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71.
Aloe plants have been widely documented in artists’ treatises dating from the sixteenth to the nineteenth century as a source
of colorant to achieve lustrous golden glazes on tin- and silver-foiled objects and warm-toned finishes on musical instruments,
such as violins. Aloe extracts contain characteristic anthraquinone and phenolic components which impart a distinctive orange
tone and fluorescence to mixtures containing them. Because of the low concentration of colorant in the coatings and its probable
degradation by high temperature during manufacture, the identification of aloe in heated oil–resin mixtures represents an
analytical challenge. For this reason, the possible presence of aloe in glazes and coatings has been largely overlooked. This
paper describes various analytical approaches to the identification of aloe in historic samples, from comparison with results
obtained from reference standards and mock-up samples. Complementary analytical techniques including thermally assisted hydrolysis
and methylation–gas chromatography–mass spectrometry, high-performance liquid chromatography, laser desorption–mass spectrometry,
matrix-assisted laser desorption-ionization-mass spectrometry and surface-enhanced Raman scattering were used. Different chemical
markers were identified by the individual methods and the advantages and limitations of each technique for the identification
of aloe in oil–resin varnishes are discussed. 相似文献
72.
UV and IR spectroscopic studies of cold alkali metal ion-crown ether complexes in the gas phase 总被引:1,自引:0,他引:1
Inokuchi Y Boyarkin OV Kusaka R Haino T Ebata T Rizzo TR 《Journal of the American Chemical Society》2011,133(31):12256-12263
We report UV photodissociation (UVPD) and IR-UV double-resonance spectra of dibenzo-18-crown-6 (DB18C6) complexes with alkali metal ions (Li(+), Na(+), K(+), Rb(+), and Cs(+)) in a cold, 22-pole ion trap. All the complexes show a number of vibronically resolved UV bands in the 36,000-38,000 cm(-1) region. The Li(+) and Na(+) complexes each exhibit two stable conformations in the cold ion trap (as verified by IR-UV double resonance), whereas the K(+), Rb(+), and Cs(+) complexes exist in a single conformation. We analyze the structure of the conformers with the aid of density functional theory (DFT) calculations. In the Li(+) and Na(+) complexes, DB18C6 distorts the ether ring to fit the cavity size to the small diameter of Li(+) and Na(+). In the complexes with K(+), Rb(+), and Cs(+), DB18C6 adopts a boat-type (C(2v)) open conformation. The K(+) ion is captured in the cavity of the open conformer thanks to the optimum matching between the cavity size and the ion diameter. The Rb(+) and Cs(+) ions sit on top of the ether ring because they are too large to enter the cavity of the open conformer. According to time-dependent DFT calculations, complexes that are highly distorted to hold metal ions open the ether ring upon S(1)-S(0) excitation, and this is confirmed by extensive low-frequency progressions in the UVPD spectra. 相似文献
73.
Kristensen K Kauczor J Thorvaldsen AJ Jørgensen P Kjærgaard T Rizzo A 《The Journal of chemical physics》2011,134(21):214104
Damped response theory is applied to the calculation of two-photon absorption (TPA) spectra, which are determined directly, at each frequency, from a modified damped cubic response function. The TPA spectrum may therefore be evaluated for selected frequency ranges, making the damped TPA approach attractive for calculations on large molecules with a high density of states, where the calculation of TPA using standard theory is more problematic. Damped response theory can also be applied to the case of intermediate state resonances, where the standard TPA expression is divergent. Both exact damped response theory and its application within density functional theory are discussed. The latter is implemented using an atomic-orbital based density matrix formulation, which makes the approach especially suitable for studies on large systems. A test preliminary study is presented for the TPA spectrum of R-(+)-1,1'-bi(2-naphtol). 相似文献
74.
Sánchez FG Díaz AN Algarra M Lovillo J Aguilar A 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2011,83(1):88-93
The effect of different solvents on the fluorescent properties of 2-(dimethylamine)fluorene (DAF) were studied. In aprotic solvents we detected a strongly emissive intramolecular charge transfer (ICT) state that decayed by intersystem crossing to triplet. In proton-accepting solvents DAF exhibits in the excited state an intramolecular proton transfer. An ionized species is postulated, which simultaneously twists to a rotated conformation in the excited state. Thus, the specific solvent interactions supplement but do not replace the twist mechanism and accompany the charge transfer accepted as the prerequisite for twisted intramolecular charged transfer (TICT) state formation. 相似文献
75.
Radovan Bast Kenneth Ruud Antonio Rizzo Trygve Helgaker 《Theoretical chemistry accounts》2011,129(3-5):685-699
We present the first relativistic study of the electric-field-gradient induced birefringence (Buckingham birefringence), with application to the series of molecules CX2 (X?=?O, S, Se, Te). A recently developed atomic-orbital-driven scheme for the calculation of time-dependent molecular properties using one-, two- and four-component relativistic wave functions (Bast et?al. in Chem Phys 356:177, 2009) is extended to first-order frequency-dependent magnetic-field perturbations, using London atomic orbitals to ensure gauge-origin independent results and to improve basis-set convergence. Calculations are presented at the Hartree?CFock and Kohn?CSham levels of theory and results for CO2 and CS2 are compared with previous high-level coupled-cluster calculations. Except for the heaviest member of the series, relativistic effects are small??in particular for the temperature-independent contribution to the birefringence. By contrast, the effects of electron correlation are significant. However, the reliability of standard exchange-correlation functionals in describing Buckingham birefringence remains unclear based on the comparison with high-level coupled-cluster singles-and-doubles calculations. 相似文献
76.
