Negative NICS values denote efficient electron delocalization. It can be calculated in the centre of the aromatic ring (NICS(0) 36), or at 1 Р“вЂ¦ above it (NICS(1) 37). It is defined as the negative value of the absolute magnetic shielding. The nucleus-independent chemical shift (NICS) index, based on the magnetic criterion of aromaticity, is probably the most widely used probe for examination of chemical compounds aromatic properties 35. The electron density difference map (EDDM) of the most intensity of the 10 lowest excitation were prepared from Gaussian03 TDDFT output using GaussSum and visualized using Gaussian viewer 5.0. GaussSum 2.2.6.1 was used to prepare total density of state (TDOS) or density of state 34. The population analysis has also been performed by the natural bond orbital method 32 using NBO program 33 under Gaussian 2003 program package.

The 10 lowest excitation energies were computed. The electronic spectra for the studied compounds were calculated by TD-DFT 31 using the same hybrid functionals and basis sets as used for the calculation of the hyperpolarizabilities. One finally obtains the equation that has been employed: Upon calculating the individual static components The total static first hyperpolarizability p was obtained from the relation: Geometries were optimized at this level of theory without any symmetry constraints followed by the calculations of the first order hyperpolarizabilities. A vibrational analysis was performed at each stationary point found, that confirm its identity as an energy minimum. The results of calculations for transition metal complexes show that MPW1PW91 functional gives better results than B3LYP 26-29. Geometry optimization was performed utilizing with Modified Perdew-Wang Exchange and Correlation (mpw1pw91) 25.

For Os element standard LANL2DZ basis set 22-24 is used and Os described by effective core potential (ECP) of Wadt and Hay pseudopotential 22 with a doublet-РћС• valence using the LANL2DZ. The calculations of systems contain C, Br, O, N, Cl, P, F and H described by the standard 6-31G(d,p) basis set 15-21. These calculations are valuable for providing insight into structure and properties in osmabenzyne.Р”Р„ll calculations were carried out with the Gaussian 03 suite of program 14. In the present work, we have attempted to study the structure, molecular orbital analysis, reactivity, electronic spectra, hyperpolarizability, and natural bond orbital analysis in osmabenzyne and substituted osmabenzyne. Also, the electronic explanation for the stabilization of osmabenzyne has been studied with the aid of orbital interaction analysis and density functional theory calculations 13. For instance, osmabenzynes, such as metallabenzenes and aromatic compounds, can undergo electrophilic substitution reactions 9. However, in recent years, numerous stable osmabenzynes have been isolated 8-12 and their interesting chemical properties have been determined. The first stable metallabenzyne was obtained unexpectly during effort to prepare osmium vinylidene complexes of the type 7. At initial view, one might expect that metallabenzynes may be too unstable to be isolated, because organic compounds with a C=C bond in the six-membered ring, for example, benzyne and cyclohexyne, are thermally highly unstable because of ring strain. Metallabenzynes, 1-6 which are organometallic compounds derived from formal replacement of a carbon atom or a C-H group in benzyne with an isolobal transition metal fragment, are closely related to metallabenzenes. Non linear optical (NLO) behavior of title compounds is investigated by the computed value of first hyperpolarizability (РћР† total). Time dependent density functional theory (TD-DFT) is used to calculate the energy, oscillatory strength and wavelength absorption maxima ( РћВ» max) of various electronic transitions and their nature within molecules. Frontier orbital analysis indicates the HOMO and LUMO are distributed on the ring carbon atoms, Os and Cl ligands. * e-mail: structures and properties of osmabenzyne and para-substituted osmabenzynes (X=F, Cl, Br, Me, NH 2, OH, NO 2, CHO, COOH) have been explored using theoretical methods. REZA GHIASI *, a, HODA PASDAR b, FAROKH IRAJIZADEH bР”Р„ Department of Chemistry, Basic science faculty, East Tehran Branch, Islamic Azad University, Qiam Dasht, Tehran, IRAN.Р”В« Chemistry faculty, North Tehran Branch, Islamic Azad University, Tehran, IRAN. UNDERSTANDING THE STRUCTURE, SUBSTITUENT EFFECT, NATURAL BOND ANALYSIS AND AROMATICITY OF OSMABENZYNE: A DFT STUDY

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