161 research outputs found
Para-ortho Hydrogen Conversion; Solving A 90-year Old Mystery
It is well known among spectroscopists that hydrogen has two modifications: para-H and ortho-H. Pure para-H can be produced by leading ``normal'' H, a 3:1 ortho:para mixture, over a catalyst at low temperature. It is perhaps less well known that para-ortho H conversion is also catalyzed by collisions with paramagnetic molecules, such as O.
Almost ninety years ago Farkas and Sachsse measured the rate coefficient of para-ortho H conversion in gas mixtures with O.[1] In the same year, 1933, it was proposed by Wigner [2] that it is the magnetic dipole-dipole coupling between the electron spin of O and the nuclear spins of the two protons in H that is responsible for the conversion. In asymmetric collisions this coupling makes the two H-nuclei inequivalent and mixes the nuclear spin functions of para- and ortho-H, as well as their rotational states with even and odd values. Another mechanism, suggested to be much more effective, was proposed later: the exchange interaction with the open-shell O induces spin density into the electronic wavefunction of H. In most collisions the spin density is different at the two H-nuclei, which makes them inequivalent by different hyperfine interactions through the Fermi contact term.
An important application of para-H is in NMR spectroscopy and its imaging variant, MRI. By adding para-H to the sample the sensitivity of NMR can be increased by four orders of magnitude by a phenomenon called para-hydrogen induced polarization (PHIP). Para-ortho H conversion by O in the gas phase was remeasured in 2014 in view of this application. A detailed and quantitative understanding of the conversion process was still lacking, however.
We theoretically investigated the para-ortho H conversion by collisions with O in a first principles approach.[3] Both mechanisms were taken into account and the corresponding coupling terms were quantitatively evaluated as functions of the geometry of the O-H collision complex by means of \textit{ab initio} electronic structure calculations. Then they were included in nearly exact quantum mechanical coupled-channels scattering calculations for the collisions between O and H, which yielded the para-ortho H conversion cross sections and the rate coefficients for temperatures up to 400\,K. The conversion rate and its temperature dependence are in good agreement with the values measured in H-O gas mixtures. The calculations provide detailed insight into the conversion process.
[1] L. Farkas and H. Sachsse, Z. Phys. Chem. B {\bf 23}, 1 (1933). [2] E. Wigner, Z. Phys. Chem. B {\bf 23}, 28 (1933). [3] X. Zhang, T. Karman, G.~C. Groenenboom, and A. van der Avoird, Nat. Sci. (2021); https://doi.org/10.1002/ntls.10002
PARA-ORTHO H2 CONVERSION BY COLLISIONS WITH O2; A FIRST PRINCIPLES APPROACH
It is well known among spectroscopists that two modifications of hydrogen exist: para-H and ortho-H. Pure para-H can be produced by leading `normal' H, a 1:3 para:ortho mixture, over an iron-containing catalyst at low temperature, and can be kept for a long time also at higher temperature in specially prepared gas cylinders. It is perhaps less well known that para-ortho H conversion is also accelerated by interactions with paramagnetic molecules, such as O.
An important application of para-H is in NMR spectroscopy and its imaging variant, MRI. By adding para-H to the sample the sensitivity of NMR can be increased by four orders of magnitude through a phenomenon called para-hydrogen induced polarization (PHIP). The para-ortho H conversion by O was recently measured in view of this application.[1]
Two mechanisms have been suggested for the para-ortho H conversion by collisions with O. The first one, proposed in 1933 by Eugene Wigner,[2] is the magnetic dipole-dipole coupling between the electron spin of O and the nuclear spins of the two protons in H. In asymmetric collisions this coupling makes the two H-nuclei inequivalent and mixes the nuclear spin functions of para- and ortho-H, as well as their rotational states with even and odd values. Another mechanism, proposed by Minaev and {\AA}gren[3] in 1995, is that the overlap of the O and H wavefunctions in a collision complex transfers some of the spin density of O to the wavefunction of H. The spin densities induced at the two H-nuclei may be different, which causes a different hyperfine interaction through the Fermi contact term. Wigner made a crude estimate of the para-ortho H conversion rate with the use of some kinetic gas data. Minaev and {\AA}gren suggested, however, that the second mechanism is much more effective.
We investigated the para-ortho H conversion by collisions with O by a first principles approach. Both mechanisms are included: the corresponding coupling terms are quantitatively evaluated as a function of the geometry of the O-H collision complex by means of \textit{ab initio} electronic structure calculations. Then they are included in nearly exact quantum mechanical coupled-channels scattering calculations for the collisions between O and H, which yield the para-ortho H conversion cross sections and the rate coefficients for a range of temperatures. The conversion rate at room temperature is compared with the value measured in H-O gas mixtures.[1]
[1] S. Wagner, Magn. Reson. Mater. Phys., Biol. Med. {\bf 27}, 195 (2014). [2] E. Wigner, Z. Phys. Chem. B {\bf 23}, 28 (1933).
