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981.
Judith C. Gallucci Yukiko Tamura Leo A. Paquette 《Acta Crystallographica. Section C, Structural Chemistry》2004,60(9):o656-o658
In the Diels–Alder reaction, the preferred addition of dienes syn to the O atom in cross‐conjugated cyclohexadienones containing an oxa‐spiro ring system is observed. The two structures reported here, namely rel‐(1R,4aR,9S,9aS,10R)‐4a,9,9a,10‐tetrahydro‐9,10‐diphenylspiro[9,10‐epoxyanthracene‐1(4H),2′‐oxiran]‐4‐one, C27H20O3, and rel‐(1R,4aS,9R,9aS,10S)‐4a,9,9a,10‐tetrahydro‐9,10‐diphenylspiro[9,10‐epoxyanthracene‐1(4H),2′‐oxetane]‐4‐one, C28H22O3, are the minor and sole products, respectively, of the reactions of diphenylisobenzofuran with two slightly different cyclohexadienones. These structures differ in the size of the oxa‐spiro ring, by one C atom, and in the relative configuration at the spirocyclic ring C atom, leading to some minor conformational differences between the two compounds. 相似文献
982.
Dr. Volodymyr Baran Prof. Dr. Leo van Wüllen Prof. Dr. Thomas F. Fässler 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(19):6598-6609
An investigation into the substitution effects in Li15Si4, which is discussed as metastable phase that forms during electrochemical charging and discharging cycles in silicon anode materials, is presented. The novel partial substitution of lithium by magnesium and zinc is reported and the results are compared to those obtained for aluminum substitution. The new lithium silicides Li14MgSi4 ( 1 ) and Li14.05Zn0.95Si4 ( 2 ) were synthesized by high‐temperature reactions and their crystal structures were determined from single‐crystal data. The magnetic properties and thermodynamic stabilities were investigated and compared with those of Li14.25Al0.75Si4 ( 3 ). The substitution of a small amount of Li in metastable Li15Si4 for more electron‐rich metals, such as Mg, Zn, or Al, leads to a vast increase in the thermodynamic stability of the resulting ternary compounds. The 6,7Li NMR chemical shift and spin relaxation time T1‐NMR spectroscopy behavior at low temperatures indicate an increasing contribution of the conduction electrons to these NMR spectroscopy parameters in the series for 1 – 3 . However, the increasing thermal stability of the new ternary phases is accompanied by a decrease in Li diffusivity, with 2 exhibiting the lowest activation energy for Li mobility with values of 56, 60, and 62 kJ mol?1 for 2 , Li14.25Al0.75Si14, and 1 , respectively. The influence of the metastable property of Li15Si4 on NMR spectroscopy experiments is highlighted. 相似文献
983.
Ab initio molecular orbital theory with triple-zeta-valence plus polarization basis sets and with electron correlation incorporated at the fourth-order Møller-Plesset level has been used to study the tetrahydridoonium dications, OH4
2+, SH4
2+, and SeH4
2+. The tetrahydridoselenonium dication SeH4
2+ is predicted to have a tetrahedral (T
d
)structure, similar to OH4
2+ and SH4
2+, with short bonds to hydrogen (1.483 Å). Although deprotonation of SeH4
2+ is thermodynamically favored Cby 104 kJ mol–1), such a reaction is inhibited by a large barrier (240kJmol–1]. Thus, SeH4
2+ lies in a deep potential well and as an isolated species should have a long lifetime in the gas phase. The estimated heat of formation, H°
f
, for SeH4
2+ is very high (2483 kJ mol–1], as is the case for OH4
2+ and SH4
2+. Of the group IV onium dications (OH4
2+, SH4
2+, and SeH4
2+), SeH4
2+ displays the greatest kinetic and thermodynamic stability toward proton loss. Substantial solvent stabilization is required in order to generate SeH4
2+ in solution. 相似文献
984.
