Direct Chemical Dynamics Simulations of H3+ + CO Bimolecular Reaction

Intramolecular Vibrational Energy Redistribution in the Reaction H3+ + CO → H2 + HCO. J Phys Chem A 2024;128:8886-8896. [PMID: 39374191 DOI: 10.1021/acs.jpca.4c03818] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

An ab initio direct molecular dynamics (MD) calculation at the RS2/aug-cc-pVQZ level, followed by vibration mapping, has been applied to the H3+ + CO → H2 + HCO+ reaction to elucidate the intramolecular vibrational energy redistribution (IVR) processes during the reaction. Direct MD calculations were carried out for 20 K (a typical temperature for interstellar dark clouds) and 330 K (a typical translational temperature for ions in a glow discharge). Under the Cs symmetry constraint, the approach of H3+ turned out to be the H-C stretching mode of the [H···CO]+ part, which invoked the C-O stretching and then the H-C-O bending modes. Under no symmetry constraint, a strong bending mode was first invoked, and the intensities of the subsequent H-C and C-O stretching modes were kept relatively small. The detailed analyses of the IVR during the reaction, in terms of vibration mixing, gave a clue to understanding experimentally observed anomalies in the bending modes, such as population inversion at some bending states. In the MD simulation at 20 K, less than two-thirds of the reaction energy was converted to the vibrational energy of the resultant HCO+ part and one-third to the translational and rotational energies of the leaving H2 molecule. These direct MD simulations, when combined with the experimental spectroscopy data, shed light on a clear understanding of the reaction mechanism, including the IVR during the reaction.

Ring-Polymer Molecular Dynamics Calculations of Thermal Rate Coefficients and Branching Ratios for the Interstellar H3+ + CO → H2 + HCO+/HOC+ Reaction and Its Deuterated Analogue

The reaction between H₃⁺ and CO is important in understanding the H₃⁺ destruction mechanism in the interstellar medium. In this work, thermal rate coefficients for the H₃⁺ + CO and D₃⁺ + CO reactions are calculated using ring-polymer molecular dynamics (RPMD) on a high-level machine-learning potential energy surface. The RPMD results agree well with the classical molecular dynamics results, where nuclear quantum effects are completely ignored, whereas the agreement between the RPMD results and the previous quasi-classical trajectory is good only at low temperatures. The calculated [HCO⁺]/[HOC⁺] product branching ratios decrease as the temperature increases, and the product branching is exclusively determined by the initial collisional orientation, which governs the formation of an ion-dipole complex, H₃⁺···CO or H₃⁺···OC, that dissociates into H₂ + HCO⁺ or H₂ + HOC⁺, respectively, via a direct mechanism. However, the contribution of the indirect mechanism via the rearrangement between H₃⁺···CO and H₃⁺···OC increases as the temperature increases, although its absolute fraction is small.

Direct dynamics in a proton transfer reaction of isomer product competition. Insight into the suppressed formation of the isoformyl cation

Proton transfer between HOCO⁺ and CO produces the formyl cation HCO⁺ and isoformyl cation HOC⁺ isomers initiating multiple astrochemical reaction networks. Here, the direct chemical dynamics simulations are performed to uncover the underlying atomistic dynamics of the above reaction. The simulations reproduce the measured product energy and scattering angle distributions and reveal that the reaction proceeds predominantly through a direct stripping mechanism which results in the prominent forward scattering observed in experiments. The reaction dynamics show propensity for the HCO⁺ product even at a collision energy larger than the threshold for HOC⁺ formation. This is a consequence of the larger opacity and impact parameter range for HCO⁺. In accordance with the revealed direct mechanistic feature, the reaction can be controlled by orienting the reactants into a reactive H-C orientation that also favors HCO⁺ formation. Considering the lack of equilibrated reactant complexes and the on the fly migration of the proton, the CO₂-catalyzed isomerization is assumed to have insignificant impact on the isomer ratios. This work provides insights of dynamical effects besides energetics into the interesting finding of strongly suppressed formation of the metastable isoformyl cation for related proton transfer reactions in the measurements.

Investigation of corona discharge ionization of barbituric acid using ion mobility spectrometry along with quantum chemical calculations

This study aimed at examining atmospheric-pressure chemical ionization of barbituric acid through the corona discharge ion mobility spectrometry (CD-IMS) and the quantum chemical calculations. The results indicated two product ion peaks in the IMS spectrum of barbituric acid. The thermal decomposition of the barbituric acid sample was investigated by scanning the temperature of the injection port and analyzing the temporal evolution of the IMS peaks over elapsed time. It was found that the barbituric acid sample was not thermally decomposed in the injection port of the instrument. Experimental evidences were collected by changing the reactant ions, concentration of barbituric acid sample, and IMS cell temperature. The two observed peaks were then assigned to cationic form and oxygen protonated isomers of barbituric acid. The positions of the product ion peaks were explicated considering the dipole moments of the product ions.

Final-State-Resolved Dynamics of the H3+ + CO → H2 +HCO+/HOC+ Reaction: A Quasi-Classical Trajectory Study

The ion-molecule reaction H₃⁺ + CO → H₂ + HCO⁺/HOC⁺, which initiates the formation of crucial organic molecules, plays a key role in interstellar and circumstellar environments. In this work, the quasi-classical trajectory method is employed to study the reaction dynamics on a recently developed full-dimensional global potential energy surface (PES). The calculated product internal energy distributions and relative internal excited fractions agree reasonably well with the experimental measurements. For the two reaction channels, most of the available energy flows into the vibrational modes of HCO⁺ or HOC⁺ at low collision energies, followed by the translational mode and the rotational modes of HCO⁺ or HOC⁺. As the collision energy increases, the proportion of the product translational energy increases while the proportion of the product vibrational energy decreases. Furthermore, the CH and CO stretching modes and their combination bands are effectively excited for the product HCO⁺ while the bending mode is remarkably excited for the product HOC⁺.

Ab Initio and Quasiclassical Trajectory Study of the O(3P) + 2-Propanol Hydrogen Abstraction Reaction

We present a theoretical study of the hydrogen abstraction reaction from 2-propanol by ground-state oxygen atoms. First, ab initio calculations are used to characterize the stationary points of the potential energy surface. Rotation around the C-C-O-H dihedral affords two conformers in 2-propanol, which gives rise to 13 hydrogen abstraction reaction pathways grouped into three channels, Cα, Cβ, and O, depending on the abstraction site. Reaction at Cα exhibits the lowest barrier and largest exothermicity, followed by reaction at Cβ, and at 2-propanol's oxygen atom. Additional ab initio calculations beyond the stationary points are employed to obtain a grid of energies with which a specific-reaction-parameters (SRP) PM6 semiemipirical Hamiltonian is derived for the title reaction. The SRP-PM6 model captures the energetics of the reaction with higher accuracy than some conventional first-principles methods but is efficient enough to allow for extensive reaction dynamics calculations. Quasiclassical trajectories are subsequently propagated with the SRP-PM6 Hamiltonian to obtain reaction dynamics properties that are compared to experiments. Product translational energy and angular distributions for reaction at Cα with the two conformers of 2-propanol are in good agreement with recent molecular-beam measurements, and they exhibit largely backward scattering with modest energy release to relative translation. Most of the energy is deposited into the organic product, substantiating a reaction mechanism dominated by rebound dynamics.