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Molecular Orbital Theory (molecular + orbital_theory)
Kinds of Molecular Orbital Theory Selected AbstractsEnthalpies of Formation of Gas-Phase N3, N - 3, N+5, and N - 5 from ab initio Molecular Orbital Theory, Stability Predictions for N+5N - 3 and N+5N - 5, and Experimental Evidence for the Instability of N+5N - 3.CHEMINFORM, Issue 17 2004David A. Dixon No abstract is available for this article. [source] Mechanisms of H2, H2C=CH2, and O=CH2 Insertion into Cp2Zr(,2 -SiMe2=NtBu)(PMe3)EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 14 2007Siwei Bi Abstract In this paper, the mechanisms for the insertion of H2, H2C=CH2, and O=CH2 into the Zr,Si bond of Cp2Zr(,2 -SiMe2=NtBu)(PMe3) (R) are theoretically investigated with the aid of density functional theory (DFT) calculations. The structure of the H2 insertion product P is discussed on the basis of our calculations, and its bonding features are rationalized in terms of molecular orbital theory. The regiochemistry for insertion of O=CH2 has also been theoretically investigated. It is found that the relative stabilities of the three insertion products of R are in the order P < P, < P,. For the reactions of R with H2 and CH2=CH2, the rate-determining steps are the insertions of H2 and CH2=CH2 into the Zr,Si bond of Cp2Zr(,2 -SiMe2=NtBu) (Int1), whereas PMe3 dissociation is the rate-determining step for the reaction of R with O=CH2. Only the precursor Int2,, formed by the coordination of O=CH2 to the Zr atom, is located; those formed by the coordination of H2 and CH2=CH2 to Int1 are not found.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] A Quantum-Chemical Study on Understanding the Dehydrogenation Mechanisms of Metal (Na, K, or Mg) Cation Substitution in Lithium Amide NanoclustersADVANCED FUNCTIONAL MATERIALS, Issue 12 2010Lanlan Li Abstract The hydrogen-releasing activity of (LiNH2)6,LiH nanoclusters and metal (Na, K, or Mg)-cation substituted nanoclusters (denoted as (NaNH2)(LiNH2)5, (KNH2)(LiNH2)5, and (MgNH)(LiNH2)5) are studied using ab initio molecular orbital theory. Kinetics results show that the rate-determining step for the dehydrogenation of the (LiNH2)6,LiH nanocluster is the ammonia liberation from the amide with a high activation energy of 167.0,kJ,mol,1 (at B3LYP/6-31,+,G(d,p) level). However, metal (Na, K, Mg)-cation substitution in amide,hydride nanosystems reduces the activation energies for the rate-determining step to 156.8, 149.6, and 144.1,kJ,mol,1 (at B3LYP/6-31,+,G(d,p) level) for (NaNH2)(LiNH2)5, (KNH2)(LiNH2)5, and (MgNH)(LiNH2)5, respectively. Furthermore, only the ,NH2 group bound to the Na/K cation is destabilized after Na/K cation substitution, indicating that the improving effect from Na/K-cation substitution is due to a short-range interaction. On the other hand, Mg-cation substitution affects all ,NH2 groups in the nanocluster, resulting in weakened N,H covalent bonding together with stronger ionic interactions between Li and the ,NH2 group. The present results shed light on the dehydrogenation mechanisms of metal-cation substitution in lithium amide,hydride nanoclusters and the application of (MgNH)(LiNH2)5 nanoclusters as promising hydrogen-storage media. [source] Length-dependence of electron transfer coupling matrix in polyene wires: Ab initio molecular orbital theory study,INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2009Govind Mallick Abstract The electron transfer (ET) properties of ,-electron conjugated quasi-one-dimensional molecular wires, consisting of polyene, [>CC<]n (n = 1,11), including ,-carotene, is investigated using ab initio molecular orbital theory within Koopmans theorem (KT) approach. The ET coupling matrix element, VDA, for 1,3- trans -butadiene molecule calculated with the KT approach shows excellent agreement with the corresponding results obtained with two-state model. The calculated values of VDA for the polyene oligomers exhibit exponential decrease in magnitude with increasing length of the wire. However, the decay curve exhibits three different regimes. The magnitude of the decay constant, ,, decreases with the increase in length of the wire. A highly delocalized ,-electron cloud in the polyene chain appears to facilitate retention of the electronic coupling at large separations between the donor