Home About us Contact
|
Home About us Contact | ||
|
|
||
AM1 Method (am1 + method)
Selected AbstractsGlobal optimization of SixHy at the ab initio level via an iteratively parametrized semiempirical methodINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2003Yingbin Ge Abstract Previously we searched for the ab initio global minima of several SixHy clusters by a genetic algorithm in which we used the AM1 semiempirical method to facilitate a rapid energy calculation for the many different cluster geometries explored. However, we found that the AM1 energy ranking significantly differs from the ab initio energy ranking. To better guarantee locating the ab initio global minimum while retaining the efficiency of the AM1 method, we present an improved iterative global optimization strategy. The method involves two separate genetic algorithms that are invoked consecutively. One is the cluster genetic algorithm (CGA), mentioned above, to find the semiempirical SixHy cluster global minimum. A second and separate parametrization genetic algorithm (PGA) is used to reparametrize the AM1 method using some of the ab initio data generated from the CGA to form a training set of different reference clusters but with fixed SixHy stoichiometry. The cluster global optimization search (CGA) and the semiempirical parametrization (PGA) steps are performed iteratively until the semiempirical GA reparametrized AM1 (GAM1) calculations give low-energy optimized structures that are consistent with the globally optimized ab initio structure. We illustrate the new global optimization strategy by attempting to find the ab initio global minima for the Si6H2 and Si6H6 clusters. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003 [source] Modeling the semi-empirical electrotopological index in QSPR studies for aldehydes and ketonesJOURNAL OF CHEMOMETRICS, Issue 5 2009Érica Silva Souza Abstract The semi-empirical electrotopological index, ISET, used for quantitative structure,retention relationship (QSRR) models firstly developed for alkanes and alkenes, was remodeled for organic functions such as ketones and aldehydes. The ISET values for hydrocarbons are calculated through the atomic charge values obtained from a Mulliken population analysis using the semi-empirical AM1 method and their correlation with the SETi values attributed to the different types of carbon atoms according to experimental data. For ketones and aldehydes the interactions between the molecules and the stationary phase are slowly increased relative to the hydrocarbons, due to the charge redistribution that occurs in the presence of heteroatoms. For these polar molecules the increase in the interactions was included in the calculation of the ISET values through the dipole moment of the whole molecule and also through an equivalent local dipole moment related to the net charges of the atoms of the CO and HCO functional groups. Our findings show that the best definition of an equivalent local dipole moment is clearly dependent on the specific features of the charge distribution in the polar region of the molecules (e.g. ketones and aldehydes), which allows them to be distinguished. Thus, the QSRR models for 15 aldehydes and 42 ketones obtained using the remodeled ISET were of good quality as shown by the statistical parameters. The ability of this remodeled index to include charge distribution and structural details opens a new way to study the correlations between the molecular structure and retention indices in gas chromatography. Copyright © 2009 John Wiley & Sons, Ltd. [source] Synthesis and alkylation of 5-(3-chlorobenzo[b]thien-2-yl)-4H -1,2,4-triazole-3-thiol under classical and microwave conditions.JOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 6 2006AM1 semiemperical calculations for investigating the regioselectivity of alkylation Under microwave irradiation (MWI), 5-(3-chlorobenzo[b]thien-2-yl)-4H -1,2,4-triazole-3-thiol (3) was synthesized in a good yield by intramolecular cyclization of the carbonyl thiosemicarbazide 2. A regioselective S-alkylation of 3 with benzyl chloride or allyl bromide has been achieved by using triethylamine as a base, while other different basic conditions led to a mixture of bis(alkylated) derivatives N4, S- and S, N2 -, under both of classical and MWI conditions. The relative stabilities, charge densities, dipole moments and electronic energies of reactants, transition states and intermediates were calculated by the AM1 method and used for investigating the regioselectivity. [source] Systematic conformational search analysis of the SRR and RRR epimers of 7-hydroxymatairesinolJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 2 2010Giovanni Li Manni Abstract An extensive and systematic conformational search was performed on the two epimers of the natural lignan 7-hydroxymatairesinol (HMR), by means of a home-made Systematic Conformational Search Analysis (SCSA) code, designed to select more and more stable conformers through sequential geometry optimization of trial structures at increasing levels of calculation theory. In the present case, the starting molecular structures were