Absorption spectra and photolysis of methyl peroxide in liquid and frozen water

Methyl peroxide (CH ₃OOH) is commonly found in atmospheric waters and ices in significant concentrations. It is the simplest organic peroxide and an important precursor to hydroxyl radical. Many studies have examined the photochemical behavior of gaseous CH ₃OOH; however, the photochemistry of liquid and frozen water solutions is poorly understood. We present a series of experiments and theoretical calculations designed to elucidate the photochemical behavior of CH ₃OOH dissolved in liquid water and ice over a range of temperatures. The molar extinction coefficients of aqueous CH ₃OOH are different from the gas phase, and they do not change upon freezing. Between -12 and 43 °C, the quantum yield of CH ₃OOH photolysis is described by the following equation: Φ(T) = exp((-2175 ± 448)1/T) + 7.66 ± 1.56). We use on-the-fly ab initio molecular dynamics simulations to model structures and absorption spectra of a bare CH ₃OOH molecule and a CH ₃OOH molecule immersed inside 20 water molecules at 50, 200, and 220 K. The simulations predict large sensitivity in the absorption spectrum of CH ₃OOH to temperature, with the spectrum narrowing and shifting to the blue under cryogenic conditions because of constrained dihedral motion around the O-O bond. The shift in the absorption spectrum is not observed in the experiment when the CH ₃OOH solution is frozen suggesting that CH ₃OOH remains in a liquid layer between the ice grains. Using the extinction coefficients and photolysis quantum yields obtained in this work, we show that under conditions with low temperatures, in the presence of clouds with a high liquid-water content and large solar zenith angles, the loss of CH ₃OOH by aqueous photolysis is responsible for up to 20% of the total loss of CH ₃OOH due to photolysis. Gas phase photolysis of CH ₃OOH dominates under all other conditions.