Temperature dependent near UV molar absorptivities of several small aldehydes in aqueous solution

Temperature-dependent molar absorptivities are reported for benzaldehyde, acrolein, isobutyraldehyde, pentanal, and hexanal in aqueous solution. With the exception of acrolein, 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 acrolein, data are given over a more limited range of temperatures and wavelengths. For benzaldehyde and acrolein, no hydration occurs, as expected, and only small changes in the shape and intensity of the spectrum with temperature are observed. For the other three aldehydes, the hydration reaction RCHO+H₂O⇄RCH(OH)₂; K_hyd=[RCH(OH)₂]/[RCHO] is the major source for the observed change in intensity with temperature. Based on these changes, the following thermodynamic parameters are obtained for the hydration reaction at T = 25°C: isobutyraldehyde, K_hyd = 0.51 ± 0.06, ΔH = - 26.8 ± 1.9 kJ mol^-1, ΔS = - 95 ± 7 J mol^-1 K^-1; pentanal, K_hyd = 0.55 ± 0.05, ΔH = - 24.6 ± 1.6 kJ mol^-1, ΔS = - 87 ± 8 J mol^-1 K^-1; hexanal, K_hyd = 0.49 ± 0.05, ΔH = - 25.3 ± 2.2 kJ mol^-1, ΔS = - 91 ± 8 J mol^-1 K^-1. The present results are compared to data obtained previously for smaller aliphatic aldehydes, and the implications of the results for aqueous phase atmospheric chemistry are discussed.