Search results for "Orbital"
showing 10 items of 679 documents
Molecular orbital studies on the mechanism of catalytic isomerization of xylenes II. The photochemical process
1981
Abstract The feasibility of an intermediate step in the mechanism of photochemical isomerization of xylenes in acidic media is explored by using CNDO molecular orbital methods. Along the reaction path from 2,6-dimethylbenzenium ion to dimethylbicyclo[3.1.0]hexenyl cation, the energy surfaces for this intermediate step have been drawn both for the first singlet S1 and triplet T1 excited states. The energies of these excited states have been calculated by adding to the ground-state energy calculated by CNDO/2 method the excitation energy calculated by CNDO/S-CI method. An optimization of the saddle point energy is achieved by introducing the angle between methyl groups and the ring plane as a…
A CNDO/2 study on the additivity and the nature of the non-additivity of the substituent effects on13C NMR shifts in chlorobenzenes and chlorophenols
1980
The general correlation between the electron densities and the 13C NMR chemical shifts is found to be quite poor in the cases discussed. The non-additivities of the substituent effects on the chemical shifts and the CNDO/2 electron densities correlate only weakly. However, when the electron densities are made specific to different types of atomic orbitals, the s electrons have a pronounced effect in all the models tested. This is explained by an indirect effect on the 〈1/r3〉 term of the p electrons. Good correlations are found between the sums of the chemical shifts and the corresponding sums of the substituent charge excesses. The different behaviour of OH and Cl substituents in the additi…
ChemInform Abstract: Experimental and Theoretical Studies on the Electronic Spectra of Indole-3-acetic Acid and Its Anionic and Protonated Species.
2010
Abstract The geometrical and electronic structure of indole-3-acetic acid (IAH) have been investigated using the MINDO/3 molecular orbital method, the geometries being optimized by Rinaldi's method. The UV spectrum of IAH has been recorded in different solvents. The corresponding spectra of IAH + 2 and IA − were also respectively obtained, from IAH in different H 2 SO 4 /water mixtures and NaOH solutions. The agreement of experimental spectra with the ones theoretically calculated by the CNDO/M-Cl method after optimization of the κ parameter, is discussed. The p K a values of IAH have also been determined from the IAH + 2 spectra and are compared with literature data.
Color Doppler ultrasonography of the eye and orbit
1993
Color Doppler imaging is the recent development in ultrasonography that allows simultaneous two-dimensional structural imaging in Doppler evaluation of blood flow. With this technique, it is possible for the first time to indirectly display the fine orbital vessels such as the ophthalmic artery and its branches, the central retinal artery, the posterior ciliary artery, and the lacrimal artery. Also, the display of venous structures such as the superior ophthalmic vein, the vortex vein, and the central retinal vein is possible. In addition to the qualitative display of those vessels, the hemodynamics in those vessels can be quantitatively assessed by looking at the Doppler spectrum and deter…
Donor–anion interactions at the charge localization and charge ordering transitions of (TMTTF)2AsF6 probed by NEXAFS
2015
High-resolution near-edge X-ray absorption fine structure (NEXAFS) measurements at the As M-edge, F K-edge and S L-edge of the Fabre salt (TMTTF)2AsF6 were performed from room temperature (RT) to 90 K, allowing to reach the charge localization regime below Tρ ≈ 230 K and to cross the charge ordering (CO) transition at TCO ≈ 102 K. The F K-edge and S L-edge spectra exhibit several transitions which have been indexed on the basis of first-principles DFT calculations. Upon cooling from RT significant energy shifts up to +0.8 eV and -0.4 eV were observed in transitions exhibited by the F 1s and S 2p spectra respectively, while the As 3p doublet does not show a significant shift. Opposite energy…
Generalized Many-Body Expanded Full Configuration Interaction Theory
2019
Facilitated by a rigorous partitioning of a molecular system's orbital basis into two fundamental subspaces - a reference and an expansion space, both with orbitals of unspecified occupancy - we generalize our recently introduced many-body expanded full configuration interaction (MBE-FCI) method to allow for electron-rich model and molecular systems dominated by both weak and strong correlation to be addressed. By employing minimal or even empty reference spaces, we show through calculations on the one-dimensional Hubbard model with up to 46 lattice sites, the chromium dimer, and the benzene molecule how near-exact results may be obtained in a entirely unbiased manner for chemical and physi…
Anomalous High-Pressure Jahn-Teller Behavior inCuWO4
2012
High-pressure optical-absorption measurements performed in CuWO4 up to 20 GPa provide experimental evidence of the persistence of the Jahn-Teller (JT) distortion in the whole pressure range both in the low-pressure triclinic and in the highpressure monoclinic phase. The electron-lattice coupling associated with the eg(Exe) and t2g(Txe) orbitals of Cu2+ in CuWO4 are obtained from correlations between the JT distortion of the CuO6 octahedron and the associated structure of Cu2+ d-electronic levels. This distortion and its associated JT energy (EJT) decrease upon compression in both phases. However, both the distortion and associated EJT increase sharply at the phase transition pressure (PT = …
Theoretical study of new acceptor and donor molecules based on polycyclic aromatic hydrocarbons
2011
Functionalized polcyclic aromatic hydrocarbons (PAHs) are an interesting class of molecules in which the electronic state of the graphene-like hydrocarbon part is tuned by the functional group. Searching for new types of donor and acceptor molecules, a set of new PAHs has recently been investigated experimentally using ultraviolet photoelectron spectroscopy (UPS). In this work, the electronic structure of the PAHs is studied numerically with the help of B3LYP hybrid density functionals. Using the DELTA-SCF method, electron binding energies have been determined which affirm, specify and complement the UPS data. Symmetry properties of molecular orbitals are analyzed for a categorization and a…
Coupled-cluster theory for atoms and molecules in strong magnetic fields
2015
An implementation of coupled-cluster (CC) theory to treat atoms and molecules in finite magnetic fields is presented. The main challenges for the implementation stem from the magnetic-field dependence in the Hamiltonian, or, more precisely, the appearance of the angular momentum operator, due to which the wave function becomes complex and which introduces a gauge-origin dependence. For this reason, an implementation of a complex CC code is required together with the use of gauge-including atomic orbitals to ensure gauge-origin independence. Results of coupled-cluster singles-doubles-perturbative-triples (CCSD(T)) calculations are presented for atoms and molecules with a focus on the depende…
Virtual Orbital Many-Body Expansions: A Possible Route towards the Full Configuration Interaction Limit
2017
In the present letter, it is demonstrated how full configuration interaction (FCI) results in extended basis sets may be obtained to within sub-kJ/mol accuracy by decomposing the energy in terms of many-body expansions in the virtual orbitals of the molecular system at hand. This extension of the FCI application range lends itself to two unique features of the current approach, namely that the total energy calculation can be performed entirely within considerably reduced orbital subspaces and may be so by means of embarrassingly parallel programming. Facilitated by a rigorous and methodical screening protocol and further aided by expansion points different from the Hartree-Fock solution, al…