Contribution of Charge-Transfer Complexes to Absorptivity of Primary Brown Carbon Aerosol

Light-absorbing organic aerosol, or brown carbon (BrC), has significant but poorly constrained effects on climate. A large fraction of the absorptivity of ambient BrC is unassigned, and organic charge-transfer (CT) complexes have the potential to contribute to this fraction. Here, the contributions of CT complexes to the absorptivity of laboratory-generated BrC and ambient aerosol material influenced by biomass burning have been investigated, using a wide range of chemical, spectroscopic, and physical analyses. Chemical functionalization experiments are inconclusive about the role of CT complexes, whereas fluorescence spectra exhibit distinct spectral features indicative of individual chromophores. Determinations of the concentration and temperature dependences of absorbance are more conclusive. In particular, for laboratory-generated BrC extracted in either water or methanol, absorbance scaled linearly with orders-of-magnitude changes in concentration, indicating that intermolecular complexes do not contribute to the absorptivity. Furthermore, whereas the absorbance of BrC extracts in dimethyl sulfoxide exhibited a slight temperature dependence, consistent with a 15% contribution from intramolecular CT complexes at 15 °C, the complete temperature independence of absorbance of water-soluble extracts from surrogate and ambient BrC indicates a negligible role for CT complexes. Overall, our results find little evidence for CT complexes in the primary BrC studied, suggesting that they do not contribute significantly to the missing absorptivity of ambient BrC

β‑Boron Effect Enables Regioselective and Stereospecific Electrophilic Addition to Alkenes

Electrophilic addition to alkenes is a textbook-taught reaction, yet it is not always possible to control the regioselectivity of addition to unsymmetrical 1,2-disubstituted substrates. We report the observation and applications of the β-boron effect that accounts for high regioselectivity in electrophilic addition reactions to allylic MIDA (N-methyliminodiacetic acid) boronates. While the well-established β-silicon effect bears partial resemblance to the observed reactivity, the silyl group is typically lost during functionalization. In contrast, the boryl moiety is retained in the product when B­(MIDA) is used as the nucleophilic stabilizer. Mechanistic studies elucidate the origin of this effect and demonstrate how σ­(C–B) hyperconjugation helps stabilize the incipient carbocation. This transformation represents a rare example of the stereospecific hydrohalogenation of secondary allyl MIDA-boronates that proceeds in a syn-fashion