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1.
Kato H Brink M Möllerstedt H Piqueras MC Crespo R Ottosson H 《The Journal of organic chemistry》2005,70(23):9495-9504
[Figure: see text]. A quantum chemical study has been performed to assess changes in aromaticity along the T1 state Z/E-isomerization pathways of annulenyl-substituted olefins. It is argued that the point on the T1 energy surface with highest substituent aromaticity corresponds to the minimum. According to Baird (J. Am. Chem. Soc. 1972, 94, 4941), aromaticity and antiaromaticity are interchanged when going from S0 to T1. Thus, olefins with S0 aromatic substituents (set A olefins) will be partially antiaromatic in T1 and vice versa for olefins with S0 antiaromatic substituents (set B olefins). Twist of the C=C bond to a structure with a perpendicular orientation of the 2p(C) orbitals (3p*) in T1 should lead to regaining substituent aromaticity in set A and loss of aromaticity in set B olefins. This hypothesis is verified through quantum chemical calculations of T1 energies, geometries (bond lengths and harmonic oscillator measure of aromaticity), spin densities, and nucleus independent chemical shifts whose differences along the T1 PES display zigzag dependencies on the number of -electrons in the annulenyl substituent of the olefin. Aromaticity changes are reflected in the profiles of the T1 potential energy surfaces (T1 PESs) for Z/E-isomerizations because olefins in set A have minima at 3p* whereas those in set B have maxima at such structures. The proper combination (fusion) of the substituents of set A and B olefins could allow for design of novel optical switch compounds that isomerize adiabatically with high isomerization quantum yields. 相似文献
2.
[n.n.n]Propellanes and bicyclo[n.n.n]alkanes (1=n=3) in which the bridgeheads and bridges are composed of different Group 14 elements will not be ideal homologues of the all-carbon propellanes and bicycloalkanes. We used quantum-chemical calculations to conclude which of these could possibly be stable at ambient temperature. Structures, strain, and dimerization energies of [n.n.n]propellanes and bicyclo[n.n.n]alkanes with C-based tethers and with Si, Ge, or Sn at the bridgehead positions were calculated. The aptitude to add water was probed in selected cases. Comparisons were made with the analogous all-C propellanes and bicycloalkanes. All heavy 1,(n+2)-dimetallabicyclo[n.n.n]alkanes should be thermally stable and suitable synthetic targets, even though the moisture sensitivity of 1,3-dimetallabicyclo[1.1.1]pentanes increases as Group 14 is descended. For 1,(n+2)-dimetalla[n.n.n]propellanes, the incompatibility between the tether and M--M bond lengths impacts strongly on structure and stability. At ambient temperature 1,3-disila[1.1.1]propellanes should oligomerize instantly upon formation, whereas 1,3-digerma- and 1,3-distanna[1.1.1]propellanes should be less prone to oligomerize and possibly isolable. The 1,4-dimetalla[2.2.2]propellanes with M=Si-Sn have biradical ground states and can presumably only be observed under inert conditions at cryogenic temperatures. Finally, we predict that 1,5-disila[3.3.3]propellane will be persistent at ambient temperature, but 1,5-distanna[3.3.3]propellane should rapidly oligomerize. 相似文献
3.
Ottosson N Børve KJ Spångberg D Bergersen H Sæthre LJ Faubel M Pokapanich W Öhrwall G Björneholm O Winter B 《Journal of the American Chemical Society》2011,133(9):3120-3130
The local electronic structure of glycine in neutral, basic, and acidic aqueous solution is studied experimentally by X-ray photoelectron spectroscopy and theoretically by molecular dynamics simulations accompanied by first-principle electronic structure and spectrum calculations. Measured and computed nitrogen and carbon 1s binding energies are assigned to different local atomic environments, which are shown to be sensitive to the protonation/deprotonation of the amino and carboxyl functional groups at different pH values. We report the first accurate computation of core-level chemical shifts of an aqueous solute in various protonation states and explicitly show how the distributions of photoelectron binding energies (core-level peak widths) are related to the details of the hydrogen bond configurations, i.e. the geometries of the water solvation shell and the associated electronic screening. The comparison between the experiments and calculations further enables the separation of protonation-induced (covalent) and solvent-induced (electrostatic) screening contributions to the chemical shifts in the aqueous phase. The present core-level line shape analysis facilitates an accurate interpretation of photoelectron spectra from larger biomolecular solutes than glycine. 相似文献
4.
