Electronic Spectroscopy of the Halogenated Criegee Intermediate, ClCHOO: Experiment and Theory.

A chlorine-substituted Criegee intermediate, ClCHOO, is photolytically generated using a diiodo precursor, detected by VUV photoionization at 118 nm, and spectroscopically characterized via ultraviolet-visible (UV-vis)-induced depletion of / = 80 under jet cooled conditions. UV-vis excitation resonant with a π* ← π transition yields a significant ground state depletion, indicating a strong electronic transition and rapid photodissociation. The broad absorption spectrum peaks at 350 nm and is attributed to contributions from both (∼70%) and (∼30%) conformers of ClCHOO based on spectral simulations using a nuclear ensemble method.

The role of solar photolysis in the atmospheric removal of methacrolein oxide and the methacrolein oxide-water van-der Waals complex in pristine environments.

Biogenic hydrocarbons are emitted into the Earth's atmosphere by terrestrial vegetation as by-products of photosynthesis. Isoprene is one such hydrocarbon and is the second most abundant volatile organic compound emitted into the atmosphere (after methane). Reaction with ozone represents an important atmospheric sink for isoprene removal, forming carbonyl oxides (Criegee intermediates) with extended conjugation.

Vibrational spectroscopy and dissociation dynamics of cyclohexyl hydroperoxide.

Organic hydroperoxides (ROOH) are ubiquitous in the atmospheric oxidation of volatile organic compounds (VOCs) as well as in low-temperature oxidation of hydrocarbon fuels. The present work focuses on a prototypical cyclic hydroperoxide, cyclohexyl hydroperoxide (CHHP). The overtone OH stretch (2) spectrum of jet-cooled CHHP is recorded by IR multiphoton excitation with UV laser-induced fluorescence detection of the resulting OH products.

Modeling the Ground- and Excited-State Unimolecular Decay of the Simplest Fluorinated Criegee Intermediate, HFCOO, Formed from the Ozonolysis of Hydrofluoroolefin Refrigerants.

Hydrofluoroolefins (HFO) are fourth-generation refrigerants designed to function as efficient refrigerants with no ozone depletion potential and zero global warming potential. Despite extensive studies on their chemical and physical properties, the ground- and excited-state chemistry of their atmospheric oxidation products is less well understood. This study focuses on the ground- and excited-state chemistry of the simplest fluorinated Criegee intermediate (CI), fluoroformaldehyde oxide (HFCOO), which is the simplest fluorinated CI formed from the ozonolysis of HFOs.

UV photodissociation dynamics of the acetone oxide Criegee intermediate: experiment and theory.

The photodissociation dynamics of the dimethyl-substituted acetone oxide Criegee intermediate [(CH)COO] is characterized following electronic excitation to the bright ππ* state, which leads to O (D) + acetone [(CH)CO, S] products. The UV action spectrum of (CH)COO recorded with O (D) detection under jet-cooled conditions is broad, unstructured, and essentially unchanged from the corresponding electronic absorption spectrum obtained using a UV-induced depletion method. This indicates that UV excitation of (CH)COO leads predominantly to the O (D) product channel.

A review of PFAS adsorption from aqueous solutions: Current approaches, engineering applications, challenges, and opportunities.

Per- and polyfluoroalkyl substances (PFAS) have drawn great attention due to their wide distribution in water bodies and toxicity to human beings. Adsorption is considered as an efficient treatment technique for meeting the increasingly stringent environmental and health standards for PFAS. This paper systematically reviewed the current approaches of PFAS adsorption using different adsorbents from drinking water as well as synthetic and real wastewater.

Electronic Spectroscopy and Dissociation Dynamics of Vinyl-Substituted Criegee Intermediates: 2-Butenal Oxide and Comparison with Methyl Vinyl Ketone Oxide and Methacrolein Oxide Isomers.

The 2-butenal oxide Criegee intermediate [(CHCH═CH)CHOO], an isomer of the four-carbon unsaturated Criegee intermediates derived from isoprene ozonolysis, is characterized on its first π* ← π electronic transition and by the resultant dissociation dynamics to O (D) + 2-butenal [(CHCH═CH)CHO] products. The electronic spectrum of 2-butenal oxide under jet-cooled conditions is observed to be broad and unstructured with peak absorption at 373 nm, spanning to half maxima at 320 and 420 nm, and in good accord with the computed vertical excitation energies and absorption spectra obtained for its lowest energy conformers. The distribution of total kinetic energy released to products is ascertained through velocity map imaging of the O (D) products.

Insights into the Ultrafast Photodissociation Dynamics of Isoprene-Derived Criegee Intermediates.

Isoprene is the most abundant nonmethane volatile organic compound emitted into the troposphere by terrestrial vegetation. Reaction with ozone represents an important isoprene removal process from the troposphere and is a well-known source of Criegee intermediates (CIs), which are reactive carbonyl oxides. Three CIs, formaldehyde oxide (CH OO), methyl vinyl ketone oxide (MVK-oxide) and methacrolein oxide (MACR-oxide) are formed during isoprene ozonolysis.

