Evan John Bieske

Iodine chemistry has an outsized role in the atmosphere and environment due to its highly reactive nature. Unlike chlorine and bromine, around 80% of atmospheric iodine is emitted directly from the ocean through the reaction of iodide and ozone at the sea surface. Uncertainties remain, however, in the effects of hydration, salinity, temperature, and other factors on this globally significant reaction. Using linear ion-trap mass spectrometry, a single water molecule is shown here to provide a 23 (±6)-fold acceleration to the rate of iodide oxidation. The first step in reaction of both I– and I–·H2O with ozone is oxidation to IO– with subsequent fast reactions to yield IO3–. No evidence is found for direct routes from I–·H2O to IO2– nor any incorporation of the water oxygen into ionic products. Ab initio calculations show that the initial rate-determining step for both reactions proceeds on the singlet surface with water playing an active role to lower the barrier through hydrogen bonding. Kinetic modeling based on the computed energetics is in excellent agreement with experimental measurements and enables extrapolation of this reaction to lower temperatures relevant to polar regions and the ozone layer. This analysis shows that (i) I– may outcompete I• for ozone destruction as temperature decreases, and (ii) I–·H2O reacts more than an order of magnitude faster than I• over 350 to 230 K. This work demonstrates that iodide has a previously underappreciated role in multiphase ozone destruction in the atmosphere and lays the foundation for understanding microscopic environmental influences in salt clusters and nanodroplets.

Electronic and infrared spectra of HC_2nO⁺ (n = 3–7) chains contained in a cryogenically cooled ion trap are measured using resonance-enhanced photodissociation spectroscopy. The HC_2nO⁺ chains, which possess X̃³Σ^– ground electronic states, are prepared through spin-forbidden association reactions between HC_2n–1⁺ chains and CO molecules, both of which have X̃¹Σ⁺ ground states. Infrared spectra of N₂-tagged HC_2nO⁺ (n = 4–7) chains measured over the 550–1820 cm^–1 range exhibit weak bands associated with chain stretch and bend vibrational modes, with more intense C≡C and CO stretch bands predicted to occur in the 2200–2300 cm^–1 region. Electronic spectra of bare HC_2nO⁺ (n = 3–6) chains measured over the 400–900 nm range are dominated by the C̃³Σ^– ← X̃³Σ^– band systems, with the origin band’s wavelength shifting by ≈+80 nm for each added C₂ subunit, consistent with theoretical predictions. The origin bands of HC₆O⁺, HC₈O⁺, and HC₁₀O⁺ chains coincide in wavelength with λ4762.2, λ5828.6, and λ6862.6 diffuse interstellar bands (DIBs) observed along the sightline toward HD 183143, although bands in the laboratory spectra are broader than the DIBs.

Carbon clusters containing between 10 and 30 atoms predominantly exist as rings, a structure that persists for clusters containing up to at least 60 carbon atoms. Although carbon rings were once regarded as fleeting entities, only observable as charged species via mass spectrometry, their recent synthesis and atomic-scale imaging on surfaces have reignited interest in their extraordinary properties. Here, we present infrared spectra of charged cyclocarbons (C_4n⁺, n = 5-11) showing that they exhibit intense infrared resonances over the 550-1800 cm^-1 range that are at least 50 times stronger than infrared transitions of coexisting fullerene and bi-ring isomers, and which, density functional theory calculations suggest, are associated with a localized Kekulé-type distortion oscillating back and forth across the ring. The resonances should serve as distinctive spectroscopic signatures for cyclocarbon cations in terrestrial and extraterrestrial environments, and may expedite radiative cooling of isolated charged cyclocarbons, facilitating their formation through bimolecular association reactions in low pressure environments, including in interstellar space.

Recent atmospheric measurements indicate that bromine and iodine may be responsible for up to 72% of halogen-induced ozone loss near the tropopause, yet there is ongoing uncertainty regarding the multiphase chemistry of bromide and iodide anions in ozone depletion. Here, we demonstrate the unequivocal ozone-dependence of the archetype ¹Br^- + ¹O₃ reaction, which proceeds with an experimental rate constant of 8.9 (±4.4) × 10^-15 cm³ molecule^-1 s^-1. The reaction mechanism is revised to proceed via a singlet transition state with a rate-limiting barrier of +22.1 kJ mol^-1 -half that of prior estimates -prior to facile spin-crossing to yield ¹BrO^- + ³O₂. Statistical rate modeling using this new barrier height predicts a rate constant of 5.7 × 10^-15 cm³ molecule^-1 s^-1, which is in excellent agreement with the experiment. This reconciliation of the kinetics for the intrinsic gas-phase reaction will enable systematic evaluation of temperature, pressure, and solvation effects on this ion-molecule chemistry and thus inform the impact of halide anion chemistry on atmospheric ozone.

