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Search Results: 1 - 10 of 1662 matches for " MO Calculations "
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On the origin of the regioselective hydrolysis of a naphthoquinone diacetate: a molecular orbital study
Longo, Ricardo L.;Nunes, Ruth L.;Bieber, Lothar W.;
Journal of the Brazilian Chemical Society , 2001, DOI: 10.1590/S0103-50532001000100007
Abstract: the regioselectivity found in the mild basic hydrolysis of the 2,5-dimethyl-1,4-naphthohydroquinone diacetate (nunes, r.l.; bieber, l.w.; longo, r.l. j. nat. prod. 1999, 62, 1600) has been studied with ab initio and semiempirical molecular orbital methods. in the gas phase (isolated systems), these methods were not able to provide results that could explain the observed selectivity. however, when the solvent effects were included in the am1 method using the discrete solvation model it was possible to establish that this selectivity is due to the relative stability of the tetrahedral intermediates and their transitions states. the origin of this relative stability and thus of the observed selectivity is due to the repulsive interactions between the 2-methyl substituent in the naphthalene ring and the methyl group in the 4-acetate substituent, as well as their hindrance towards the hydration of the ionic group in the tetrahedral intermediates.
On the origin of the regioselective hydrolysis of a naphthoquinone diacetate: a molecular orbital study
Longo Ricardo L.,Nunes Ruth L.,Bieber Lothar W.
Journal of the Brazilian Chemical Society , 2001,
Abstract: The regioselectivity found in the mild basic hydrolysis of the 2,5-dimethyl-1,4-naphthohydroquinone diacetate (Nunes, R.L.; Bieber, L.W.; Longo, R.L. J. Nat. Prod. 1999, 62, 1600) has been studied with ab initio and semiempirical molecular orbital methods. In the gas phase (isolated systems), these methods were not able to provide results that could explain the observed selectivity. However, when the solvent effects were included in the AM1 method using the discrete solvation model it was possible to establish that this selectivity is due to the relative stability of the tetrahedral intermediates and their transitions states. The origin of this relative stability and thus of the observed selectivity is due to the repulsive interactions between the 2-methyl substituent in the naphthalene ring and the methyl group in the 4-acetate substituent, as well as their hindrance towards the hydration of the ionic group in the tetrahedral intermediates.
An Adjusted Model for Simple 1,2-Dyotropic Reactions. Ab Initio MO and VB Considerations  [PDF]
Henk M. Buck
Open Journal of Physical Chemistry (OJPC) , 2013, DOI: 10.4236/ojpc.2013.33015
Abstract:

With an adjusted model, we reconsider simple 1,2-dyotropic reactions with the introduction of a concept based on the intramolecular dynamics of a tetrahedron (van ’t Hoff modeling). In fact the dyotropic reactions are strongly related to conversions originated from neighbouring group participation or anchimeric assistance, defined as the interaction of a center with a lone pair of electrons in an atom and the electrons present in aδor π bond. The researchful 1,2-dyotropic reactions, based on the 1,2-interchange of halogens, methyl and hydrogen taking place in a concerted fashion, are in competition with the two-step reaction in which the neighbouring group participation or anchimeric assistance comes to full expression by ionic dissociation of the other exchangeable (halogen) atom. As to be expected there is an essential difference between halogen or methyl exchange regarding the number of electrons participating in the transition state. This aspect becomes evident in the geometries of the corresponding transition state geometries. In this paper we refer to ab initio MO calculations and VB considerations. We consider the 1,2-halogen exchange as a combination of two SN2 reactions each containing four electrons. The van ’t Hoff dynamics appears a useful model in order to illustrate the computations in a straightforward manner.

Molecular Structure of Heterocycles: 6. Solvent Effects on the 17O Nmr Chemical Shifts of 5-Trichloromethylisoxazoles
Martins, Marcos A. P.;Freitag, Rogério A.;Zimmermanna, Nilo E. K.;Sinhorin, Adilson P.;Cúnico, Wilson;Bastos, Giovani P.;Zanatta, Nilo;Bonacorso, Helio G.;
Journal of the Brazilian Chemical Society , 2001, DOI: 10.1590/S0103-50532001000600019
Abstract: a multilinear-regression analysis using the kamlet-abboud-taft (kat) solvatochromic parameters in order to elucidate and quantify the solvent effects on the 17o chemical shifts of three 5-trichloromethylisoxazoles [(1a) non-, (1b) 3-methyl- and (1c) 4-methyl-substituted] is reported. the chemical shifts of ring oxygen atom, o1, of compounds 1a-c show dependencies (in ppm) on the solvent polarity-polarizability of -4.8p*, -3.2p*, -8.9p*, on the solvent hydrogen-bond-donor (hbd) acidities 0.9a, -0.2a, -2.7a and the solvent hydrogen-bond-acceptor (hba) basicities -0.4b, 1.9b, 0.9b, respectively. the data of net charges of o1 and dipole moment, obtained from mo calculations (am1), are compared with the solvent effect parameters obtained for compounds 1a-c.