We have applied crystal structure prediction methods to understand and predict the formation of a DMSO solvate of the anti-convulsant drug carbamazepine (CBZ), in which the DMSO molecules are disordered. Crystal structure prediction calculations on the 1:1 CBZ:DMSO solvate revealed the generation of two similar low energy structures which differ only in the orientation of the DMSO molecules. Analysis of crystal energy landscapes generated at 0 K suggests the possibility of solvent disorder. A combined computational and experimental study of the changes in the orientation of the DMSO within the crystal structure revealed that the nature of the disorder changes with temperature. At low temperature, the DMSO disorder is static whilst at high temperature the DMSO configurations can interconvert by a 180° rotation of the DMSO molecules within the lattice. This 180° rotation of the DMSO molecules drives a phase change from a high temperature dynamically disordered phase to a low temperature phase with static disorder. Crystallisation of a DMSO solvate of the related molecule epoxycarbamazepine resulted in a different degree of DMSO disorder in the crystal structure, despite the similarity of the carbamazepine and epoxycarbamazepine molecules. We believe consideration of disorder and its contribution to entropy and crystal free energies at temperature other than 0 K is fundamental for the accuracy of future energy rankings in crystal structure prediction calculations of similar solvated structures. 相似文献
77.
Dr. Valérie Alain‐Rizzo Delphine Drouin‐Kucma Cédric Rouxel Dr. Issa Samb Jérémy Bell Dr. Patrick Y. Toullec Dr. Véronique Michelet Dr. Isabelle Leray Dr. Mireille Blanchard‐Desce 《化学:亚洲杂志》2011,6(4):1080-1091
A series of rod‐shaped and related three‐branched push–pull derivatives containing phosphane oxide or phosphane sulfide (PO or PS)—as an electron‐withdrawing group conjugated to electron‐donating groups, such as amino or ether groups, with a conjugated rod consisting of arylene–vinylene or arylene–ethynylene building blocks—were prepared. These compounds were efficiently synthesized by a Grignard reaction followed by Sonogashira coupling. Their photophysical properties including absorption, emission, time‐resolved fluorescence, and two‐photon absorption (TPA) were investigated with special attention to structure–property relationships. These fluorophores show high fluorescence quantum yields and solvent‐dependent experiments reveal that efficient intramolecular charge transfer occurs upon excitation, thereby leading to highly polar excited states, the polarity of which can be significantly enhanced by playing on the end groups and conjugated linker. Rod‐shaped and related three‐branched systems show similar fluorescence properties in agreement with excitation localization on one of the push–pull branches. By using stronger electron donors or replacing the arylene–ethynylene linkers with an arylene–vinylene one induces significant redshifts of both the low‐energy one‐photon absorption and TPA bands. Interestingly, a major enhancement in TPA responses is observed, whereas OPA intensities are only weakly affected. Similarly, phosphane oxide derivatives show similar OPA responses than the corresponding sulfides but their TPA responses are significantly larger. Finally, the electronic coupling between dipolar branches promoted by common PO or PS acceptor moieties induces either slight enhancement of the TPA responses or broadening of the TPA band in the near infrared (NIR) region. Such behavior markedly contrasts with triphenylamine‐core‐mediated coupling, which gives evidence for the different types of interactions between branches. 相似文献
78.
Moment redistribution in continuous reinforced concrete beams strengthened with carbon-fiber-reinforced polymer laminates 总被引:1,自引:0,他引:1
The results of tests on continuous steel-fiber-reinforced concrete (RC) beams, with and without an external strengthening,
are presented. The internal flexural steel reinforcement was designed so that to allow steel yielding before the collapse
of the beams. To prevent the shear failure, steel stirrups were used. The tests also included two nonstrengthened control
beams; the other specimens were strengthened with different configurations of externally bonded carbon-fiber-reinforced polymer
(CFRP) laminates. In order to prevent the premature failure from delamination of the CFRP strengthening, a wrapping was also
applied. The experimental results obtained show that it is possible to achieve a sufficient degree of moment redistribution
if the strengthening configuration is chosen properly, confirming the results provided by two simple numerical models.
Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 43, No. 5, pp. 667–686, September–October, 2007. 相似文献
79.
Summary Analytic equations of the multiconfigurational SCF (MCSCF) response theory are combined with the finite-field (FF) approach to compute static and frequency dependent electric and magnetic properties of the Argon atom. A complete active space (CAS SCF) function including the 3s, 3p, 3d, 4s and 4p orbitals in the active space and a large (17s 13p 7d 5f 3g) basis set are employed. This permits an accurate determination of various linear and non-linear response properties such as e.g. electric dipole polarisability and second hyperpolarisability, Verdet constant, magnetisability and second hyperpolarisability. The results, both for the static values and for the frequency dependence of these properties, compare well with other most recent experimental and theoretical data.Dedicated to Jan Linderberg on the occasion of his 60th birthday 相似文献
80.
3-Hydroxy-1-phenyl-2-pyrazolin-5-one 1 reacts with primary, secondary and allylic alcohols under catalytic acidic conditions and in the presence of 3 Å molecular sieves, to afford 3-alkoxy-1-phenyl-2-pyrazolin-5-ones 3 . 相似文献