Kinetics and thermodynamics of association reactions of CO+• and HCO+ with CO and of N2+• and N2H+ with N2 between 120 and 650 °K

Abstract

We conducted a comparative study of the kinetic and thermodynamic parameters of the association reactions of the radical ions CO^+• and ${\mathrm{N}}_2^{\mathrm{+•}}$ and of the even electron ions HCO⁺ and N₂H⁺ with Co and N₂, respectively. The studies were conducted by the technique of pulsed high‐pressure mass spectrometry. We obtained the values of ΔH=11.7 kcal/mole and ΔS=20.9 e.u. for the dissociation of the complex HCO⁺ · CO and ΔH=14.5 kcal/mole, ΔS=20.4 e.u. for the dissociation of the complex N₂H⁺ · N₂. Both enthalpies are significantly smaller than the binding energies in the analogous complexes of the radical ions CO^+• and ${\mathrm{N}}_2^{\mathrm{+•}}$ . We measured the rate constants for the association reaction leading to these ion‐molecule complexes and obtained the following results: HCO⁺+2CO→HCO⁺ · CO+CO, k₃₂₈=0.61×10⁻²⁹ cc²/molecule² · sec; N₂H⁺+2N₂→N₄H⁺+N₂, k₃₂₉=0.54×10⁻²⁹; ${\mathrm{CO}}^{+}\text{+2}\mathrm{CO}\to {\mathrm{C}}_2{\mathrm{O}}_2^{\mathrm{+•}}+\mathrm{CO}$ , k₃₂₈=19.8×10⁻²⁹ and ${\mathrm{N}}_2^{\mathrm{+•}}\text{+ 2}{\mathrm{N}}_2\to {\mathrm{N}}_4^{\mathrm{+•}}+{\mathrm{N}}_2$ , k₃₂₈=7.9×10⁻²⁹. A direct correlation between the thermodynamic and kinetic parameters is seen in that the more exothermic reactions of the radical ions proceed significantly faster than the less exothermic reactions of the even‐electron ions. The temperature dependences of the reactions reported over the wide temperature ranges applied demonstrates unequivocally that the rate constants follow the functional form k =CTⁿ. n was found to be ≈−1.5 for the reactions of the radical ions, and ≈−3 for the reactions of the even‐electron ions. A small negative deviation from linearity in the plots of log k vs log T for some of the reactions indicates that a small positive activation energy (≤0.2 kcal/mole) may be present in these highly exothermic reactions. The meaning of the results of our kinetic studies are discussed in the framework of energy‐transfer theory and transition‐state theory concepts.