The Importance of NO+(H2O)4 in the Conversion of NO+(H2O)n to H3O+(H2O)n: I. Kinetics Measurements and Statistical Rate Modeling

The kinetics for conversion of NO+(H2O)(n) to H3O+(H2O)(n) has been investigated as a function of temperature from 150 to 400 K. In contrast to previous studies, which show that the conversion goes completely through a reaction of NO+(H2O)(3,) the present results show that NO+(H2O)(4) plays an increasing role in the conversion as the temperature is lowered. Rate constants are derived for the clustering of H2O to NO+(H2O)(1-3) and the reactions of NO+(H2O)(3,4) with H2O to form H3O+(H2O)(2,3), respectively. In addition, thermal dissociation of NO+(H2O)(4) to lose HNO2 was also found to be important. The rate constants for the clustering increase substantially with the lowering of the temperature. Flux calculations show that NO+(H2O)(4) accounts for over 99% of the conversion at 150 K and even 20% at 300 K, although it is too small to be detectable. The experimental data are complimented by modeling of the falloff curves for the clustering reactions. The modeling shows that, for many of the conditions, the data correspond to the falloff regime of third body association.