219,968 research outputs found
The KCAL VERA 22 GHz calibrator survey
We observed at 22 GHz with the VLBI array VERA a sample of 1536 sources with
correlated flux densities brighter than 200 mJy at 8 GHz. One half of target
sources has been detected. The detection limit was around 200 mJy. We derived
the correlated flux densities of 877 detected sources in three ranges of
projected baseline lengths. The objective of these observations was to
determine the suitability of given sources as phase calibrators for dual-beam
and phase-referencing observations at high frequencies. Preliminary results
indicate that the number of compact extragalactic sources at 22 GHz brighter
than a given correlated flux density level is twice less than at 8 GHz.Comment: Accepted for publication by the Astronomical Journal. 6 pages, 3
figures, 3 table. The machine readable catalogue file, kcal_cat.txt can be
extracted from the source of this submissio
Fully ab initio atomization energy of benzene via W2 theory
The total atomization energy at absolute zero, (TAE) of benzene,
CH, was computed fully {\em ab initio} by means of W2h theory as 1306.6
kcal/mol, to be compared with the experimentally derived value 1305.7+/-0.7
kcal/mol. The computed result includes contributions from inner-shell
correlation (7.1 kcal/mol), scalar relativistic effects (-1.0 kcal/mol), atomic
spin-orbit splitting (-0.5 kcal/mol), and the anharmonic zero-point vibrational
energy (62.1 kcal/mol). The largest-scale calculations involved are
CCSD/cc-pV5Z and CCSD(T)/cc-pVQZ; basis set extrapolations account for 6.3
kcal/mol of the final result. Performance of more approximate methods has been
analyzed. Our results suggest that, even for systems the size of benzene,
chemically accurate molecular atomization energies can be obtained from fully
first-principles calculations, without resorting to corrections or parameters
derived from experiment.Comment: J. Chem. Phys., accepted. RevTeX, 12 page
A definitive heat of vaporization of silicon through benchmark ab initio calculations on SiF_4
In order to resolve a significant uncertainty in the heat of vaporization of
silicon -- a fundamental parameter in gas-phase thermochemistry -- [Si(g)] has been determined from a thermochemical cycle involving
the precisely known experimental heats of formation of SiF_4(g) and F(g) and a
benchmark calculation of the total atomization energy (TAE_0) of SiF_4 using
coupled-cluster methods. Basis sets up to on Si and
on F have been employed, and extrapolations for residual basis
set incompleteness applied. The contributions of inner-shell correlation (-0.08
kcal/mol), scalar relativistic effects (-1.88 kcal/mol), atomic spin-orbit
splitting (-1.97 kcal/mol), and anharmonicity in the zero-point energy (+0.04
kcal/mol) have all been explicitly accounted for. Our benchmark TAE_0=565.89
\pm 0.22 kcal/mol leads to [Si(g)]=107.15 \pm 0.38
kcal/mol ([Si(g)]=108.19 \pm 0.38 kcal/mol): between
the JANAF/CODATA value of 106.5 \pm 1.9 kcal/mol and the revised value proposed
by Grev and Schaefer [J. Chem. Phys. 97, 8389 (1992}], 108.1 \pm 0.5 kcal/mol.
