Solutions, formaldehyde Free energy

Liu et aL (1999a) studied the hydrolysis of CI2 in water, while Meijer and Sprik (1998a) investigated the reaction mechanism of water with formaldehyde in sulfuric acid. Laasonen and Klein (1997) studied the hydrolysis of HCl upon addition to water, and Meijer and Sprik (1998b) studied the addition of H2O to SO3 in solution. Finally, Trout and Parrinello (1998) evaluated the mechanism and free energy profile of the dissociation of H2O in water. 

Fig. 7. Effect of temperature on the free energy of activation, AF+, of the reaction of O.IOM solutions of acrolein, formaldehyde, and acetaldehyde with rattail tendon as measured by effects on T..

The Sj 2 reaction, X + RY XR + Y", has been simulated with MC equilibrium calculations by Jorgensen and coworkers [81, 82]. The procedure used by these authors involves three steps i) the lowest energy reaction path is determined for the in vacuo system by using ab initio molecular orbital calculations ii) inter-molecular potential functions are obtained to describe the interactions between the substrate and a solvent molecule these potentials depend on the internal structure of the substrate iii) MC simulations are carried out to determine the free energy profile for the reaction in solution. This is a difficult computational task since importance sampling methods are required to explore all the values of the reaction coordinate. A similar technique was used by Madura and Jorgensen [83] in simulating the nucleophilic addition of hydroxide ion to formaldehyde in the gas phase and in aqueous solution. 

Heat of Formation and Free Energy. The heat of formation of aqueous formaldehyde is eqiuA alent to the heat, of formation of the anhydrous gas plus its heat of solution in water. As lias been prei iou.sly pointed out (page 20), the most acceptable I alue for the heat of formation of -anhydrous formaldehyde at 18°C is 28 kcal per mol. According to DeLepine , the heat of solution of fonnaldeliyde gas in water is 15 kcal per mol. Since the thermal energi of dissoh ed formaldehyde is reported to be independent of concentration up to approximately 30 per cent (page 37), the heat of formation of di.ssolved foimaldehyde at concentrations of 30 per cent or lees may be taken as approximately 43 kcal per mol. 

The earliest indications that water could be photolyzed by 1850-A. radiation came from the experiments of Fricke and Hart, who used a quartz-mercury arc and condensed sparks. Hydrogen and formaldehyde were produced by irradiation of aqueous methanol solutions. It was improbable that, at the low concentrations of methanol used, the reactions could be accounted for by direct photoactivation of methanol instead, they were attributed to the absorption of light by the solvent, followed by transfer of the absorbed energy to the methanol. By the use of filters of potassium sulfate, it was shown that the spectral limit for the water-photosensitized reaction is in the neighborhood of 2000 A. Although Fricke and Hart did not interpret their reactions in terms of free radicals, formed according to reaction (2), these provide the most probable mechanism. 

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