320 research outputs found
New quantum chemical computations of formamide deuteration support a gas-phase formation of this prebiotic molecule
Based on recent work, formamide might be a potentially very important
molecule in the emergence of terrestrial life. Although detected in the
interstellar medium for decades, its formation route is still debated, whether
in the gas phase or on the dust grain surfaces. Molecular deuteration has
proven to be, in other cases, an efficient way to identify how a molecule is
synthesised. For formamide, new published observations towards the
IRAS16293-2422 B hot corino show that its three deuterated forms have all the
same deuteration ratio, 2--5%, and that this is a factor 3--8 smaller than that
measured for H2CO towards the IRAS16293-2422 protostar. Following a previous
work on the gas-phase formamide formation via the reaction NH2 + H2CO -> HCONH2
+ H, we present here new calculations of the rate coefficients for the
production of monodeuterated formamide through the same reaction, starting from
monodeuterated NH2 or H2CO. Some misconceptions regarding our previous
treatment of the reaction are also cleared up. The results of the new
computations show that, at the 100 K temperature of the hot corino, the rate of
deuteration of the three forms is the same, within 20%. On the contrary, the
reaction between non-deuterated species proceeds three times faster than that
with deuterated ones. These results confirm that a gas-phase route for the
formation of formamide is perfectly in agreement with the available
observations.Comment: MNRAS in pres
Gas phase formation of the prebiotic molecule formamide: insights from new quantum computations
New insights into the formation of interstellar formamide, a species of great
relevance in prebiotic chemistry, are provided by electronic structure and
kinetic calculations for the reaction NH2 + H2CO -> NH2CHO + H. Contrarily to
what previously suggested, this reaction is essentially barrierless and can,
therefore, occur under the low temperature conditions of interstellar objects
thus providing a facile formation route of formamide. The rate coefficient
parameters for the reaction channel leading to NH2CHO + H have been calculated
to be A = 2.6x10^{-12} cm^3 s^{-1}, beta = -2.1 and gamma = 26.9 K in the range
of temperatures 10-300 K. Including these new kinetic data in a refined
astrochemical model, we show that the proposed mechanism can well reproduce the
abundances of formamide observed in two very different interstellar objects:
the cold envelope of the Sun-like protostar IRAS16293-2422 and the molecular
shock L1157-B2. Therefore, the major conclusion of this Letter is that there is
no need to invoke grain-surface chemistry to explain the presence of formamide
provided that its precursors, NH2 and H2CO, are available in the gas-phase.Comment: MNRAS Letters, in pres
Spatial correlations in sheared isothermal liquids : From elastic particles to granular particles
Spatial correlations for sheared isothermal elastic liquids and granular
liquids are theoretically investigated. Using the generalized fluctuating
hydrodynamics, correlation functions for both the microscopic scale and the
macroscopic scale are obtained. The existence of the long-range correlation
functions obeying power laws has been confirmed. The validity of our
theoretical predictions have been verified from the molecular dynamics
simulation.Comment: 34 pages, 12 figure
Quantum chemical computations of gas-phase glycolaldehyde deuteration and constraints to its formation route
Despite the detection of numerous interstellar complex organic molecules
(iCOMs) for decades, it is still a matter of debate whether they are
synthesized in the gas-phase or on the icy surface of interstellar grains. In
the past, molecular deuteration has been used to constrain the formation paths
of small and abundant hydrogenated interstellar species. More recently, the
deuteration degree of formamide, one of the most interesting iCOM, has also
been explained in the hypothesis that it is formed by the gas-phase reaction
NH + HCO. In this article, we aim at using molecular deuteration to
constrain the formation of another iCOM, glycolaldehyde, which is an important
prebiotic species. More specifically, we have performed dedicated electronic
structure and kinetic calculations to establish the glycolaldehyde deuteration
degree in relation to that of ethanol, which is its possible parent species
according to the suggestion of Skouteris et al. (2018). We found that the
abundance ratio of the species containing one D-atom over the all-protium
counterpart depends on the produced D isotopomer and varies from 0.9 to 0.5.
These theoretical predictions compare extremely well with the monodeuterated
isotopomers of glycolaldehyde and that of ethanol measured towards the
Solar-like protostar IRAS 16293-2422, supporting the hypothesis that
glycolaldehyde could be produced in the gas-phase for this source. In addition,
the present work confirms that the deuterium fractionation of iCOMs cannot be
simply anticipated based on the deuterium fractionation of the parent species
but necessitates a specific study, as already shown for the case of formamide.Comment: Accepted by Ap
Fluctuating magnetic moments in liquid metals
We re-analyze literature data on neutron scattering by liquid metals to show
that non-magnetic liquid metals possess a magnetic moment that fluctuates on a
picosecond time scale. This time scale follows the motion of the cage-diffusion
process in which an ion rattles around in the cage formed by its neighbors. We
find that these fluctuating magnetic moments are present in liquid Hg, Al, Ga
and Pb, and possibly also in the alkali metals.Comment: 17 pages, 5 figures, submitted to PR
Phonon-like and single particle dynamics in liquid lithium
The dynamic structure factor, S(Q,E), of liquid lithium (T=475 K) has been
determined by inelastic x-ray scattering (IXS) in the momentum transfer region
(Q = 1.4-110 nm-1). These data allow to observe how, in a simple liquid, a
phonon-like collective mode evolves towards the single particle dynamics. As a
function of Q, one finds: i) at low Q's, a sound mode with a positive
dispersion of the sound velocity, ii) at intermediate Q's, excitations whose
energy oscillates similarly to phonons in the crystal Brillouin zones, and iii)
at high Q's, the S(Q,E) approaches a Gaussian shape, indicating that the single
particle dynamics has been reached.Comment: 3 pages and 5 figure
Simple theory for spin-lattice relaxation in metallic rare earth ferromagnets
The spin-lattice relaxation time is a key quantity both for the
dynamical response of ferromagnets excited by laser pulses and as the speed
limit of magneto-optical recording. Extending the theory for the electron
paramagnetic resonance of magnetic impurities to spin-lattice relaxation in
ferromagnetic rare earths we calculate for Gd and find a value of
48 ps in very good agreement with time-resolved spin-polarized photoemission
experiments. We argue that the time scale for in metals is
essentially given by the spin-orbit induced magnetocrystalline anisotropy
energy.Comment: 18 pages revtex, 5 uuencoded figure
Two liquid states of matter: A new dynamic line on a phase diagram
It is generally agreed that the supercritical region of a liquid consists of
one single state (supercritical fluid). On the other hand, we show here that
liquids in this region exist in two qualitatively different states: "rigid" and
"non-rigid" liquid. Rigid to non-rigid transition corresponds to the condition
{\tau} ~ {\tau}0, where {\tau}is liquid relaxation time and {\tau}0 is the
minimal period of transverse quasi-harmonic waves. This condition defines a new
dynamic line on the phase diagram, and corresponds to the loss of shear
stiffness of a liquid at all available frequencies, and consequently to the
qualitative change of many important liquid properties. We analyze the dynamic
line theoretically as well as in real and model liquids, and show that the
transition corresponds to the disappearance of high-frequency sound,
qualitative changes of diffusion and viscous flow, increase of particle thermal
speed to half of the speed of sound and reduction of the constant volume
specific heat to 2kB per particle. In contrast to the Widom line that exists
near the critical point only, the new dynamic line is universal: it separates
two liquid states at arbitrarily high pressure and temperature, and exists in
systems where liquid - gas transition and the critical point are absent
overall.Comment: 21 pages, 8 figure
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