The Ring-Closing Reaction of Cyclopentadiene Probed with Ultrafast X-ray Scattering.

The dynamics of cyclopentadiene (CP) following optical excitation at 243 nm was investigated by time-resolved pump-probe X-ray scattering using 16.2 keV X-rays at the Linac Coherent Light Source (LCLS). We present the first ultrafast structural evidence that the reaction leads directly to the formation of bicyclo[2.1.0]pentene (BP), a strained molecule with three- and four-membered rings. The bicyclic compound decays via a thermal backreaction to the vibrationally hot CP with a time constant of 21 ± 3 ps. A minor channel leads to ring-opened structures on a subpicosecond time scale.

Peroxide-Selective Reduction of O2 at Redox-Inactive Rare-Earth(III) Triflates Generates an Ambiphilic Peroxide.

Metal peroxides are key species involved in a range of critical biological and synthetic processes. Rare-earth (group III and the lanthanides; Sc, Y, La-Lu) peroxides have been implicated as reactive intermediates in catalysis; however, reactivity studies of isolated, structurally characterized rare-earth peroxides have been limited. Herein, we report the peroxide-selective (93-99% O22-) reduction of dioxygen (O2) at redox-inactive rare-earth triflates in methanol using a mild metallocene reductant, decamethylferrocene (Fc*). The first molecular praseodymium peroxide ([PrIII2(O22-)(18C6)2(EG)2][OTf]4; 18C6 = 18-crown-6, EG = ethylene glycol, -OTf = -O3SCF3; 2-Pr) was isolated and characterized by single-crystal X-ray diffraction, Raman spectroscopy, and NMR spectroscopy. 2-Pr displays high thermal stability (120 °C, 50 mTorr), is protonated by mild organic acids [pKa1(MeOH) = 5.09 ± 0.23], and engages in electrophilic (e.g., oxygen atom transfer) and nucleophilic (e.g., phosphate-ester cleavage) reactivity. Our mechanistic studies reveal that the rate of oxygen reduction is dictated by metal-ion accessibility, rather than Lewis acidity, and suggest new opportunities for differentiated reactivity of redox-inactive metal ions by leveraging weak metal-ligand binding events preceding electron transfer.

Effects of Coadsorbed Water on the Heterogeneous Photochemistry of Nitrates Adsorbed on TiO2.

Nitric acid, a well-known sink of NO x gases in the atmosphere, has been found to be photoactive while adsorbed on tropospheric particles. When adsorbed onto semiconductive metal oxides, nitrate's photochemical degradation can be interpreted as a photocatalytic process. Yet, the photolysis of nitrate ions on the surface of aerosols can also be initiated by changes in the symmetry of the ion upon adsorption. In this study, we use quantum chemistry to model the vibrational spectra of adsorbed nitrate on TiO2, a semiconductor component of atmospheric aerosols, and determine the kinetics of the heterogeneous photochemical degradation of nitrate under simulated solar light. Frequencies and geometry calculations suggest that the symmetry of chemisorbed nitrate ion depends strongly on coadsorbed water, with water changing the reactive surface of TiO2. Upon irradiation, surface nitrate undergoes photolysis to yield nitrogen-containing gaseous products including NO2, NO, HONO, and N2O, in proportions that depend on relative humidity (RH). In addition, the heterogeneous photochemistry rate constant decreases an order of magnitude, from (5.7 ± 0.1) × 10-4 s-1 on a dry surface to (7.1 ± 0.8) × 10-5 s-1 when nitrate is coadsorbed with water above monolayer coverage. Little is known about the roles of coadsorbed water on the heterogeneous photochemistry of nitrates on TiO2, along with its impact on the chemical balance of the atmosphere. This work discusses the roles of water in the photolysis of surface nitrates on TiO2 and the concomitant renoxification of the atmosphere.