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When volatile alkenes are emitted into the atmosphere, they are rapidly removed by oxidizing agents such as hydroxyl radicals and ozone. The latter reaction is termed ozonolysis and is an important source of Criegee intermediates (CIs), i.e., carbonyl oxides, that are implicated in enhancing the oxidizing capacity of the troposphere. These CIs aid in the formation of lower volatility compounds that typically condense to form secondary organic aerosols. CIs have attracted vast attention over the past two decades. Despite this, the effect of their substitution on the ground and excited state chemistries of CIs is not well studied. Here, we extend our knowledge obtained from prior studies on CIs by CF3 substitution. The resulting CF3CHOO molecule is a CI that can be formed from the ozonolysis of hydrofluoroolefins (HFOs). Our results show that the ground state unimolecular decay should be less reactive and thus more persistent in the atmosphere than the non-fluorinated analog. The excited state dynamics, however, are predicted to occur on an ultrafast timescale. The results are discussed in the context of the ways in which our study could advance synthetic chemistry, as well as processes relevant to the atmosphere. 

The photodissociation dynamics of the dimethyl-substituted acetone oxide Criegee intermediate [(CH 3 ) 2 COO] is characterized following electronic excitation on the π*←π transition, which leads to O ( 1 D) + acetone [(CH 3 ) 2 CO, S0] products. The UV action spectrum of (CH 3 ) 2 COO recorded with O ( 1 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 3 ) 2 COO leads predominantly to the O ( 1 D) product channel. A higher energy O ( 3 P) + (CH3)2CO (T1) product channel is not observed, although it is energetically accessible. This is attributed to the relatively weak absorption cross section at UV excitation energies above the threshold. In addition, complementary MS-CASPT2 trajectory surface-hopping (TSH) simulations indicate minimal population leading to the O ( 3 P) channel and non-unity overall probability for dissociation (within 100 fs). Velocity map imaging of the O ( 1 D) products is utilized to reveal the total kinetic energy release (TKER) distribution upon photodissociation of (CH 3 ) 2 COO at various UV excitation energies. Simulation of the TKER distributions is performed using a hybrid model that combines an impulsive model with a statistical component, the latter reflecting the longer-lived (> 100 fs) trajectories identified in the TSH calculations. The impulsive model accounts for vibrational activation of (CH 3 ) 2 CO arising from geometrical changes between the Criegee intermediate and the carbonyl product, indicating the importance of CO stretch, CCO bend, and CC stretch along with activation of hindered rotation and rock of the methyl groups in the (CH 3 ) 2 CO product. Detailed comparison is also made with the TKER distribution arising from photodissociation dynamics of CH 2 OO upon UV excitation. 

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. This manuscript reports a multi-state direct dynamics (full-dimensional) study of the photoinduced fragmentation of the simplest Criegee intermediate, CH2OO, using state-of-the-art CASPT2 trajectory surface hopping. Energy- (excitation-) transfer is observed between the separating O and H2CO fragments at separations that, traditionally, might be viewed as the classically asymptotic region of the potential energy surface. We suggest that such long-range energy exchange accounts for the unusual and non-intuitive total kinetic energy distribution (TKER) in the O(1D) + H2CO(S0) products observed following photoexcitation of CH2OO. The present results also reveal the interplay between seven singlet electronic states and dissociation to yield the experimentally observed O(1D) + H2CO(S0) and O(3P) + H2CO(T1) products. The former (singlet) products are favored, with a branching ratio of ca. 80 %, quantifying the hitherto unknown product branching ratios observed in velocity map imaging experiments. To the best of our knowledge, such long-range energy transfer between fragment pairs originating from a common parent – at classically asymptotic separations approaching those more typically reminiscent of Forster-type Energy Transfer – has not been recognized hitherto in the case of a molecular photodissociation. 

Molecular spectroscopy and photochemistry constitute an integral field in modern chemistry. However, undergraduate level classes provide limited opportunities for hands-on experimentation of photochemistry and photophysics. For this reason, a simple laboratory experiment was designed that may be easily implemented into undergraduate teaching laboratories with the aim of introducing undergraduate students to UV/visible spectroscopy and photochemistry/photophysics and its possible applications. Samples of three unknown sunscreen formulations are given to students and they are asked to use a set of techniques to identify their molecular composition and to test their efficacy using basic laboratory equipment available to them. In particular, the students are asked to complete the following tasks: (i) sample preparation using solvent extraction to extract active ingredients from the sunscreen lotion, (ii) identify the extracted molecular sunscreen constituents by Thin Layer Chromatography (TLC) and UV/visible spectroscopy, and finally (iii) study their photostability by means of steady state irradiation coupled with UV/visible spectroscopy. The students were provided with the following tools for data collection: silica-backed TLC plates, a short-wave lamp (254 nm, for TLC analysis), a UV-Vis spectrophotometer with an associated computer and software, and an LED lamp (315 nm) to irradiate the samples. Combined TLC and UV-Vis spectroscopy allowed the students to identify the extracted ingredients. UV irradiation confirmed the photostability of sunscreens. 

Molecular spectroscopy and photochemistry constitute an integral field in modern chemistry. However, undergraduate level classes provide limited opportunities for hands-on experimentation of photochemistry and photophysics. For this reason, a simple laboratory experiment was designed that may be easily implemented into undergraduate teaching laboratories with the aim of introducing undergraduate students to UV/visible spectroscopy and photochemistry/photophysics and its possible applications. Samples of three unknown sunscreen formulations are given to students and they are asked to use a set of techniques to identify their molecular composition and to test their efficacy using basic laboratory equipment available to them. In particular, the students are asked to complete the following tasks: (i) sample preparation using solvent extraction to extract active ingredients from the sunscreen lotion, (ii) identify the extracted molecular sunscreen constituents by Thin Layer Chromatography (TLC) and UV/visible spectroscopy, and finally (iii) study their photostability by means of steady state irradiation coupled with UV/visible spectroscopy. The students were provided with the following tools for data collection: silica-backed TLC plates, a short-wave lamp (254 nm, for TLC analysis), a UV-Vis spectrophotometer with an associated computer and software, and an LED lamp (315 nm) to irradiate the samples. Combined TLC and UV-Vis spectroscopy allowed the students to identify the extracted ingredients. UV irradiation confirmed the photostability of sunscreens. 

Criegee intermediates (CIs) 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 CIs has been the focus of several experimental and computational studies, and it now recognized that CIs undergo unimolecular decay to form OH radicals. In this contribution we reveal insight into the chemistry of CIs by highlighting the hitherto neglected multi-state contribution to the ground state unimolecular decay dynamics of the Criegee intermediate products. The two key intermediates of present focus are dioxirane and vinylhydroperoxide – known to be active intermediates that mediate the unimolecular decay of CH2OO and CH3CHOO, respectively. In both cases the unimolecular decay path encounters conical intersections, which may play a pivotal role in the ensuing dynamics. This hitherto unrecognized phenomenon may be vital in the way in which the reactivity of CIs are modelled and is likely to affect the ensuing dynamics associated with the unimolecular decay of a given CI.