Search results for " quantum chemistry"

Ab initio calculations and vibrational spectroscopy on the phenylenediamine isomers

1998

Molecular orbital calculations at HF and MP2 levels have been performed using the 6-3IG** basis set for full geometry optimization of the phenylenediamine isomers. Our results show that only a transoid conformer is found for o-phenylenediamine, whereas cis and trans conformers exist for m- and p-phenylenediamine. Vibrational normal modes have been also analyzed for the gas phase and in chloroform solution, and compared with experimental data we have obtained using FTIR spectroscopy. © 1998 Elsevier Science B.V.

Tautomeric preferences of phthalones and related compounds

2007

Abstract Multinuclear magnetic resonance and IR spectra prove that although 2-(diacylmethyl)pyridines and 2-(diacylmethyl)quinolines are β-diketones, their proton transfer product present in chloroform solution is not ketoenol but enaminone (earlier opinions were contradictory). Quinoline derivatives are less zwitterionic by character than the respective pyridyl congeners. The β-diketone form itself may also be rarely present in the solution. X-ray data show that 2-(2(1H)-pyridinylidene)-1H-indene-1,3(2H)-dione, i.e., enaminone tautomer of 2-(pyridin-2-yl)-2H-indene-1,3-dione, is also the only form present in crystal. Ab initio calculations show that the enaminone is usually more stable tha…

(1Z,3Z)-3-[Quinolin-2(1H)-ylidene]-1-(quinolin-2-yl)prop-1-en-2-ol: An unexpected most stable tautomer of 1,3-bis(quinolin-2-yl)acetone

2009

Abstract 1 H, 13 C and 15 N NMR spectra reveal that CDCl 3 solution of 1,3-bis(quinolin-2-yl)acetone contains only ( 1Z , 3Z )-3-[quinolin-2(1 H )-ylidene]-1-(quinolin-2-yl)prop-1-en-2-ol. The proton transfer takes place between two basic centers of the molecule, which means that the process is an identity reaction by character. The situation is completely different from that detected in chloroform solution of 1,3-bis(pyridin-2-yl)acetone where three different tautomers are in equilibrium with each other. Although the proton transfers in both ( 1Z , 3Z )-3-[quinolin-2(1 H )-ylidene]-1-(quinolin-2-yl)prop-1-en-2-ol and ( 1Z , 3Z )-3-hydroxy-1-[quinolin-2(1 H )-ylidene-4-quinolin-2-yl]but-3-e…

Quantum chemical simulation of excited states of chlorophylls, bacteriochlorophylls and their complexes

2006

The present review describes the use of quantum chemical methods in estimation of structures and electronic transition energies of photosynthetic pigments in vacuum, in solution and imbedded in proteins. Monomeric Mg-porphyrins, chlorophylls and bacteriochlorophylls and their solvent 1:1 and 1:2 complexes were studied. Calculations were performed for Mg-porphyrin, Mg-chlorin, Mg-bacteriochlorin, mesochlorophyll a, chlorophylls a, b, c(1), c(2), c(3), d and bacteriochlorophylls a, b, c, d, e, f, g, h, plus several homologues. Geometries were optimised with PM3, PM3/CISD, PM5, ab initio HF (6-31G*/6-311G**) and density functional B3LYP (6-31G*/6-311G**) methods. Spectroscopic transition energ…

Correction of cavity-induced errors in polarization charges of continuum solvation models

1998

The molecular structure of N-hydroxyurea

1996

Ab initio calculations were performed on the tautomers and conformers of N-hydroxyurea using a 6-31G** basis set. The minimum-energy structures have been found and the importance of the intramolecular hydrogen bond as the stabilizing factor was pointed out. © 1996 John Wiley & Sons, Inc.

Using Applications and Tools to Visualize ab initio Calculations Performed in VASP

2018

Visualization of the results of the ab initio calculations is important for the analysis of these results. It improves the quality of the analysis by supplementing the plain numbers received as the output of the calculations with various graphical images and facilitates the analysis of the results. In addition to that visualization helps avoiding some mistakes or inconsistencies. Various tools have been used in this work to construct the unit cell models of the calculated lattices, to check and analyze the calculated lattice structure before and after the relaxation, to plot total and difference electron charge density maps.

A PH-functionalized polyphosphazene: a macromolecule with a highly flexible backbone.

2006

Comparative classical and ab initio Molecular Dynamics study of molten and glassy germanium dioxide

2008

A Molecular Dynamics (MD) study of static and dynamic properties of molten and glassy germanium dioxide is presented. The interactions between the atoms are modelled by the classical pair potential proposed by Oeffner and Elliott (OE) [Oeffner R D and Elliott S R 1998, Phys. Rev. B, 58, 14791]. We compare our results to experiments and previous simulations. In addition, an ab initio method, the so-called Car-Parrinello Molecular Dynamics (CPMD), is applied to check the accuracy of the structural properties, as obtained by the classical MD simulations with the OE potential. As in a similar study for SiO2, the structure predicted by CPMD is only slightly softer than that resulting from the cl…

Chemisorption of a molecular oxygen on the UN(001) surface: Ab initio calculations

2010

The results of DFT GGA calculations on oxygen molecules adsorbed upon the (0 0 1) surface of uranium mononitride (UN) are presented and discussed. We demonstrate that O2 molecules oriented parallel to the substrate can dissociate either (i) spontaneously when the molecular center lies above the surface hollow site or atop N ion, (ii) with the activation barrier when a molecule sits atop the surface U ion. This explains fast UN oxidation in air.