The revision will be relevant for future computational studies on heats of
formation of silicon compounds.Comment: J. Phys. Chem. A, submitted Feb 1, 199
Fluid or fuel? The context of consuming a beverage is important for satiety
Energy-containing beverages have a weak effect on satiety, limited by their fluid characteristics and perhaps because they are not considered ‘food’. This study investigated whether the context of consuming a beverage can influence the satiating power of its nutrients. Eighty participants consumed a lower- (LE, 75 kcal) and higher-energy (HE, 272 kcal) version of a beverage (covertly manipulated within-groups) on two test days, in one of four beverage contexts (between-groups): thin versions of the test-drinks were consumed as a thirst-quenching drink (n = 20), a filling snack (n = 20), or without additional information (n = 20). A fourth group consumed subtly thicker versions of the beverages without additional information (n = 20). Lunch intake 60 minutes later depended on the beverage context and energy content (p = 0.030): participants who consumed the thin beverages without additional information ate a similar amount of lunch after the LE and HE versions (LE = 475 kcal, HE = 464 kcal; p = 0.690) as did those participants who believed the beverages were designed to quench-thirst (LE = 442 kcal, HE = 402 kcal; p = 0.213), despite consuming an additional 197 kcal in the HE beverage. Consuming the beverage as a filling snack led participants to consume less at lunch after the HE beverage compared to the LE version (LE = 506 kcal, HE = 437 kcal; p = 0.025). This effect was also seen when the beverages were subtly thicker, with participants in this group displaying the largest response to the beverage’s energy content, consuming less at lunch after the HE version (LE = 552 kcal, HE = 415 kcal; p<0.001). These data indicate that beliefs about the consequences of consuming a beverage can affect the impact of its nutrients on appetite regulation and provide further evidence that a beverage’s sensory characteristics can limit its satiating power
Structures, Energetics, and Reaction Barriers for CH_x Bound to the Nickel (111) Surface
To provide a basis for understanding and improving such reactive processes on nickel surfaces as the catalytic
steam reforming of hydrocarbons, the decomposition of hydrocarbons at fuel cell anodes, and the growth of
carbon nanotubes, we report quantum mechanics calculations (PBE flavor of density functional theory, DFT)
of the structures, binding energies, and reaction barriers for all CH_x species on the Ni(111) surface using
periodically infinite slabs. We find that all CH_x species prefer binding to μ3 (3-fold) sites leading to bond
energies ranging from 42.7 kcal/mol for CH_3 to 148.9 kcal/mol for CH (the number of Ni-C bonds is not
well-defined). We find reaction barriers of 18.3 kcal/mol for CH_(3,ad) → CH_(2,ad) + H_(ad) (with ΔE = +1.3 kcal/
mol), 8.2 kcal/mol for CH_(2,ad) → CH_(ad) + H_(ad) (with ΔE = -10.2 kcal/mol) and 32.3 kcal/mol for CH_(ad) → C_(ad)
+ H_(ad) (with ΔE = 11.6 kcal/mol). Thus, CH_(ad) is the stable form of CH_x on the surface. These results are in
good agreement with the experimental data for the thermodynamic stability of small hydrocarbon species
following dissociation of methane on Ni(111) and with the intermediates isolated during the reverse methanation
process
Accurate ab initio anharmonic force field and heat of formation for silane, SiH_4
From large basis set coupled cluster calculations and a minor empirical
adjustment, an anharmonic force field for silane has been derived that is
consistently of spectroscopic quality ( on vibrational
fundamentals) for all isotopomers of silane studied. Inner-shell polarization
functions have an appreciable effect on computed properties and even on
anharmonic corrections. From large basis set coupled cluster calculations and
extrapolations to the infinite-basis set limit, we obtain TAE_0=303.80 \pm 0.18
kcal/mol, which includes an anharmonic zero-point energy (19.59 kcal/mol),
inner-shell correlation (-0.36 kcal/mol), scalar relativistic corrections
(-0.70 kcal/mol), and atomic spin-orbit corrections (-0.43 kcal/mol). In
combination with the recently revised \HVSI{0}, we obtain , in between the two established
experimental values.Comment: Mol. Phys., in pres
The heats of formation of the haloacetylenes XCCY [X, Y = H, F, Cl]: basis set limit ab initio results and thermochemical analysis
The heats of formation of haloacetylenes are evaluated using the recent W1
and W2 ab initio computational thermochemistry methods. These calculations
involve CCSD and CCSD(T) coupled cluster methods, basis sets of up to spdfgh
quality, extrapolations to the one-particle basis set limit, and contributions
of inner-shell correlation, scalar relativistic effects, and (where relevant)
first-order spin-orbit coupling. The heats of formation determined using W2
theory are: \hof(HCCH) = 54.48 kcal/mol, \hof(HCCF) = 25.15 kcal/mol,
\hof(FCCF) = 1.38 kcal/mol, \hof(HCCCl) = 54.83 kcal/mol, \hof(ClCCCl) = 56.21
kcal/mol, and \hof(FCCCl) = 28.47 kcal/mol. Enthalpies of hydrogenation and
destabilization energies relative to acetylene were obtained at the W1 level of
theory. So doing we find the following destabilization order for acetylenes:
FCCF ClCCF HCCF ClCCCl HCCCl HCCH. By a combination of W1
theory and isodesmic reactions, we show that the generally accepted heat of
formation of 1,2-dichloroethane should be revised to -31.80.6 kcal/mol, in
excellent agreement with a very recent critically evaluated review. The
performance of compound thermochemistry schemes such as G2, G3, G3X and CBS-QB3
theories has been analyzed.Comment: Mol. Phys., in press (E. R. Davidson issue
Quantum chemical studies of redox properties and conformational changes of a four-center iron CO2 reduction electrocatalyst.
