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81.
Krogh-Jespersen K Stibrany RT John E Westbrook JD Emge TJ Clarke MJ Potenza JA Schugar HJ 《Inorganic chemistry》2008,47(21):9813-9827
Five distorted-octahedral complexes containing (NH3)5Ru(III)L ions, where L = 2,4-dihydroxybenzoate or a xanthine, have been studied using a combination of X-ray crystallography, solution and polarized single-crystal electronic absorption spectroscopy, and first principles electronic structure computational techniques. Both yellow (2) and red (3) forms of the complex (NH3)5Ru(III)L, where L = 2,4-dihydroxybenzoate, as well as three xanthine complexes in which L = hypoxanthine-kappaN(7) (4), 7-methylhypoxanthine-kappaN(9) (5), and 1,3,9-trimethylxanthine-kappaN(7) (6) were examined. In the solid state, some of these complexes exhibit split low-energy ligand-to-metal charge-transfer (LMCT) bands. Traditional solid-state effects, such as ligand pi-pi overlap or hydrogen bonding that might lead to splitting of electronic absorption bands, were probed in an attempt to identify the origins of these unusual observations. For comparison, companion studies were carried out for spectroscopically normal reference complexes of the same ligands. Time-dependent density-functional theory (TD-DFT) calculations, employing modified B3LYP-type functionals with increased contributions of exact exchange, attribute the color change in the crystalline complexes 2 and 3 to pi(ligand) --> Ru[d(pi)] LMCT bands which, in the red form (3), arise from ligand donor pi-orbitals split by strongly overlapping phenyl moieties in centrosymmetric (NH3)5Ru(III)(2,4-dihydroxybenzoate) dimers. Complex 5 does not show split visible absorptions, whereas both the polarizations and energies of the split visible absorptions shown by 4 and 6 also suggest assignment as LMCT. No support is found for relating the split absorptions of 4 and 6 to the details of pi-pi xanthine overlap in the solid state; indeed, complex 4 enjoys considerably less pi-stacking overlap than does 5. We feel compelled to attribute the split absorptions in crystalline 4 and 6 to the emergence of a LMCT transition originating in the carbonyl lone pair, potentially deriving intensity from the significant intramolecular N-H...O hydrogen bonding present in both 4 and 6 (but not in 5). The electronic structure calculations suggest an O(n) --> Ru[d(sigma*)] LMCT transition; however, this novel assignment must be considered tentative. 相似文献
82.
We present what we believe to be the first direct measurements of enhanced nonlinearities in large-mode-area fibers due to bend induced reductions in effective area. Both Raman scattering and self-phase modulation are observed to increase in tightly coiled fibers. The measured increase in nonlinearity compares well with predictions from simulations of the modal effective area. 相似文献
83.
We show that fiber Bragg gratings can extend an optical continuum to spectral regions where continuum generation is very weak. Highly nonlinear fibers with Bragg grating resonances at 700, 750, and 800 nm were pumped with 70 fs pulses at 1580 nm and exhibited enhancement peaks up to 25 dB above the extremely weak continuum at these wavelengths, normally more than 40 dB below the average power in the continuum. We show that the grating peaks may be computed by treating the continuum pulse as an undepleted pump and including the grating dispersion as a phase-matching term. 相似文献
84.
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86.
M.F. Campbell D.F. Davidson R.K. Hanson C.K. Westbrook 《Proceedings of the Combustion Institute》2013,34(1):419-425
Ignition delay times for methyl oleate (C19H36O2, CAS: 112-62-9) and methyl linoleate (C19H34O2, CAS: 112-63-0) were measured for the first time behind reflected shock waves, using an aerosol shock tube. The aerosol shock tube enabled study of these very-low-vapor-pressure fuels by introducing a spatially-uniform fuel aerosol/4% oxygen/argon mixture into the shock tube and employing the incident shock wave to produce complete fuel evaporation, diffusion, and mixing. Reflected shock conditions covered temperatures from 1100 to 1400 K, pressures of 3.5 and 7.0 atm, and equivalence ratios from 0.6 to 2.4. Ignition delay times for both fuels were found to be similar over a wide range of conditions. The most notable trend in the observed ignition delay times was that the pressure and equivalence ratio scaling were a strong function of temperature, and exhibited cross-over temperatures at which there was no sensitivity to either parameter. Data were also compared to the biodiesel kinetic mechanism of Westbrook et al. (2011) [10], which underpredicts ignition delay times by about 50%. Differences between experimental and computed ignition delay times were strongly related to existing errors and uncertainties in the thermochemistry of the large methyl ester species, and when these were corrected, the kinetic simulations agreed significantly better with the experimental measurements. 相似文献
87.
