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141.
Lewis WK Lindsay CM Bemish RJ Miller RE 《Journal of the American Chemical Society》2005,127(19):7235-7242
Electron impact ionization of a helium atom in a helium nanodroplet is followed by rapid charge migration, which can ultimately result in the localization of the charge on an atomic or molecular solute. This process is studied here for the cases of hydrogen cyanide, acetylene, and cyanoacetylene in helium, using a new experimental method we call optically selected mass spectrometry (OSMS). The method combines infrared laser spectroscopy with mass spectrometry to separate the contributions to the overall droplet beam mass spectrum from the various species present under a given set of conditions. This is done by vibrationally exciting a specific species that exists in a subset of the droplets (for example, the droplets containing a single HCN molecule). The resulting helium evaporation leads to a concomitant reduction in the ionization cross sections for these droplets. This method is used to study the charge migration in helium and reveals that the probability of charge transfer to a solvated molecule does not approach unity for small droplets and depends on the identity of the solvated molecule. The experimental results are explained quantitatively by considering the effect of the electrostatic potential (between the charge and the embedded molecule) on the trajectory of the migrating charge. 相似文献
142.
Two new sesquiterpenoids of the unprecedented neoiemnane ring system, and one eremophflane-derived sesquiterpene diol have been isolated from the pacific soft-coral Lemnalia africana. The structure of the novel neolemnane compounds, 4 and 5, were confirmed by X-ray crystallography, and the diol, 6, has been described based upon spectral analysis and chemical modification. 相似文献
143.
Abdel-Magid AF Carson KG Harris BD Maryanoff CA Shah RD 《The Journal of organic chemistry》1996,61(11):3849-3862
Sodium triacetoxyborohydride is presented as a general reducing agent for the reductive amination of aldehydes and ketones. Procedures for using this mild and selective reagent have been developed for a wide variety of substrates. The scope of the reaction includes aliphatic acyclic and cyclic ketones, aliphatic and aromatic aldehydes, and primary and secondary amines including a variety of weakly basic and nonbasic amines. Limitations include reactions with aromatic and unsaturated ketones and some sterically hindered ketones and amines. 1,2-Dichloroethane (DCE) is the preferred reaction solvent, but reactions can also be carried out in tetrahydrofuran (THF) and occasionally in acetonitrile. Acetic acid may be used as catalyst with ketone reactions, but it is generally not needed with aldehydes. The procedure is carried out effectively in the presence of acid sensitive functional groups such as acetals and ketals; it can also be carried out in the presence of reducible functional groups such as C-C multiple bonds and cyano and nitro groups. Reactions are generally faster in DCE than in THF, and in both solvents, reactions are faster in the presence of AcOH. In comparison with other reductive amination procedures such as NaBH(3)CN/MeOH, borane-pyridine, and catalytic hydrogenation, NaBH(OAc)(3) gave consistently higher yields and fewer side products. In the reductive amination of some aldehydes with primary amines where dialkylation is a problem we adopted a stepwise procedure involving imine formation in MeOH followed by reduction with NaBH(4). 相似文献
144.
The crystal structures of the high-temperature modifications of sodium and silver orthophosphates have been determined using powder neutron diffraction (PND) data. II-Na3PO4 adopts the space group Fm3m with
at 400°C. The PO3−4 group is centered around the origin, but it shows high orientational disorder. The sodium ions occupy the
and
sites. II-Ag3PO4, at 650°C, is similar with
. The structure of I-Ag3PO4 at room temperature
has been re-examined by single-crystal X-ray diffraction. The derived model, with R=0.019 for 116 independent reflections, is in agreement with the latest work reported in the literature. The structure of I-Ag3PO4 at 375°C, as determined by PND, has
, and displays no gross modifications from that observed at 25°C, although the anisotropic nature of the silver sites is markedly more pronounced at this higher temperature. The cation mobility is discussed in relation to the high-temperatures structures. 相似文献
Full-size image (<1K)
145.