The CO2 reduction electrocatalyst [Fe4N(CO)12]- (abbrev. 1-) reduces CO2 to HCO2- in a two-electron, one-proton catalytic cycle. Here, we employ ab initio calculations to estimate the first two redox potentials of 1- and explore the pathway of a side reaction involving CO dissociation from 13-. Using the BP86 density functional approximation, the redox potentials were computed with a root mean squared error of 0.15 V with respect to experimental data. High temperature Born-Oppenheimer molecular dynamics was employed to discover a reaction pathway of CO dissociation from 13- with a reaction energy of +10.6 kcal mol-1 and an activation energy of 18.8 kcal mol-1; including harmonic free energy terms, this yields ΔGsep = 1.4 kcal mol-1 for fully separated species and ΔG‡ = +17.4 kcal mol-1, indicating CO dissociation is energetically accessible at ambient conditions. The analogous dissociation pathway from 12- has a reaction energy of 22.1 kcal mol-1 and an activation energy of 22.4 kcal mol-1 (ΔGsep = 12.8 kcal mol-1, ΔG‡ = +18.1 kcal mol-1). Our computed harmonic vibrational analysis of [Fe4N(CO)11]3- or 23- reveals a distinct CO-stretching peak red-shifted from the main CO-stretching band, pointing to a possible vibrational signature of dissociation. Multi-reference CASSCF calculations are used to check the assumptions of the density functional approximations that were used to obtain the majority of the results
Thermal Decomposition of Some Linear Perfluoroalkanes in an Inconel Tube
The products of the pyrolysis reactions of perfluoropropane, perfluoroethane, and carbon tetrafluoride in an Inconel bomb are described. The values for the energy of activation and frequency factor for the first-order pyrolysis reactions are respectively: (1) 80 kcal. per mole and 2x10(exp14) sec.(exp-1) for perfluoropropane; (2) 53 kcal. per mole and 3x10(exp7) sec.(exp-1) for perfluoroethane; and (3) 96 kcal. per mole and 4x10(exp13) sec.(exp-1) for carbon tetrafluoride
Explanation of the colossal detonation sensitivity of silicon pentaerythritol tetranitrate (Si-PETN) explosive
DFT calculations have identified the novel rearrangement shown here for decomposition of the Si derivative of the PETN explosive [pentaerythritol tetranitrate (PETN), C(CH_2ONO_2)_4] that explains the very dramatic increase in sensitivity observed experimentally. The critical difference is that Si-PETN allows a favorable five-coordinate transition state in which the new Si−O and C−O bonds form simultaneously, leading to a transition state barrier of 33 kcal/mol (it is 80 kcal/mol for PETN) and much lower than the normal O−NO_2 bond fission observed in other energetic materials (40 kcal/mol). In addition this new mechanism is very exothermic (45 kcal/mol) leading to a large net energy release at the very early stages of Si-PETN decomposition that facilitates a rapid temperature increase and expansion of the reaction zone
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