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31.
Pickard FC Shepherd RL Gillis AE Dunn ME Feldgus S Kirschner KN Shields GC Manoharan M Alabugin IV 《The journal of physical chemistry. A》2006,110(7):2517-2526
We present a detailed theoretical study of geometries, electronic structure, and energies of transition states and intermediates completing the full Bergman cycloaromatization pathway of ortho-substituted enediynes with a focus on polar and steric contributions to the kinetics and thermodynamics of hydrogen abstraction. This study provides a rare unambiguous example of remote substitution that affects reactivity of a neutral reactive intermediate through an sigma framework. 相似文献
32.
P. Atkinson M.B. Ward S.P. Bremner D. Anderson T. Farrow G.A.C. Jones A.J. Shields D.A. Ritchie 《Physica E: Low-dimensional Systems and Nanostructures》2006,32(1-2):21
Conventional electron-beam lithographic patterning of GaAs substrates followed by reactive-ion etching of small holes has been successfully used to control the nucleation of InAs dots. We have observed >50% single dot occupancy for holes wide and deep and show that the dot occupancy and dot size can be varied by changing the size of the holes. Luminescence from an array of these site-controlled dots has been demonstrated. Thus this use of substrate patterning is a viable technique to controllably place single dots at pre-determined positions in devices. 相似文献
33.
Patel RB Bennett AJ Cooper K Atkinson P Nicoll CA Ritchie DA Shields AJ 《Physical review letters》2008,100(20):207405
We report an electrically driven semiconductor single-photon source capable of emitting photons with a coherence time of up to 400 ps under fixed bias. It is shown that increasing the injection current causes the coherence time to reduce, and this effect is well explained by the fast modulation of a fluctuating environment. Hong-Ou-Mandel-type two-photon interference using a Mach-Zehnder interferometer is demonstrated using this source to test the indistinguishability of individual photons by postselecting events where two photons collide at a beam splitter. Finally, we consider how improvements in our detection system can be used to achieve a higher interference visibility. 相似文献
34.
Caitlin E. Shields Dr. Xue Wang Dr. Thomas Fellowes Rob Clowes Dr. Linjiang Chen Prof. Graeme M. Day Prof. Anna G. Slater Dr. John W. Ward Dr. Marc A. Little Prof. Andrew I. Cooper 《Angewandte Chemie (International ed. in English)》2023,62(34):e202303167
Hydrogen-bonded organic frameworks (HOFs) with low densities and high porosities are rare and challenging to design because most molecules have a strong energetic preference for close packing. Crystal structure prediction (CSP) can rank the crystal packings available to an organic molecule based on their relative lattice energies. This has become a powerful tool for the a priori design of porous molecular crystals. Previously, we combined CSP with structure-property predictions to generate energy-structure-function (ESF) maps for a series of triptycene-based molecules with quinoxaline groups. From these ESF maps, triptycene trisquinoxalinedione (TH5) was predicted to form a previously unknown low-energy HOF (TH5-A) with a remarkably low density of 0.374 g cm−3 and three-dimensional (3D) pores. Here, we demonstrate the reliability of those ESF maps by discovering this TH5-A polymorph experimentally. This material has a high accessible surface area of 3,284 m2 g−1, as measured by nitrogen adsorption, making it one of the most porous HOFs reported to date. 相似文献
35.
Dunn ME Shields GC Takahashi K Skodje RT Vaida V 《The journal of physical chemistry. A》2008,112(41):10226-10235
In this study we present the gas-phase vibrational spectrum of vinylacetic acid with a focus on the nu = 1-5 vibrational states of the OH stretching transitions. Cross sections for nu = 1, 2, 4 and 5 of the OH stretching vibrational transitions are derived on the basis of the vapor pressure data obtained for vinylacetic acid. Ab initio calculations are used to assist in the band assignments of the experimental spectra, and to determine the threshold for the decarboxylation of vinylacetic acid. When compared to the theoretical energy barrier to decarboxylation, it is found that the nu OH = 4 transition with thermal excitation of low frequency modes or rotational motion and nu OH = 5 transitions have sufficient energy for the reaction to proceed following overtone excitation. 相似文献
36.