Mononuclear gold complexes bonded to TiO(2) were synthesized from Au(CH(3))(2)(C(5)H(7)O(2)), and their decomposition and conversion into gold nanoclusters on the TiO(2) surface were characterized by time-resolved X-ray absorption and infrared spectroscopies as the temperature of the sample in flowing helium was ramped up. Mass spectra of the evolved gases were also measured during this process. The results show (a) the onset of formation of CH(4) as a decomposition product, (b) the reduction of Au(III) to Au(0), and (c) the formation of Au-Au bonds, all occurring in approximately the same temperature range (about 335-353 K), indicating that the reduction and aggregation of the supported gold are simultaneous processes facilitated by the removal of methyl ligands initially bonded to the gold. IR spectra recorded during the treatment indicate that water on the TiO(2) surface may be involved in the process by reacting with methyl groups bonded to Au(III) to give CH(4). 相似文献
146.
We have synthesized a 22 residue alanine-based peptide with a tris(bipyridyl)ruthenium(II) amino acid near the middle of the peptide which can act as a photoinducible electron donor. Two histidines spaced i, i + 4 near the C-terminus of the peptide were then cross-linked with a tetraammineruthenium(III) moiety to prenucleate the helix and provide an electron acceptor site. Introduction of the cross-link enhances the average helix content from 67% to 84% at 0 degrees C, as judged by circular dichroism spectroscopy. The temperature dependence of the mean molar residue ellipticity at 222 nm, [THETAV;](222), for the bimetalated peptide was fit to a modified Lifson-Roig helix-coil model to permit extraction of the population of helical conformation at each residue separating the electron donor and acceptor. On average, the residues between the donor and acceptor are 92% helical. Photoinduced electron transfer with a driving force of -1.0 eV and an estimated reorganization energy of 0.82 eV was measured by fluorescence quenching methods in H(2)O and D(2)O, yielding rate constants, k(ET), of 7 +/- 3 x 10(6) s(-)(1) and 5 +/- 1 x 10(6) s(-)(1) at 0 degrees C. Calculation of the electronic coupling matrix element, H(ab), with the Marcus equation yields a value of 0.19 +/- 0.4 cm(-)(1). Analysis in terms of the pathway model for electronic coupling indicates that this magnitude of H(ab) is consistent with the participation of hydrogen bonds in electronic coupling for an isolated alpha-helix. 相似文献
147.
The interaction of aspartic acid with kaolinite was studied by potentiometric titrations and by adsorption measurements both at constant aspartate concentration (but varying pH) and at a constant pH of 5.5. The temperature was 25 degrees C, and the ionic medium 5 mM KNO3. Aspartic acid dissociation constants estimated from titrations agreed with those from the literature. The adsorption of aspartic acid to kaolinite was weak and varied only slightly with pH; 10-18% of 100 microM aspartic acid adsorbed to kaolinite at 100 m(2)L(-1) between pH 3 and 10. Data from the titrations and adsorption experiments were fitted closely by an extended constant-capacitance surface complexation model, in which monodentate outer-sphere complexes formed between deprotonated aspartic acid molecules and protonated sites on the variable-charge edges of the kaolinite crystals. There appeared to be no adsorption to the permanently charged crystal faces. 相似文献
148.
The nickelacarboranes [NEt(4)][2-(eta(3)-C(3)H(4)R)-closo-2,1,7-NiC(2)B(9)H(11)] (R = H (1a), Ph (1b)) have been synthesized via reaction between [Na](2)[nido-7,9-C(2)B(9)H(11)] and [Ni(2)(micro-Br)(2)(eta(3)-C(3)H(4)R)(2)] in THF (THF = tetrahydrofuran), followed by addition of [NEt(4)]Cl. Protonation of 1a in the presence of a donor ligand L affords the complexes [2,2-L(2)-closo-2,1,7-NiC(2)B(9)H(11)] (L = CO (2), CNBu(t) (3)). Addition of PEt(3) (1 equiv) to 2 produces quantitative conversion to [2-CO-2-PEt(3)-closo-2,1,7-NiC(2)B(9)H(11)], 4. Species 2-4 exhibit in solution hindered rotation of the NiL(2) fragment with respect to the eta(5)-C(2)B(9) cage unit. Protonation of 1a in the presence of a diene affords the neutral complexes [2-(eta(2):eta(2)-diene)-closo-2,1,7-NiC(2)B(9)H(11)] (diene = C(5)Me(5)H (5), dcp (6), cod (7), nbd (8), chd (9), and cot (10a); dcp = dicyclopentadiene, cod = 1,5-cyclooctadiene, nbd = norbornadiene, chd = 1,3-cyclohexadiene, and cot = cyclooctatetraene). Variable temperature (1)H NMR experiments show that the [Ni(diene)] fragments are freely rotating even at 193 K. A small quantity of the di-cage species [2,2'-micro-(1,2:5,6-eta-3,4:7,8-eta-cot)-(closo-2,1,7-NiC(2)B(9)H(11))(2)] (10b) is formed as a coproduct in the synthesis of 10a. This species can be rationally synthesized by protonation of 1a and subsequent addition of 10a. 相似文献
149.
