We present direct experimental evidence for the existence of a surface-bound state of partially hydrated SO2 on water. Surface second harmonic generation (SHG) and static surface tension measurements are used to examine the SO2 surface coverage as a function of its bulk aqueous concentration. The results indicate a Langmuir-type adsorption of SO;? at the air-water interface. These experiments represent the first report of the application of surface SHG techniques to detect gas adsorption onto a liquid surface.

The photodissociation of gaseous acetyl cyanide has been examined following excitation at 193 nm. CN X2Σ+ photofragments were probed via laser fluorescence excitation to determine their rotational, vibrational, and translational energy distributions. CN was produced in v‘‘ = 0 and 1 with mean rotational energy (13.5 ± 2) kJ mol-1, and v‘‘ = 2 with mean rotational energy (10 ± 4) kJ mol-1. Mean translational energies of the CN fragments were (32 ± 10) kJ mol-1. Ab initio electronic structure theory has been used to characterize the heat of formation for acetyl cyanide along with its geometries and vibrational frequencies. The acetyl cyanide heat of formation, ΔH 0 f,0, is predicted to be (−0.4 ± 8) kJ mol-1 using Gaussian-2 theory (G2). The theoretical results are used to compute bond dissociation energies of acetyl cyanide for further interpretation of the experimental photodissociation data. Evidence is presented that the majority of CN fragments are produced via dissociation of the parent acetyl cyanide to CH3CO + CN, with subsequent decomposition of the acetyl fragment. The alternate possible primary α-cleavage pathway to CH3 + OCCN is proposed as a possible source for the OCCN radical.

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essentially identical results because the former method also yields an experimentally determined C2H3 entropy (55.9 (±2.6) cal mol-1 K"1) which is virtually the same as the theoretical value (55.5 (±0.5) cal mol"1 K"1). Since the enthalpy of formation and entropy determinations are coupled, high accuracy in the determination of one of these thermochemical quantities is a strong indication that the other is also accurate. In our prior study of the CH3 + HC1 reaction, a similar kinetic analysis, one which coupled our measured CH3 + HC1 rate constants with those for the Cl + CH4 reaction reported by Dobis and Benson and others,28 we also obtained exactly the correct entropy for CH3 and a heat of formation which was within ±0.2 kcal mol"1 of the most accurate prior determinations.1Recent studies using neutral reactants and products have obtained very similar values for the heat of formation of the C2H3 radical. Parmar and Benson12 obtained a C2H3 heat of formation within 1 kcal mol"1 of our value, in spite of the fact that their values for k] are lower than ours by a factor of 4. Wodtke and Lee have observed the velocity distribution of DF(u=4) from the F + C2D4 -» DF + C2D3 reaction.32 Since this information is a very sensitive function of the energetics of the reaction, it was possible to use the results of their experiments to determine a C2H3 heat of formation. They report a value of 65.6 (±0.5) kcal mol"1.