Miljanić S Frkanec L Meić Z Zinić M 《Langmuir : the ACS journal of surfaces and colloids》2005,21(7):2754-2760
Oxalyl amide derivatives bearing 4-dodecyloxy-stilbene as a cis-trans photoisomerizing unit were synthesized. The trans derivative acted as a versatile gelator of various organic solvents, whereas the corresponding cis derivative showed a poor gelation ability or none at all. In diluted solution (c = 2.0 x10(-5) mol dm(-3), ethanol), the cis isomer was photochemically converted into the trans isomer within 4 min. Depending on the radiation wavelength, the trans isomer was stable or liable to photodecomposition. When exposed to irradiation, a concentrated solution of the cis isomer (c = 2.0 x 10(-2) mol dm(-3), ethanol) turned into a gel. The FT-Raman, FT-IR, and 1H NMR spectra demonstrated that the gelation process occurred because of a rapid cis --> trans photoisomerization followed by a self-assembly of the trans molecules. Apart from the formation of hydrogen bonding between the oxalyl amide parts of the molecules, confirmed by FT-IR spectroscopy, it was assumed that the pi-pi stacking between the trans-stilbene units of the molecule and a lipophilic interaction between long alkyl chains were the interactions responsible for gelation. 相似文献
985.
Christopher J. Parkinson Paul M. Mayer Leo Radom 《Theoretical chemistry accounts》1999,102(1-6):92-96
Stabilization energies for the 1-cyanovinyl radical (CH2=CCN) have been calculated using a variety of conventional ab initio (M?ller–Plesset, quadratic configuration interaction
and coupled-cluster) and density functional theory (B-LYP, B3-LYP) procedures, as well as with a range of compound methods.
Compared with a high-level benchmark value (that predicts a stabilization energy of 17.1 kJ mol−1), UMP2 and UMP4 give the wrong sign and magnitude of the stabilization energy (both methods predicting desta- bilization
instead of stabilization), while B-LYP and B3-LYP overestimate the degree of stabilization. The RMP2, RMP4, QCISD(T) and CCSD(T)
techniques, and several, but not all, variants of G2 and CBS theories give radical stabilization energies in good agreement
with the benchmark value.
Received: 15 June 1998 / Accepted: 19 August 1998 / Published online: 15 February 1999 相似文献
986.
Sanna Jääskeläinen Chandra S. Verma Roderick E. Hubbard Leo S. D. Caves 《Theoretical chemistry accounts》1999,101(1-3):175-179
The conformational change associated with the interfacial activation of Rhizomucor miehei lipase involves the displacement of an α-helical lid (residues 82–96) away from the active site on moving from water (high
dielectric) to lipid (low dielectric). The presence of two media of very different dielectric properties suggests that electrostatic
interactions play an important role in this process. We have used linearized Poisson–Boltzmann calculations to examine the
key electrostatic interactions which contribute to lid stability in the closed and open states. It is the two charged residues
of the lid, Arg86 and Asp91, that form the strongest electrostatic interactions with the rest of the protein. We identify
key residues whose interactions with the lid are significantly perturbed by the change in the dielectric of the medium: Asp61,
Arg80, Lys109, Glu117 and the active-site residues Asp203 and Asp256, all of which lie within approximately 20 ? of the lid.
We suggest that these residues are good candidates for site-specific mutation studies, which could help elucidate their role
in the lipase activation mechanism.
Received: 27 May 1998 / Accepted: 17 September 1998 / Published online: 7 December 1998 相似文献
987.
AbstractSelective binding of fluoride anion and its alkali metal cation ion pairs by a fluorescent calix[4]arene (L) derivative bearing tryptophan moieties was studied in acetonitrile at 25 °C. It was found that LF? and LF22? complexes were formed and their stability constants were determined by means of UV and 1H NMR titrations. Both amide and indole NH groups were involved in the stabilisation of the fluoride complexes, i.e. L afforded two anion-binding sites. 1H NMR titrations provided evidence of simultaneous complexation of both Na+ cation and fluoride anion resulting in the formation of a ternary NaLF complex. In this ion pair complex, the Na+ cation was most probably bound primarily by the calixarene ether and amide oxygen atoms, whereas the indole NH groups interacted with the fluoride ion. A highly favourable ion pairing between Na+ and F– in acetonitrile was studied using potentiometric measurements. The results pointed out the importance of fluoride pairing with alkali metal cations in aprotic organic solvents and the necessity of taking these reactions into account in the course of speciation studies of such solutions. 相似文献
988.