We report significant enhancement (+24 dB) of the optical beat note between a 657 nm cw laser and the second-harmonic generation of the tailored continuum at 1314 nm generated with a femtosecond Cr:forsterite laser and a nonlinear fiber Bragg grating. The same continuum is used to stabilize the carrier-envelope offset frequency of the Cr:forsterite femtosecond laser and permits improved optical stabilization of the frequency comb from 1.0 to 2.2 microm. Using a common optical reference at 657 nm, a relative fractional frequency instability of 2.0 x 10(-15) is achieved between the repetition rates of Cr:forsterite and Ti:sapphire laser systems in 10 s averaging time. The fractional frequency offset between the optically stabilized frequency combs of the Cr:forsterite and Ti:sapphire lasers is +/-(0.024 +/- 6.1) x 10(-17). 相似文献
88.
Jaskula JC Bonneau M Partridge GB Krachmalnicoff V Deuar P Kheruntsyan KV Aspect A Boiron D Westbrook CI 《Physical review letters》2010,105(19):190402
We demonstrate sub-Poissonian number differences in four-wave mixing of Bose-Einstein condensates of metastable helium. The collision between two Bose-Einstein condensates produces a scattering halo populated by pairs of atoms of opposing velocities, which we divide into several symmetric zones. We show that the atom number difference for opposing zones has sub-Poissonian noise fluctuations, whereas that of nonopposing zones is well described by shot noise. The atom pairs produced in a dual number state are well adapted to sub-shot-noise interferometry and studies of Einstein-Podolsky-Rosen-type nonlocality tests. 相似文献
89.
John D. Westbrook Ronald M. Levy Karsten Krogh-Jespersen 《Journal of computational chemistry》1992,13(8):979-989
Procedures have been developed to generate molecular electrostatic potentials based on correlated wave function from ab initio or semiempirical electronic structure programs. A new algorithm for point-wise sampling of the potential is described and used to obtain partial atomic charges via a linear, least squares fit between classical and quantum mechanical electrostatic potentials. The proposed sampling algorithm is efficient and promises to introduce less rotational variance in the potential derived partial charges than algorithms applied previously. Electrostatic potentials and fitted atomic charges from ab initio (HF/6–31G* and MP2/6-31G*) and semiempirical (INDO/S; HF, SECI, and SDCI) wave functions are presented for the electronic ground (S0) and excited (1Lb, 1La) states of 3-methylindole. © 1992 by John Wiley & Sons, Inc. 相似文献
90.
Douglas B. Kitchen Fumio Hirata John D. Westbrook Ronald Levy David Kofke Martin Yarmush 《Journal of computational chemistry》1990,11(10):1169-1180
Molecular dynamics simulations have been carried out for a series of systems of increasing complexity including: pure water, a model polypeptide (α-helical decaglycine) in vacuo, a protein (Pancreatic Trypsin Inhibitor, PTI) in vacuo, and a fully solvated protein (PTI in water). The equations of motion were integrated using Andersen's velocity version of the Verlet algorithm with internal contraints (the RATTLE algorithm). The accuracy with which the equations of motion are integrated has been analyzed for several different simulation conditions. The effects of various nonbonded interaction truncation schemes on the conservation of energy have been examined, including the use of atomic cutoffs, and (neutral group) residue cutoffs. The use of a smoothing function to eliminate the discontinuities in the potential at the cutoff leads to a significant improvement in the accuracy of the integration for each of the systems studied. The accuracy with which the equations of motion are integrated using the RATTLE algorithm for pure water and for the solvated protein are found to be comparable when the nonbonded interactions are tapered with a smoothing function at the cutoff. 相似文献