Comparing the Excited State Dynamics of CHOO, the Simplest Criegee Intermediate, Following Vertical versus Adiabatic Excitation.

molecular dynamics studies of CHOO molecules following excitation to the minimum-energy geometry of the strongly absorbing S (ππ*) state reveal a much richer range of behaviors than just the prompt O-O bond fission, with unity quantum yield and retention of overall planarity, identified in previous vertical excitation studies from the ground (S) state. Trajectories propagated for 100 fs from the minimum-energy region of the S state show a high surface hopping (nonadiabatic coupling) probability between the near-degenerate S and S (nπ*) states at geometries close to the S minimum, which enables population transfer to the optically dark S state. Greater than 80% of the excited population undergoes O-O bond fission on the S or S potential energy surfaces (PESs) within the analysis period, mostly from nonplanar geometries wherein the CH moiety is twisted relative to the COO plane.

A direct dynamics study of the exotic photochemistry of the simplest Criegee intermediate, CHOO.

Criegee intermediates are amongst the most fascinating molecules in modern-day chemistry. They are highly reactive intermediates that find vital roles that range from atmospheric chemistry to organic synthesis. Their excited state chemistry is exotic and complicated, and a myriad of electronic states can contribute to their photodissociation dynamics.

Electronic Absorption Spectroscopy and Photochemistry of Criegee Intermediates.

Interest in Criegee intermediates (CIs), often termed carbonyl oxides, and their role in tropospheric chemistry has grown massively since the demonstration of laboratory-based routes to their formation and characterization in the gas phase. This article reviews current knowledge regarding the electronic spectroscopy of atmospherically relevant CIs like CH OO, CH CHOO, (CH ) COO and larger CIs like methyl vinyl ketone oxide and methacrolein oxide that are formed in the ozonolysis of isoprene, and of selected conjugated carbene-derived CIs of interest in the synthetic chemistry community. Of the aforementioned atmospherically relevant CIs, all except CH OO and (CH ) COO exist in different conformers which, under tropospheric conditions, can display strikingly different thermal loss rates via unimolecular and bimolecular processes.

Modeling the Conformer-Dependent Electronic Absorption Spectra and Photolysis Rates of Methyl Vinyl Ketone Oxide and Methacrolein Oxide.

Criegee intermediates are important atmospheric oxidants, formed via the reaction of ozone with volatile alkenes emitted into the troposphere. Small Criegee intermediates (e.g.

Achieving high selectivity towards electro-conversion of CO using In-doped Bi derived from metal-organic frameworks.

Metal-organic frameworks (MOFs) and their derivatives have shown great potential as electrocatalysts, in virtue of their ease of functionalization and abundance of active sites. Here, we report a series of indium-doped bismuth MOF-derived composites (BiIn-Y@C) for the direct conversion of carbon dioxide (CO) to hydrocarbon derivatives. Amongst the catalysts studied, BiIn-500@C demonstrated high selectivity for the production of formate and intrinsic activity in a wide potential window, ranging from - 1.

The states that hide in the shadows: the potential role of conical intersections in the ground state unimolecular decay of a Criegee intermediate.

Criegee intermediates are of great significance to Earth's troposphere - implicated in altering the tropospheric oxidation cycle and in forming low volatility products that typically condense to form secondary organic aerosols (SOAs). As such, their chemistry has attracted vast attention in recent years. In particular, the unimolecular decay of thermal and vibrationally-excited Criegee intermediates has been the focus of several experimental and computational studies, and it is now recognized that Criegee intermediates undergo unimolecular decay to form OH radicals.

Insights into the Ultrafast Dynamics of CH OO and CH CHOO Following Excitation to the Bright ππ* State: The Role of Singlet and Triplet States.

Criegee intermediates make up a class of molecules that are of significant atmospheric importance. Understanding their electronically excited states guides experimental detection and provides insight into whether solar photolysis plays a role in their removal from the troposphere. The latter is particularly important for large and functionalized Criegee intermediates.

Photodissociation dynamics of methyl vinyl ketone oxide: A four-carbon unsaturated Criegee intermediate from isoprene ozonolysis.

The electronic spectrum of methyl vinyl ketone oxide (MVK-oxide), a four-carbon Criegee intermediate derived from isoprene ozonolysis, is examined on its second π* ← π transition, involving primarily the vinyl group, at UV wavelengths (λ) below 300 nm. A broad and unstructured spectrum is obtained by a UV-induced ground state depletion method with photoionization detection on the parent mass (m/z 86). Electronic excitation of MVK-oxide results in dissociation to O (D) products that are characterized using velocity map imaging.

Photodissociation Dynamics of CHOO on Multiple Potential Energy Surfaces: Experiment and Theory.

UV excitation of the CHOO Criegee intermediate across most of the broad span of the (B A')-(X A') spectrum results in prompt dissociation to two energetically accessible asymptotes: O (D) + HCO (X A) and O (P) + HCO (a A''). Dissociation proceeds on multiple singlet potential energy surfaces that are coupled by two regions of conical intersection (CoIn). Velocity map imaging (VMI) studies reveal a bimodal total kinetic energy release (TKER) distribution for the O (D) + HCO (X A) products with the major and minor components accounting for ca.

