Search results for "computational chemistry"
showing 10 items of 1209 documents
Ab Initio Study on the Mechanism of the Reactions of the Nitrate Radical with Haloalkenes: 1,2-Dichloroethene, 1,1-Dichloroethene, Trichloroethene, …
2000
A general mechanism for the reactions of the NO3 radical with 1,2-dichloroethene, 1,1-dichloroethene, trichloroethene, and tetrachloroethene is proposed from ab initio DFT calculations. The calculated mechanism shows three main parallel reaction pathways. For the systems where the two carbon atoms are differently substituted, the study includes both the attacks with Markownikoff and contra-Markownikoff orientation. The first reaction pathway leads to the formation of an epoxide along with the NO2 radical, the second one to the formation of carbonyl compounds, and the third one leads, through the cleavage of the C−C bond, to the formation of carbonyl compounds with a lower number of carbon a…
Mechanochemical Synthesis, Photophysical Properties, and X-ray Structures of N-Heteroacenes
2016
The described mechanochemical methodology is an example of a proof-of-concept in which solution-based tedious, poor yielding, and difficult syntheses of pyrazaacenes are achieved under solvent-free ball-milling conditions; the method is easy, high yielding, time-efficient, and environmentally benign. The synthesized compounds also include pyrazaacenes (N-heteroacenes) that are octacene analogues containing pyrene building blocks. The compounds were sparingly soluble in common solvents, and column chromatographic purifications could be avoided after the solvent-free syntheses. The UV/Vis absorption spectra of the pyrazaacenes show intense absorption bands in the near-IR region. The single-cr…
Carbon-13 chemical shifts of bicyclic compounds
1970
13C NMR absorption spectra of 50 bicyclic hydrocarbons, alcohols and ketones have been measured, in addition to some terpenes. The 13C chemical shifts are approximately additive for similar compounds and can be used for the determination of molecular structure; they differ for endo- and exo-isomers, just as in proton spectra. These quite regular and predictable 13C shift differences are much larger and are caused by the 1,4-nonbonded interaction between atoms heavier than hydrogen, not by magnetic anisotropy effects.
Ab Initio Studies of Triplet-State Properties for Organic Semiconductor Molecules
2012
Triplet–triplet annihilation (TTA) leads to a reduced efficiency of organic light-emitting diodes (OLEDs) at high current densities. Spacial confinement of the triplet excitons, which is mainly dependent on triplet energy differences, can reduce the TTA rate. Therefore, a deliberate choice of the organic semiconductor materials with particular attention to their triplet energies can help to considerably increase the device efficiency. Organic solid-state lasers are, on the other hand, efficiently quenched by singlet–triplet annihilation (STA), which is closely related to the triplet–triplet absorption of the organic semiconductors. To establish a useful set of parameters related to the proc…
A butterfly like motion as a clue to the photophysics of thioxanthone
2006
Abstract A theoretical, quantum-chemical study of the thioxanthone (TX) molecule has been performed at the MP2 and CASPT2 levels of theory. Geometries, absorption energies, and transition and state properties have been investigated. Two conformers have been found very close in energy with planar (P) and non-planar (NP) structures, the latter characterized by a dihedral angle θ = 173.3° representing the bend of the side benzene rings and an out-of-plane angle ϕ = 4° of the C O bond. Large changes in the relative positions and properties of the two low-lying electronic absorption bands, ππ * and nπ * , are computed when changing the conformation. As a consequence of the analysis of the TX a…
Theoretical Study of the Electronic Excited States of Tetracyanoethylene and Its Radical Anion
2005
The low-lying electronic states of tetracyanoethylene (TCNE) and its radical anion were studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. The results obtained yield a full interpretation of the electronic absorption spectra, explain the spectral changes undergone upon reduction, give support to the occurrence of a bound excited state for the anionic species, and provide valuable information for the rationalization of the experimental data obtained with electron transmission spectroscopy.
Probing NiO nanocrystals by EXAFS spectroscopy
2010
Abstract The structure relaxation in nanocrystalline NiO (nano-NiO, 13 nm crystallite size) has been studied by X-ray absorption spectroscopy at the Ni K-edge at 300 K. Conventional single-scattering analysis of the EXAFS signals from the first two coordination shells showed a lattice volume expansion by about 1% and a contraction of the Ni–O bonds by about 0.5% in nano-NiO compared to microcrystalline NiO. A more sophisticated approach, based on a combination of classical molecular dynamics and ab initio multiple-scattering EXAFS theory, allowed us to interpret both static relaxation and lattice dynamics in nano-NiO.
Incorporation of Li dopant into Cu2ZnSnSe4 photovoltaic absorber: hybrid-functional calculations
2015
We have studied the formation of Li extrinsic defects in CuZnSnSe by first-principles hybrid functional calculations. Li atoms in the Cu site (Li) and Li atoms in the Se site (Li) are the most and the least stable point defect, respectively. The formation energies of two Li interstitial defects with different numbers of nearest neighbors are the same. These interstitial point defects act as a donor but do not create gap states. Formation of the acceptor point defects (Li and Li) is less likely in p-type CuZnSnSe compared with n-type CuZnSnSe. In contrast to Li which does not create gap states, the formation of Li creates two charge transition levels in the middle of the bandgap which might …
A series expansion of the extended Debye-H�ckel equation and application to linear prediction of stability constants
1996
The Debye-Hückel semiempirical extended equation is frequently used to calculate activity coefficients of chemical species and equilibrium constants at ionic strengths different from those used in their experimental evaluation. A series expansion of the extended Debye-Hückel equation is proposed here and checked with experimental data taken from the literature. The expansion is linear in the ionic parameters and yields a geometrical series which converges rapidly and that enables the accurate calculation of interpolated and extrapolated activity coefficients and equilibrium constants by simple and multiple linear regression without previous knowledge of the ionic parameters.
A quantum mechanics/molecular mechanics study of the acylation reaction of TEM1 β-lactamase and penicillanate
2000
The acylation step in β-lactamase catalyzed hydrolysis of β-lactams has been explored by means of a quantum mechanics/molecular mechanics approach (AM1/CHARMM). The TEM1 enzyme, a class A β-lactamase, and the penicillanate constitute the system employed in our study. The entire molecular system is divided into a quantum and a classical region: the quantum part is composed by the substrate, the serine Ser70 and the essential moieties of key active site residues, Lys73, Ser130 and Glu166, as well as a water molecule present in the active site region, while the classical part is formed by the remaining residues and structural waters of the enzyme. In particular, the sequence of steps proposed …