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$_2$ and ortho-H$_2$. Pure para-H$_2$ can be produced by leading `normal' H$_2$, 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$_2$ conversion is also accelerated by interactions with paramagnetic molecules, such as O$_2$. An important application of para-H$_2$ is in NMR spectroscopy and its imaging variant, MRI. By adding para-H$_2$ 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$_2$ conversion by O$_2$ was recently measured in view of this application.[1] Two mechanisms have been suggested for the para-ortho H$_2$ conversion by collisions with O$_2$. The first one, proposed in 1933 by Eugene Wigner,[2] is the magnetic dipole-dipole coupling between the electron spin of O$_2$ and the nuclear spins of the two protons in H$_2$. In asymmetric collisions this coupling makes the two H-nuclei inequivalent and mixes the nuclear spin functions of para- and ortho-H$_2$, as well as their rotational states with even and odd $j$ values. Another mechanism, proposed by Minaev and {\AA}gren[3] in 1995, is that the overlap of the O$_2$ and H$_2$ wavefunctions in a collision complex transfers some of the spin density of O$_2$ to the wavefunction of H$_2$. 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$_2$ 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$_2$ conversion by collisions with O$_2$ 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$_2$-H$_2$ 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$_2$ and H$_2$, which yield the para-ortho H$_2$ 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$_2$-O$_2$ 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$_2$ with He atoms in which CO$_2$ 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$_2$-He potential surface including the CO$_2$ bend ($\nu_2$) 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$^{-1}$, 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

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