An unpublished letter of beaumarchais

Mechanisms of radical removal by SO2

“…At elevated temperatures an "inner" transition state containing a tight entropy bottleneck comes into play. From ab initio calculations [18] we estimate the 1000-1500 K rate constant to be of the order of 10 11 cm 3 /mol·s and decreasing with temperature. These values were obtained from a simple transition state theory approach and are expected to represent an upper limit.…”

“…The reaction of 3 SO with O2 (R5) is well characterized experimentally [14]. This is not the case for the reaction of 3 SO with HO2 (R6), but ab initio calculations [18] indicate that this reaction is relatively fast and proceeds without barriers. Recombination of SO with H atoms is a potential radical sink [14], but according to ab initio calculations [18] the reaction is slow.…”

“…For the singlet SO species introduced in the present mechanism, we have estimated a heat of formation of ∆ f H298 = 23.90 kcal /mol and an entropy of S298 = 50.89 cal /mol·K. From ab initio calculations [18] the excitation energy for the first singlet state was estimated to 22.4 kcal /mol. This value is somewhat higher than the estimate from Huber and Herzberg [19] of 18.1 kcal /mol, but it is in agreement with the corresponding value for O2 (22.7 kcal /mol).…”

“…However, a number of rate constants were modified according to ab initio calculations [18], and the S2Hx subset was updated with data from Sendt et al [20]. Data for some important reactions in the sulfur subset are found in Table 1.…”

“…A recent ab initio study [15,18] of the HOSO+H reaction indicates a very fast reaction yielding >95 % 1 SO+H2O (R26) following an addition/elimination mechanism. Formation of SO in the singlet state conserves spin; however it is possible that intersystem crossing occurs during the course of reaction (R26) and that the triplet state occurs directly.…”