Site-Selective Dissociation Processes of Cationic Ethanol Conformers: The Role of Hyperconjugation

In present report, we explored hyperconjugation effects on the site- and bond-selective dissociation processes of cationic ethanol conformers by the use of theoretical methods (including configuration optimizations, natural bond orbital (NBO) analysis, and density of states (DOS) calculations, etc.) and the tunable synchrotron vacuum ultraviolet (SVUV) photoionization mass spectrometry. The dissociative mechanism of ethanol cations, in which hyperconjugative interactions and charge-transfer processes were involved, was proposed. The results reveal C_α–H and C–C bonds are selectively weakened, which arise as a result of the hyperconjugative interactions σ_Cα‑H → p in the trans-conformer and σ_C–C → p in gauche-conformer after being ionized. As a result, the selective bond cleavages would occur and different fragments were observed

Measurements of Secondary Organic Aerosol Formed from OH-initiated Photo-oxidation of Isoprene Using Online Photoionization Aerosol Mass Spectrometry

Isoprene is a significant source of atmospheric organic aerosol; however, the secondary organic aerosol (SOA) formation and involved chemical reaction pathways have remained to be elucidated. Recent works have shown that the photo-oxidation of isoprene leads to form SOA. In this study, the chemical composition of SOA from the OH-initiated photo-oxidation of isoprene, in the absence of seed aerosols, was investigated through the controlled laboratory chamber experiments. Thermal desorption/tunable vacuum-ultraviolet photoionization time-of-flight aerosol mass spectrometry (TD-VUV-TOF-PIAMS) was used in conjunction with the environmental chamber to study SOA formation. The mass spectra obtained at different photon energies and the photoionization efficiency (PIE) spectra of the SOA products can be obtained in real time. Aided by the ionization energies (IE) either from the ab initio calculations or the literatures, a number of SOA products were proposed. In addition to methacrolein, methyl vinyl ketone, and 3-methyl-furan, carbonyls, hydroxycarbonyls, nitrates, hydroxynitrates, and other oxygenated compounds in SOA formed in laboratory photo-oxiadation experiments were identified, some of them were investigated for the first time. Detailed chemical identification of SOA is crucial for understanding the photo-oxidation mechanisms of VOCs and the eventual formation of SOA. Possible reaction mechanisms will be discussed

Cl-Loss Dynamics of Vinyl Chloride Cations in the B²A″ State: Role of the C²A′ State

The dissociative photoionization of vinyl chloride (C₂H₃Cl) in the 11.0–14.2 eV photon energy range was investigated using threshold photoelectron photoion coincidence (TPEPICO) velocity map imaging. Three electronic states, namely, A²A′, B²A″, and C²A′, of the C₂H₃Cl⁺ cation were prepared, and their dissociation dynamics were investigated. A unique fragment ion, C₂H₃⁺, was observed within the excitation energy range. TPEPICO three-dimensional time-sliced velocity map images of C₂H₃⁺ provided the kinetic energy release distributions (KERD) and anisotropy parameters in dissociation of internal-energy-selected C₂H₃Cl⁺ cations. At 13.14 eV, the total KERD showed a bimodal distribution consisting of Boltzmann- and Gaussian-type components, indicating a competition between statistical and non-statistical dissociation mechanisms. An additional Gaussian-type component was found in the KERD at 13.65 eV, a center of which was located at a lower kinetic energy. The overall dissociative photoionization mechanisms of C₂H₃Cl⁺ in the B²A″ and C²A′ states are proposed based on time-dependent density functional theory calculations of the Cl-loss potential energy curves. Our results highlight the inconsistency of previous conclusions on the dissociation mechanism of C₂H₃Cl⁺