A Simple and Efficient Method for Simulating the Electronic Absorption Spectra of Criegee Intermediates: Benchmarking on CHOO and CHCHOO.

Criegee intermediates (CIs) play a vital role in the atmosphere-known most prominently for enhancing the oxidizing capacity of the troposphere. Knowledge of their electronic absorption spectra is of vital importance for two reasons: (1) to aid experimentalists in detecting CIs and (2) in deciding if their removal is affected by solar photolysis. In this article we report a simple and efficient method based on the nuclear ensemble method that may be effectively used to compute the electronic absorption spectra of Criegee intermediates without the need for extensive computation of preparing the initial configurations of the starting geometry.

The Role of Norrish Type-I Chemistry in Photoactive Drugs: An Study of a Cyclopropenone-Enediyne Drug Precursor.

We present a contemporary mechanistic description of the light-driven conversion of cyclopropenone containing enediyne (CPE) precusors to ring-opened species amenable to further Bergman cyclization and formation of stable biradical species that have been proposed for use in light-induced cancer treatment. The transformation is rationalized in terms of (purely singlet state) Norrish type-I chemistry, wherein photoinduced opening of one C-C bond in the cyclopropenone ring facilitates non-adiabatic coupling to high levels of the ground state, subsequent loss of CO and Bergman cyclization of the enediyne intermediate to the cytotoxic target biradical species. Limited investigations of substituent effects on the ensuing photochemistry serve to vindicate the experimental choices of Popik and coworkers (.

Hydrogen-Deuterium Exchange in Basic Near-Critical and Supercritical Media: An Experimental and Theoretical Study.

Treatment of homo- and heterocyclic aromatic substrates with basic deuterium oxide under near- or supercritical conditions results in rapid base-catalyzed hydrogen-deuterium exchange (HDE) in aromatic and benzylic positions. It has been postulated that HDE follows a simple deprotonation-reprotonation mechanism, but little evidence has been provided to date. This study correlates experimentally observed proton exchanges in -butylbenzene with calculations of the acidities and potential energy (PE) profiles.

Comparative study of methodologies for calculating metastable states of small to medium-sized molecules.

We present a benchmarking study on the performance of two methods at the forefront of studying electronic metastable states of molecules: the orbital stabilization method and the method of complex absorbing potential augmented Hamiltonians. The performance of the two methods is compared for the calculation of shape resonances in small to medium-sized molecules (up to 15 atoms) at the equation of motion coupled cluster with singles and doubles for the electron attachment level of methodology using even-tempered Gaussian basis sets. The theoretical positions and widths of shape resonances obtained from both methods are compared to the experimentally determined electron affinities and lifetimes.

Oxidative Addition of Singlet Oxygen to Model Building Blocks of the Aerucyclamide A Peptide: A First-Principles Approach.

Singlet oxygen (O) is a significant source of biodamage in living organisms. O is a highly reactive excited electronic-state spin-configuration of molecular oxygen and is usually prepared via organic molecule sensitization. Despite the wealth of experimental studies on the O-induced oxidation of several bio-organic molecules, the detailed mechanism of the oxidation process is largely unknown.

Quantifying rival bond fission probabilities following photoexcitation: C-S bond fission in -butylmethylsulfide.

We illustrate a new, collision-free experimental strategy that allows determination of the absolute probabilities of rival bond fission processes in a photoexcited molecule - here -butylmethylsulfide (BSM). The method combines single photon ('universal') ionization laser probe methods, simultaneous imaging of all probed fragments (multi-mass ion imaging) and the use of an appropriate internal calibrant (here dimethylsulfide). Image analysis allows quantification of the dynamics of the rival B-SM and BS-M bond fission processes following ultraviolet (UV) excitation of BSM and shows the former to be twice as probable, despite the only modest (∼2%) differences in the respective ground state equilibrium C-S bond lengths or bond strengths.

Exploring Norrish type I and type II reactions: an ab initio mechanistic study highlighting singlet-state mediated chemistry.

Norrish reactions are important photo-induced reactions in mainstream organic chemistry and are implicated in many industrially and biologically relevant processes and in the processing of carbonyl molecules in the atmosphere. The present study reports multi-reference electronic structure calculations designed to assess details of the potential energy profiles associated with the Norrish type-I and type-II reactions of a prototypical ketone 5-methyl-hexan-2-one. We show that the well-established 'triplet state mediated' reaction pathways following initial population of a singlet excited state can be complemented by (hitherto rarely recognized) 'singlet state only' Norrish type-I and type-II reaction mechanisms that involve no spin-forbidden transitions along the respective reaction paths, and suggest how the efficiencies of such reactions might be affected by strategic substitutions at selected sites within the parent ketone.

Photoinduced C-H bond fission in prototypical organic molecules and radicals.

Recent experimental and computational advances have heralded huge progress in the range and the detail of the database pertaining to photoinduced C-H bond fission processes. This Perspective provides a snapshot of the current state of knowledge as determined via gas phase (i.e.