CHEMICAL BASIS FOR THE ANTIMICROBIAL ACTIVITY OF ACETANILIDES
Bravo,Hector R.; Weiss-López,Boris; Lamborot,Madeleine; Copaja,Sylvia;
Journal of the Chilean Chemical Society , 2003, DOI: 10.4067/S0717-97072003000400005
Abstract: in vitro antimicrobial activity of a series of substituted acetanilides against s. aureus, e. coli and c. albicans were measured at two concentrations, 250 mg/ml and 500 mg/ml. only the structures substituted with halogens in ca and electron acceptors in the aromatic ring are bioactive. the results are rationalized in terms of the acid properties of the n-h bond, as calculated using am1-mo theory
Molecular Structure of Heterocycles: 6. Solvent Effects on the 17O Nmr Chemical Shifts of 5-Trichloromethylisoxazoles
Martins Marcos A. P.,Freitag Rogério A.,Zimmermanna Nilo E. K.,Sinhorin Adilson P.
Journal of the Brazilian Chemical Society , 2001,
Abstract: A multilinear-regression analysis using the Kamlet-Abboud-Taft (KAT) solvatochromic parameters in order to elucidate and quantify the solvent effects on the 17O chemical shifts of three 5-trichloromethylisoxazoles [(1a) non-, (1b) 3-methyl- and (1c) 4-methyl-substituted] is reported. The chemical shifts of ring oxygen atom, O1, of compounds 1a-c show dependencies (in ppm) on the solvent polarity-polarizability of -4.8pi*, -3.2pi*, -8.9pi*, on the solvent hydrogen-bond-donor (HBD) acidities 0.9alpha, -0.2alpha, -2.7alpha and the solvent hydrogen-bond-acceptor (HBA) basicities -0.4beta, 1.9beta, 0.9beta, respectively. The data of net charges of O1 and dipole moment, obtained from MO calculations (AM1), are compared with the solvent effect parameters obtained for compounds 1a-c.
CHEMICAL BASIS FOR THE ANTIMICROBIAL ACTIVITY OF ACETANILIDES
Hector R. Bravo,Boris Weiss-López,Madeleine Lamborot,Sylvia Copaja
Journal of the Chilean Chemical Society , 2003,
Abstract: In vitro antimicrobial activity of a series of substituted acetanilides against S. aureus, E. coli and C. albicans were measured at two concentrations, 250 mug/ml and 500 mug/ml. Only the structures substituted with halogens in Calpha and electron acceptors in the aromatic ring are bioactive. The results are rationalized in terms of the acid properties of the N-H bond, as calculated using AM1-MO theory
Structural Investigation of Methanol {6-[(2-oxidopropyl)iminomethyl] phenolato} dioxidomolybdenum(VI) by X-Ray Crystallography and DFT Calculations
Iran SHEIKHSHOAIE,Yousef EBRAHIMIPOUR,Mahdiyeh SHEIKHSHOAEE
Walailak Journal of Science and Technology , 2012, DOI: 10.2004/vol10iss1ppaccepted
Abstract: This article presents the computational calculations of a cis-dioxomolybdenum(VI) complex by using density functional theory (DFT) with a DZP basis set (double zeta polarized basis set). The Schiff base 2-((E)-(2-hydroxypropylimino)methyl)-6-methoxyphenol was treated with MoO2(acac)2 in dry methanol to produce the mononuclear complex methanol{2-methoxy-6-[(2-oxidopropyl) iminomethyl]phenolato} dioxidomolybdenum(VI), whose structure has been solved and successfully refined in the monoclinic space group P21/c, with a = 6.755 , b = 15.835 , c = 13.119 , V = 1388.79 3, and Z = 4.
用环戊酮巯基化反应检测硫化钼基催化剂上的硫氢基团
王军,李文钊
催化学报 , 1997,
Abstract: ?
Push-pull alkenes: structure and p-electron distribution
ERICH KLEINPETER
Journal of the Serbian Chemical Society , 2006,
Abstract: Push-pull alkenes are substituted alkenes with one or two electron-donating substituents on one end of C=C double bond and with one or two electron-accepting substituents at the other end. Allowance for p-electron delocalization leads to the central C=C double bond becoming ever more polarized and with rising push-pull character, the p-bond order of this double bond is reduced and, conversely, the corresponding p-bond orders of the C–Don and C–Acc bonds are accordingly increased. This push-pull effect is of decisive influence on both the dynamic behavior and the chemical reactivity of this class of compounds and thus it is of considerable interest to both determine and to quantify the inherent push-pull effect. Previously, the barriers to rotation about the C=C, C–Don and/or C–Acc partial double bonds (DG±, as determined by dynamic NMR spectroscopy) or the 13C chemical shift difference of the polarized C=C partial double bond (DdC=C) were employed for this purpose. However, these parameters can have serious limitations, viz. the barriers can be immeasurable on the NMR timescale (either by being too high or too low; heavily-biased conformers are present, etc.) or DdC=C behaves in a non-additive manner with respect to the combination of the four substituents. Hence, a general parameter to quantify the push-pull effect is not yet available. Ab initio MO calculations on a collection of compounds, together with NBO analysis, provided valuable information on the structure, bond energies, electron occupancies and bonding/antibonding interactions. In addition to DG±C=C (either experimentally determined or theoretically calculated) and DdC=C, the bond length of the C=C partial double bond was also examined and it proved to be a reliable parameter to quantify the push-pull effect. Equally so, the quotient of the occupation numbers of the antibonding andbonding p orbitals of the central C=C partial double bond ( p*C=C/ pC=C) could also be employed for this purpose.
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