and acceptor centers. Published 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Development of analytic energy gradient method in nuclear orbital plus molecular orbital theoryINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 14 2007Minoru Hoshino Abstract This study formulates the analytic energy gradients in the Hartree-Fock calculations of the NOMO theory, which simultaneously determines nuclear and electronic wave functions without the Born-Oppenheimer approximation. The formulations correspond to the translation- and rotation-contaminated (TRC), translation-free (TF), and translation- and rotation-free (TRF) treatments. The optimizations of the orbital centers for several diatomic molecules, which have been performed by using the analytic energy gradients, have given the averaged nuclear distances {R0} reflecting the quantum effects of nuclei and the anharmonicity of the potential energy surfaces. The numerical assessments have clarified that the effects of eliminating the translational and rotational contaminations, i.e., the TRF effects are important to improve the evaluations of {R0}, especially for the molecules including hydrogen atoms. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Unified quantum molecular dynamics method based on centroid molecular dynamics and semiempirical molecular orbital theoryINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2003Yasuhito Ohta Abstract We have developed a unified quantum molecular dynamics method, in which the centroid molecular dynamics including the quantum effect of atomic nuclei is unified with the semiempirical molecular orbital calculation for the atomic configuration generated in the molecular dynamics. This unified method has successfully been applied to a couple of diatomic molecular systems for examining its performance. The method has yielded excellent agreement with exact quantum results and taken sufficiently short CPU time for practical use. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003 [source] Parameterization of charge model 3 for AM1, PM3, BLYP, and B3LYPJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 11 2003Jason D. Thompson Abstract We have recently developed a new Class IV charge model for calculating partial atomic charges in molecules. The new model, called Charge Model 3 (CM3), was parameterized for calculations on molecules containing H, Li, C, N, O, F, Si, S, P, Cl, and Br by Hartree,Fock theory and by hybrid density functional theory (DFT) based on the modified Perdew,Wang density functional with several basis sets. In the present article we extend CM3 to semiempirical molecular orbital theory, in particular Austin Model 1 (AM1) and Parameterized Model 3 (PM3), and to the popular BLYP and B3LYP DFT and hybrid DFT methods, respectively. For the BLYP extension, we consider the 6-31G(d) basis set, and for the B3LYP extension, we consider three basis sets: 6-31G(d), 6-31+G(d), and MIDI!6D. We begin with the previous CM3 strategy, which involves 34 parameters for 30 pairs of elements. We then refine the model to improve the charges in compounds that contain N and O. This modification, involving two new parameters, leads to improved dipole moments for amides, bifunctional H, C, N, O compounds, aldehydes, ketones, esters, and carboxylic acids; the improvement for compounds not containing N results from obtaining more physical parameters for carbonyl groups when the OCN conjugation of amides is addressed in the parameterization. In addition, for the PM3 method, we added an additional parameter to improve dipole moments of compounds that contain bonds between C and N. This additional parameter leads to improved accuracy in the dipole moments of aromatic nitrogen heterocycles with five-membered rings. