TEMPERATURE-DEPENDENT NEAR UV MOLAR ABSORPTIVITIES OF SEVERAL SMALL ALDEHYDES IN AQUEOUS-SOLUTION

Temperature-dependent molar absorptivities are reported for benzaldehy de, acrolein, isobutyraldehyde, pentanal, and hexanal in aqueous solut ion. With the exception of acrolein, molar absorptivities are given at eight temperatures in the range 6.5-69.5-degrees-C for wavelengths gr eater 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 tempe ratures and wavelengths. For benzaldehyde and acrolein, no hydration o ccurs, as expected, and only small changes in the shape and intensity of the spectrum with temperature are observed. For the other three ald ehydes, the hydration reaction RCHO + H2O reversible RCH(OH)2; K(hyd) = [RCH(OH)2]/[RCHO] is the major source for the observed change in int ensity with temperature. Based on these changes, the following thermod ynamic parameters are obtained for the hydration reaction at T = 25-de grees-C: isobutyraldehyde, K(hyd) = 0.51 +/- 0.06, DELTAH = -26.8 +/- 1.9 kJ mol-1, DELTAS = -95 +/- 7 J mol-1 K-1; pentanal, K(hyd) = 0.55 +/- 0.05, DELTAH = -24.6 +/- 1.6 kJ mol-1, DELTAS = -87 +/- 8 J mol-1 K-1; hexanal, K(hyd) = 0.49 +/- 0.05, DELTAH = 25.3 +/- 2.2 kJ mol-1, DELTAS = -91 +/- 8 J mol-1 K-1. The present results are compared to da ta obtained previously for smaller aliphatic aldehydes, and the implic ations of the results for aqueous phase atmospheric chemistry are disc ussed.