The Na⁺-pyrene and K⁺-pyrene complexes are investigated using two-color resonance enhanced photodissociation action spectroscopy in a cryogenic ion trap and through complementary density functional theory calculations. Both complexes are predicted to have π-bound structures in which the metal cation lies above the plane of the pyrene molecule, with calculated binding energies of 10570 and 8150 cm^-1, respectively. Electronic spectra of Na⁺-pyrene and K⁺-pyrene over the 26,000-33,000 cm^-1 range exhibit S₁(B_2u) ← S₀(A_g) and S₂(B_1u) ← S₀(A_g) band systems associated with excitation of the pyrene chromophore. The S₁(B_2u) ← S₀(A_g) band systems of Na⁺-pyrene and K⁺-pyrene are dominated by progressions in Franck-Condon active a_g vibrational modes, whereas transitions involving b_3g vibrational modes, which are prominent in the spectrum of the bare pyrene molecule by virtue of Herzberg-Teller coupling, are relatively weak or absent altogether. The S₁ ← S₀ origin transitions are shifted to lower energy from the corresponding pyrene transitions by 273 cm^-1 for Na⁺-pyrene and 246 cm^-1 for K⁺-pyrene. Density functional theory calculations predict that an attached Na⁺ or K⁺ cation has a minor effect on the pyrene vibrational frequencies but enhances the intensity of the weak S₁ ← S₀ transition while reducing slightly the intensity of the S₂ ← S₀ transition, explaining the relative dominance of vibronic transitions involving a_g vibrational modes in the S₁ ← S₀ spectra of Na⁺-pyrene and K⁺-pyrene. The strong, broad S₂(B_1u) ← S₀(A_g) vibronic bands of Na⁺-pyrene and K⁺-pyrene are also displaced to lower energy compared to the corresponding bands of the bare pyrene molecule.

Recent atmospheric measurements have indicated that bromine and iodine may be responsible for up to 72% of halogen-induced ozone loss near the tropopause. Whilst the neutral bromine and iodine radicals have well-described reaction pathways with ozone, there is ongoing uncertainty regarding the multiphase chemistry of the corresponding bromide and iodide anions and thus the potential role of these ionic species in ozone depletion. Here, we demonstrate the unequivocal ozone-dependence of the archetype Br¯ + O3 reaction, which proceeds at ambient temperatures with an experimental rate constant of 8.9 (±4.4) × 10-15 cm3 molecule-1 s-1 (0.001% collision efficiency). Using a state-of-the-art computational approach, the reaction mechanism is revised to proceed via a singlet transition state with a calculated barrier of +22.1 kJ mol-1, substantially lower than prior estimates and not on the triplet surface as previously claimed. Statistical rate modelling using this new barrier height predicts a rate constant of 5.7 × 10-15 cm3 molecule-1 s-1 from canonical transition state theory, which is in excellent agreement with the experiment. This reconciliation of the kinetics for the intrinsic gas-phase reaction will enable systematic evaluation of temperature, pressure and solvation effects on this ion-molecule chemistry and thus inform the impact of both bromide and iodide anion chemistry on atmospheric ozone.

Electronic spectra are measured for cryogenically cooled HC_2n+1H⁺ chains containing between 5 and 13 carbon atoms.

The bond dissociation energy of FeCr+ is measured using resonance enhanced photodissociation spectroscopy in a cryogenic ion trap. The onset for FeCr+ → Fe + Cr+ photodissociation occurs well above the lowest Cr+(6S, 3d5) + Fe(5D, 3d64s2) dissociation limit. In contrast, the higher energy FeCr+ → Fe+ + Cr photodissociation process exhibits an abrupt onset at the energy of the Cr(7S, 3d54s1) + Fe+(6D, 3d64s1) limit, enabling accurate dissociation energies to be extracted: D(Fe–Cr+) = 1.655 ± 0.006 eV and D(Fe+–Cr) = 2.791 ± 0.006 eV. The measured D(Fe–Cr+) bond energy is 10%–20% larger than predictions from accompanying CAM (Coulomb Attenuated Method)-B3LYP and NEVPT2 and coupled cluster singles, doubles, and perturbative triples electronic structure calculations, which give D(Fe–Cr+) = 1.48, 1.40, and 1.35 eV, respectively. The study emphasizes that an abrupt increase in the photodissociation yield at threshold requires that the molecule possesses a dense manifold of optically accessible, coupled electronic states adjacent to the dissociation asymptote. This condition is not met for the lowest Cr+(6S, 3d5) + Fe(5D, 3d64s2) dissociation limit of FeCr+ but is satisfied for the higher energy Cr(7S, 3d54s1) + Fe+(6D, 3d64s1) dissociation limit.

Electronic transitions are observed for the O2+–Ar and O2+–N2 complexes over the 225–350 nm range. The transitions are not associated with recognized electronic band systems of the respective atomic and diatomic constituents (Ar+, Ar, O2+, O2, N2+, and N2) but rather are due to charge transfer transitions. Onsets of the O2+–Ar and O2+–N2 band systems occur at 3.68 and 3.62 eV, respectively, corresponding to the difference in the ionization potentials of Ar and O2 (3.69 eV), and N2 and of O2 (3.51 eV), suggesting the band systems arise from intramolecular charge transfer transitions to states correlating with O2(X3Σg−) + Ar+ (2Pu) and O2(X3Σg−) + N2+(X2Σg+) limits, respectively. The dominant vibronic progressions have ωe values of 1565 cm−1 for O2+–Ar and 1532 cm−1 for O2+–N2, reasonably close to the value for the neutral O2 molecule in its X3Σg− state (1580 cm−1). Higher energy band systems for O2+–Ar and O2+–N2 are assigned to transitions to states correlating with the O2 (a1Δg) + Ar+ (2Pu) and O2 (a1Δg) + N2+(X2Σg+) limits, respectively.