2,523 research outputs found
Effect of quantum nuclear motion on hydrogen bonding
This work considers how the properties of hydrogen bonded complexes,
D-H....A, are modified by the quantum motion of the shared proton. Using a
simple two-diabatic state model Hamiltonian, the analysis of the symmetric
case, where the donor (D) and acceptor (A) have the same proton affinity, is
carried out. For quantitative comparisons, a parametrization specific to the
O-H....O complexes is used. The vibrational energy levels of the
one-dimensional ground state adiabatic potential of the model are used to make
quantitative comparisons with a vast body of condensed phase data, spanning a
donor-acceptor separation (R) range of about 2.4-3.0 A, i.e., from strong to
weak bonds. The position of the proton and its longitudinal vibrational
frequency, along with the isotope effects in both are discussed. An analysis of
the secondary geometric isotope effects, using a simple extension of the
two-state model, yields an improved agreement of the predicted variation with R
of frequency isotope effects. The role of the bending modes in also considered:
their quantum effects compete with those of the stretching mode for certain
ranges of H-bond strengths. In spite of the economy in the parametrization of
the model used, it offers key insights into the defining features of H-bonds,
and semi-quantitatively captures several experimental trends.Comment: 12 pages, 8 figures. Notation clarified. Revised figure including the
effect of bending vibrations on secondary geometric isotope effect. Final
version, accepted for publication in Journal of Chemical Physic
Energy dependence on fractional charge for strongly interacting subsystems
The energies of a pair of strongly-interacting subsystems with arbitrary
noninteger charges are examined from closed and open system perspectives. An
ensemble representation of the charge dependence is derived, valid at all
interaction strengths. Transforming from resonance-state ionicity to ensemble
charge dependence imposes physical constraints on the occupation numbers in the
strong-interaction limit. For open systems, the chemical potential is evaluated
using microscopic and thermodynamic models, leading to a novel correlation
between ground-state charge and an electronic temperature.Comment: 4 pages, 3 figs.; as accepted (Phys. Rev. Lett.
Sub-millimeter images of a dusty Kuiper belt around eta Corvi
We present sub-millimeter and mid-infrared images of the circumstellar disk
around the nearby F2V star eta Corvi. The disk is resolved at 850um with a size
of ~100AU. At 450um the emission is found to be extended at all position
angles, with significant elongation along a position angle of 130+-10deg; at
the highest resolution (9.3") this emission is resolved into two peaks which
are to within the uncertainties offset symmetrically from the star at 100AU
projected separation. Modeling the appearance of emission from a narrow ring in
the sub-mm images shows the observed structure cannot be caused by an edge-on
or face-on axisymmetric ring; the observations are consistent with a ring of
radius 150+-20AU seen at 45+-25deg inclination. More face-on orientations are
possible if the dust distribution includes two clumps similar to Vega; we show
how such a clumpy structure could arise from the migration over 25Myr of a
Neptune mass planet from 80-105AU. The inner 100AU of the system appears
relatively empty of sub-mm emitting dust, indicating that this region may have
been cleared by the formation of planets, but the disk emission spectrum shows
that IRAS detected an additional hot component with a characteristic
temperature of 370+-60K (implying a distance of 1-2AU). At 11.9um we found the
emission to be unresolved with no background sources which could be
contaminating the fluxes measured by IRAS. The age of this star is estimated to
be ~1Gyr. It is very unusual for such an old main sequence star to exhibit
significant mid-IR emission. The proximity of this source makes it a perfect
candidate for further study from optical to mm wavelengths to determine the
distribution of its dust.Comment: 22 pages, 4 figures. Scheduled for publication in ApJ 10 February
2005 issu
An integrated study of earth resources in the state of California using remote sensing techniques
The University of California has been conducting an investigation which seeks to determine the usefulness of modern remote sensing techniques for studying various components of California's earth resources complex. Most of the work has concentrated on California's water resources, but with some attention being given to other earth resources as well and to the interplay between them and California's water resources
Electronic polarization at surfaces and thin films of organic molecular crystals: PTCDA
The electronic polarization energies, P = (P+) + (P-), of a PTCDA
(perylenetetracarboxylic acid dianhydride) cation and anion in a crystalline
thin film on a metallic substrate are computed and compared with measurements
of the PTCDA transport gap on gold and silver. Both experiments and theory show
that P is 500 meV larger in a PTCDA monolayer than in 50 A films. Electronic
polarization in systems with surfaces and interfaces are obtained
self-consistently in terms of charge redistribution within molecules.Comment: 5 pages, 4 postscript figures embedde
CO emission from discs around isolated HAeBe and Vega-excess stars
We describe results from a survey for J=3-2 12CO emission from visible stars
with an infrared excess. The line is clearly detected in 21 objects, with
molecular gas (>10^-3 Jupiter masses) common in targets with infrared excesses
>0.01 (>56% of objects). Such high excesses indicate the presence of a disc of
opening angle >12 degrees; within this, the optically thick disc prevents CO
photodissociation. Two or three stars with associated CO have an excess <0.01,
implying a disc opening angle <1 degree. Most line profiles are double-peaked
or relatively broad. Model fits, assuming a Keplerian disc, indicate outer
radii, R_out, of ~20-300 au. As many as 5 discs have outer radii smaller than
the Solar System (50 au), and a further 4 have gas at radii <20 au. R_out is
independent of the stellar spectral type (from K through to B9), but is
correlated with total dust mass. R_out appears to decrease with time: discs
around stars of age 3-7 Myr have a mean radius of ~210 au, whereas discs of age
7-20 Myr are a factor of 3 smaller. The only bona fide debris disc with
detected CO is HD9672; this has a double peaked line profile and is the most
compact gas disc observed, with a modelled radius 17 au). A fit to HD141569
suggests the gas lies in two rings of radii 90 and 250 au, similar to the
scattered light structure. In both AB Aur and HD163296 the sizes of the
molecular and dust scattering discs are also similar, suggesting that the gas
and small dust grains are co-located.Comment: 16 pages, 5 figures MNRAS - accepte
Polarization forces in water deduced from single molecule data
Intermolecular polarization interactions in water are determined using a
minimal atomic multipole model constructed with distributed polarizabilities.
Hydrogen bonding and other properties of water-water interactions are
reproduced to fine detail by only three multipoles , , and
and two polarizabilities and , which
characterize a single water molecule and are deduced from single molecule data.Comment: 4 revtex pages, 3 embedded color PS figure
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