Dao-Fu Yuan

The elementary reaction N(2D) + C6H5CH3 (toluene), of potential relevance in the atmospheric chemistry of Titan, has been investigated using a combined experimental/theoretical approach. CMB (crossed-molecular-beam) experiments employing mass-spectrometric detection and TOF (time-of-flight) analysis at collision energy of 34.3 kJ/mol were used for exploring primary products, branching fractions (BFs), and reaction micromechanism. Kinetic experiments in a continuous supersonic flow reactor were used for determining rate constants as a function of temperature (50, 75, 127, 177, and 296 K). Synergistically, electronic structure computations of the C7H8N PES (potential energy surface) and related statistical (Rice-Ramsperger-Kassel-Marcus/Master-Equation, RRKM/ME) calculations of product BFs were carried out to facilitate interpretation of experiments and elucidate the reaction micromechanism. It is found that barrierless “bridge” additions of N(2D) to the different positions of the aromatic ring form seven-, six-, and five-membered ring isomeric C7H8N intermediates that lead to a variety of product channels. Rate constants are fast, ranging between 2.1 and 2.4 × 10–10 cm3 s–1 over the 50–296 K range. The BFs of channels leading to “ring-contraction” via HCN/C2H2/H eliminations and to “ring-expansion” via CH3 elimination were quantified. Statistical simulations of isomeric product BFs on the computed PES were performed in CMB conditions and at 94, 175, and 296 K. Agreement is noted between experimental and theoretical BFs. The results are expected to be useful for improved modeling of Titan’s atmosphere and to provide molecular-level insight into N-incorporation at methyl-substituted aromatic carbon sites, with potential relevance to plasma-assisted functionalization of carbon nanomaterials.

Nuclear spin symmetry imposes rigorous selection rules on the rotational levels accessible to molecules with identical nuclei, potentially influencing nonadiabatic dynamical pathways. Here, we investigate this role in the state-selected photodissociation of H2S+ via the A2A1 (2, 9, 0) state using time-sliced velocity map imaging of ortho- and para-enriched parent cations. Our results show strict conservation of nuclear spin symmetry, with H2 co-fragments selectively populating odd- and even-JH2 states. Critically, fragment angular distributions exhibit a pronounced dependence on the parent’s nuclear spin symmetry. This highlights competition between the A2A1 → B2B2 → 14A2 and 12A′ → 14A″ dissociation pathways, governed strongly by nuclear spin, independent of rotational excitation. Furthermore, co-activating the symmetric stretching mode (v1) alongside the bending mode (v2) amplifies these symmetry-dependent differences. These findings reveal a vibrational-mode-specific coupling that actively steers nonadiabatic transition probabilities in molecular dissociation.

Due to the intrinsic electron-donating effect of chalcogen lone pairs to the aromatic rings, well-characterized radicals, including phenoxy and thiophenoxy, demonstrate that sulfur substitution for oxygen generally raises the electron affinity (EA). Here we report a counterintuitive reversal using high-resolution photoelectron imaging of 4-pyridoxide (4-PyO–) and 4-pyridylthiolate (4-PyS–). Well-resolved vibronic features are obtained for their corresponding neutral radicals. The EAs are determined to be 23941 ± 7 cm–1 (2.9683 ± 0.0009 eV) for 4-PyO and 22975 ± 6 cm–1 (2.8485 ± 0.0007 eV) for 4-PyS, with EA(4-PyO) > EA(4-PyS). Term energies of the A2B2 excited state also follow this order (0.6599 vs. 0.2219 eV). Combined quantum chemical calculations and molecular orbital analyses show that the electron-withdrawing ring nitrogen stabilizes the entire π-manifold, and the compact oxygen 2p orbitals undergo stronger conjugation-induced stabilization than diffuse sulfur 3p orbitals. This heterocycle-mediated p-π conjugation modulation overrides the classical donating effect and provides a critical design principle for tuning frontier orbitals in heteroaromatic radicals.

Photodissociation dynamics of deuterium sulfide cations (D2S+) via the A2A1 state were investigated using the time-sliced velocity map ion imaging technique. High-resolution images of S+(4S) from the S+-loss channel, D2(X1Σg+) + S+(4S), were acquired at five wavelengths near 320 nm. From the high-resolution images, we derived the total product kinetic energy releases, angular distributions, and rovibrational-state populations of the D2(X1Σg+) co-products. The product angular distributions are nearly isotropic across all photolysis wavelengths studied. While the available energy distributions in internal and translational energy show only weak variations, the internal state populations of products exhibit clear wavelength dependence. Based on MRCI+Q/aug-cc-pV5Z calculations, the energy correlation diagram of D2S+ was constructed. The excitation photon energies employed in this study lie near the dissociation threshold of the SD+(3Σ−) + D(2S) channel. The results reveal rich photofragmentation dynamics arising from complex non-adiabatic couplings among several electronic states. Multiple dissociation pathways, including possible roaming mechanisms, contribute to the formation of D2(X1Σg+) + S+(4S) products at the excitation energy near the dissociation threshold of the SD+(3Σ−) + D(2S) channel.

