S_N2-type halide substitution and hydrolysis are two of the most ubiquitous reactions in chemistry. The interplay between these processes is fundamental in atmospheric chemistry through reactions of N₂O₅ and seawater. N₂O₅ plays a major role in regulating levels of O₃, OH, NO _x , and CH₄. While the reactions of N₂O₅ and seawater are of central importance, little is known about their mechanisms. Of interest is the activation of Cl in seawater by the formation of gaseous ClNO₂, which occurs despite the fact that hydrolysis (to HNO₃) is energetically more favorable. We determine key features of the reaction landscape that account for this behavior in a theoretical study of the cluster N₂O₅/Cl^-/H₂O. This was carried out using ab initio molecular dynamics to determine reaction pathways, structures, and time scales. While hydrolysis of N₂O₅ occurs in the absence of Cl^-, results here reveal that a low-lying pathway featuring halide substitution intermediates enhances hydrolysis.

Acetaldehyde cations (CH₃CHO⁺) were prepared using single-photon vacuum ultraviolet ionization of CH₃CHO in a molecular beam and the fragmentation dynamics explored over the photolysis wavelength range 390-210 nm using velocity-map ion imaging and photofragment yield (PHOFY) spectroscopy. Four fragmentation channels are characterized: CH₃CHO⁺→ C₂H₃O⁺ + H (I), CH₃CHO⁺→ HCO⁺ + CH₃ (II), CH₃CHO⁺→ CH₃⁺ + HCO (III), CH₃CHO⁺→ CH₄⁺ + CO (IV). Channels (I), (II), and (IV) are observed across the full photolysis wavelength range while channel (III) is observed only at λ < 317 nm. Maximum fragment ion yields are obtained at ∼250 nm. Ion images were recorded over the range 316-228 nm, which corresponds to initial excitation to the B[combining tilde]²A' and C[combining tilde]²A' states of CH₃CHO⁺. The speed and angular distributions are distinctly different for each detected ion and show evidence of both statistical and dynamical fragmentation pathways. At longer wavelengths, fragmentation via channel (I) leads to modest translational energies (E_T), consistent with dissociation over a small barrier and production of highly internally excited CH₃CO⁺. Additional components with E_INT greater than the CH₃CO⁺ secondary dissociation threshold appear at shorter wavelengths and are assigned to fragmentation products of vinyl alcohol cation or oxirane cation formed by isomerization of energized CH₃CHO⁺. The E_T distribution observed for channel (III) products peaks at zero but is notably colder than that predicted by phase space theory, particularly at longer photolysis wavelengths. The colder-than-statistical E_T distributions are attributed to contributions from secondary fragmentation of energized CH₃CO⁺ formed via channel (I), which are attenuated by CH₃CHO⁺ isomerization at shorter wavelengths. Fragmentation via channels (II) and (IV) results in qualitatively similar outcomes, with evidence of isotropic statistical components at low-E_T and anisotropic components due to excited state dynamics at higher E_T.

In typical carbonyl-containing molecules, bond dissociation events follow initial excitation to nπ_C═O^* states. However, in acetyl iodide, the iodine atom gives rise to electronic states with mixed nπ_C═O^* and nσ_C-I^* character, leading to complex excited-state dynamics, ultimately resulting in dissociation. Using ultrafast extreme ultraviolet (XUV) transient absorption spectroscopy and quantum chemical calculations, we present an investigation of the primary photodissociation dynamics of acetyl iodide via time-resolved spectroscopy of core-to-valence transitions of the I atom after 266 nm excitation. The probed I 4d-to-valence transitions show features that evolve on sub-100-fs time scales, reporting on excited-state wavepacket evolution during dissociation. These features subsequently evolve to yield spectral signatures corresponding to free iodine atoms in their spin-orbit ground and excited states with a branching ratio of 1.1:1 following dissociation of the C-I bond. Calculations of the valence excitation spectrum via equation-of-motion coupled cluster with single and double substitutions (EOM-CCSD) show that initial excited states are of spin-mixed character. From the initially pumped spin-mixed state, we use a combination of time-dependent density functional theory (TDDFT)-driven nonadiabatic ab initio molecular dynamics and EOM-CCSD calculations of the N_4,5 edge to reveal a sharp inflection point in the transient XUV signal that corresponds to rapid C-I homolysis. By examining the molecular orbitals involved in the core-level excitations at and around this inflection point, we are able to piece together a detailed picture of C-I bond photolysis in which d → σ* transitions give way to d → p excitations as the bond dissociates. We also report theoretical predictions of short-lived, weak 4d → 5d transitions in acetyl iodide, validated by weak bleaching in the experimental transient XUV spectra. This joint experimental-theoretical effort has thus unraveled the detailed electronic structure and dynamics of a strongly spin-orbit coupled system.