COMPUTATIONAL KINETICS STUDY OF ATMOSPHERIC RING-CLOSURE AND DEHYDRATION REACTIONS
OF 1,4-HYDROXYCARBONYLS IN THE GAS PHASEa
PARANDAMAN ARATHALA, CHANIN B. TANGTARTHARAKUL, AMITABHA SINHA, Department
of Chemistry and Biochemistry, UC San Diego, San Diego, CA, USA.
Several experimental studies have shown that 1,4-hydroxycarbonyls can undergo sequential reactions involving cy-
clization followed by dehydration to form dihydrofurans.b As dihydrofurans contain a double bond, they are highly reactive
towards OH, O3, and NO3 in the atmosphere. In this work, we investigate the energetics and kinetics of the cyclization and
dehydration reaction steps associated with 4-hydroxybutanal (4-OH-BL), a prototypical 1,4-hydroxycarbonyl molecule us-
ing ab initio calculations. The cyclization step transforms 4-OH-BL into 2-hydroxytetrahydrofuran (2-OH-THF), which
can subsequently undergo dehydration to form 2,3-dihydrofuran. Since the barriers associated with the cyclization and
dehydration steps for 4-OH-BL are respectively 34.8 and 63.0 kcal/mol in the absence of any catalyst, both reaction steps
are not feasible under atmospheric conditions. However, the presence of a suitable catalyst can significantly reduce the
reaction barriers. Therefore, we investigate the effect of a single molecule of H2O, HO2 radical, HC(O)OH, HNO3, and
H2SO4 as catalysts on the reaction. We find that H2SO4 lowers the reaction barriers the greatest, with the barrier for the
cyclization step being reduced to -13.1 kcal/mol and that for the dehydration step going down to 9.2 kcal/mol, below their
respective separated starting reactants. Interestingly, our rate calculations shows that HNO3 provides the fastest rate due the
combined effects of larger atmospheric concentration and reduced barrier. Thus, our study suggests that with acid catalysis
the cyclization reaction step can readily occur for 1,4-hydroxycarbonyls in the gas phase. The 2-OH-THF products, once
formed, likely undergo reaction with OH radicals in the atmosphere because the dehydration step involves a large barrier
even with acid catalysis. The reaction pathways and rate constant for this reaction in the presence of molecular oxygen
(3O2) were also investigated using computational chemistry over the 200-300K temperature range. The main products
found from the 2-OH-THF + OH/3O2 reactions are succinaldehyde + HO2 and 2,3-dihydro-2-furanol + HO2.
aThe authors are grateful to NSF for support of this work.
bAtkinson, R. et al. Atmos. Environ. 2008, 42, 5859; Ranney, A. P.; Ziemann, P. J. J. Phys. Chem. A 2016, 120, 2561.