215 research outputs found
HAB79: A New Molecular Dataset for Benchmarking DFT and DFTB Electronic Couplings Against High-Level Ab-initio Calculations
A new molecular dataset called HAB79 is introduced to provide ab-initio reference values for electronic couplings (transfer integrals) and to benchmark density functional theory (DFT) and density functional tight-binding (DFTB) calculations. The HAB79 dataset is comprised of 79 planar heterocyclic polyaromatic hydrocarbon molecules frequently encountered in organic (opto)electronics, arranged to 921 structurally diverse dimer configurations. We show that CASSCF/NEVPT2 with a minimal active space provides a robust reference method that can be applied to the relatively large molecules of the dataset. Electronic couplings are largest for cofacial dimers, in particular sulfur-containing polyaromatic hydrocarbons, with values in excess of 0.5 eV, followed by parallel displaced cofacial dimers. V-shaped dimer motifs, often encountered in the herringbone layers of organic crystals, exhibit medium-sized couplings whereas T-shaped dimers have the lowest couplings. DFT values obtained from the projector operator-based diabatization (POD) method are initially benchmarked against the smaller databases HAB11 (HAB7-) and found to systematically improve when climbing Jacob's ladder, giving mean relative unsigned errors (MRUE) of 27.7% (26.3%) for the GGA functional BLYP, 20.7% (15.8%) for hybrid functional B3LYP and 5.2% (7.5%) for the long-range corrected hybrid functional omega-B97X. Cost effective POD in combination with a GGA functional (PBE) and very efficient DFTB calculations on the dimers of the HAB79 database give a good linear correlation with the CASSCF/NEVPT2 reference data, which, after scaling with a multiplicative constant, gives reasonably small MRUEs of 17.9% and 40.1%, respectively, bearing in mind that couplings in HAB79 vary over 4 orders of magnitude
Nature of the bonding in metal-silane Ï-complexes
The nature of metal silane Ï-bond interaction has been investigated in several key systems by a range of experimental and computational techniques. The structure of [CpâČMn(CO)2(η2-HSiHPh2)] 1 has been determined by single crystal neutron diffraction, and the geometry at the Si atom is shown to approximate a trigonal bipyramid; salient bond distances and angles are MnâH(1) 1.575(14), SiâH(1) 1.806(14), SiâH(2) 1.501(13) Ă
, and H(1)âSiâH(2) 148.5(8)°. This complex is similar to [CpâČMn(CO)2(η2-HSiFPh2)] 2, whose structure and bonding characteristics have recently been determined by charge density studies based on high-resolution X-ray and neutron diffraction data. The geometry at the Si atom in these Ï-bond complexes is compared with that in other systems containing hypercoordinate silicon. The MnâH distances for 1 and 2 in solution have been estimated using NMR T1 relaxation measurements, giving a value of 1.56(3) Ă
in each case, in excellent agreement with the distances deduced from neutron diffraction. Density functional theory calculations have been employed to explore the bonding in the MnâHâSi unit in 1 and 2 and in the related system [CpâČMn(CO)2(η2-HSiCl3)] 3. These studies support the idea that the oxidative addition of a silane ligand to a transition metal center may be described as an asymmetric process in which the MnâH bond is formed at an early stage, while both the establishment of the MnâSi bond and also the activation of the η2-coordinated SiâH moiety are controlled by the extent of Mn â Ï*(XâSiâH) back-donation, which increases with increasing electron-withdrawing character of the X substituent trans to the metal-coordinated SiâH bond. This delocalized molecular orbital (MO) approach is complemented and supported by combined experimental and theoretical charge density studies: the source function S(r,Ω), which provides a measure of the relative importance of each atomâs contribution to the density at a specific reference point r, clearly shows that all three atoms of the Mn(η2-SiH) moiety contribute to a very similar extent to the density at the MnâSi bond critical point, in pleasing agreement with the MO model. Hence, we advance a consistent and unifying concept which accounts for the degree of SiâH activation in these silane Ï-bond complexes
Discovery and Mass Measurements of a Cold, 10-Earth Mass Planet and Its Host Star
We present the discovery and mass measurement of the cold, low-mass planet
MOA-2009-BLG-266Lb, made with the gravitational microlensing method. This
planet has a mass of m_p = 10.4 +- 1.7 Earth masses and orbits a star of mass
M_* = 0.56 +- 0.09 Solar masses at a semi-major axis of a = 3.2 (+1.9 -0.5) AU
and an orbital period of P = 7.6 (+7.7 -1.5} yrs. The planet and host star mass
measurements are enabled by the measurement of the microlensing parallax
effect, which is seen primarily in the light curve distortion due to the
orbital motion of the Earth. But, the analysis also demonstrates the capability
to measure microlensing parallax with the Deep Impact (or EPOXI) spacecraft in
a Heliocentric orbit. The planet mass and orbital distance are similar to
predictions for the critical core mass needed to accrete a substantial gaseous
envelope, and thus may indicate that this planet is a "failed" gas giant. This
and future microlensing detections will test planet formation theory
predictions regarding the prevalence and masses of such planets.Comment: 38 pages with 7 figure
Catalysis Research of Relevance to Carbon Management: Progress, Challenges, and Opportunities
Composition of initial mixtures for production of the gaseous CîžHîžO fuel cells with molten alkaline carbonates
Improvement of melting and finishing technology of semi-killed steels
22.00; Translated from Polish (Wiad. Hutn. 1987 v. 43(9) p. 223-225)Available from British Library Document Supply Centre- DSC:9022.06(BISI-Trans--26758)T / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo
Technical and technological concept of deoxidising selected kinds of steel with aluminium rod in the ladle
26.00; Translated from Polish (Hutnik (Katowice) 1987 v. 54(4) p. 93-97)Available from British Library Document Supply Centre- DSC:9022.06(BISI-Trans--26542)T / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo
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