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81.
Summary Energy extrapolation techniques in conjunction with individual configuration selection are applied to averaged coupled-pair functional expansions. In order to test the quality of this approach, benchmark calculations have been performed for N2, the open and ring forms of O3, and for the ground and several excited states of CuH and PdH. Reliable energy estimates are obtained for N2 and the two transition metal hydrides and spectroscopic properties are in close agreement with the values for the non-truncated expansions. In the case of O3 the perturbation corrections substantially underestimate the complete singles and doubles results. These deviations cancel to a large extent, however, in the calculated isomerization energy. The accuracy of the one-particle density matrix is examined by computing dipole moments for several electronic states of CuH and PdH. Deviations are significant in some cases. For the evaluation of properties the current approach requires modifications.Dedicated to Prof. W. Kutzelnigg on the occasion of his sixtieth birthday 相似文献
82.
Klaus Schaefers Christel Schneider 《Journal of polymer science. Part A, Polymer chemistry》1985,23(12):2879-2895
The monomer initiator t-butyl-p-vinylperbenzoate (TBVP) was synthesized and its homo- and copolymerization with styrene, methacrylonitrile, isoprene and phenylmethacrylate was investigated. TBVP is preponderantly incorporated in all four systems; values of Q = 2.27 and e = ?0.13 characterize TBVP as a weak donor with a high monomer reactivity. Kinetic considerations show that all propagation steps in which TBVP takes part are faster or at least as fast as the self-propagation steps of the comonomers used. Rate constants and activation energies of the thermal decomposition of poly(t-butyl-p-vinylperbenzoate) and poly(t-butyl-p-vinylperbenzoate-co-styrene) were determined. The polymer initiators were used for grafting experiments with styrene, acrylonitrile, and methyl methacrylate. The data evaluated for the grafting efficiency, the success of grafting, and the degree of grafting show that these polymers of p-vinylperbenzoate are efficient initiators. 相似文献
83.
Maurizot V Dolain C Leydet Y Léger JM Guionneau P Huc I 《Journal of the American Chemical Society》2004,126(32):10049-10052
A new strategy is proposed to control the relative orientation of two folded helical oligomers in such a way that they diverge from an aromatic linker and have opposite helical handedness. Mutual steric exclusion between the two helices results from the fact that they cannot be at the same time folded and on the same side of the linker. The concept is validated using the helical conformations of oligoamides of 8-amino-2-quinolinecarboxylic acid, but it should be applicable to many families of oligomers and leads to the first designed meso-helices. 相似文献
84.
Starting from the equilibrium mixture of cis- and trans-1-bromo-1-propene, isomeric mixtures of compounds Men Sn(CH=GHMe)4-n (n = 0–3) have been prepared and studied. While proton NMR only allows distinction between the methyltin signals of the various isomers (except where n = 3), the 13C spectra show separate signals for almost all isomeric carbons even when n = 0. In the 119Sn spectra the signals due to the various isomers are separated by ca. 20 ppm for a given value of n; the peak areas can be used to estimate the proportions of cis- and trans-propenyl residues present in the mixtures. Addition of 2-bromo-propene to the starting 1-bromo-1-propenes leads to the formation of further isomers, which can in all cases be observed and identified in the 119Sn spectra; 119Sn shifts can be calculated using the shifts for the Me3SnC3H5 isomers as increments. 相似文献
85.
Bernd A. Hess Robert J. Buenker Christel M. Marian Sigrid D. Peyerimhoff 《Chemical physics letters》1982,89(6):459-462
First-order 2Π zero-field splittings are computed for the H2S+, BO, NO, CH, CO+ and OH ground electronic states by employing ab initio multi-configuration wavefunctions and by including all one-and two-electron spin-orbit interactions in a gaussian AO basis explicitly in the theoretical treatment. 相似文献
86.
We present first-principles molecular dynamics simulations of azobenzene and a sterically hindered derivative in the first excited state. The restricted open-shell Kohn-Sham (ROKS) approach is employed to describe the motion in the lowest excited state. The rotational pathway is observed in the molecular dynamics simulations for both azobenzene and its azacrown ether capped derivative. 相似文献
87.
We present molecular-dynamics simulations for 1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and cyclopentadiene at finite temperature using periodic boundary conditions. These systems form weakly bound aggregates in the ground state and exhibit bond formation in the excited state. Monomeric excitation of an ensemble of butadiene molecules leads to a transfer of the excitation between two molecules in the excited state with an intermediate delocalization of the wave function over both moieties. 相似文献
88.
