On the Symmetry and Degeneracy of H<sub>3</sub><sup>+</sup>

Crabtree, Kyle N.; McCall, Benjamin J.

doi:10.1021/jp400080j

Cited by 4 publications

(3 citation statements)

References 54 publications

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“…The determination of the symmetries of the nuclear spin wave functions is explained lucidly in Bunker (1979) and in Bunker & Jensen (2006). In this work, where we do not consider the rovibronic states of the molecules, it is sufficient to classify the nuclear spin functions in the complete nuclear spin permutation group, which does not include the inversion operation (see also Crabtree & McCall 2013). Most of the statistical branching ratio tables needed in the present study are available in Tables 3 and 4 of Hugo et al (2009).…”

Section: Theorymentioning

confidence: 99%

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Spin-state chemistry of deuterated ammonia

Sipilä

Harju

Caselli

et al. 2015

A&A

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Aims. We aim to develop a chemical model that contains a consistent description of spin-state chemistry in reactions involving chemical species with multiple deuterons. We apply the model to the specific case of deuterated ammonia, to derive values for the various spin-state ratios.Methods. We applied symmetry rules in the context of the complete scrambling assumption to calculate branching ratio tables for reactions between chemical species that include multiple protons and/or deuterons. New reaction sets for both gas-phase and grainsurface chemistry were generated using an automated routine that forms all possible spin-state variants of any given reaction with up to six H/D atoms, using the predetermined branching ratios. Both a single-point and a modified Bonnor-Ebert model were considered to study the density and temperature dependence of ammonia and its isotopologs, and the associated spin-state ratios. Results. We find that the spin-state ratios of the ammonia isotopologs are, at late times, very different from their statistical values. The ratios are rather insensitive to variations in the density, but present strong temperature dependence. We derive high peak values (∼0.1) for the deuterium fraction in ammonia, in agreement with previous (gas-phase) models. The deuterium fractionation is strongest at high density, corresponding to a high degree of depletion, and also presents temperature dependence. We find that in the temperature range 5 K to 20 K, the deuterium fractionation peaks at ∼15 K, while most of the ortho/para (and meta/para for ND 3 ) ratios present a minimum at 10 K (ortho/para NH 2 D has instead a maximum at this temperature). Conclusions. Owing to the density and temperature dependence found in the abundances and spin-state ratios of ammonia and its isotopologs, it is evident that observations of ammonia and its deuterated forms can provide important constraints on the physical structure of molecular clouds.

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Section: Theorymentioning

confidence: 99%

“…We calculate first the characters, χ(R), of the representations generated by the nuclear spin functions of H 6 and D 6 under the operations R of S 6 . Crabtree & McCall (2013) give a convenient formula for this:…”

Section: A1 Three Identical Nucleimentioning

confidence: 99%

Spin-state chemistry of deuterated ammonia

Sipilä

Harju

Caselli

et al. 2015

A&A

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“…This branching ratio is due to spin statistics (see e.g. Crabtree & McCall 2013;Okumura et al 2013). Contrary to Sipilä et al (2013), we allow reactions without H 3 + or H 2 + as reactants to form not only para-H 2 , but also ortho−H 2 with an energy barrier of γ = 170 K. The final network consists of 1 300 species connected by 40 000 reactions.…”

Section: Chemical Networkmentioning

confidence: 99%

FIRST TIME-DEPENDENT STUDY OF H₂AND H$_3^+$ORTHO-PARACHEMISTRY IN THE DIFFUSE INTERSTELLAR MEDIUM: OBSERVATIONS MEET THEORETICAL PREDICTIONS

Albertsson¹,

Indriolo²,

Kreckel³

et al. 2014

ApJ

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The chemistry in the diffuse interstellar medium initiates the gradual increase of molecular complexity during the life cycle of matter. A key molecule that enables build-up of new molecular bonds and new molecules via proton-donation is H + 3 . Its evolution is tightly related to molecular hydrogen and thought to be well understood. However, recent observations of ortho and para lines of H 2 and H + 3 in the diffuse ISM showed a puzzling discrepancy in nuclear spin excitation temperatures and populations between these two key species. H + 3 , unlike H 2 , seems to be out of thermal equilibrium, contrary to the predictions of modern astrochemical models. We conduct the first time-dependent modeling of the para-fractions of H 2 and H + 3 in the diffuse ISM and compare our results to a set of line-of-sight observations, including new measurements presented in this study. We isolate a set of key reactions for H + 3 and find that the destruction of the lowest rotational states of H + 3 by dissociative recombination largely control its ortho/para ratio. A plausible agreement with observations cannot be achieved unless a ratio larger than 1:5 for the destruction of (1, 1)− and (1, 0)−states of H + 3 is assumed. Additionally, an increased CR ionization rate to 10 −15 s −1 further improves the fit whereas variations of other individual physical parameters, such as density and chemical age, have only a minor effect on the predicted ortho/para ratios. Thus our study calls for new laboratory measurements of the dissociative recombination rate and branching ratio of the key ion H + 3 under interstellar conditions.

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