Pokapanich W Kryzhevoi NV Ottosson N Svensson S Cederbaum LS Öhrwall G Björneholm O 《Journal of the American Chemical Society》2011,133(34):13430-13436
Auger electron spectroscopy combined with theoretical calculations has been applied to investigate the decay of the Ca 2p core hole of aqueous Ca(2+). Beyond the localized two-hole final states on the calcium ion, originating from a normal Auger process, we have further identified the final states delocalized between the calcium ion and its water surroundings and produced by core level intermolecular Coulombic decay (ICD) processes. By applying the core-hole clock method, the time scale of the core level ICD was determined to be 33 ± 1 fs for the 2p core hole of the aqueous Ca(2+). The comparison of this time constant to those associated with the aqueous K(+), Na(+), Mg(2+), and Al(3+) ions reveals differences of 1 and up to 2 orders of magnitude. Such large variations in the characteristic time scales of the core level ICD processes is qualitatively explained by different internal decay mechanisms in different ions as well as by different ion-solvent distances and interactions. 相似文献
5.
The absorption spectra and excited state dipole moments of four differently substituted fulvenes have been investigated both experimentally and computationally. The results reveal that the excited state dipole moment of fulvenes reverses in the first excited singlet state when compared to the ground state. The oppositely polarized electron density distributions, which dominate the ground state and the first excited singlet state of fulvenes, respectively, reflect the reversed π-electron counting rules for aromaticity in the two states (4n + 2 vs. 4n, respectively). The results show that substituents indeed influence the polarity of fulvenes in the two states, however, cooperative interactions between the substituents and the fulvene moiety are most pronounced in the ground state. 相似文献
6.
Toom L Kütt A Kaljurand I Leito I Ottosson H Grennberg H Gogoll A 《The Journal of organic chemistry》2006,71(19):7155-7164
Basicity constants for a series of 3,7-diazabicyclo[3.3.1]nonane derivatives in acetonitrile with a variation over 13 orders of magnitude have been determined using a spectrophotometric titration technique. An excellent correlation between basicity and calculated proton affinities obtained at PCM-B3LYP/6-31+G(d)//B3LYP/6-31G(d) level was found. The results are discussed in terms of substituent effects and compared to (15)N NMR chemical shifts. 相似文献
7.
Epitaxial NiO (1 1 1) and NiO (1 0 0) films have been grown by atomic layer deposition on both MgO (1 0 0) and α-Al2O3 (0 0 l) substrates at temperatures as low as 200 °C by using bis(2,2,6,6-tetramethyl-3,5-heptanedionato)Ni(II) and water as precursors. The films grown on the MgO (1 0 0) substrate show the expected cube on cube growth while the NiO (1 1 1) films grow with a twin rotated 180° on the α-Al2O3 (0 0 l) substrate surface. The films had columnar microstructures on both substrate types. The single grains were running throughout the whole film thickness and were significantly smaller in the direction parallel to the surface. Thin NiO (1 1 1) films can be grown with high crystal quality with a FWHM of 0.02–0.05° in the rocking curve measurements. 相似文献
8.
The strain energies and through-space distances between the two bridgehead E atoms of a selection of 1,3-dimethyl-1,3-ditetrelbicyclo[1.1.1]pentanes
(tetrel E = Si, Ge or Sn) were examined by quantum chemical calculations at MP2 and B3LYP levels. The aim is to identify which
bridges lead to short through-space E,E distances, and simultaneously, to as low strain as possible. A short E,E distance
should improve through-space interaction, and a low strain should promote the thermal stability and possibly also facilitate
their synthesis. The bridges examined included CH2, CMe2, CtBu2, C(CH2)n (n = 2–4), O, NMe, S, PMe, SiMe2, GeMe2, and SnMe2. The calculations indicate that the phospha bridge is a good compromise providing reasonably low strain as well as E,E through-space
distances which are only longer than normal E–E single bonds by factors of 1.06–1.10.
