Simplified Protocol for the Calculation of Multiconformer Transition State Theory Rate Constants Applied to Tropospheric OH-Initiated Oxidation Reactions

Chemical kinetics plays a fundamental role in the understanding and modeling of tropospheric chemical processes, one of the most important being the atmospheric degradation of volatile organic compounds. These potentially harmful molecules are emitted into the troposphere by natural and anthropogenic sources and are chemically removed by undergoing oxidation processes, most frequently initiated by reaction with OH radicals, the atmosphere's "detergent". Obtaining the respective rate constants is therefore of critical importance, with calculations based on transition state theory (TST) often being the preferred choice. However, for molecules with rich conformational variety, a single-conformer method such as lowest-conformer TST is unsuitable while state-of-the-art TST-based methodologies easily become unmanageable. In this Feature Article, the author reviews his own cost-effective protocol for the calculation of bimolecular rate constants of OH-initiated reactions in the high-pressure limit based on multiconformer transition state theory. The protocol, which is easily extendable to other oxidation reactions involving saturated organic molecules, is based on a variety of freeware and open-source software and tested against a series of oxidation reactions of hydrofluoropolyethers, computationally very challenging molecules with potential environmental relevance. The main features, advantages and disadvantages of the protocol are presented, along with an assessment of its predictive utility based on a comparison with experimental rate constants.

Elucidation of the mechanisms governing the thermal diastereomerization of bioactive chiral 1,3,4-thiadiazoline spiro-cyclohexyl derivatives towards their anancomeric stereoisomers

Synthesis of bioactive 1,3,4-thiadiazoline-spiro-cyclohexyl-alkyl-substituted diastereomers by effective switching between kinetic and thermodynamic control.

Atmospheric Vinyl Alcohol to Acetaldehyde Tautomerization Revisited

The atmospheric oxidation of vinyl alcohol (VA) produced by photoisomerization of acetaldehyde (AA) is thought to be a source of formic acid (FA). Nevertheless, a recent theoretical study predicted a high rate coefficient k1(298 K) of ≈10(-14) cm(3) molecule(-1) s(-1) for the FA-catalyzed tautomerization reaction 1 of VA back into AA, which suggests that FA buffers its own production from VA. However, the unusually high frequency factor implied by that study prompted us to reinvestigate reaction 1 . On the basis of a high-level ab initio potential energy profile, we first established that transition state theory is applicable, and derived a k1(298 K) of only ≈2 × 10(-20) cm(3) molecule(-1) s(-1), concluding that the reaction is negligible. Instead, we propose and rationalize another important VA sink: its uptake by aqueous aerosol and cloud droplets followed by fast liquid-phase tautomerization to AA; global modeling puts the average lifetime by this sink at a few hours, similar to oxidation by OH.

Theoretical design of multi-nitroxyl organocatalysts with enhanced reactivity for aerobic oxidation

Higher catalytic performances of N,N',N''-trihydroxyisocyanuric acid (THICA), N,N-dihydroxypyromellitimide (NDHPI), and N-hydroxynaphthalimide (NHNI) than that of N-hydroxyphthalimide (NHPI) have been demonstrated recently in aerobic oxidation. Herein, the rational design of reactive multi-nitroxyl organocatalysts has been addressed theoretically by using systematic analysis of some important properties and catalytic activities of yet-to-be-synthesized catalysts. Our results show that 1) NHNI and its analogue, similar to THICA, unlike NHPI and others, are unsuitable for solvent- or mediator-free catalysis due to their strong intramolecular hydrogen-bonding interactions; 2) increasing the reactive hydroxyimide groups on the same aromatic ring, or doped N atoms or ionic-pair groups onto the aromatic ring, can improve catalytic reactivity, whereas appropriate enlargement of conjugated aromatic systems results in unchanged activity; 3) the newly designed catalysts are more active than NHPI and NHNI and have catalytic activities comparable to NDHPI and THICA; 4) the ionic-pair supported case is suggested to be a very active catalyst, even towards inert propane, and can be used as a novel model catalyst for further improvements. The present work will be helpful in designing reactive hydroxyimide organocatalysts.

Metal-free aerobic oxidations mediated by N-hydroxyphthalimide. A concise review

Since the beginning of the century, N-hydroxyphthalimide and related compounds have been revealed to be efficient organocatalysts for free-radical processes and have found ample application in promoting the aerobic oxidation of a wide range of organic substrates. When combined with different co-catalysts, they are activated to the corresponding N-oxyl radical species and become able to promote radical chains, involving molecular oxygen, directly or indirectly. Most of the examples reported in the literature describe the use of these N-hydroxy derivatives in the presence of transition-metal complexes. However, eco-friendly standards, including the demand for highly selective transformations, impose the development of metal-free processes, especially for large-scale productions, as in the case of the oxygenation of hydrocarbons. For this reason, many efforts have been devoted in the past decade to the design of new protocols for the activation of N-hydroxy imides in the presence of nonmetal initiators. Herein we provide a concise overview of the most significant and successful examples in this field, with the final aim to furnish a useful instrument for all scientists actively involved in the O₂-mediated selective oxidation of organic compounds and looking for environmentally safe alternatives to metal catalysis.