A study of the recombination of IO with NO2 and the stability of INO3:: Implications for the atmospheric chemistry of iodine

The recombination reaction between IO and NO2 was studied over a large range of temperature (216-474 K) and pressure (20-760 Torr). IO, produced by the pulsed photolysis at 193 nm of NO2 to yield O in the presence of CF3I, was probed at 445.0 nm [IO(A(2)Pi(3/2) - X(2)Pi(3/2)), (2,0)] and monitored by nonresonant LIF at 458.6 nm [10(A(2)Pi(3/2) - X(2)Pi(3/2)), (2, 1)]. The resulting rate coefficients were then fitted by RRKM theory, using molecular parameters and a heat of formation for INO3 (DeltaH(f,0)degrees = 70 +/- 16 kJ mol(-1), best fit value 80 kJ mol(-1)) derived from quantum calculations at the B3LYP/6-311+g(2d,p) level. This yielded k(rec,0) = (1.3 +/- 0.2) x 10(-30) (T/300 K)(-(4.5+/-0.6)) cm(6) molecule(-2) s(-1), k(rec,infinity) = (6.5 +/- 1.0) x 10(-12) (T/300 K)(-(1.3+/-0.8)) cm(3) molecule(-1) s(-1), and F-c = 0.57, over the experimental range of temperature and pressure. The RRKM calculations, in conjunction with measurements made at 474 K, were used to determine the rate of thermal dissociation of INO3 for the conditions of the lower atmosphere: k(diss)(240-305 K, 760 Torr) = 1.1 x 10(15) exp(-12060/T) s(-1), with an upper limit about a factor of 2 higher. The atmospheric significance of these results is discussed briefly.