排序方式: 共有102条查询结果,搜索用时 15 毫秒
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da Costa LM Hayaki S Stoyanov SR Gusarov S Tan X Gray MR Stryker JM Tykwinski R Carneiro JW Sato H Seidl PR Kovalenko A 《Physical chemistry chemical physics : PCCP》2012,14(11):3922-3934
We applied a multiscale modeling approach that involves the statistical-mechanical three-dimensional reference interaction site model with the Kovalenko-Hirata closure approximation (3D-RISM-KH molecular theory of solvation) as well as density functional theory (DFT) of electronic structure to study the role of water in aggregation of the asphaltene model compound 4,4'-bis(2-pyren-1-yl-ethyl)-2,2'-bipyridine (PBP) [X. Tan, H. Fenniri and M. R. Gray, Energy Fuels, 2008, 22, 715]. The solvation free energy and potential of mean force predicted by 3D-RISM-KH reveal favorable pathways for disaggregation of PBP dimers in pure versus water-saturated chloroform solvent. The water density distribution functions elucidate hydrogen bonding preferences and water bridge formation between PBP monomers. The ΔG(298) values of -5 to -7 kcal mol(-1) for transfer of water molecules in chloroform to a state interacting with PBP molecules are in agreement with experimental results. Geometry optimization and thermochemistry analysis of PBP dimers with and without water bridges using WB97Xd/6-31G(d,p) predict that both PBP dimerization and dimer stabilization by water bridges are spontaneous (ΔG(298) < 0). The (1)H NMR chemical shifts of PBP monomers and dimers predicted using the gauge-independent atomic orbital method and polarizable continuum model for solvation in chloroform are in an excellent agreement with the experimental results for dilute and concentrated PBP solutions in chloroform, respectively [X. Tan, H. Fenniri and M. R. Gray, Energy Fuels, 2009, 23, 3687]. The DFT calculations of PBP dimers with explicit water show that bridges containing 1-3 water molecules lead to stabilization of PBP dimers. Additional water molecules form hydrogen bonds with these bridges and de-shield the PBP protons, negating the effect of water on the (1)H(C3) NMR chemical shift of PBP, in agreement with experiment. The ΔG(298) results show that hydrogen bonding to water and water-promoted polynuclear assembly bridging is as important as π-π interactions for asphaltene aggregation. 相似文献
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VG Adlakha RR Vemuganti 《International Journal of Mathematical Education in Science & Technology》2013,44(5):708-713
In many practical situations scaling the data is necessary to solve linear programs. This note explores the relationships in translating the sensitivity analysis between the original and the scaled problems. 相似文献
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Slepkov AD Hegmann FA Tykwinski RR Kamada K Ohta K Marsden JA Spitler EL Miller JJ Haley MM 《Optics letters》2006,31(22):3315-3317
We present ultrafast z-scan measurements of the two-photon absorption (TPA) spectra of a pair of two-dimensionally conjugated quadrupolar donor/acceptor (D/A) chromophores. The all-donor substituted species displays a peak TPA cross section [sigma(2)=520+/-30 GM] that is more than twice that of the D-A species [sigma(2)=240+/-20 GM]. Unlike previous structure-property studies that have evaluated TPA behavior for D/A molecules through the comparison of dipolar and quadrupolar compounds, both molecules investigated herein are quadrupolar, ultimately providing a more consistent evaluation of the effects of donor and/or acceptor substitution on the TPA of conjugated chromophores. 相似文献
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Sarah N. Spisak Martina U. Bühringer Zheng Wei Zheng Zhou Rik R. Tykwinski Marina A. Petrukhina 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(7):2045-2050
The chemical reduction of a [3]cumulene ([3]TrTol) has been explored using alkali metals. Mono‐ and doubly reduced forms of [3]TrTol were isolated as solvent‐separated ion pairs with {Na(18‐crown‐6)THF2}+ and {K(18‐crown‐6)THF2}+ counterions and crystallographically characterized. This allowed analysis of structural parameters of the “naked” anions of [3]TrTol without interference from metal binding. The dianion of [3]TrTol was also isolated as a contact‐ion complex with {Cs(18‐crown‐6)}+ cations, thereby adding the effect of metal coordination to the core. Structural comparisons of anions to the neutral molecule, [3]TrTol, outline monotonic increases in bond‐length alternation (BLA) upon stepwise reduction. The greatest BLA value is found for the contact‐ion complex, which shows an alternating sequence of short and long carbon–carbon bonds, consistent with the structure of an alkyne. In contrast to studies on tetraphenyl[3]cumulene, the cumulenic framework of [3]TrTol remains planar in all the derivatives. 相似文献
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Eisler S Chahal N McDonald R Tykwinski RR 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(11):2542-2550
The synthesis of conjugated polyyne structures via a modification of the Fritsch-Buttenberg-Wiechell (FBW) rearrangement is reported. Our adaptation provides for the 1,2-migration of an alkyne in a carbene/carbenoid intermediate that is conveniently effected via lithium-halogen exchange with the appropriate dibromo-olefinic precursor. This rearrangement is quite rapidly accomplished under mild conditions (hexane solution, -78 degrees C), and the seemingly high migratory aptitude of the alkynyl moiety provides for efficient rearrangement. This, in turn, allows for multiple rearrangements in a single molecule, greatly facilitating the construction of highly unsaturated substrates. This procedure is exploited for the rapid synthesis of symmetrical and unsymmetrical 1,3,5-hexatriynes, extended polyynes, and aryl polyyne building blocks. Most significantly, many of these structures have been or would be difficult to access via more traditional transition metal catalyzed homo- or cross-coupling techniques. 相似文献
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Michael Franz Dr. Johanna A. Januszewski Dominik Wendinger Dr. Christian Neiss Dr. Levon D. Movsisyan Dr. Frank Hampel Prof. Dr. Harry L. Anderson Prof. Dr. Andreas Görling Prof. Dr. Rik R. Tykwinski 《Angewandte Chemie (International ed. in English)》2015,54(22):6645-6649
The stabilization of long [n]cumulenes has traditionally been achieved by placing sterically bulky “protecting groups” at the termini, which shield the reactive carbon chain from unwanted reactions. Herein, we present an alternative strategy: stabilization through threading the sp‐hybridized carbon chain through a phenanthroline‐based macrocycle. The result is stable [9]cumulene rotaxanes that enable the study of properties as a function of length for [n]cumulenes in unprecedented detail, including by quantitative UV/Vis spectroscopy, cyclic voltammetry, and differential scanning calorimetry. The experimental results are supported by DFT calculations. 相似文献
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A divergent, one-pot synthesis of functionalized polyynes has been developed. Beginning with the appropriately substituted dibromoolefinic precursor, a carbenoid Fritsch-Buttenberg-Wiechell (FBW) rearrangement is used to generate the lithium acetylide of a conjugated polyyne framework, and subsequent trapping with carbon-based electrophiles provides for in situ formation of a wide range of di- and triynes. The lithium acetylide formed from the FBW reaction can also undergo transmetalation to provide the corresponding zinc, copper, tin, or platinum acetylides, leading to the divergent formation of symmetrical and unsymmetrical conjugated acetylenes, as well as ynones. 相似文献