Singlet O₂ from Ultraviolet Excitation of the Quinoline-O₂ Complex

Abstract

We report results from experiments with the quinoline-O 2 complex, which was photodissociated using light near 312 nm. Photodissociation resulted in formation of the lowest excited state of oxygen, O 2 a 1 Δ g , which we detected using resonance enhanced multiphoton ionization and velocity map ion imaging. The O 2 + ion image allowed for a determination of the center-of-mass translational energy distribution, P(E T ), following complex dissociation. We also report results of electronic structure calculations for the quinoline singlet ground state and lowest energy triplet state. From the CCSD/aug-cc-pVDZ//(U)MP2/cc-pVDZ calculations, we determined the lowest energy triplet state to have ππ* electronic character and to be 2.69 eV above the ground state. We also used electronic structure calculations to determine the geometry and binding energy for several quinoline-O 2 complexes. The calculations indicated that the most strongly bound complex has a well depth of about 0.11 eV and places the O 2 moiety above and approximately parallel to the quinoline ring system. By comparing the experimental P(E T ) with the energy for the singlet ground state and the lowest energy triplet state, we concluded that the quinoline product was formed in the lowest energy triplet state. Finally, we found the experimental P(E T ) to be in agreement with a Prior translational energy distribution, which suggests a statistical dissociation for the complex.

Ultraviolet Excitation of M-O2 (M = Phenalenone, Fluorenone, Pyridine, & Acridine) Complexes Resulting in 1O2

“…For β-ionone-O 2 photodissociation at 312 nm, the available energy is then in the range of 0.53–0.87 eV and our observed experimental P ( E T ) was somewhat colder than statistical . Thus, for low available energies, the experimental P ( E T ) appears close to statistical as exhibited by quinoline (0.11 eV), fluorenone (0.13–0.14 eV), and pyridine (0.19–0.20 eV) where the number in parentheses is the available energy. Alternatively, for higher available energies, the experimental P ( E T ) appears to be colder than statistical as observed with β-ionone (0.53–0.87 eV), acridine (0.49 eV), and phenalenone (0.52–0.54 eV).…”

“…We now consider the discrepancy between the experimental and statistical P ( E T ), observed particularly for phenalenone-O 2 and acridine-O 2 , Figures and . We first compare these observations with our previous work for the quinoline-O 2 and β-ionone-O 2 complexes, which were photodissociated at 312 nm (3.97 eV). , For quinoline, the adiabatic triplet energy was reported as 2.79 eV, and for quinoline-O 2 , the available energy was 0.11 eV and the experimental and statistical P ( E T ) showed good agreement. , For β-ionone, the lowest triplet energy was measured from solution phase experiments to be 2.38 eV. − Our previous electronic structure calculations gave the adiabatic triplet energy of 2.04 eV and the β-ionone-O 2 complex binding energy was calculated as 0.08 eV . For β-ionone-O 2 photodissociation at 312 nm, the available energy is then in the range of 0.53–0.87 eV and our observed experimental P ( E T ) was somewhat colder than statistical .…”

“…Since no 1 O 2 was observed without an organic chromophore seeded into the molecular beam, we conclude the signal originates from a cluster of the form (C 13 H 8 O) n -O 2 with n ≥ 1 and the unobserved partner fragment in Figure must contain phenalenone. For previous experiments with quinoline-O 2 , we recorded a mass spectrum of the molecular beam using the focused 312 nm laser for ionization and observed a weak signal for the bimolecular complex with no signal observed for higher-order clusters . For quinoline, 312 nm photons are nearly resonant with the S 2 (ππ*) excited state, which may enhance the ionization of quinoline-containing clusters.…”

“…To determine the available energy using eq , we need the ground state complex binding energy, D 0 ″. We have previously determined such binding energies using ab initio MP2 electronic structure calculations. ,, Our previous MP2/aug-cc-pVDZ calculations for pyridine-O 2 found the most stable geometry with the O 2 moiety above the ring with the complex having approximate C s symmetry and a BSSE-corrected binding energy of 456 cm –1 (0.056 eV); however, MP2 calculations with large basis sets may become cost prohibitive for complexes including larger organic molecules. Therefore, we performed a series of calculations for the pyridine-O 2 complex to evaluate four DFT methods: BLYP, B3LYP, PBE, and M06 with the goal of determining which methods provide binding energies consistent with MP2 results.…”

“…How energy is partitioned between CM translation and rotational and vibrational motions of the chromophore is indicative of the photodissociation mechanism. In our previous work, photodissociation was analyzed in terms of a statistical mechanism; , however, the current work gives evidence for photodissociation via vibrational predissociation.…”