1,263 research outputs found
An extended hybrid density functional (X3LYP) with improved descriptions of nonbond interactions and thermodynamic properties of molecular systems
We derive here the form for the exact exchange energy density for a density that decays with Gaussian-type behavior at long range. This functional is intermediate between the B88 and the PW91 exchange functionals. Using this modified functional to match the form expected for Gaussian densities, we propose the X3LYP extended functional. We find that X3LYP significantly outperforms Becke three parameter Lee–Yang–Parr (B3LYP) for describing van der Waals and hydrogen bond interactions, while performing slightly better than B3LYP for predicting heats of formation, ionization potentials, electron affinities, proton affinities, and total atomic energies as validated with the extended G2 set of atoms and molecules. Thus X3LYP greatly enlarges the field of applications for density functional theory. In particular the success of X3LYP in describing the water dimer (with Re and De within the error bars of the most accurate determinations) makes it an excellent candidate for predicting accurate ligand–protein and ligand–DNA interactions
A candidate LiBH4 for hydrogen storage: Crystal structures and reaction mechanisms of intermediate phases
First-principles calculation and x-ray diffraction simulation methods have been used to explore crystal structures and reaction mechanisms of the intermediate phases involved in dehydriding of LiBH4. LiBH4 was found to dehydride via two sequential steps: first dehydriding through LiBH, followed by the dehydriding of LiBH through LiB. The first step, which releases 13.1 wt. % hydrogen, was calculated to have an activation barrier of 2.33 eV per formula unit and was endothermic by 1.28 eV per formula unit, while the second step was endothermic by 0.23 eV per formula unit. On the other hand, if LiBH4 and LiBH each donated one electron, possibly to the catalyst doped on their surfaces, it was found that the barrier for the first step was reduced to 1.50 eV. This implies that the development of the catalyst to induce charge migration from the bulk to the surface is essential to make LiBH4 usable as a hydrogen storage material in a moderate temperature range, which is also important to stabilize the low-temperature structure of Pnma (no. 62) LiBH on dehydrogenation. Consequently, the high 13.1 wt. % hydrogen available from the dehydriding of LiBH4 and LiBH and their phase stability on Pnma when specific catalysts were used suggest that LiBH4 has good potential to be developed as the hydrogen storage medium capable of releasing the Department of Energy target of 6.5 wt. % for a hydrogen fuel cell car in a moderate temperature range
Deep ALMA imaging of the merger NGC1614 - Is CO tracing a massive inflow of non-starforming gas?
Observations of the molecular gas over scales of 0.5 to several kpc provide
crucial information on how gas moves through galaxies, especially in mergers
and interacting systems, where it ultimately reaches the galaxy center,
accumulates, and feeds nuclear activity. Studying the processes involved in the
gas transport is an important step forward to understand galaxy evolution.
12CO, 13CO and C18O1-0 high-sensitivity ALMA observations were used to assess
properties of the large-scale molecular gas reservoir and its connection to the
circumnuclear molecular ring in NGC1614. The role of excitation and abundances
were studied in this context. Spatial distributions of the 12CO and 13CO
emission show significant differences. 12CO traces the large-scale molecular
gas reservoir, associated with a dust lane that harbors infalling gas. 13CO
emission is - for the first time - detected in the large-scale dust lane. Its
emission peaks between dust lane and circumnuclear molecular ring. A
12CO-to-13CO1-0 intensity ratio map shows high values in the ring region (~30)
typical for the centers of luminous galaxy mergers and even more extreme values
in the dust lane (>45). This drop in ratio is consistent with molecular gas in
the dust lane being in a diffuse, unbound state while being funneled towards
the nucleus. We find a high 16O-to-18O abundance ratio in the starburst region
(>900), typical of quiescent disk gas - by now, the starburst is expected to
have enriched the nuclear ISM in 18O relative to 16O. The massive inflow of gas
may be partially responsible for the low 18O/16O abundance since it will dilute
the starburst enrichment with unprocessed gas from greater radii. The
12CO-to-13CO abundance is consistent with this scenario. It suggests that the
nucleus of NGC1614 is in a transient phase of evolution where starburst and
nuclear growth are fuelled by returning gas from the minor merger event.Comment: 10 pages, 9 figures, accepted for publication in A&
Molecular tendrils feeding star formation in the Eye of the Medusa - The Medusa merger in high resolution 12CO 2-1 maps
Studying molecular gas properties in merging galaxies gives us important
clues to the onset and evolution of interaction-triggered starbursts. NGC4194
is particularly interesting to study since its FIR-to-CO luminosity ratio
rivals that of ULIRGs,despite its lower luminosity compared to ULIRGs, which
indicates a high star formation efficiency that is relative to even most
spirals and ULIRGs.We study the molecular medium at an angular resolution of
0.65"x .52" through our observations of CO2-1 emission using the SMA. We
compare our CO2-1 maps with optical HST and high angular resolution radio
continuum images to study the relationship between molecular gas and other
components of the starburst region. The molecular gas is tracing the
complicated dust lane structure of NGC4194 with the brightest emission being
located in an off-nuclear ring-like structure with ~320pc radius, the Eye of
the Medusa. The bulk CO emission of the ring is found south of the kinematical
center of NGC4194. The northern tip of the ring is associated with the galaxy
nucleus, where the radio continuum has its peak. A prominent, secondary
emission maximum in the radio continuum is located inside the molecular ring.
