2,201 research outputs found
Ab Initio Molecular Dynamics Study of Aqueous Solvation of Ethanol and Ethylene
The structure and dynamics of aqueous solvation of ethanol and ethylene are
studied by DFT-based Car-Parrinello molecular dynamics. We did not find an
enhancement of the structure of the hydrogen bonded network of hydrating water
molecules. Both ethanol and ethylene can easily be accommodated in the
hydrogen-bonded network of water molecules without altering its structure. This
is supports the conclusion from recent neutron diffraction experiments that
there is no hydrophobic hydration around small hydrophobic groups. Analysis of
the electronic charge distribution using Wannier functions shows that the
dipole moment of ethanol increases from 1.8 D to 3.1 D upon solvation, while
the apolar ethylene molecule attains an average dipole moment of 0.5 D. For
ethylene, we identified configurations with -H bonded water molecules,
that have rare four-fold hydrogen-bonded water coordination, yielding
instantaneous dipole moments of ethylene of up to 1 D. The results provide
valuable information for the improvement of empirical force fields, and point
out that for an accurate description of the aqueous solvation of ethanol, and
even of the apolar ethylene, polarizable force fields are required.Comment: 15 pages, 10 figures, 4 tables, revtex4, submitted to J. Chem. Phy
Investigation of room temperature multispin-assisted bulk diamond 13C hyperpolarization at low magnetic fields
In this work we investigated the time behavior of the polarization of bulk
13C nuclei in diamond above the thermal equilibrium. This nonthermal nuclear
hyperpolarization is achieved by cross relaxation between two nitrogen related
paramagnetic defect species in diamond in combination with optical pumping. The
decay of the hyperpolarization at four different magnetic fields is measured.
Furthermore, we use the comparison with conventional nuclear resonance
measurements to identify the involved distances of the nuclear spin with
respect to the defects and therefore the coupling strengths. Also, a careful
look at the linewidth of the signal give valuable information to piece together
the puzzle of the hyperpolarization mechanism
A dynamic neural model of localization of brief successive stimuli in saltation
Somatosensory saltation is an illusion robustly generated using short tactile stimuli [1,2]. There is a perceived displacement of a first stimulus if followed by a subsequent nearby stimulus with a short stimulus onset asynchrony (SOA). Experimental reports suggest that this illusion results from spatiotemporal integration in early processing stages, but the exact neural mechanism is unknown. The neuronal mechanism involved is probably quite generic as similar phenomena occur in other modalities, audition for example [3]
Dynamic interplay between defective UiO‐66 and protic solvents in activated processes
UiO-66, composed by Zr-oxide inorganic bricks [Zr-6(mu(3)-O)(4)(mu(3)-OH)(4)] and organic terephthalate linkers, is one of the most studied metal-organic frameworks (MOFs) due to its exceptional thermal, chemical, and mechanical stability. Thanks to its high connectivity, the material can withstand structural deformations during activation processes such as linker exchange, dehydration, and defect formation. These processes do alter the zirconium coordination number in a dynamic way, creating open metal sites for catalysis and thus are able to tune the catalytic properties. In this work, it is shown, by means of first-principle molecular-dynamics simulations at operating conditions, how protic solvents may facilitate such changes in the metal coordination. Solvent can induce structural rearrangements in the material that can lead to undercoordinated but also overcoordinated metal sites. This is demonstrated by simulating activation processes along well-chosen collective variables. Such enhanced MD simulations are able to track the intrinsic dynamics of the framework at realistic conditions
Acidity of edge surface sites of montmorillonite and kaolinite
Peer reviewedPreprin
Single-photon emission from Ni-related color centers in CVD diamond
Color centers in diamond are very promising candidates among the possible
realizations for practical single-photon sources because of their long-time
stable emission at room temperature. The popular nitrogen-vacancy center shows
single-photon emission, but within a large, phonon-broadened spectrum (~100nm),
which strongly limits its applicability for quantum communication. By contrast,
Ni-related centers exhibit narrow emission lines at room temperature. We
present investigations on single color centers consisting of Ni and Si created
by ion implantation into single crystalline IIa diamond. We use systematic
variations of ion doses between 10^8/cm^2 and 10^14/cm^2 and energies between
30keV and 1.8MeV. The Ni-related centers show emission in the near infrared
spectral range (~770nm to 787nm) with a small line-width (~3nm FWHM). A
measurement of the intensity correlation function proves single-photon
emission. Saturation measurements yield a rather high saturation count rate of
77.9 kcounts/s. Polarization dependent measurements indicate the presence of
two orthogonal dipoles.Comment: 8 pages, published in conference proceedings of SPIE Photonics Europe
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