Roaming mediated nonadiabatic dynamics in molecular hydrogen elimination from propane at 157 nm

Direct tracking of H2 roaming reaction in real time

Roaming is an unconventional type of molecular reaction where fragments, instead of immediately dissociating, remain weakly bound due to long-range Coulombic interactions. Due to its prevalence and ability to form new molecular compounds, roaming is fundamental to photochemical reactions in small molecules. However, the neutral character of the roaming fragment and its indeterminate trajectory make it difficult to identify experimentally. Here, we introduce an approach to image roaming, utilizing intense, femtosecond IR radiation combined with Coulomb explosion imaging to directly reconstruct the momentum vector of the neutral roaming H2, a precursor toH 3 + formation, in acetonitrile, CH3CN. This technique provides a kinematically complete picture of the underlying molecular dynamics and yields an unambiguous experimental signature of roaming. We corroborate these findings with quantum chemistry calculations, resolving this unique dissociative process.

Photodissociation Dynamics of Methyl Hydroperoxide at 193 nm: A Trajectory Surface-Hopping Study

The photodissociation of methyl hydroperoxide (CH₃OOH) at 193 nm has been studied using a direct dynamics trajectory surface-hopping (TSH) method. The potential energies, energy gradients, and nonadiabatic couplings are calculated on the fly at the MRCIS(6,7)/aug-cc-pVDZ level of theory. The hopping of a trajectory from one electronic state to another is decided on the basis of Tully's fewest switches algorithm. An analysis of the trajectories reveals that the cleavage of the weakest O-O bond leads to major products CH₃O(²E) + OH(²Π), contributing about 72.7% of the overall product formation. This OH elimination was completed in the ground degenerate product state where both the ground singlet (S₀) and first excited singlet (S₁) states become degenerate. The O-H bond dissociation of CH₃OOH is a minor channel contributing about 27.3% to product formation, resulting in products CH₃OO + H. An inspection of the trajectories indicates that unlike the major channel OH elimination, the H-atom elimination channel makes a significant contribution (∼3% of the overall product formation) through the nonadiabatic pathway via conical intersection S₁/S₀ leading to ground-state products CH₃OO(X ²A″) + H(²S) in addition to adiabatic dissociation in the first excited singlet state, S₁, correlating to products CH₃OO(1 ²A') + H(²S). The computed translational energy of the majority of the OH products is found to be high, distributed in the range of 70 to 100 kcal/mol, indicating that the dissociation takes place on a strong repulsive potential energy surface. This finding is consistent with the nature of the experimentally derived translational energy distribution of OH with an average translational energy of 67 kcal/mol after the excitation of CH₃OOH at 193 nm.

H2 roaming chemistry and the formation of H3+ from organic molecules in strong laser fields

Roaming mechanisms, involving the brief generation of a neutral atom or molecule that stays in the vicinity before reacting with the remaining atoms of the precursor, are providing valuable insights into previously unexplained chemical reactions. Here, the mechanistic details and femtosecond time-resolved dynamics of H₃⁺ formation from a series of alcohols with varying primary carbon chain lengths are obtained through a combination of strong-field laser excitation studies and ab initio molecular dynamics calculations. For small alcohols, four distinct pathways involving hydrogen migration and H₂ roaming prior to H₃⁺ formation are uncovered. Despite the increased number of hydrogens and possible combinations leading to H₃⁺ formation, the yield decreases as the carbon chain length increases. The fundamental mechanistic findings presented here explore the formation of H₃⁺, the most important ion in interstellar chemistry, through H₂ roaming occurring in ionic species.

H₂ roaming is associated with H₃⁺ formation when certain organic molecules are exposed to strong laser fields. Here, the mechanistic details and time-resolved dynamics of H₃⁺ formation from a series of alcohols were obtained and found that the product yield decreases as the carbon chain length increases.

Trajectory surface hopping study of propane photodissociation dynamics at 157 nm

The photodissociation dynamics of propane molecules has been studied using the quasiclassical trajectory surface hopping (TSH) method in conjunction with Tully's fewest switches algorithm. The trajectories are propagated on potential energy surfaces computed on-the-fly using the multiconfiguration and multireference ab initio method starting in the lowest excited singlet state (HOMO → 3s Rydberg state) of propane at 157 nm with the emphasis on the site specificity of atomic hydrogen elimination, molecular hydrogen elimination, and their product branching ratios. Our dynamics simulation revealed that there are three primary dissociation channels: the atomic hydrogen elimination, the molecular hydrogen elimination, and the C-C bond scission. The trajectories indicate that the H₂ elimination from the internal carbon atom (2,2-H₂ elimination) and terminal carbon atom (1,1-H₂ elimination) is the major process and follows a three centred synchronous concerted mechanism. 1,2-H₂ and 1,3-H₂ eliminations on the other hand are minor processes and exclusively follow the roaming mediated nonadiabatic dynamics. The probability of elimination of the hydrogen atom from two terminal groups (terminal hydrogen elimination) is greater than that from the internal CH₂ group (internal hydrogen elimination). Almost 83% of atomic hydrogen elimination occurs through the asynchronous concerted mechanism from the terminal carbon atom via triple dissociation leading to CH₃ + C₂H₄ + H products. This finding is in good agreement with a recent experimental observation. The present TSH study indicates that approximately one-third of the trajectories those resulted in a triple dissociation channel, CH₃ + C₂H₄ + H completed in the ground singlet state following a nonadiabatic path (hopping from the first excited singlet S₁ to the ground state S₀) via the C-C and C-H dissociation coordinate conical intersection S₁/S₀. The products CH₃(1 ²A₂^″) + C₂H₄(¹A_g) + H, obtained are ground state methyl radicals and ground state ethylene. The trajectories those ended in a triple dissociation channel CH₃ + C₂H₄ + H adiabatically in the S₁ state lead to CH₃(1 ²A₂^″) + C₂H₄ (1 ³B₁) + H, where singlet methyl radicals and triplet ethylene are formed in their corresponding lowest electronic state via a spin conserving route. Two channels, CH₄ + CH₃CH and C₂H₆ + CH_2, are found to have minor contributions. In the case of methane elimination, the trajectories that follow an adiabatic path lead to CH₃CH(1 ¹A″) + CH_4,(1 ¹A₁), where ethylidene is in the excited state and methane is in the ground state. Methane elimination via nonadiabatic path leads to CH₃CH(1¹A^') + CH₄(1 ¹A₁), where both ethylidene and methane are in the ground electronic state. Ethane eliminations follow the adiabatic path leading to C₂H₆(1 ¹A_1g) + CH₂(1 ¹B₁) where ethane is in the ground state and methylene is in the first excited state.