Marian CM 《The Journal of chemical physics》2005,122(10):104314
Combined density functional and multireference configuration interaction methods have been used to calculate the electronic spectrum of 9H-adenine, the most stable tautomer of 6-aminopurine. In addition, constrained minimum energy paths on excited potential energy hypersurfaces have been determined along several relaxation coordinates. The minimum of the first (1)[n-->pi*] state has been located at an energy of 4.54 eV for a nuclear arrangement in which the amino group is pyramidal whereas the ring system remains planar. Close by, another minimum on the S(1) potential energy hypersurface has been detected in which the C(2) center is deflected out of the molecular plane and the electronic character of S(1) corresponds to a nearly equal mixture of (1)[pi-->pi*] and (1)[n-->pi*] configurations. The adiabatic excitation energy of this minimum amounts to 4.47 eV. Vertical and adiabatic excitation energies of the lowest n-->pi* and pi-->pi* transitions as well as transition moments and their directions are in very good agreement with experimental data and lend confidence to the present quantum chemical treatment. On the S(1) potential energy hypersurface, an energetically favorable path from the singlet n-->pi* minimum toward a conical intersection with the electronic ground state has been identified. Close to the conical intersection, the six-membered ring of adenine is strongly puckered and the electronic structure of the S(1) state corresponds to a pi-->pi* excitation. The energetic accessibility of this relaxation path at about 0.1 eV above the singlet n-->pi* minimum is presumably responsible for the ultrafast decay of 9H-adenine after photoexcitation and explains why sharp vibronic peaks can only be observed in a rather narrow wavelength range above the origin. The detected mechanism should be equally applicable to adenosine and 9-methyladenine because it involves primarily geometry changes in the six-membered ring whereas the nuclear arrangement of the five-membered ring (including the N(9) center) is largely preserved. 相似文献
89.
Sension RJ Harris DA Stickrath A Cole AG Fox CC Marsh EN 《The journal of physical chemistry. B》2005,109(38):18146-18152
Femtosecond to nanosecond transient absorption spectroscopy is used to investigate the photolysis of 5'-deoxyadenosylcobalamin (coenzyme B12, AdoCbl) bound to glutamate mutase. The photochemistry of AdoCbl is found to be inherently dependent upon the environment of the cofactor. Excitation of AdoCbl bound to glutamate mutase results in formation of a metal-to-ligand charge transfer intermediate state which decays to form cob(II)alamin with a time constant of 105 ps. This observation is in contrast to earlier measurements in water where the photohomolysis proceeds through an intermediate state in which the axial dimethylbenzimidazole ligand appears to have dissociated, and measurements in ethylene glycol where prompt bond homolysis is observed (Yoder, L. M.; Cole, A. G.; Walker, L. A., II; Sension, R. J. J. Phys. Chem. B 2001, 105, 12180-12188). The quantum yield for formation of stable radical pairs in the enzyme is found to be phi = 0.05 +/- 0.03, and the resulting intrinsic rate constants for geminate recombination and "cage escape" are 1.0 +/- 0.1 and 0.05 +/- 0.03 ns(-1), respectively. The rate constant for geminate recombination is 30% less than that observed for AdoCbl in water or ethylene glycol. This reduction is insufficient to account for the 10(12)-fold increase in the homolysis rate observed when substrate is bound to the protein. Finally, the protein provides a cage to prevent diffusive loss of the adenosyl radical; however, the ultimate yield for long-lived radicals is determined by the evolution from a singlet to a triplet radical pair as proposed for AdoCbl in ethylene glycol. 相似文献
90.
Stephan Schulz Tillmann Bauer Jörn Schmedt auf der Günne Christel M. Marian 《Journal of solid state chemistry》2008,181(3):530-538
Ammonolysis of the monomeric, base-stabilized trisaminoalane Me3N-Al[N(H)Dipp)]3 (Dipp=2,6-iPr2-C6H3) yielded Al-N oligomers, which were characterized in detail by solid state NMR spectroscopy (1H, 13C, 15N, 27Al) and TGA/DTA. Pyrolysis of as-prepared oligomers at different temperatures in an argon steam yielded carbon-containing black solids, whereas pyrolysis under a steady flow of NH3 produced pure aluminum nitride (AlN). The role of the pyrolysis temperature and the influence of NH3 on the formation of crystalline materials were investigated. As-prepared AlN was characterized by solid state NMR spectroscopy (15N, 27Al), X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Theoretical calculations were performed in order to identify potential reaction intermediates. 相似文献