iso-heterocyclic mononuclear rearrangement of some 3-aroylamino-5-methyl-1,2,4-ozadiazoles was carefully examined under largely variable acidic or alkaline conditions. This reaction may proceed via two different mechanistic pathways (an uncatalyzed and a base-catalyzed one), as accounted for also by the evaluation of the relevant activation parameters. Substituent effects, as quantified by means of the Hammett’s equation, appear relatively modest; however, they reveal some interesting anomalies, which enabled us to draw a very precise picture of the intimate reaction course.

Origin of Enzymatic Kinetic Isotope Effects in Human Purine Nucleoside Phosphorylase

2017

Here we report a study of the effect of heavy isotope labeling on the reaction catalyzed by human purine nucleoside phosphorylase (hPNP) to elucidate the origin of its catalytic effect and of the enzymatic kinetic isotope effect (EKIE). Using quantum mechanical and molecular mechanical (QM/MM) molecular dynamics (MD) simulations, we study the mechanism of the hPNP enzyme and the dynamic effects by means of the calculation of the recrossing transmission coefficient. A free energy surface (FES), as a function of both a chemical and an environmental coordinate, is obtained to show the role of the environment on the chemical reaction. Analysis of reactive and nonreactive trajectories allows us …

A quantum dynamics study of the benzopyran ring opening guided by laser pulses

2014

Abstract The ring-opening photoisomerization of benzopyran, which occurs via a photochemical route involving a conical intersection, has been studied with quantum dynamics calculations using the multi-configuration time-dependent Hartree method (MCTDH). We introduce a mechanistic strategy to control the conversion of benzopyran to merocyanine with laser pulses. We use a six-dimensional model developed in a previous work for the potential energy surfaces (PES) based on an extension of the vibronic-coupling Hamiltonian model (diabatization method by ansatz), which depends on the most active degrees of freedom. The main objective of these quantum dynamics simulations is to provide a set of str…

Quantum dynamics of 16O in collision with ortho- and para-17O17O

2017

Abstract We report full quantum dynamical observables, such as integral and differential cross sections and rate constants, for the 16 O +  17 O 17 O reactive collision process. We particularly emphasize the effect coming from the nonzero nuclear spin of 17 O, leading to two nuclear spin isomers of 34 O 2 , ortho- and para- 34 O 2 which can be studied independently and behave differently. A comparison with the 16 O +  18 O 18 O collision is given. We find that processes involving 17 O 17 O are always faster than with 18 O 18 O.

Full-dimensional control of the radiationless decay in pyrazine using the dynamic Stark effect

2014

We present a full quantum-mechanical study of the laser control of the radiationless decay between the B3u(nπ(*)) and B2u(ππ(*)) states of pyrazine using the dynamic Stark effect. In contrast to our previous study [Sala et al., J. Chem. Phys. 140, 194309 (2014)], where a four-dimensional model was used, all the 24 degrees of freedom are now included in order to test the robustness of the strategy of control. Using a vibronic coupling Hamiltonian model in a diabatic representation, the multi-layer version of the multi-configuration time-dependent Hartree method is exploited to propagate the corresponding wave packets. We still observe a trapping of the wavepacket on the B2u(ππ(*)) potential …

Heavy enzymes and the rational redesign of protein catalysts

2019

Abstract An unsolved mystery in biology concerns the link between enzyme catalysis and protein motions. Comparison between isotopically labelled “heavy” dihydrofolate reductases and their natural‐abundance counterparts has suggested that the coupling of protein motions to the chemistry of the catalysed reaction is minimised in the case of hydride transfer. In alcohol dehydrogenases, unnatural, bulky substrates that induce additional electrostatic rearrangements of the active site enhance coupled motions. This finding could provide a new route to engineering enzymes with altered substrate specificity, because amino acid residues responsible for dynamic coupling with a given substrate present…

The substituent effect of π-electron delocalization in N-methylamino-nitropyridine derivatives: crystal structure and DFT calculations

2020

AbstractThe crystal and molecular structures of 3-(N-methylamino)-2-nitropyridine, 5-(N-methylamino)-2-nitropyridine and 2-(N-methylamino)-5-nitropyridine have been characterized by X-ray diffraction. To perform conformational analysis, the geometries of the compounds as well as their conformers and rotamers were optimized at the B3LYP/6-311++G(3df,3pd) level. The resulting data were used to analyze the π-electron delocalization effect in relation to the methylamino group rotation in ortho-, meta- and para-substitution positions. Quantitative aromaticity indices were calculated based on which we estimated the electronic structures of the analyzed compounds. The substituent effect of the met…

2020

The course of organic chemical reactions is efficiently modelled through the concepts of “electrophiles” and “nucleophiles” (meaning electron-seeking and nucleus-seeking reactive species). On the one hand, an advanced approach of the correlation of the nucleophilicity parameters N and electrophilicity E has been delivered from the linear free energy relationship log k (20 °C) = s(N + E). On the other hand, the general influence of the solvent mixtures, which are very often employed in preparative synthetic chemistry, has been poorly explored theoretically and experimentally, to date. Herein, we combined experimental and theoretical studies of the solvent influence on pyrrolidine nucleophili…