Nuclear magnetic resonance techniques were used to study the kinetics and products of the reaction of a variety of epoxides with various amines under varying pH conditions. In agreement with a previous finding, the amine-epoxide reactions were found to be water-catalyzed and not directly dependent on the pH of the reaction environment. At pH values higher than the pK(a) of the particular amine, the amine-epoxide reactions were extremely efficient, outcompeting hydrolysis reactions even for conditions where water was the solvent and the amine was a relatively low-concentration solute. This finding was rationalized by measurements that showed that the relative nucleophilic strength of amines relative to water was on the order of 1000, while the nucleophilic strength of protonated amines (which are predominant when pH < pK(a)) was negligible. The epoxide carbon substitution environment was found to have a large effect on the measured rate constants (more substituted epoxides had slower rate constants), while the amine carbon substitution environment had a much more limited effect. While the amine-epoxide rate constants are large enough such that amine-epoxide reactions may be kinetically feasible for isoprene-derived epoxides and high amine secondary organic aerosol (SOA) concentrations, most atmospheric amines are expected to be present in protonated form on the largely acidic SOA found in the atmosphere and thus are expected to be largely unreactive toward epoxides.

Laboratory and field measurements have demonstrated that isoprene epoxydiol (IEPOX) is the base component of a wide range of chemical species found in isoprene-derived secondary organic aerosol (SOA). To address newly raised questions concerning the chemical identities of IEPOX-derived SOA, the results of laboratory experiments carried out in bulk aqueous and organic media and analyzed via nuclear magnetic resonance spectroscopy and computed free energies of possible products are reported. The IEPOX nucleophilic addition product 2-methyltetrol was found to react too slowly in aqueous solution to explain the previous observation of tetrahydrofuran-based species. The IEPOX isomerization reactions in organic media were shown to mainly produce 3-methyltetrahydrofuran-2,4-diols, which were also established by the computational results as one of the most thermodynamically favorable possible IEPOX reaction products. However, these isomerization reactions were found to be relatively slow as compared to nucleophilic addition reactions, indicating that their occurrence on ambient SOA might be limited to low water content situations. No evidence was found for the production of the C alkene triols or 3-methyltetrahydrofuran-3,4-diols previously reported for IEPOX reaction on SOA as analyzed via the gas chromatography/electron ionization-quadrupole mass spectrometry with prior trimethylsilyl derivatization method.

Extensive studies of secondary organic aerosol (SOA) formation have identified isoprene epoxydiol (IEPOX) intermediates as key species in the formation of isoprene-derived SOA. Recent work has suggested that isoprene-derived dimers and oligomers may constitute a significant fraction of SOA, but a mechanism for the formation of such abundant SOA components has yet to be established. The potential for dimer formation from the nucleophilic addition of 2-methyltetrol to trans-β-IEPOX was assessed through a series of model epoxide−nucleophile experiments using nuclear magnetic resonance (NMR) spectroscopy. These experiments helped establish a rigorous understanding of structural, stereochemical, and NMR chemical shift trends, which were used along with nucleophilic strength calculations to interpret the results of the trans-β-IEPOX + 2-methyltetrol reaction and evaluate its relevance in the atmosphere. A preference for less sterically hindered nucleophiles was observed in all model systems. In all addition products, a significant increase in NMR chemical shift was observed directly adjacent to the epoxide−nucleophile linkage, with smaller decreases in chemical shift at all other sites. A partial NMR assignment of a single trans-β-IEPOX + 2-methyltetrol nucleophilic addition product was obtained, but nucleophilic strength calculations suggest that 2-methyltetrol is a poor nucleophile. Therefore, this reaction is unlikely to significantly contribute to dimer and oligomer formation on SOA. Nevertheless, the structural and stereochemical considerations, NMR assignments, and NMR chemical shift trends reported here will prove useful in future attempts to synthesize dimer and oligomer analytical standards.

We report vibrational spectra of the H2-tagged, cryogenically cooled X−∙HOCl (X = Cl, Br and I) ion-molecule complexes and analyze the resulting band patterns with electronic structure calculations and an anharmonic theoretical treatment of nuclear motions on extended potential energy surfaces. The complexes are formed by "ligand exchange" reactions of X−∙(H2O) n clusters with HOCl molecules at low pressure (~10−2 mbar) in a radio-frequency ion guide. The spectra generally feature many bands in addition to the fundamentals expected at the double harmonic level. These "extra bands" appear in patterns that are similar to those displayed by the X−∙HOD analogues, where they are assigned to excitations of nominally IR forbidden overtones and combination bands. The interactions driving these features include mechanical and electronic anharmonicities. Particularly intense bands are observed for the v = 0 → 2 transitions of the out-of-plane bending soft modes of the HOCl molecule relative to the ions. These involve displacements that act to break the strong H-bond to the ion, which give rise to large quadratic dependences of the electric dipoles (electronic anharmonicities) that drive the transition moments for the overtone bands. On the other hand, overtone bands arising from the intramolecular OH bending modes of HOCl are traced to mechanical anharmonic coupling with the v = 1 level of the OH stretch (Fermi resonances). These interactions are similar in strength to those reported earlier for the X‾∙HOD complexes.

In the presence of a halide ion, hypohalous acids can adopt two binding motifs upon formation of the ion–molecule complexes [XHOY]− (X, Y = Cl, Br, I): a hydrogen (HB) bond to the acid OH group and a halogen (XB) bond between the anion and the acid halogen. Here we isolate the X-bonded Cl–·IOH ion–molecule complex by collisions of I–·(H2O) n clusters with HOCl vapor and measure its vibrational spectrum by IR photodissociation of the H2-tagged complex. Anharmonic analysis of its vibrational band pattern reveals that formation of the XB complex results in dramatic lowering of the HOI bending fundamental frequency and elongation of the O–I bond (by 168 cm–1 and 0.13 Å, respectively, relative to isolated HOI). The frequency of the O–I stretch (estimated 436 cm–1) is also encoded in the spectrum by the weak v = 0 → 2 overtone transition at 872 cm–1.