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Hydrofluoroolefins are being adopted as sustainable alternatives to long-lived fluorine- and chlorine-containing gases and are finding current or potential mass-market applications as refrigerants, among a myriad of other uses. Their olefinic bond affords relatively rapid reaction with hydroxyl radicals present in the atmosphere, leading to short lifetimes and proportionally small global warming potentials. However, this type of functionality also allows reaction with ozone, and whilst these reactions are slow, we show that the products of these reactions can be extremely long-lived. Our chamber measurements show that several industrially important hydrofluoroolefins produce CHF3 (fluoroform, HFC-23), a potent, long-lived greenhouse gas. When this process is accounted for in atmospheric chemical and transport modeling simulations, we find that the total radiative effect of certain compounds can be several times that of the direct radiative effect currently recommended by the World Meteorological Organization. Our supporting quantum chemical calculations indicate that a large range of exothermicity is exhibited in the initial stages of ozonolysis, which has a powerful influence on the CHF3 yield. Furthermore, we identify certain molecular configurations that preclude the formation of long-lived greenhouse gases. This demonstrates the importance of product quantification and ozonolysis kinetics in determining the overall environmental impact of hydrofluoroolefin emissions. [ABSTRACT FROM AUTHOR]

In this article, we describe an experiment for the undergraduate physical chemistry laboratory in which students measure the compressibility factor of two gases, helium and carbon dioxide, as a function of pressure at constant temperature. The experimental apparatus is relatively inexpensive to construct and is described and diagrammed in detail. Students analyze their data to determine values for the second virial coefficient for each gas, which may then be compared with the literature. The experiment demonstrates the nonideality of real gases. (Contains 2 tables and 2 figures.)

The acetylperoxy + HO2 reaction has multiple impacts on the troposphere, with a triplet pathway leading to peracetic acid + O2 (reaction (1a)) competing with singlet pathways leading to acetic acid + O3 (reaction (1b)) and acetoxy + OH + O2 (reaction (1c)). A recent experimental study has reported branching fractions for these three pathways (α1a, α1b, and α1c) from 229 K to 294 K. We constructed a theoretical model for predicting α1a, α1b, and α1c using quantum chemical and Rice–Ramsperger–Kassel–Marcus/master equation (RRKM/ME) simulations. Our main quantum chemical method was Weizmann-1 Brueckner Doubles (W1BD) theory; we combined W1BD and equation-of-motion spin-flip coupled cluster (SF) theory to treat open-shell singlet structures. Using RRKM/ME simulations that included all conformers of acetylperoxy–HO2 pre-reactive complexes led to a 298 K triplet rate constant, k1a = 5.11 × 10−12 cm3 per molecule per s, and values of α1a in excellent agreement with experiment. Increasing the energies of all singlet structures by 0.9 kcal mol−1 led to a combined singlet rate constant, k1b+1c = 1.20 × 10−11 cm3 per molecule per s, in good agreement with experiment. However, our predicted variations in α1b and α1c with temperature are not nearly as large as those measured, perhaps due to the inadequacy of SF theory in treating the transition structures controlling acetic acid + O3 formation vs. acetoxy + OH + O2 formation. [ABSTRACT FROM AUTHOR]

The B 2[uppercase_phi_synonym]5/2-X1 2Δ3/2(0,0) band at 778 nm and the C 2Δ3/2-X1 2Δ3/2(0,0) band at 737 nm of tantalum oxide (TaO) were recorded by laser excitation spectroscopy using a hollow cathode sputtering source to generate the molecules. The hyperfine structure arising from the 181Ta (I=7/2) nucleus was measured at sub-Doppler resolution using the technique of intermodulated fluorescence spectroscopy. The hyperfine structure was assigned and fitted in order to derive accurate values for the magnetic dipole and electric quadrupole interactions. The magnetic hyperfine constant for the ground electronic state was also calculated using the density functional theory as h3/2=625 MHz, in good agreement with the experimental value of 647±10 MHz. This result suggests that the X 2Δ ground state of TaO is well described by a pure δσ2 electronic configuration, where the unpaired electron is located in a Ta 5dδ orbital. [ABSTRACT FROM AUTHOR]

Infrared spectra of mass-selected clusters NO+(H2O)n for n=1 to 5 were recorded from 2700 to 3800 cm-1 by vibrational predissociation spectroscopy. Vibrational frequencies and intensities were also calculated for n=1 and 2 at the second-order Mo\ller–Plesset (MP2) level, to aid in the interpretation of the spectra, and at the singles and doubles coupled cluster (CCSD) level energies of n=1 isomers were computed at the MP2 geometries. The smaller clusters (n=1 to 3) were complexes of H2O ligands bound to a nitrosonium ion NO+ core. They possessed perturbed H2O stretch bands and dissociated by loss of H2O. The H2O antisymmetric stretch was absent in n=1 and gradually increased in intensity with n. In the n=4 clusters, we found evidence for the beginning of a second solvation shell as well as the onset of an intracluster reaction that formed HONO. These clusters exhibited additional weak, broad bands between 3200 and 3400 cm-1 and two new minor photodissociation channels, loss of HONO and loss of two H2O molecules. The reaction appeared to go to completion within the n=5 clusters. The primary dissociation channel was loss of HONO, and seven vibrational bands were observed. From an analysis of the spectrum, we concluded that the n=5 cluster rearranged to form H3O+(H2O)3(HONO), i.e., an adduct of the reaction products. [ABSTRACT FROM AUTHOR]

