Xingan Wang

Bis(trifluoromethyl) sulfide (CF3SCF3) is a candidate insulating gas whose atmospheric persistence is poorly constrained: the structure–activity relationship cannot be reliably extrapolated to this perfluorinated species, and the only available theoretical estimate has not yet been tested against experiment. We measured the rate coefficient of its dominant atmospheric loss, reaction with OH, by pulsed laser photolysis–laser-induced fluorescence over 272–328 K at 50 Torr. The reaction is slow and nearly temperature independent, with k1(298 K) = (2.15 ± 0.34) × 10–15 cm3 molecule–1 s–1 and a slight positive trend (apparent Ea = (0.30 ± 0.14) kcal mol–1). No O2 dependence was detectable at 50 Torr. The measured k1 exceeds the theoretical value by approximately an order of magnitude, implying an OH-initiated lifetime of ∼14.7 years and a 100-year GWP of ∼2570, well below that of SF6 (24300), supporting further evaluation of CF3SCF3 as an alternative dielectric gas.

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.

CO ₂ photodissociation dynamics in the low-energy ¹ Δ _u ← X ¹ Σ _g ⁺ band are investigated by VMI with state-selective detection of O( ¹ D).

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.

Photodissociation of discharge-prepared CO(a′ ³ Σ ⁺ , a ³ Π) was studied using velocity-map imaging. Results suggest access to a high-lying triplet Rydberg–valence manifold and predissociation via nonadiabatic coupling to repulsive ³ Π states.

Two-photon dissociation dynamics of N2O were investigated by means of the time-sliced velocity map ion imaging technique. The N(2PJ) + NO(X2Π) and N(2DJ) + NO(X2Π) channels were accessed via two-photon excitation in the wavelength range of 218–236 nm. The four lobes in the angular distributions were clearly observed in both N-atom product channels, implying that the coherence between the two sequential transitions in the overall two-photon photodissociation process at the energies investigated has a significant influence on the angular distribution of the products. In addition, the translational energy distributions have been determined from the recorded images, revealing highly inverted vibrational populations of both NO products. In the N(2PJ) channel, initial vibrational excitation has a significant impact on the photodissociation dynamics of N2O via 3pσ1Π states. According to experimental results, the N(2PJ) channel may proceed via a bent configuration dissociation pathway involving coupling to an A′ symmetry potential energy surface that has not yet been theoretically reported. While the N(2DJ) channel may be preferentially formed via triplet and singlet Rydberg states that couple to the 43A′ and 51A′ states, followed by dissociation in a bent geometry.