Graham A. Bowmaker Prof. Dr. Michael I. Bruce Brian W. Skelton Neil Somers Allan H. White 《无机化学与普通化学杂志》2007,633(7):1024-1030
The complexes [Cu2Br4]2?, [Cu2I4]2?, [Cu2I2Br2]2?, [Cu2I3Cl]2?, [Ag2Cl4]2? have been characterized as their isomorphous bis(triphenylphosphoranylidene)ammonium ([Ph3PNPPh3]+ = PNP+) salts by single crystal structural determinations. All anions show the centrosymmetric doubly halogen‐bridged forms [XM(μ‐X)2MX]2? with three‐coordinate metal atoms that have been observed in [M2X4]2? complexes with other large organic cations. In [Cu2I2Br2]2? the iodide ligands occupy the bridging positions and the bromide the terminal positions, while in [Cu2I3Cl]2?, obtained in an attempt to prepare [Cu2I2Cl2]2?, two of the iodide ligands occupy the bridging positions with the third iodide and the chloride ligand occupying two statistically disordered terminal positions. In [Ag2Cl4]2? the distortion from ideal trigonal coordination of the metal atom is greater than in the copper complexes, but less than in other previously reported [Ag2Cl4]2? complexes with organic cations. The ν(MX) bands have been assigned in the far‐IR spectra, and confirm previous observations regarding the unexpectedly simple IR spectra of [Cu2X4]2? complexes. 相似文献
150.
Paul RL Argent SP Jeffery JC Harding LP Lynam JM Ward MD 《Dalton transactions (Cambridge, England : 2003)》2004,(21):3453-3458
Reaction of the bis-bidentate bridging ligand L(3), in which two bidentate chelating 3(2-pyridyl)pyrazole units are separated by a 3,3'-biphenyl spacer, with Co(II) salts affords tetranuclear cage complexes of composition [Co(4)(L(3))(6)]X(8)(X =[BF(4)](-), [ClO(4)](-), [PF(6)](-) or I(-)) in which four 6-coordinate Co(II) ions in an approximately tetrahedral array are connected by six bis-bidentate bridging ligands, one spanning each of the six edges of the Co(4) tetrahedron. In every case, X-ray crystallography reveals that the 'apical' Co(II) ion has a fac tris-chelate geometry, whereas the other three Co(II) ions have mer tris-chelate geometries, resulting in (non-crystallographic)C(3) symmetry for the cages; that this structure is retained in solution is confirmed by (1)H NMR spectroscopy of the paramagnetic cages. In every case one of the anions is located inside the central cavity of the cage, with the remaining seven outside. We found no clear evidence for an anion-based templating effect. The cage superstructure is sufficiently large to leave gaps in the centres of the faces through which the internal and external anions can exchange. Variable-temperature (19)F NMR spectroscopy was used to investigate the dynamic behaviour of the cages with X =[BF(4)](-) and [PF(6)](-) in MeCN solution: in both cases two separate signals, corresponding to external and internal anions, are clear at 233 K which have coalesced to a single signal at room temperature. Analysis of the linewidth of the minor signal (for the internal anion) at various temperatures below coalescence gave an activation energy for anion exchange of ca. 50 kJ mol(-1) in each case, a figure which suggests that anion exchange can occur via a conformational rearrangement of the cage superstructure in solution rather than opening of the cavity by cleavage of metal-ligand bonds. 相似文献