RESUMO
Nitrogen heterocycles are known to be important light-absorbing chromophores in a newly discovered class of aerosols, commonly referred to as "brown carbon" (BrC) aerosols. Due to their significant absorption and spectral overlap with the solar actinic flux, these BrC chromophores steer the physical and optical properties of aerosols. To model the local aqueous solvation environment surrounding BrC chromophores, we generated cold molecular complexes with water and a prototypical BrC chromophore, 1-phenylpyrrole (1PhPy), using supersonic jet-cooling and explored their intermolecular interactions using single-conformation spectroscopy. Herein, we utilized resonant two-photon ionization (R2PI) and UV holeburning (UV HB) double-resonance spectroscopies to obtain a molecular-level understanding of the role of water microsolvation in charge transfer upon photoexcitation of 1PhPy. Quantum chemical calculations and one-dimensional discrete variable representation simulations revealed insights into the charge transfer efficacy of 1PhPy with and without addition of a single water molecule. Taken together, our results indicate that the intermolecular interactions with water guide the geometry of 1PhPy to adopt a more twisted intramolecular charge transfer (TICT) configuration, thus facilitating charge transfer from the pyrrole donor to the phenyl ring acceptor. Furthermore, the water network surrounding 1PhPy reports on the charge transfer such that the H2O solvent primarily interacts with the pyrrole ring donor in the ground state, whereas it preferentially interacts with the phenyl ring acceptor in the excited state. Large Franck-Condon activity is evident in the 1PhPy + 1H2O excitation spectrum for the water-migration vibronic bands, supporting H2O solvent reorganization upon excitation of the 1PhPy chromophore. Fluorescence measurements with increasing H2O % volume corroborated our gas-phase studies by indicating that a polar water solvation environment stabilizes the TICT configuration of 1PhPy in the excited electronic state, from which emission is observed at a lower energy compared to the locally excited configuration.
RESUMO
Nitric oxide (NO) radicals are ubiquitous chemical intermediates present in the atmosphere and in combustion processes, where laser-induced fluorescence is extensively used on the NO (A2Σ+ â X2Π) band to report on fuel-burning properties. However, accurate fluorescence quantum yields and NO concentration measurements are impeded by electronic quenching of NO (A2Σ+) to NO (X2Π) with colliding atomic and molecular species. To improve predictive combustion models and develop a molecular-level understanding of NO (A2Σ+) quenching, we report the velocity map ion images and product state distributions of NO (X2Π, vâ³ = 0, Jâ³, Fn, Λ) following nonreactive collisional quenching of NO (A2Σ+) with molecular oxygen, O2 (X3Σg -). A novel dual-flow pulse valve nozzle is constructed and implemented to carry out the NO (A2Σ+) electronic quenching studies and to limit NO2 formation. The isotropic ion images reveal that the NO-O2 system evolves through a long-lived NO3 collision complex prior to formation of products. Furthermore, the corresponding total kinetic energy release distributions support that O2 collision coproducts are formed primarily in the c1Σu - electronic state with NO (X2Π, vâ³ = 0, Jâ³, Fn, Λ). The product state distributions also indicate that NO (X2Π) is generated with a propensity to occupy the Π(Aâ³) Λ-doublet state, which is consistent with the NO π* orbital aligned perpendicular to nuclear rotation. The deviations between experimental results and statistical phase space theory simulations illustrate the key role that the conical intersection plays in the quenching dynamics to funnel population to product rovibronic levels.