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1291,1304, 2003 [source] A computational study of conformational interconversions in 1,4-dithiacyclohexane (1,4-dithiane)JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 8 2003Fillmore Freeman Abstract Ab initio molecular orbital theory with the 6-31G(d), 6-31G(d,p), 6-31+G(d), 6-31+G(d,p), 6-31+G(2d,p), 6-311G(d), 6-311G(d,p), and 6-311+G(2d,p) basis sets and density functional theory (BLYP, B3LYP, B3P86, B3PW91) have been used to locate transition states involved in the conformational interconversions of 1,4-dithiacyclohexane (1,4-dithiane) and to calculate the geometry optimized structures, relative energies, enthalpies, entropies, and free energies of the chair and twist conformers. In the chair and 1,4-twist conformers the CHax and CHeq bond lengths are equal at each carbon, which suggest an absence of stereoelectronic hyperconjugative interactions involving carbon,hydrogen bonds. The 1,4-boat transition state structure was 9.53 to 10.5 kcal/mol higher in energy than the chair conformer and 4.75 to 5.82 kcal/mol higher in energy than the 1,4-twist conformer. Intrinsic reaction coordinate (IRC) calculations showed that the 1,4-boat transition state structure was the energy maximum in the interconversion of the enantiomers of the 1,4-twist conformer. The energy difference between the chair conformer and the 1,4-twist conformer was 4.85 kcal/mol and the chair-1,4-twist free energy difference (,G°c-t) was 4.93 kcal/mol at 298.15 K. Intrinsic reaction coordinate (IRC) calculations connected the transition state between the chair conformer and the 1,4-twist conformer. This transition state is 11.7 kcal/mol higher in energy than the chair conformer. The effects of basis sets on the 1,4-dithiane calculations and the relative energies of saturated and unsaturated six-membered dithianes and dioxanes are also discussed. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 909,919, 2003 [source] Experimental and theoretical investigation of a new rapid switching near-infrared electrochromic conjugated polymerJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 18 2010Han-Yu Wu Abstract A new rapid switching near-IR electrochromic conjugated propeller-shape polymer (PBTPAFL) with lower oxidation potential containing a di-triarylamine group was synthesized via Suzuki coupling approach. The observed UV-vis-NIR absorption changes in the PBTPAFL film at various potentials are fully reversible and associated with strong color changes from the original light green to dark green and then to a Prussian blue. Excellent continuous cyclic stability of the electrochromic characteristics with a rapid color switching time 2.58 s and bleaching time 1.76 s was found as well. Compared with P1 and P2, the introduction of more electron-donating propyl phenyl group in the para position of PBTPAFL lowered the oxidative potential and prevented coupling reaction during the electrochromic procedure. The high molecular weight conjugated polymer having high thermal stability with Td10 more than 450 °C has excellent solubility in common organic solvents such as NMP, THF, chloroform, toluene, xylene, and benzene at room temperature (25 °C) due to the propeller-shape structure and long alkyl chain on fluorene. Herein, from the combination of the experimental and computational study, we proposed a mechanism on the basis of the molecular orbital theory to explain the electrochromic oxidation behavior. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3913,3923, 2010. [source] Substituent effects on the photofading of disperse azo dyes on poly(ethylene terephthalate) substrateCOLORATION TECHNOLOGY, Issue 3 2010Yasuyo Okada The relationships between the chemical structures and oxidative fading of the disperse azo dyes, p -nitrophenylazo- and benzothiazoleazo-anilines, on poly(ethylene terephthalate) substrate are discussed in terms of the parameters k0,i (rate constants of reaction towards 1O2) and fi (photosensitivity), the molecular parameters of molecular orbital theory and substituents in the diazo and coupling components, on the assumption that the initial rates of oxidative fading are proportional to the product of k0,i and fi. 2-Methoxy-5-acetylamino- N -substituted aniline couplers exhibited large fi values. 2-Chloro and 4-nitro substituents of aniline diazo components exhibited small fi values or high quantum yields of internal conversion, while 4-nitro substituent did not. A close correlation between N -substituents and light fastness, proposed by Müller and supplemented by Dawson, demonstrates the applicability of frontier orbital theory, through the highest occupied molecular orbital (HOMO) energy of the dyes, to the analysis of oxidative fading. Dyes with N -2-cyanoethyl substituents, which gave a lower HOMO energy, also exhibited superior light fastness compared with N -2-hydroxyethyl substituents. [source] | |||||||||||||