Gas phase structures of peroxides: experiments and computational problems

There and back again: the role of hyperconjugation in the fluorine gauche effect

The origin of the fluorine gauche effect has been debated for decades and recently different interpretations have been raised in the scientific community as new computational methods emerged and were applied to rationalize 1,2-difluoroethane (DFE) gauche preference. In this context, we revisited 1,2-difluoroethane (DFE) and its chlorine and bromine derivative conformational preferences through a comparative approach: the conformational behavior and hyperconjugative, steric and electrostatic contributions for the internal rotational barrier of DFE were compared with several analogue backbones, such as peroxides, disulfides and ammonia boranes. By using the Natural Bond Orbital (NBO) analysis it was found that hyperconjugation is the driving force of the conformational preference in DFE and its chlorine and bromine analogues. Electrostatics was found to be negligible and steric effects played a minor role in general, but are important in ClCH₂CH₂Cl and BrCH₂CH₂Br to counterbalance gauche stabilization by hyperconjugation and for the preference of the anti conformer.

Tight electrostatic regulation of the OH production rate from the photolysis of hydrogen peroxide adsorbed on surfaces

Recently, experimental and theoretical works have reported evidence indicating that photochemical processes may significantly be accelerated at heterogeneous interfaces, although a complete understanding of the phenomenon is still lacking. We have carried out a theoretical study of interface and surface effects on the photochemistry of hydrogen peroxide (H2O2) using high-level ab initio methods and a variety of models. Hydrogen peroxide is an important oxidant that decomposes in the presence of light, forming two OH radicals. This elementary photochemical process has broad interest and is used in many practical applications. Our calculations show that it can drastically be affected by heterogeneous interfaces. Thus, compared to gas phase, the photochemistry of H2O2 appears to be slowed on the surface of apolar or low-polar surfaces and, in contrast, hugely accelerated on ionic surfaces or the surface of aqueous electrolytes. We give particular attention to the case of the neat air-water interface. The calculated photolysis rate is similar to the gas phase, which stems from the compensation of two opposite effects, the blue shift of the n→σ* absorption band and the increase of the absorption intensity. Nevertheless, due to the high affinity of H2O2 for the air-water interface, the predicted OH production rate is up to five to six orders of magnitude larger. Overall, our results show that the photochemistry of H2O2 in heterogeneous environments is greatly modulated by the nature of the surface, and this finding opens interesting new perspectives for technological and biomedical applications, and possibly in various atmospheres.

The preferred geometry of hydroperoxides is the result of an interplay between electrostatic and hyperconjugative effects

Steric, electrostatic and hyperconjugative effects were evaluated in the natural bond orbital framework as candidate sources of the preferred geometry of hydroperoxides. Stabilising 1,2-δ+H-δ-O electrostatic interactions and nO → σ*OR hyperconjugative interactions were found to be the driving force for the preferred anticlinal or perpendicular geometries, respectively. There is an interesting interplay between these two effects: when one increases, the other decreases. On the other hand, steric effects showed negligible contributions to the conformational behaviour of the studied molecules. Also, HO-OR bond dissociation energy appeared to be dependent on RO˙ radical stability, which is governed by hyperconjugation.

Dichlorine peroxide (ClOOCl), chloryl chloride (ClCl(O)O) and chlorine chlorite (ClOClO): very accurateab initiostructures and actinic degradation

The structural parameters of the three most stable isomers with formula Cl₂O₂, dichlorine peroxide, chloryl chloride and chlorine chlorite, were determined by high-levelab initiotheory. The photodissociation pathways were investigated.

Multi-slit-type interference in carbon 2s photoionization of polyatomic molecules: from a fundamental effect to structural parameters

In molecular photoemission, the analogue of the celebrated Young's double slit experiment is coherent electron emission from two equivalent atomic centers, giving rise to an interference pattern. Here multi-slit interference is investigated in inner-valence photoionization of propane, n-butane, isobutane and methyl peroxide. A more complex pattern is observed due to molecular orbital delocalization in polyatomic molecules, blurring the distinction between interference and diffraction. The potential to extract geometrical information is emphasized, as a more powerful extension of the EXAFS technique. Accurate reproduction of experimental features is obtained by simulations at the static Density Functional Theory level.

Accurate theoretical characterization of dioxygen difluoride: a problem resolved

Determination of the minimum structure on a very flat surface is problematic. If r_e(OO) is changed by 0.0025 Å around the minimum, r_e(OF) is changed by 0.01 Å. Vibrational averaging is required.

Reaction of Methyl Fluoroformyl Peroxycarbonate (FC(O)OOC(O)OCH3) with Cl Atoms: Formation of Hydro-ChloroFluoro-Peroxides

The products following Cl atom initiated reactions of FC(O)OOC(O)OCH₃ in 50-760 Torr of N₂ at 296 K were investigated using FTIR. Reaction of Cl atoms with methyl fluoroformyl peroxycarbonate proceeds mainly via attack at the methyl group, forming FC(O)OOC(O)OCH₂^• radicals. Further reaction of this kind of radical with Cl₂ forms three new compounds: FC(O)OOC(O)OCH₂Cl, FC(O)OOC(O)OCHCl₂, and FC(O)OOC(O)OCCl₃, whose existence was characterized experimentally by FTIR spectroscopy assisted by ab initio calculations at the B3LYP/6-31++G(d,p) level. Relative rate techniques were used to measure k_{(Cl+FC(O)OOC(O)OCH3)} = (4.0 ± 0.4) × 10^-14 cm³ molecule^-1 s^-1 and k_{(Cl+FC(O)OOC(O)OCH2Cl)} = (3.2 ± 0.3) × 10^-14 cm³ molecule^-1 s^-1. When the reaction is run in the presence of oxygen, the paths giving chlorinated peroxide formation are suppressed, and oxidation to (mainly) CO₂ and HCl takes place through highly oxidized intermediates with lifetimes long enough to be detected by FTIR spectroscopy.

The gas-phase structure of dimethyl peroxide

To understand the spectroscopic properties of a molecule that performs large-amplitude motion, distinction must be made between the statical and dynamical structures.