Experiments and Calculations on Rate Coefficients for Pyrolysis of SO₂ and the Reaction O + SO at High Temperatures

Abstract

Rate coefficients for the pyrolysis of SO 2 in Ar in the temperature range 2188-4249 K were determined using a diaphragmless shock tube. The concentration of O atoms was monitored with resonance absorption. Rate coefficients determined in this work show Arrhenius behavior, with k 1a (T) ) (4.86 ( 1.31) × 10 -9 exp [-(50450 ( 730)/T] cm 3 molecule -1 s -1 ; listed errors represent one standard deviation in fitting. These values are consistent with some previous measurements that show a preexponential factor and activation energy greater than other measurements. Theoretical calculations at the G2M(RCC2) level, using geometries optimized with the B3LYP/6-311+G(3df) method, yield energies of transition states and products relative to those of the reactants. Rate coefficients predicted with a microcannonical variational RRKM theory agree well with experimental observations; contributions from electronically excited states of SO 2 are significant. Rate coefficients for the recombination O + SO f SO 2 are predicted to decrease with temperature with k 10a (T) ) (4.82 ( 0.05) × 10 -31 (T/298) -2.17(0.03 cm 6 molecule -2 s -1 for the temperature range 298-3000 K. In some experiments, S atoms were monitored with resonance absorption. With detailed chemical modeling, we found that S atoms were mainly produced from the secondary reaction O + SO f S + O 2 rather than from direct pyrolysis of SO 2 or from further pyrolysis of the SO product. Rate coefficients for this secondary reaction, determined to be k 10b (T) ) (3.0 ( 0.3) × 10 -11 exp [-(6980 ( 280)/T] cm 3 molecule -1 s -1 , agree closely with the theoretically predicted value comprising three product-channels via one triplet and two singlet SOO intermediates.

Experimental and theoretical investigations of rate coefficients of the reaction S(3P)+O2 in the temperature range 298–878 K

“…The total rate coefficient agrees satisfactorily with experimental results in this work, and also with those at high temperatures reported by Miyoshi et al 7 and by Woiki and Roth. 9 In our previous experiments using a diaphragmless shock tube to investigate pyrolysis of SO 2 , 38 we observed formation of S atoms from the secondary reaction,…”

“…Predicted vibrational wave numbers and rotational constants as well as available experimental data [43][44][45][46][47] for the species involved are discussed and summarized in Table 4 of Ref. 38. The results indicate that the predicted vibrational wave numbers are underestimated by 0.2%-2.3% from experimental values.…”

“…The Lennard-Jones ͑LJ͒ parameters required for the RRKM calculations are the same as those used for the reaction OϩSO previously. 38 The exponential-down model with ␣ϭ1500 cm Ϫ1 is employed for collisional deactivation. Because the electronic ground state of the S atom is split into three ͑quintuplet, triplet, and singlet͒ degenerate levels separated by 396.1 and 573.6 cm Ϫ1 , the electronic partition function of these low-lying excited states are also taken into account in calculations of the rate constant.…”

Potential energy surfaces for low-lying electronic states of SO2

“…1 [24] , and can be compared with those obtained by B3LYP/6-311G(3df) [9] (0.1437 nm, 119.2º) and CAS-AQCC/ ANO-Ⅱ [14] (0.1439 nm, 119.9º). [23]; b) ref.…”

“…Because of its importance in atmospheric photochemistry, as well as in atmospheric dynamics, this molecule has been the subject of much experimental [2][3][4][5][6][7][8][9][10] and theoretical [11][12][13][14][15][16][17][18][19] photochemical study for many years. They provide a wealth of information about the SO 2 spectrum, predissociation mechanism, vibration including vibration-rotation interaction and the onset of classical chaos, as well as normal-to-local mode transitions.…”