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Electronic structure calculationshave been used to investigatepossible gas-phase decomposition pathways of α-hydroxyalkylhydroperoxides (HHPs), an important source of tropospheric hydrogenperoxide and carbonyl compounds. The uncatalyzed as well as water-and acid-catalyzed decomposition of multiple HHPs have been examinedat the M06-2X/aug-cc-pVTZ level of theory. The calculations indicatethat, compared to an uncatalyzed or water-catalyzed reaction, thefree-energy barrier of an acid-catalyzed decomposition leading toan aldehyde or ketone and hydrogen peroxide is dramatically lowered.The calculations also find a direct correlation between the catalyticeffect of an acid and the distance separating its hydrogen acceptorand donor sites. Interestingly, the catalytic effect of an acid onthe HHP decomposition resulting in the formation of carboxylic acidand water is relatively much smaller. Moreover, since the free-energybarrier of the acid-catalyzed aldehyde- or ketone-forming decompositionis ∼25% lower than that required to break the O–OH linkageof the HHP leading to the formation of hydroxyl radical, these resultssuggest that HHP decomposition is likely not an important source oftropospheric hydroxyl radical. Finally, transition state theory estimatesindicate that the effective rate constants for the acid-catalyzedaldehyde- or ketone-forming HHP decomposition pathways are 2–3orders of magnitude faster than those for the water-catalyzed reaction,indicating that an acid-catalyzed HHP decomposition is kineticallyfavored as well. [ABSTRACT FROM AUTHOR]

Density functional theory calculationspredict that the gas-phasedecomposition of carbonic acid, a high-energy, 1,3-hydrogen atom transferreaction, can be catalyzed by a monocarboxylic acid or a dicarboxylicacid, including carbonic acid itself. Carboxylic acids are found tobe more effective catalysts than water. Among the carboxylic acids,the monocarboxylic acids outperform the dicarboxylic ones whereinthe presence of an intramolecular hydrogen bond hampers the hydrogentransfer. Further, the calculations reveal a direct correlation betweenthe catalytic activity of a monocarboxylic acid and its pKa, in contrast to prior assumptions about carboxylic-acid-catalyzedhydrogen-transfer reactions. The catalytic efficacy of a dicarboxylicacid, on the other hand, is significantly affected by the strengthof an intramolecular hydrogen bond. Transition-state theory estimatesindicate that effective rate constants for the acid-catalyzed decompositionare four orders-of-magnitude larger than those for the water-catalyzedreaction. These results offer new insights into the determinants ofgeneral acid catalysis with potentially broad implications. [ABSTRACT FROM AUTHOR]

Density functional theory and transitionstate theory rate constantcalculations have been performed to gain insight into the bimolecularreaction of the Criegee intermediate (CI) with carbon monoxide (CO)that is proposed to be important in both atmospheric and industrialchemistry. A new mechanism is suggested in which the CI acts as anoxidant by transferring an oxygen atom to the CO, resulting in theformation of a carbonyl compound (aldehyde or ketone depending uponthe CI) and carbon dioxide. Fourteen different CIs, including onesresulting from biogenic ozonolysis, are considered. Consistent withprevious reports for other CI bimolecular reactions, the anticonformers are found to react faster than the synconformers. However, this can be attributed to steric effects andnot hyperconjugation as generally invoked. The oxidation reactionis slow, with barrier heights between 6.3 and 14.7 kcal/mol and estimatedreaction rate constants 6–12 orders-of-magnitude smaller thanpreviously reported literature estimates. The reaction is thus expectedto be unimportant in the context of tropospheric oxidation chemistry.However, the reaction mechanism suggests that CO could be exploitedin ozonolysis to selectively obtain industrially important carbonylcompounds. [ABSTRACT FROM AUTHOR]

The crystal structures are reported of trans-dioxocyclam dihydrate, C10H20N4O2·2H2O, a structural isomer of the well known cis-dioxocyclam, and of its novel Ni complex, (1,4,8,11-tetra­aza­cyclo­tetra­decane-2,9-dionato-κ4 N)­nickel(II) dihydrate, [Ni(C10H18N4O2)]·2H2O, the first example of a trans­ition metal complex of this ligand. Both mol­ecules lie on crystallographic centres of inversion. The free ligand has two of its N atoms turned outwards from the ring and hydrogen bonded to water mol­ecules. A major conformational change takes place in the complex in which the ligand binds in a trans tetradentate fashion, as suggested by the electronic spectrum. The nickel(II) ion is low spin, although the electronic spectrum of the complex in water indicates an equilibrium mixture of low-spin and high-spin species. The irreversible electrochemical oxidation of [Ni L1] ( L1 is deprotonated trans-dioxocyclam, C10H18N4O2) in water occurs at a potential of 0.964 V [ versus SHE (standard hydrogen electrode)], which is very similar to that for the Ni- cis-dioxocyclam complex. [ABSTRACT FROM AUTHOR]

Abstract: Among industrial chemical processes, ethylene oxide manufacture emits the largest amount of CO2 (∼2–3milliontons/yr), as byproduct from the burning of both the ethylene (feed) and ethylene oxide. Further, the conventional silver-based catalytic process presents safety challenges due to the formation of explosive ethylene oxide/O2 mixtures in the gas phase. By judicious choice of the catalyst (methyltrioxorhenium), oxidant (H2O2) and reaction medium (methanol/water), a homogeneous liquid phase catalytic system has been demonstrated that eliminates CO2 formation while producing ethylene oxide at >90% selectivity at near-ambient temperatures. Given its high volatility, the ethylene oxide is easily recovered from the reaction phase by distillation. The vicinity of the gaseous ethylene feed to its critical temperature (9°C) is exploited to significantly increase its solubility in the liquid reaction phase by facile compression beyond the critical pressure of ethylene (∼50bar). Since H2O2 is stable at typical reaction temperatures (40°C or less), potentially explosive ethylene oxide/O2 mixtures are avoided in the gas phase. In addition to the potential of arresting the carbon footprint of a large-scale industrial process, the demonstrated process concept shows how gas-expanded liquids can be generally exploited in homogeneous catalysis to enhance productivity. [Copyright &y& Elsevier]