Isoprene-Derived Organosulfates: Vibrational Mode Analysis by Raman Spectroscopy, Acidity-Dependent Spectral Modes, and Observation in Individual Atmospheric Particles.

Isoprene, the most abundant biogenic volatile organic compound (BVOC) in the atmosphere, and its low-volatility oxidation products lead to secondary organic aerosol (SOA) formation. Isoprene-derived organosulfates formed from reactions of isoprene oxidation products with sulfate in the particle phase are a significant component of SOA and can hydrolyze forming polyols. Despite characterization by mass spectrometry, their basic structural and spectroscopic properties remain poorly understood. Herein, Raman microspectroscopy and density functional theory (DFT) calculations (CAM-B3LYP level of theory) were combined to analyze the vibrational modes of key organosulfates, 3-methyltetrol sulfate esters (racemic mixture of two isomers), and racemic 2-methylglyceric acid sulfate ester, and hydrolysis products, 2-methyltetrols, and 2-methylglyceric acid. Two intense vibrational modes were identified, ν(RO-SO3) (846 ± 4 cm-1) and νs(SO3) (1065 ± 2 cm-1), along with a lower intensity δ(SO3) mode (586 ± 2 cm-1). For 2-methylglyceric acid and its sulfate esters, deprotonation of the carboxylic acid at pH values above the pKa decreased the carbonyl stretch frequency (1724 cm-1), while carboxylate modes grew in for νs(COO-) and νa(COO-) at 1413 and 1594 cm-1, respectively. The ν(RO-SO3) and νs(SO3) modes were observed in individual atmospheric particles and can be used in future studies of complex SOA mixtures to distinguish organosulfates from inorganic sulfate or hydrolysis products.