Stereodynamics Study of the Reaction of O(3P) with CH4 (v = 0, j = 0)

A new London-Eyring-Polanyi-Sato (LEPS) potential energy surface (PES) is used in the O + CH4 → OH + CH3 reaction via the quasiclassical trajectory method (QCT). Comparing with the experiments and the former ab initio calculations, the new LEPS PES describes the actual potential energy surface of the O + CH4 reaction successfully. The four polarization dependent “generalized” differential cross sections (PDDCS) are presented in the center of mass frame. In the meantime, the distribution of dihedral angle [P(φr), the distribution of angle between k and j′ (P(θr)] and the angular distribution of product rotational vectors in the form of polar plots in θr and φr (P(θr, φr) are calculated. The isotope effect for the reactions O + CD4 is also calculated. These results are in good agreement with the experiments

Computational Study of Competition Between Direct Abstraction and Addition-Elimination in the Reaction of Cl Atoms with Propene

Quasi-classical trajectory calculations on a newly constructed and full-dimensionality potential energy surface (PES) examine the dynamics of the reaction of Cl atoms with propene. The PES is an empirical valence bond (EVB) fit to high-level ab initio energies and incorporates deep potential energy wells for the 1-chloropropyl and 2-chloropropyl radicals, a direct H atom abstraction route to HCl + allyl radical (CH2CHCH2(•)) products (Δ(r)H(298K)(⊖) = −63.1 kJ mol(-1)), and a pathway connecting these regions. In total, 94 000 successful reactive trajectories were used to compute distributions of angular scattering and HCl vibrational and rotational level populations. These measures of the reaction dynamics agree satisfactorily with available experimental data. The dominant reaction pathway is direct abstraction of a hydrogen atom from the methyl group of propene occurring in under 500 fs. Less than 10% of trajectories follow an addition–elimination route via the two isomeric chloropropyl radicals. Large amplitude motions of the Cl about the propene molecular framework couple the addition intermediates to the direct abstraction pathway. The EVB method provides a good description of the complicated PES for the Cl + propene reaction despite fitting to a limited number of ab initio points, with the further advantage that dynamics specific to certain mechanisms can be studied in isolation by switching off coupling terms in the EVB matrix connecting different regions of the PES.status: publishe

Exploring conformationally and state-resolved ionic and neutral reactions

Understanding the impact of the molecular structure of reactants and the initial reaction conditions, arising from their quantum states, is crucial for gaining control over chemical reactions and their outcomes. Recent advances in physical chemistry allow for detail exploration of fundamental effects behind the reaction mechanisms by combining advanced theoretical calculations with specifically designed experiments. This dissertation presents a complementary theoretical and experimental investigation of structural and quantum-state effects in bimolecular ionic and neutral reactions. Quantum-chemical calculations of the potential energy surface (PES) were employed to examine the impact of the molecular structure (conformation) on the reaction mechanisms of three conformationally-resolved ionic reactions, experimentally studied in a gas phase using molecular beams and ion-trapping methods. In the ion-molecule reaction of s-trans- and gauche-dibromobutadiene (DBB) with Ca$^+$, the theoretical results revealed a conformer-specific reaction mechanism likely originating from the anisotropic interactions in the entrance channel. For the polar cycloaddition of propene ions with DBB, the PES calculations showed the competition of a stepwise mechanism with a concerted process for the reaction with gauche-DBB, whereas a pure stepwise mechanism was identified for DBB in its s-trans conformation. Combining this with experimental kinetic data, it was found that both conformers are reactive with capture-limited reaction rates, indicating that the s-trans pathway is not inhibited by steric hindrance, which is typical in concerted mechanisms. Lastly, for the polar cycloaddition reaction of propene ions with methyl vinyl ketone, the stepwise reaction pathways were identified for both s-cis and s-trans conformers. A novel crossed-molecular-beam experiment was employed to study the effects of rotational quantum states in the neutral-neutral chemi-ionisation reaction between carbonyl sulfide (OCS) and metastable neon. It was found that the branching ratio between the Penning- and dissociative-ionisation channels strongly depends on the initial rotational state of the OCS molecule. These results were compared to the analytical interaction potential constructed using semi-empirical methods to explain differences in reactivity between the rotational states, and a tentative explanation was proposed. As a first step towards the investigation of radical reactions, two pulsed discharge sources, a high-voltage plate discharge and a dielectric-barrier discharge, generating supersonic beams of fluorine radicals, were developed. Finally, a potential reaction system for examining steric effects in hydrogen-atom abstraction, a mechanism typical for radical reactions, is suggested

Probing the dynamics of radical reactions with polyatomic hydrocarbons by crossed-beam dc slice imaging