Gaussian-3 and MP2/aug-cc-pVnZ methods have been used to calculate geometries and thermochemistry of CS(2)(H2O)n, where n=1-4. An extensive molecular dynamics search followed by optimization using these two methods located two dimers, six trimers, six tetramers, and two pentamers. The MP2/aug-cc-pVDZ structure matched best with the experimental result for the CS(2)(H2O) dimer, showing that diffuse functions are necessary to model the interactions found in this complex. For larger CS(2)(H2O)n clusters, the MP2/aug-cc-pVDZ minima are significantly different from the MP2(full)6-31G* structures, revealing that the G3 model chemistry is not suitable for investigation of sulfur containing van der Waals complexes. Based on the MP2/aug-cc-pVTZ free energies, the concentration of saturated water in the atmosphere and the average amount of CS(2) in the atmosphere, the concentrations of these clusters are predicted to be on the order of 10(5) CS(2)(H2O) clusters.cm(-3) and 10(2) CS(2)(H2O)(2) clusters.cm(-3) at 298.15 K. The MP2/aug-cc-pVDZ scaled harmonic and anharmonic frequencies of the most abundant dimer cluster at 298 K are presented, along with the MP2/aug-cc-pVDZ scaled harmonic frequencies for the CS(2)(H(2)O)(n) structures predicted to be present in a low-temperature molecular beam experiment. 相似文献
37.
The middle levels problem is to find a Hamilton cycle in the middle levels, M2k+1, of the Hasse diagram of (the partially-ordered set of subsets of a 2k+1-element set ordered by inclusion). Previously, the best known, from [I. Shields, C.D. Savage, A Hamilton path heuristic with applications to the middle two levels problem, in: Proceedings of the Thirtieth Southeastern International Conference on Combinatorics, Graph Theory, and Computing (Boca Raton, FL, 1999), vol. 140, 1999], was that M2k+1 is Hamiltonian for all positive k through k=15. In this note we announce that M33 and M35 have Hamilton cycles. The result was achieved by an algorithmic improvement that made it possible to find a Hamilton path in a reduced graph (of complementary necklace pairs) having 129,644,790 vertices, using a 64-bit personal computer. 相似文献
38.
Complete basis set and Gaussian‐n methods were combined with Barone and Cossi's implementation of the polarizable conductor model (CPCM) continuum solvation methods to calculate pKa values for six carboxylic acids. Four different thermodynamic cycles were considered in this work. An experimental value of ?264.61 kcal/mol for the free energy of solvation of H+, ΔGs(H+), was combined with a value for Ggas(H+) of ?6.28 kcal/mol, to calculate pKa values with cycle 1. The complete basis set gas‐phase methods used to calculate gas‐phase free energies are very accurate, with mean unsigned errors of 0.3 kcal/mol and standard deviations of 0.4 kcal/mol. The CPCM solvation calculations used to calculate condensed‐phase free energies are slightly less accurate than the gas‐phase models, and the best method has a mean unsigned error and standard deviation of 0.4 and 0.5 kcal/mol, respectively. Thermodynamic cycles that include an explicit water in the cycle are not accurate when the free energy of solvation of a water molecule is used, but appear to become accurate when the experimental free energy of vaporization of water is used. This apparent improvement is an artifact of the standard state used in the calculation. Geometry relaxation in solution does not improve the results when using these later cycles. The use of cycle 1 and the complete basis set models combined with the CPCM solvation methods yielded pKa values accurate to less than half a pKa unit. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 相似文献
39.
Fluorescence at 490 nm from the triatomic excimer Xe2Cl* has been investigated to determine the 308 nm absorption due to this species in an x-ray preionized, self-sustained gas discharge XeCl laser. The dependence of Xe2Cl* density on laser intensity (at 308 nm), buffer gas and Xe and HCl partial pressures has been determined for discharges with a peak electrical power deposition of 2.5 GWl–1. Xe2Cl* absorption is estimated to reach 0.6% cm–1 under non-lasing conditions but decreases to a non-saturable 0.2% cm–1 for intracavity laser intensity>1 MW cm–2. XeCl* and Xe2Cl* fluorescence intensities were found to be a similar for both helium and neon buffer gases but laser output was a factor of two greater with a neon buffer. 相似文献
40.