The vinyl hydroperoxide (VHP), the major isomerization product of the syn-alkyl Criegee intermediate (CI) formed in alkene ozonolysis, is a direct precursor of hydroxyl radical (OH), the most important oxidant in the troposphere. While simulations of CI reactivity have usually assumed the VHP to be a prompt and quantitative source of OH, recent quantum chemical studies have revealed subtleties in VHP reactivity such as a barrier to peroxy bond homolysis and a possible rearrangement to a hydroxycarbonyl. In this work, we use M06-L, Weizmann-1 Brueckner Doubles, and equation-of-motion spin-flip coupled-cluster theories to calculate a comprehensive reaction mechanism for the syn and anti conformers of the parent VHP formed in trans-2-butene ozonolysis and the 1-methyl VHP formed in 2,3-dimethyl-2-butene ozonolysis. We predict that for the parent VHP the anti homolysis transition structure (TS) is 3 kcal mol-1 lower in energy than the syn TS, but for the 1-methyl system, the syn TS is 2 kcal mol-1 lower in energy. Statistical rate theory simulations based on the quantum chemical data predict that the parent VHP preferentially decomposes to vinoxy and OH radicals under all tropospheric conditions, while the 1-methyl VHP preferentially decomposes to 1-methylvinoxy and OH radicals only close to 298 K; at 200 K, the 1-methyl VHP preferentially rearranges to hydroxyacetone. Lower temperatures and higher pressures favor the temporary accumulation of both the parent and the 1-methyl VHP. [ABSTRACT FROM AUTHOR]

The atmospheric oxidation of sulfurdioxide by the parent and dimethyl Criegee intermediates (CIs) maybe an important source of sulfuric acid aerosol, which has a largeimpact on radiative forcing and therefore upon climate. A number ofcomputational studies have considered how the CH2OOS(O)Oheteroozonide (HOZ) adduct formed in the CI + SO2reactionconverts SO2to SO3. In this work we use theCBS-QB3 quantum chemical method along with equation-of-motion spin-flipCCSD(dT) and MCG3 theories to reveal new details regarding the formationand decomposition of the endoand exoconformers of the HOZ. Although ∼75% of the parent CI + SO2reaction is initiated by formation of the exoHOZ, hyperconjugation preferentially stabilizes many of the endointermediates and transition structures by 1–5kcal mol–1. Our quantum chemical calculations, inconjunction with statistical rate theory models, predict a rate coefficientfor the parent CI + SO2reaction of 3.68 × 10–11cm3molecule–1s–1, in good agreement with recent experimental measurements.RRKM/master equation simulations based on our quantum chemical datapredict a prompt carbonyl + SO3yield of >95% for thereaction of both the parent and dimethyl CI with SO2. Theexistence of concerted cycloreversion transition structures 10–15kcal mol–1higher in energy than the HOZ accountsfor most of the predicted SO3formation. [ABSTRACT FROM AUTHOR]

The article focuses on the Midwest Undergraduate Computational Chemistry Consortium (MU3C), a community of undergraduate research collaborators and 14 computational chemistry professor. It says that the annual regional undergraduate research (UR) conference was the most important activity of the consortium which focused on students' poster and oral research presentations. It adds that the online conferences aims to provide student researchers with updates on research they have been conducting.

Assesses the oxygen (O)-O bond breaking reactions of peroxide and the subsequent rearrangement using CBS-QB3 and B3LYP/6-311G* hybrid density functional theory. Chemistry of peroxides; Rejection of the sigmatrophic shift hypothesis; Characterization of ground-state and transition-state structures and energies.

The mechanism of the ene reaction of singlet (¹Δ[sub g]) oxygen with simple alkenes is investigated by a combination of experimental isotope effects and several levels of theoretical calculations. For the reaction of 2,4-dimethyl-3-isopropyl-2-pentene, the olefinic carbons exhibit small and nearly equal [sup 13]C isotope effects of 1.005-1.007, while the reacting methyl groups exhibit [sup 13]C isotope effects near unity. In a novel experiment, the [sup 13]C composition of the product is analyzed to determine the intramolecular [sup 13]C isotope effects in the ene reaction of tetramethylethylene. The new [sup 13]C and literature ²H isotope effects are then used to evaluate the accuracy of theoretical calculations. RHF, CASSCF(10e, 8o), and restricted and unrestricted B3LYP calculations are each applied to the ene reaction with tetramethylethylene. Each predicts a different mechanism, but none leads to reasonable predictions of the experimental isotope effects. It is concluded that none of these calculations accurately describe the reaction. A more successful approach was to use high-level, up to CCSD(T), single-point energy calculations on a grid of B3LYP geometries. The resulting energy surface is supported by its accurate predictions of the intermolecular [sup 13]C and ²H isotope effects and a very good prediction of the reaction barrier. This CCSD(T)//B3LYP surface features two adjacent transition states without an intervening intermediate. This is the first experimentally supported example of such a surface and the first example of a valley-ridge inflection with significant chemical consequences. [ABSTRACT FROM AUTHOR]