Chemical kinetics and thermodynamics of tin ionisation in H-2-O-2-N-2 flames and the proton affinity of SnO

A small amount (less than or equal to 10(-6) mol fraction) of tin was intro duced into five, fuel-rich, H-2-O-2-N-2 flames in the temperature range 182 0-2400 K at atmospheric pressure. Ions in a flame were observed by sampling the flame along its axis through a nozzle into a mass spectrometer. The ma jor neutral tin species in these flames were SnO (>97%) and Sn (<3%). The p rincipal tin ions observed were SnOH+ and Sn+. Thermodynamic functions for SnOH+, Sn+, SnO, and Sn were calculated by statistical mechanics using publ ished data from ab initio calculations and spectroscopy. The SnOH+ ion was formed initially by proton transfer to SnO by H3O+, a natural flame ion, wi th which it is in equilibrium. It was also produced by chemi-ionization of SnO reacting with H; SnOH+ rapidly equilibrates with Sn+. Ion ratio measure ments of SnOH+/H3O+ led to the proton affinity PA(298)degrees (SnO) = 911 /- 21 kJ mol(-1) (218 +/- 5 kcal mol(-1)). A calculated equilibrium constan t provided the SnOH+/Sn+ ion ratio. When electron-ion recombination of SnOH + with free electrons was made dominant by the addition of CH4 and K, the m easured recombination coefficient of SnOH+ was (0.116 +/- 0.065)T-(1.66+/-0 .16) cm(3) molecule(-1) s(-1); the temperature dependence is in good agreem ent with the T-1.5 dependence predicted by simple theory. The rate constant for chemi-ionization could not be measured due to impurity ions from potas sium and sodium, but the equilibrium constant for chemi-ionization/recombin ation was calculated to be 0.004 863 exp (-52 070/T). Assuming detailed bal ance and the experimental recombination coefficient, the relatively small r ate constant for chemi-ionization was given by 3.27 x 10(-10) exp (-48 630/ T) cm(3) molecule(-1) s(-1) Finally, calculated values were obtained for th e bond energy D(0)degrees(HO-Sn+) = 408 +/- 21 kJ mol(-1), and the standard zero-point enthalpy of formation Delta(f)H(0)degrees(SnOH+) = 637 +/- 21 k J mol(-1).