This work presents results of crossed molecular beam imaging studies on the reaction of radicals (Cl and CN) with polyatomic hydrocarbons of different functionalities such as pentane isomers, deuterated alkanes, alkenes, and alcohols. The reactively scattered alkyl radicals are probed as a function of collision energy using single photon ionization. The scattering results for pentane are quite similar for all reactants, suggesting that the nature of the abstraction site has surprisingly little influence on the dynamics studied at ~5 and ~9 kcal/mol. The angular distributions are broad with a backscattered peak at low collision energy and a sharp forward peak at high collision energy. The similarity of the angular distributions was observed in the Cl reactions with deuterated butane albeit studied at considerably high collision energy. The presence of conformers in this target molecule likely play a major role. The reduced translational energy distributions manifested distinct dynamics showing marked variation with collision energy in the backward direction and variation in the forward direction for primary versus secondary abstraction, respectively. For alkenes, an isotropic component was observed in the angular distributions at low collision energy suggesting complex formation that survive for few rotational period, followed by HCl elimination. At increased collision energy, the distributions show a sharp forward peak superimposed on the isotropic component accounting for ~13% of the product flux. The forward translational energy distributions changed dramatically with collision energy. A sharp forward peak at ~80% of the collision energy appears at higher energy, similar to that of pentane isomers. The butanol isomers exhibit similar dynamics with that of saturated hydrocarbons although the HCl product distributions for these two systems are different. The angular distributions showed direct reaction with backscattering at low collision energy and enhanced forward scattering with respect to the alcohol beam with increased energy. This confirms that the well present in the potential energy surface is shallow to cause long-lived complexes to exist. The product translational energy distributions further support the similarity of these reactions. At high collision energy, a sharp peak of ~80% of the collision energy is seen in the forward scattered products. The sideways-scattered product showed the lowest fraction of energy appearing in translation. Hydrogen abstraction of CN radical with alkanes indicate direct reaction with the products largely backscattered and that most of the available energy (~80% - 85%) goes into internal energy the recoiling products. In the 1-pentene system, the results demonstrate the presence of H-atom abstraction channel yielding a resonantly-stabilized C5H9 radical. The results have implications for hydrocarbon growth and nitrile incorporation in formation of haze particles on Saturn\u27s moon, Titan

Highly Rotationally Excited N2 Reveals Transition-State Character in the Thermal Decomposition of N2O on Pd(110)

We employ time-slice and velocity map ion imaging methods to explore the quantum-state resolved dynamics in thermal N2O decomposition on Pd(110). We observe two reaction channels: a thermal channel that is ascribed to N2 products initially trapped at surface defects and a hyperthermal channel involving a direct release of N2 to the gas phase from N2O adsorbed on bridge sites oriented along the [001] azimuth. The hyperthermal N2 is highly rotationally excited up to J = 52 (v″ = 0) with a large average translational energy of 0.62 eV. Between 35 and 79% of the estimated barrier energy (1.5 eV) released upon dissociation of the transition state (TS) is taken up by the desorbed hyperthermal N2. The observed attributes of the hyperthermal channel are interpreted by post-transition-state classical trajectories on a density functional theory-based high-dimensional potential energy surface. The energy disposal pattern is rationalized by the sudden vector projection model, which attributes to unique features of the TS. Applying detailed balance, we predict that in the reverse Eley–Rideal reaction, both N2 translational and rotational excitation promote N2O formation

Roaming in the Dark: Deciphering the Mystery of NO3 --> NO + O2 Photolysis

The focus of this dissertation is to decipher the previously unknown reaction dynamics of NO3 photodissociation. Although the NO + O2 products are known to catalyze atmospheric ozone destruction, the mechanism by which these products are formed has remained a mystery, and no energetically accessible transition state has ever been calculated. Using velocity map ion imaging experiments to carefully study the stereochemistry of the product fragments combined with theoretical calculations performed by Drs. Xiao, Maeda, and Morokuma at Kyoto University, we have determined that the reaction proceeds exclusively via the unusual "roaming mechanism," with no evidence of a competing traditional transition state pathway. Within, the significance of this discovery is discussed in regards to both the NO3 system and roaming dynamics in general, for which this system has provided new insight

State Resolved Sliced Imaging Of Infrared Multiphoton Dissociation

This dissertation focuses on unimolecular dissociations of molecules under colissionless conditions with IRMPD. IRMPD was used as the dissociation technique in these studies since roaming type dissociations predominates from the ground electronic state. DC slice imaging was used with REMPI as the detection technique to study the products in a state selective manner to understand the nano scale dynamics of unimolecular dissociations. In the investigation of photodissociation dynamics of nitromethane and methyl nitrite with IRMPD, nitromethane show very low translational energy release of the photofragments and resemble the “roaming” pathway in the dissociation of nitromethane. The difference in the intensities of the averaged images show the presence of the lambda doublet propensity during the dissociation of nitromethane. In contrast, methyl nitrite does not show lambda doublet propensity while giving extra few kcal/mol during the dissociation into products relative to nitroethane supporting the conclusion of roaming mediated isomerization of nitromethane into methyl nitrite prior to dissociation. The studies were extended for the C-2, C-3, and C-4 systems to generalize the idea of isomerization of nitro compound to nitrite prior to dissociation. The studies suggested that the C-2 and the C-3 system predominates via the isomerization channel to give NO as a product despite the fact that the CME channel threshold is lower. Nitrobutane, however, show some distinct behavior than the C-2 and C-3 systems. The reason may be the presence of a 6-membered transition state or another pathway for the C-4 system which is not available for the C-2 and C-3 systems. Even though roaming reactions are more popular from the ground electronic state dissociations, NO3 molecule was reported roaming dissociations on an excited electronic. The two experimental sets carried out to understand the photodissociation of NO3 shows the roaming type dissociation of both D0 and D1 surfaces while suggesting a possible different dissociation pathway for the excited NO3. In the study of IRMPD of vinyl chloride, translational energy distributions suggest that vinyl chloride dissociate via the 3-C transition state while giving vibrationally and rotationally cold vinylidene as a product