This paper is dedicated to Professor Mitsuo Kira in recognition of his stimulating Si chemistry and his 2005 Wacker Award. 相似文献
9.
The extraordinary opportunities offered by integrating solution chemistry of molecular entities with the solid-state nature of the gel provide the basis for designing a number of novel molecular materials. Herein, we present a strategy based on encapsulation of suitable response active species to impart useful optical properties to sol–gel glasses. The basic concept of this molecular programming approach is based on deliberate incorporation of response-active species in the silica gel framework to elicit specific optical responses. Design of molecular materials for device applications depends on selection of molecules which exhibit well-defined electronic or optical response, and assembly of these molecular components into a geometric structure that retains the rigidity, addressability, and stability necessary for practical applications. The approach is based on using molecules as active species and sol–gel glass as structural matrix in which the molecules are selectively integrated. A designer approach that employs specific molecules for generating optical signals is described. As such the properties of these silica-based glasses can be tuned by varying the composition of encapsulated species. These modified glasses exhibit substantially altered optical properties as compared to pristine silica sol–gels. The optical response of these materials provide initial examples toward designing novel materials whose optical and/or photonic responses can be modulated by structural integration of specific dopant entities. 相似文献
10.
Gunnar Westin Annika Pohl Mikael Ottosson Koroush Lashgari Kjell Jansson 《Journal of Sol-Gel Science and Technology》2008,48(1-2):194-202
Manganate and cobaltate perovskites having the general formula ABO3 have many technologically important applications. Here we present all alkoxide based routes to manganate and coboltate perovskite films and nano-phase powders of the compositions; LCMO (La0.67Ca0.33MnO3), LSMO (La0.75Sr0.25MnO3), LNCMO (La0.33Nd0.33Ca0.33MnO3), LBSMO (La0.75Ba0.125Sr0.125MnO3) and LSCO (La0.50Sr0.50CoO3). The Mn and Co (oxo) methoxy-ethoxide precursors were prepared by reaction of MnCl2 or CoCl2 with 2 Kmoe in methoxy-ethanol-toluene. After hydrolysis of the alkoxide solutions by atmospheric air all systems produced X-ray and electron diffraction amorphous gels of high elemental homogeneity, and the IR spectra showed that they consisted of hydrated oxo-carbonates. Heating in air resulted in similar weight-loss curves for all studied gels passing; loss of H2O in the range 20–300 °C, decomposition of carbonate groups into oxide and CO2 in the temperature range 300–700 °C, and in some cases loss of a small amount of oxygen in the temperature range 700–1,000 °C. The pure perovskites were obtained at 690–770 °C with heat rates of typically 5–20 °C min?1 without annealing. Perovskites could also be obtained at 550 °C by annealing, but these perovskites are prone to be A-site ion inhomogeneous according to the TEM EDS studies, which was not the case for the perovskites heat-treated to at least the carbonate decomposition temperature. This A-site inhomogeneity is ascribed to sequential decomposition of carbonates due to their different thermal stabilities, which is probably a general feature also with other sol–gel precursors and low temperature annealing. High quality polycrystalline films were prepared on Si/SiO2/TiO2/Pt substrates with all compositions and high quality epitaxial films were prepared of LCMO (on 100 LaAlO3) and LSCO (on 100 SrTiO3). The colossal magneto resistance (CMR) of the epitaxial LCMO films of 32% (246 K) is in parity with PLD derived films. The conductivities of the epitaxial and polycrystalline LSCO films were 1.9 mΩcm (on STO) and 1.7 mΩcm (on α-Al2O3, respectively). 相似文献