More Like this

1. Branching Ratio for O + H ₃ ⁺ Forming OH ⁺ + H ₂ and H ₂ O ⁺ + H

   https://doi.org/10.3847/1538-4357/ac41ce  

   Hillenbrand, Pierre-Michel; Ruette, Nathalie de; Urbain, Xavier; Savin, Daniel W. (March 2022, The Astrophysical Journal)

   Abstract The gas-phase reaction of O + H 3 + has two exothermic product channels: OH + + H 2 and H 2 O + + H. In the present study, we analyze experimental data from a merged-beams measurement to derive thermal rate coefficients resolved by product channel for the temperature range from 10 to 1000 K. Published astrochemical models either ignore the second product channel or apply a temperature-independent branching ratio of 70% versus 30% for the formation of OH + + H 2 versus H 2 O + + H, respectively, which originates from a single experimental data point measured at 295 K. Our results are consistent with this data point, but show a branching ratio that varies with temperature reaching 58% versus 42% at 10 K. We provide recommended rate coefficients for the two product channels for two cases, one where the initial fine-structure population of the O( 3 P J ) reactant is in its J = 2 ground state and the other one where it is in thermal equilibrium. 

   more » « less
2. Mechanisms and dynamics of the NH ⁺ ₂ + H ⁺ and NH ⁺ + H ⁺ + H fragmentation channels upon single-photon double ionization of NH ₃

   https://doi.org/10.1088/1361-6455/abc3aa  

   Larsen, Kirk A; Rescigno, Thomas N; Streeter, Zachary L; Iskandar, Wael; Heck, Saijoscha; Gatton, Averell; Champenois, Elio G; Severt, Travis; Strom, Richard; Jochim, Bethany; et al (November 2020, Journal of Physics B: Atomic, Molecular and Optical Physics)

   Abstract We present state-selective measurements on the N H 2 + + H + and NH + + H + + H dissociation channels following single-photon double ionization at 61.5 eV of neutral NH 3 , where the two photoelectrons and two cations are measured in coincidence using 3D momentum imaging. Three dication electronic states are identified to contribute to the N H 2 + + H + dissociation channel, where the excitation in one of the three states undergoes intersystem crossing prior to dissociation, producing a cold N H 2 + fragment. In contrast, the other two states directly dissociate, producing a ro-vibrationally excited N H 2 + fragment with roughly 1 eV of internal energy. The NH + + H + + H channel is fed by direct dissociation from three intermediate dication states, one of which is shared with the N H 2 + + H + channel. We find evidence of autoionization contributing to each of the double ionization channels. The distributions of the relative emission angle between the two photoelectrons, as well as the relative angle between the recoil axis of the molecular breakup and the polarization vector of the ionizing field, are also presented to provide insight on both the photoionization and photodissociation mechanisms for the different dication states. 

   more » « less
3. Experimental study of the proton-transfer reaction C + H ₂ ⁺ → CH ⁺ + H and its isotopic variant (D ₂ ⁺ )

   https://doi.org/10.1039/D0CP04810K  

   Hillenbrand, Pierre-Michel; Bowen, Kyle P.; Dayou, Fabrice; Miller, Kenneth A.; de Ruette, Nathalie; Urbain, Xavier; Savin, Daniel W. (December 2020, Physical Chemistry Chemical Physics)

   We report absolute integral cross section (ICS) measurements using a dual-source merged-fast-beams apparatus to study the titular reactions over the relative translational energy range of E r ∼ 0.01–10 eV. We used photodetachment of C − to produce a pure beam of atomic C in the ground electronic 3 P term, with statistically populated fine-structure levels. The H 2 + and D 2 + were formed in an electron impact ionization source, with well known vibrational and rotational distributions. The experimental work is complemented by a theoretical study of the CH 2 + electronic system in the reactant and product channels, which helps to clarify the possible reaction mechanisms underlying the ICS measurements. Our measurements provide evidence that the reactions are barrierless and exoergic. They also indicate the apparent absence of an intermolecular isotope effect, to within the total experimental uncertainties. Capture models, taking into account either the charge-induced dipole interaction potential or the combined charge-quadrupole and charge-induced dipole interaction potentials, produce reaction cross sections that lie a factor of ∼4 above the experimental results. Based on our theoretical study, we hypothesize that the reaction is most likely to proceed adiabatically through the 1 4 A′ and 1 4 A′′ states of CH 2 + via the reaction C( 3 P) + H 2 + ( 2 Σ+g) → CH + ( 3 Π) + H( 2 S). We also hypothesize that at low collision energies only H 2 + ( v ≤ 2) and D 2 + ( v ≤ 3) contribute to the titular reactions, due to the onset of dissociative charge transfer for higher vibrational v levels. Incorporating these assumptions into the capture models brings them into better agreement with the experimental results. Still, for energies ≲0.1 eV where capture models are most relevant, the modified charge-induced dipole model yields reaction cross sections with an incorrect energy dependence and lying ∼10% below the experimental results. The capture cross section obtained from the combined charge-quadrupole and charge-induced dipole model better matches the measured energy dependence but lies ∼30–50% above the experimental results. These findings provide important guidance for future quasiclassical trajectory and quantum mechanical treatments of this reaction. 

   more » « less
4. Using Diffusion Monte Carlo Wave Functions to Analyze the Vibrational Spectra of H ₇ O ₃ ⁺ and H ₉ O ₄ ⁺

   https://doi.org/10.1021/acs.jpca.1c05025  

   DiRisio, Ryan J.; Finney, Jacob M.; Dzugan, Laura C.; Madison, Lindsey R.; McCoy, Anne B. (August 2021, The Journal of Physical Chemistry A)

   null (Ed.)
5. Proton relays in anomalous carbocations dictate spectroscopy, stability, and mechanisms: case studies on C ₂ H ₅ ⁺ and C ₃ H ₃ ⁺

   https://doi.org/10.1039/C7CP05577C  

   Sager, LeeAnn M.; Iyengar, Srinivasan S. (January 2017, Phys. Chem. Chem. Phys.)

   We present a detailed analysis of the anomalous carbocations: C 2 H 5 + and C 3 H 3 + . This work involves (a) probing electronic structural properties, (b) ab initio dynamics simulations over a range of internal energies, (c) analysis of reduced dimensional potential surfaces directed along selected conformational transition pathways, (d) dynamically averaged vibrational spectra computed from ab initio dynamics trajectories, and (e) two-dimensional time–frequency analysis to probe conformational dynamics. Key findings are as follows: (i) as noted in our previous study on C 2 H 3 + , it appears that these non-classical carbocations are stabilized by delocalized nuclear frameworks and “proton shuttles”. We analyze this nuclear delocalization and find critical parallels between conformational changes in C 2 H 3 + , C 2 H 5 + , and C 3 H 3 + . (ii) The vibrational signatures of C 2 H 5 + are dominated by the “bridge-proton” conformation, but also show critical contributions from the “classical” configuration, which is a transition state at almost all levels of theory. This result is further substantiated through two-dimensional time–frequency analysis and is at odds with earlier explanations of the experimental spectra, where frequencies close to the classical region were thought to arise from an impurity. While this is still possible, our results here indicate an additional (perhaps more likely) explanation that involves the “classical” isomer. (iii) Finally, in the case of C 3 H 3 + our explanation of the experimental result includes the presence of multiple, namely, “cyclic”, “straight”, and propargyl, configurations. Proton shuttles and nuclear delocalization, reminiscent of those seen in the case of C 2 H 3 + , were seen all through and have a critical role in all our observations. 

   more » « less