This suggests that the morphology of the ring is partially influenced by
massive supernova explosions. From the combined evidence, we propose that the
Eye of the Medusa contains a shell of swept up material where we identify a
number of giant molecular associations. We propose that the Eye may be the site
of an efficient starburst of 5-7M_sun/yr, but it would still constitute only a
fraction of the 30-50M_sun/yr SFR of NGC4194. Furthermore, we find that ~50% of
the molecular mass of NGC4194 is found in extended filamentary-like structures
tracing the minor and major axis dust lanes. We suggest that molecular gas is
transported along these lanes providing the central starburst region with fuel.Comment: accepted for publication in A&A, 12 pages, 9 figure
Mechanism of the Stoddart-Heath Bistable Rotaxane Molecular Switch
We use quantum mechanics to characterize the structure and current−voltage performance of the Stoddart−Heath rotaxane-based programmable electronic switch. We find that the current when the ring is on the DNP is 37−58 times the current when the ring is on the TTF, in agreement with experiment (ratio of 10−100). This establishes the basis for iterative experimental−theoretical efforts to optimize systems for molecule-based electronics which we illustrate by predicting the effect of adding a group such as CN to the rotaxane
Major impact from a minor merger - The extraordinary hot molecular gas flow in the Eye of the NGC 4194 Medusa galaxy
Minor mergers are important processes contributing significantly to how
galaxies evolve across the age of the Universe. Their impact on supermassive
black hole growth and star formation is profound. The detailed study of dense
molecular gas in galaxies provides an important test of the validity of the
relation between star formation rate and HCN luminosity on different galactic
scales. We use observations of HCN, HCO+1-0 and CO3-2 to study the dense gas
properties in the Medusa merger. We calculate the brightness temperature ratios
and use them in conjunction with a non-LTE radiative line transfer model. The
HCN and HCO+1-0, and CO3-2 emission do not occupy the same structures as the
less dense gas associated with the lower-J CO emission. The only emission from
dense gas is detected in a 200pc region within the "Eye of the Medusa". No HCN
or HCO+ is detected for the extended starburst. The CO3-2/2-1 brightness
temperature ratio inside "the Eye" is ~2.5 - the highest ratio found so far.
The line ratios reveal an extreme, fragmented molecular cloud population inside
"the Eye" with large temperatures (>300K) and high gas densities (>10^4 cm^-3).
"The Eye" is found at an interface between a large-scale minor axis inflow and
the Medusa central region. The extreme conditions inside "the Eye" may be the
result of the radiative and mechanical feedback from a deeply embedded, young,
massive super star cluster, formed due to the gas pile-up at the intersection.
Alternatively, shocks from the inflowing gas may be strong enough to shock and
fragment the gas. For both scenarios, however, it appears that the HCN and HCO+
dense gas tracers are not probing star formation, but instead a post-starburst
and/or shocked ISM that is too hot and fragmented to form new stars. Thus,
caution is advised in linking the detection of emission from dense gas tracers
to evidence of ongoing or imminent star formation.Comment: 10 pages, 5 figures, 2 tables, accepted for publication in A&
Creative Local Governance & Neighborhood Revitalization: Urban Informality Facilitation Inventive Community Development Strategies in Berlin, Germany and South Florida
This study identifies inventive ways of revitalizing distressed neighborhoods in post-recession, urban landscapes. It aims to introduce a new discourse in the area of community development, one that brings to light informal processes that exist in our cities and how these processes may aid in the creation of more effective ways to strengthen neighborhoods without displacing residents.Ope
Procedural Rights and Remedies under the Texas Property Tax Code - A Guide to the Code, Recent Amendments, and Developing Case Law.
Abstract Forthcoming
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