Nonadiabatic dynamics play an essential role in unraveling complex chemical processes. With intricate existing at the dissociation product channels, HONO has emerged as a classic benchmark system for investigating nonadiabatic dynamics. To elucidate the dynamic interplay between nonadiabatic transition and intramolecular hydrogen bonding in the vicinity of the product asymptotic region, the photodissociation dynamics of cis-nitrous acid (cis-HONO, ¹A″, 2₀ ²) have been investigated using the time-sliced velocity map ion imaging technique. It is found that the intramolecular hydrogen bonding in cis-HONO leads to a significant difference in the force field between cis- and trans-HONO, thereby leaving a characteristic fingerprint on their photochemical behaviors: the hydrogen bonding induces coupling between the N=O stretch vibration (v₂) and the in-plane ONO bend vibration (v₅) in the cis isomer, resulting in an increase in the "rotational temperature" of the photoejected NO fragment; meanwhile, the hydrogen bond suppresses the excitation of the out-of-plane torsional mode, leading to a slight increase in the anisotropy of the dissociation products. Furthermore, the relatively stronger intramolecular hydrogen bonding in cis-HONO renders the nonadiabatic transition probability negligible. This study provides key experimental evidence for understanding hydrogen bond-regulated nonadiabatic dynamics, and further high-level theoretical calculations are required to clarify the environmental effect of the ubiquitous intramolecular hydrogen bonding on nonadiabatic dynamics.

Quantum state-resolved studies of molecular photodissociation provide an effective view of how electronic and nuclear motion conspire to break chemical bonds. Here, we use parity-resolved photofragment imaging of trans-HONO photodissociation to show how electronic symmetry actively steers intramolecular vibrational energy redistribution (IVR) at the moment of bond cleavage. By performing Λ-doublet–resolved detection of OH­(X ²Π_3/2, υ = 0, J = 3/2, 5/2, 7/2) with velocity-map ion imaging, we simultaneously determine the vibrational state distributions and angular distributions of the NO­(X ²Π, υ) cofragment. The parity-resolved measurements reveal a striking correlation: one parity class of OH is strongly associated with highly excited NO­(υ = 2), whereas the opposite parity favors NO­(υ = 1). Analysis of the total kinetic energy release, together with ab initio potential energy and spin–orbit coupling calculations, shows that these propensities fingerprint two competing pathways on electronically distinct excited-state surfaces. A prompt, nearly adiabatic dissociation on an A″ surface preserves electronic symmetry and channels energy into a specific NO stretch, whereas nonadiabatic transfer to an A′ surface enables electron-mediated IVR via the out-of-plane mode of trans-HONO. The J dependence of the parity–vibrational correlation further reveals near-resonant coupling between the N=O stretch and a combination band. Our results demonstrate that fragment parity can map electronic symmetry onto product energy flow, offering a general strategy for disentangling electronically mediated and near-resonant vibrational dynamics in complex photochemical reactions.

Quantum interference between reaction pathways plays a crucial role in understanding the microscopic mechanisms of chemical reactions. The hydrogen exchange reaction, involving a well-characterized conical intersection, exhibits rich dynamics arising from interference between multiple reactive pathways. Here, we report high-resolution scattering measurements at a collision energy of 2.38 eV, which reveal pronounced sideways angular oscillations in the HD product distribution. Together with exact quantum theoretical calculations, these results provide unambiguous evidence of geometric phase-induced quantum interference in the H + D₂ reaction. Detailed analysis reveals that the observed sideways scattering oscillations, hallmarks of quantum interference, are strongly influenced by transient D–D bond elongation in D₂ reactants. This behavior, in contrast to the reaction with HD, highlights a pronounced isotopic effect and reflects the unique dynamic competition along the roaming insertion pathway. Collectively, these findings underscore the critical role of pathway interference and provide new mechanistic insights into how the isotopic effect modulated the geometric phase in an elementary chemical reaction.

The H + CH4 reaction and its isotopic variant reactions serve as a benchmark reaction system to understand the dynamics of polyatomic reactions. In this paper, we report a vibrationally state-resolved crossed molecular beam study on the D + CD4 → D2 + CD3 reaction using the time-sliced velocity map ion imaging method. Pronounced collision energy dependence is found in product kinetic energy distributions and angular distributions. At 0.67 eV, CD3 products are dominantly distributed in the backward direction with D2 products in the vibrational ground state. As collision energy increases, the angular distributions of CD3 products shift from the backward direction to the sideways and forward directions, and vibrational excited D2 appears and intensifies. By comparing our result of the D + CD4 → D2 + CD3 reaction with previous studies on the D + CH4 → HD + CH3 and H + CD4 → HD + CD3 reactions, isotope effects have been discussed. The product kinetic energy and angular distributions vary significantly among the three isotopic variant reactions.

How the non-reacting moiety of a molecule influences a polyatomic reaction has been a topic of much research interest. Here we present a comprehensive investigation of the D + CH₄ → HD + CH₃ reaction, a benchmark polyatomic elementary reaction with CH₃ as the non-reacting moiety, employing a high-resolution crossed molecular beams apparatus and an accurate seven-dimensional wave packet method. An interesting angular distribution of the CH₃(v’ = 1) product umbrella bending vibrational state is observed to scatter more in the sideways direction than the CH₃(v’ = 0) one. By monitoring the wave functions on a dividing surface in the transition state region, the CH₃ umbrella bending mode is established as a reporter mode that faithfully reveals how the D atom dynamically approaches CH₄ at different total angular momenta or impact parameters. This discovery of the reporter mode provides an opportunity for the detailed study of polyatomic reaction dynamics.