selected by the semi-empirical AM1 method and filtered , i.e. decreased in number by choosing the more stable species , on the basis of their energy calculated by the HF method and the 6-31G(d) basis set. The geometries obtained were further refined by performing density functional theory (DFT) optimizations, using the B3LYP functional and the 6-31G(d,p) basis set, both in vacuo and in ethanol solution. This procedure allowed us to isolate, at a high level of theory, three groups of epimer conformers characterized by open, semi-folded, and folded conformations. Moreover, the SCSA allowed us to describe a conformational space made-up by about 20 species for each of the two epimers. The corresponding energy content of these species was within 27,kJ,mol,1 from the absolute minimum found, both in vacuo and in ethanol solution. The conformational analysis, followed by the inspection of the stereochemistry of the two most stable conformers of both epimers, provides support in rationalizing the proposed reaction mechanism of the catalytic hydrogenolysis of the HMR to matairesinol (MAT). Copyright © 2009 John Wiley & Sons, Ltd. [source] Substituent effect on the [2,+,3] cycloaddition of (E)-,-Nitrostyrene with (Z)- C,N -Diarylnitrones,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 2 2002Andrzej Baranski Abstract The kinetics of the title reaction were studied in toluene at 80,°C by means of HPLC. The overall rate constant (ktotal) of the reaction of (E)-,-nitrostyrene (1) with (Z)- C -phenyl- N -arylnitrones (2a,g) was found to increase with increasing Hammett ,-constant, whereas in the case of (Z)- C -aryl- N -phenylnitrones (2g,p) and the same nitrostyrene the rate decreased. The substituent effect in these reactions is inconsistent with FMO treatments of cycloaddition rates. For both reaction series good linear relationships were obtained between log ktotal and the net charge on the carbon atom in the >CN(O), moiety of the nitrones calculated by the AM1 method. Copyright © 2001 John Wiley & Sons, Ltd. [source] Structural study of ethyl 3-arylcarbamoyl-2,3-diazabicyclo[2.2.1]hept-5-ene-2-carboxylates: conformation and transmission of substituent effects across the diazabicycloheptene ring,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 11 2001A. Perjéssy Abstract Nine new ethyl 3-arylcarbamoyl-2,3-diazabicyclo[2.2.1]hept-5-ene-2-carboxylates were prepared by a [4,+,2] cycloaddition and their FTIR, 1H, 13C and 15N NMR spectra were measured and assigned. Single crystals were grown for five compounds and their X-ray data were obtained. The electronic structure and the conformations were calculated by the semi-empirical AM1 method. Using correlations between the spectral, empirical and theoretical structural data, the transmission of substituent effects and the preferential conformation connected with the consecutive double nitrogen inversion and regarding the mutual orientation of NH and CO bonds were investigated. The results are compared with those for a previously reported series of analogous ethyl 2-arylcarbamoyl-4,5-dimethyl-1,2,3,6-tetrahydropyridazine-1-carboxylates. Copyright © 2001 John Wiley & Sons, Ltd. [source] Semi-Empirical and DFT Studies on Structures and Spectra for C78(CH2)2CHINESE JOURNAL OF CHEMISTRY, Issue 2 2007Shi Wu Abstract Eighteen possible isomers of C78(CH2)2 were investigated by the INDO method. It was indicated that the most stable isomer was 42,43,62,63-C78(CH2)2, where the ,CH2 groups were added to the 6/6 bonds located at the same hexagon passed by the longest axis of C78 (C2v), to form cyclopropane structures. Based on the most stable four geometries of C78(CH2)2 optimized at B3LYP/3-21G level, the first absorptions in the electronic spectra calculated with the INDO/CIS method and the IR frequencies of the C,C bonds on the carbon cage computed using the AM1 method were blue-shifted compared with those of C78 (C2v) because of the bigger LUMO-HOMO energy gap and the less conjugated carbon cage after the addition. The chemical shifts of 13C NMR for the carbon atoms on the added bonds calculated at B3LYP/3-21G level were moved upfield thanks to the conversion from sp2 -C to sp3 -C. [source] Enantioselective chromatography of alkyl derivatives of 5-ethyl-5-phenyl-2-thiobarbituric acid studied by semiempirical AM1 methodCHIRALITY, Issue 8 2002Krzysztof Zborowski Abstract Complexation of alkyl derivatives of 5-ethyl-5-phenyl-2-thiobarbituric acid (2-thiophenobarbital) enantiomers by ,-cyclodextrin was investigated by the AM1 method. The inclusion complexes of ,-cyclodextrin with neutral and anionic forms of these enantiomers have been modeled and energetically optimized. The chiral discrimination of enantiomers was analyzed in terms of differences in the interaction energies. The calculated interaction energies between each enantiomer of the investigated 2-thiobarbiturates and ,-cyclodextrin confirm the ability of ,-cyclodextrin to act as a mobile phase additive in reversed-phase HPLC to separate enantiomers by liquid chromatography and rationalize their order of elution. Chirality 14:632,637, 2002. © 2002 Wiley-Liss, Inc. [source] | ||||||||||