Brown carbon in aerosol remains a significant source of error in global climate modeling due to its complex nature and limited product characterization. Though significant efforts have been made in the previous decade to identify the major lightabsorbing brown carbon chromophores formed through the reactions of carbonylcontaining compounds with ammonium, substantial work is still required to identify the main absorbing species resulting from reactions of glyoxal, glycolaldehyde, and hydroxyacetone with ammonium sulfate (AS). Using tandem mass spectrometry and 15 N experiments to confirm proposed structures and support their mechanistic pathways, compelling evidence is provided for the formation of pyrazines and imidazoles in the glyoxal + AS, glycolaldehyde + AS, and hydroxyacetone + AS systems. Through density functional theory calculations, the N-containing oligomers and aromatic heterocycles formed within these reaction systems are shown to contribute to brown carbon light absorption, thus holding significant relevance toward accurately predicting their effects on global climate.

The contribution of organic aerosol from biomass burning is poorly constrained, and the lack of consensus regarding its overall contribution to global radiative forcing leads to significant uncertainties in climate modeling. Identification of potential brown carbon chromophores from common biomass burning emissions may reduce this uncertainty. Biacetyl (BA) is found in emissions from industry and biomass burning from various ecosystems and shares structural similarities with other small carbonyls that react with ammonium sulfate (AS) to produce brown carbon compounds. Like previous carbonyl + AS studies, the BA + AS system results in the formation of hundreds of different products; these were separated and identified using supercritical fluid chromatography–tandem mass spectrometry, isotopic substitution experiments, and comparisons to standards. Kinetic information was obtained through spectral decomposition of experimentally measured UV–visible absorbance data. Theoretical TDDFT calculations were utilized to extract more information on the light absorbance of identified products and to determine how these individual chromophores would contribute to the light absorbance of organic aerosol. This information could provide insight into unknown organic aerosol behavior by furthering our understanding of the reactivity of a common biomass burning emission product like biacetyl.

A series of CCSD(T) single-point calculations on MP4(SDQ) geometriesand the W1 model chemistry method have been used to calculate ⌬H°and ⌬G°values for the deprotonation of 17 gas-phase reactions where the experimental values have reported accuracies within 1 kcal/mol. These values have been compared with previous calculations using the G3 and CBS model chemistries and two DFT methods. The most accurate CCSD(T) method uses the aug-cc-pVQZ basis set. Extrapolation of the aug-ccpVTZ and aug-cc-pVQZ results yields the most accurate agreement with experiment, with a standard deviation of 0.58 kcal/mol for ⌬G°and 0.70 kcal/mol for ⌬H°. Standard deviations from experiment for ⌬G°and ⌬H°for the W1 method are 0.95 and 0.83 kcal/mol, respectively. The G3 and CBS-APNO results are competitive with W1 and are much less expensive. Any of the model chemistry methods or the CCSD(T)/ aug-cc-pVQZ method can serve as a valuable check on the accuracy of experimental data reported in the National Institutes of Standards and Technology (NIST) database.

Yunnaneic acids A-D, isolated from the roots of Salvia yunnanensis , are hexameric (A and B) and trimeric (C and D) assemblies of caffeic acid that feature an array of synthetically challenging and structurally interesting domains. In addition to being caffeic acid oligomers, yunnaneic acids A and B are formally dimeric and heterodimeric adducts of yunnaneic acids C and D. Herein we report the first total syntheses of yunnaneic acids C and D featuring the formation of their bicyclo[2.2.2]octene cores in a single step from simple precursors via an oxidative dearomatization/Diels-Alder cascade that may have biogenetic relevance. In addition, exploitation of the key intermediate resulting from this cascade reaction has enabled rapid access to the structurally related caffeic acid metabolite rufescenolide through an unexpected Lewis acid-mediated reduction. Finally, we report the results of extensive model studies toward forming the dimeric yunnaneic acids A and B. These explorations indicate that the innate reactivities of the monomeric fragments do not favor spontaneous formation of the desired dimeric linkages. Consequently, enzymatic involvement may be required for the biosynthesis of these more complex family members.

A synthesis of the 2-azatricyclo­[4.3.2.04,9]­undecane ring systema hitherto unreported bridged azatricyclic ring systembeginning from tricarbonyl­(tropone)iron and allylamine was accomplished in three steps: (1) aza-Michael addition of allylamine to tricarbonyl­(tropone)­iron; (2) Boc-protection of the resulting secondary amine; and (3) oxidative demetallation leading to a spontaneous intramolecular Diels–Alder reaction. The effect of a variety of parameters on the intramolecular Diels–Alder reaction was investigated, including diene and dienophile substitution patterns and dienophile tether length.