Temperature dependent near-UV molar absorptivities of aliphatic aldehydes and ketones in aqueous solution

Abstract

Abstract Temperature dependent molar absorptivities are reported for acetone, 2-butanone, 2-pentanone, 3-pentanone, acetaldehyde, propionaldehyde, and n -butyraldehyde in aqueous solution. Molar absorptivities are given at eight temperatures in the range 6.5–69.5°C for wavelengths greater than 200 nm, a spectral resolution of 2.0 nm, and a spacing of 2.5 nm. For both ketones and aldehydes a shift to shorter wavelengths of approximately 10 nm is observed in the aqueous phase absorption spectrum relative to that found in the gas phase. For the ketones, there is an increase in the total intensity of the spectrum of approximately 5% over the range of temperatures studied. For the aldehydes a much larger change in the intensity of the absorption spectrum is observed, due to the temperature dependence of the hydration reaction RCHO + H 2 O ⇄ RCH(OH) 2 ; K hyd = [RCH(OH) 2 /[RCHO]. The change in the spectral intensity with temperature is used to determine thermodynamic parameters for the hydration reaction, giving the following results (at 25°C): acetaldehyde, K hyd = 1.13 ± 0.06, Δ H = −19.7±0.6kJ/mol, Δ S = −65.0±2.5J/mol-K; propionaldehyde, K hyd =1.02±0.06, ΔH=-20.8±0.8kJ/mol, Δ S =-69.6±3.1J/mol-K; n -butyraldehyde, K hyd =0.50±0.05, ΔH=-27.0±2.2kJ/mol, Δ S = −96.5± 8.2 J/mol-K. The implications of these results for aqueous phase atmospheric chemistry are discussed.