Vladislav Tomii Nives Gali Branimir Bertoa Leo Frkanec Vladimir Simeon Mladen
ini 《Journal of inclusion phenomena and macrocyclic chemistry》2005,53(3):263-268
Complexation of alkali metal cations with 5,11,17,23-tetra-tert-butyl-26,28,25,27-tetrakis(O-methyl-D-α-phenylglycylcarbonylmethoxy)calix[4]arene (L) was studied by means of spectrophotometric, conductometric and potentiometric titrations at 25 °C. The solvent effect on the binding ability of L was examined by using two solvents with different affinities for hydrogen bonding, viz. methanol and acetonitrile. Despite the presence of intramolecular NH···O=C hydrogen bonds in L, which need to be disrupted to allow metal ion binding, this calix[4]arene amino acid derivative was shown to be an efficient binder for smaller Li+ and Na+ cations in acetonitrile (lg KLiL > 5, lg KNaL = 7.66), moderately efficient for K+ (lg KKL = 4.62), whereas larger Rb+ and Cs+ did not fit in its hydrophilic cavity. The complex stabilities in methanol were significantly lower (lg KNaL = 4.45, lg KKL = 2.48). That could be explained by different solvation of the cations and by competition between the cations and methanol molecules (via hydrogen bonds) for amide carbonyl oxygens. The influence of cation solvation on complex stability was most pronounced in the case of Li+ for which, contrary to the quite stable LiL+ complex in acetonitrile, no complexation was observed in methanol under the conditions used. 相似文献
989.
H. Ackermann G. Geiseler K. Harms R. Leo W. Massa F. Weller K. Dehnicke 《无机化学与普通化学杂志》1999,625(9):1500-1506
Triethylphosphanimine Complexes of the Acetates of Copper(II) and Zinc. Crystal Structures of [Zn(O2C–CH3)2(HNPEt3)], [Cu5(O2C–CH3)10(HNPEt3)2], and [Cu(O2C–CH3)2(HNPEt3)2] The title compounds originate from the anhydrous acetates of zinc and copper(II) with trimethylsilyl-triethylphosphanimine, Me3SiNPEt3, in the presence of water in dichloromethane. They form colourless ( 1 ), bluish-green ( 2 ), and blue ( 3 ), respectively, single crystals, which were characterized by IR spectroscopy and by crystal structure analyses. [Zn(O2C–CH3)2(HNPEt3)] ( 1 ): Space group P 4 21c, Z = 8, lattice dimensions at –83 °C: a = b = 1709.6(2), c = 982.4(1) pm, R = 0.0551. 1 has a polymeric chain structure in which the zinc atoms are μ2-bridged via the oxygen atoms of one of the two acetato groups, while the second acetato group and the phosphanimine are bonded terminally. [Cu5(O2C–CH3)10(HNPEt3)2]( 2 · 4 CH2Cl2): Space group P21/c, Z = 8, lattice dimensions at –80 °C: a = 1761.18(13), b = 4074.5(2), c = 1733.34(15) pm, β = 91.383(10)°, R = 0.0413. 2 consists of the two structural units [Cu2(O2C–CH3)4] and [Cu3(O2C–CH3)6(HNPEt3)2], which are connected via two of the acetato groups of the Cu3-unit along the crystallographic a-axis to form three crystallographically independent polymeric strands. [Cu(O2C–CH3)2(HNPEt3)2] ( 3 ): Space group P21/n, Z = 2, lattice dimensions at 20 °C: a = 695.49(8), b = 1217.85(10), c = 1380.05(7) pm, β = 96.451(7)°, R = 0.0291. 3 forms monomeric, centrosymmetric molecules with a square planar environment at the Cu atoms. 相似文献
990.