[3] B. F. Minaev and H. {\AA}gren, J. Phys. Chem {\bf 99}, 8936 (1995)
Jahn-Teller effect in van der Waals complexes: Ar-C6H6+ and Ar-C6D6+
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Hyperfine interactions and internal rotation in methanol
We present a rigorous derivation of the nuclear spin-rotation and spin-torsion coupling terms in the hyperfine Hamiltonian for molecules with internal rotation. Our formulas differ from the expressions derived by Heuvel and Dymanus [J. Mol. Spectrosc. 47, 363 (1973)], which these authors used and which were also applied recently by others to interpret experimental hyperfine spectra of such molecules. In the present work, our theoretical results are applied to methanol. We calculate the nuclear spin-spin magnetic dipole-dipole interactions and the nuclear contribution to the spin-torsion coupling vectors from the nuclear coordinates as functions of the internal rotation angle γ, compute the spin-rotation coupling tensors by ab initio electronic structure methods also as functions of γ, and obtain the missing parameters for the electronic contribution to the spin-torsion coupling from a fit to measured spectra. The resulting hyperfine Hamiltonian is then used to compute hyperfine transition frequencies and intensities for twelve torsion-rotation transitions in methanol. With the use of the ab initio calculated spin-rotation coupling parameters without any modification, and physically reasonable values for the spin-torsion coupling parameters from the fit, we find good agreement with all of the measured spectra
State-to-state rovibrational transition rates for CO2 in the bend mode in collisions with He atoms
Modeling environments that are not in local thermal equilibrium, such as
protoplanetary disks or planetary atmospheres, with molecular spectroscopic
data from space telescopes requires knowledge of the rate coefficients of
rovibrationally inelastic molecular collisions. Here, we present such rate
coefficients in a temperature range from 10 to 500 K for collisions of CO
with He atoms in which CO is (de)excited in the bend mode. They are
obtained from numerically exact coupled-channel (CC) calculations as well as
from calculations with the less demanding coupled-states approximation (CSA)
and the vibrational close-coupling rotational infinite-order sudden (VCC-IOS)
method. All of the calculations are based on a newly calculated accurate ab
initio four-dimensional CO-He potential surface including the CO bend
() mode. We find that the rovibrationally inelastic collision cross
sections and rate coefficients from the CSA and VCC-IOS calculations agree to
within 50% with the CC results at the rotational state-to-state level, except
for the smaller ones and in the low energy resonance region, and to within 20%
for the overall vibrational quenching rates except for temperatures below 50 K
where resonances provide a substantial contribution. Our CC quenching rates
agree with the most recent experimental data within the error bars. We also
compared our results with data from Clary et al. calculated in the 1980's with
the CSA and VCC-IOS methods and a simple atom-atom model potential based on ab
initio Hartree-Fock calculations and found that their cross sections agree
fairly well with ours for collision energies above 500 cm, but that the
inclusion of long range attractive dispersion interactions is crucial to obtain
reliable cross sections at lower energies and rate coefficients at lower
temperatures.Comment: The article has been accepted to the Journal of Chemical Physic
THE AMMONIA DIMER REVISITED
Author Institution: Missouri University of Science and Technology; Rolla, MO 65409-0010; Radboud University, 6525 AJ Nijmegen, The NetherlandsThe conclusion from microwave spectra by Nelson, Fraser, and Klemperer nderline{\textbf{83}} 6201 (1985)} that the ammonia dimer has a nearly cyclic structure led to much debate about the issue of whether (NH) is hydrogen bonded. This structure was surprising because most \textit{ab initio} calculations led to a classical, nearly linear, hydrogen-bonded structure. An obvious explanation of the discrepancy between the outcome of these calculations and the microwave data which led Nelson \textit{et al.} to their "surprising structure'' might be the effect of vibrational averaging: the electronic structure calculations focus on finding the minimum of the intermolecular potential, the experiment gives a vibrationally averaged structure. Isotope substitution studies seemed to indicate, however, that the complex is nearly rigid. Additional data became available from high-resolution molecular beam far-infrared spectroscopy in the Saykally group nderline{\textbf{97}} 4727 (1992)}. These spectra, displaying large tunneling splittings, indicate that the complex is very floppy. The seemingly contradictory experimental data were explained when it became possible nderline{\textbf{101}} 8430 (1994); E.~H.~T.~Olthof, A.~van der Avoird, P.~E.~S.~Wormer, J.~G.~Loeser, and R.~J.~Saykally \textit{J.~Chem.~Phys.} nderline{\textbf{101}} 8443 (1994)} to calculate the vibration-rotation-tunneling (VRT) states of the complex on a six-dimensional intermolecular potential surface. The potential used was a simple model potential, with parameters fitted to the far-infrared data. Now, for the first time, a six-dimensional potential was computed by high level \textit{ab initio} methods and this potential will be used in calculations of the VRT states of (NH) and (ND). So, we will finally be able to answer the question whether the conclusions from the model calculations are indeed a valid explanation of the experimental data
Characterization of methanol as a magnetic field tracer in star-forming regions
Magnetic fields play an important role during star formation. Direct magnetic
field strength observations have proven specifically challenging in the
extremely dynamic protostellar phase. Because of their occurrence in the
densest parts of star forming regions, masers, through polarization
observations, are the main source of magnetic field strength and morphology
measurements around protostars. Of all maser species, methanol is one of the
strongest and most abundant tracers of gas around high-mass protostellar disks
and in outflows. However, as experimental determination of the magnetic
characteristics of methanol has remained largely unsuccessful, a robust
magnetic field strength analysis of these regions could hitherto not be
performed. Here we report a quantitative theoretical model of the magnetic
properties of methanol, including the complicated hyperfine structure that
results from its internal rotation. We show that the large range in values of
the Land\'{e} g-factors of the hyperfine components of each maser line lead to
conclusions which differ substantially from the current interpretation based on
a single effective g-factor. These conclusions are more consistent with other
observations and confirm the presence of dynamically important magnetic fields
around protostars. Additionally, our calculations show that (non-linear) Zeeman
effects must be taken into account to further enhance the accuracy of
cosmological electron-to-proton mass ratio determinations using methanol.Comment: 23 pages, 3 figures, excluding Supplementary information. Author
manuscript version before editorial/copyediting by Nature Astronomy. Journal
version available via http://rdcu.be/FPeB . Supplementary material available
via
https://static-content.springer.com/esm/art%3A10.1038%2Fs41550-017-0341-8/MediaObjects/41550_2017_341_MOESM1_ESM.pd
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