โ Scribed by Meot-Ner, M.; Field, F. H.
No coin nor oath required. For personal study only.
Cluster ion stabilities were measured by pulsed, high-pressure mass spectrometry. Enthalpies for the reactions H,O+.( n -1)HzO + HzO + M -H,O+.nH20 + M were measured as -33.0, -21.0, and -16.0 kcal/mol for n = I , 2, and 3, respectively, in good agreement w i t h values obtained by Kebarle and co-workers. Enthalpies for analogous reactions involving H2S were -12.8, -7.2, -5.4, and -3.3 kcal/mol for n = 1,2,3, and 4. Theenthalpy for the association of H3Sf w i t h HzO was -17.0 kcal/mol. T h e results reflect the contribution o f the partial charge on the hydrogens of the protonated ion to the stabilities of the cluster ions, which can be expressed quantitatively as -AH,[,,,, = 100 X (partial charge) + 9 kcal/mol. Kinetic measurements on the association reactions yielded third-order rate constants k i = 37 X IO-2x and 0.8 l X l O-2x cmh/(mo12 s) for the formation o f H3S+.H20 and H3S+.HzS at 320 K. k3 exhibits negative temperature dependences o f T -) ' and T-4.8 for the two reactions. These are related to the positive temperature dependences o f the decomposition rates of the excited complexes H30+.H20* and H3Sf.H2S*, which are found to be kd = 0.52 X 10' and 22.9 X IO9 s-' at 320 K. Decomposition rates of the thermalized association ions H 3 0 + . H z 0 and H$+.H>S, which exhibit second-order kinetics at our pressures, are k -, / ( M ) = 8.2 X cm3/(mol s) for the t w o ions in methane at 400 K. The thermal decomposition reactions have Arrhenius activation energies lower b y -3 kcal/mol than the endothermicities o f the reactions, w i t h preexponential factors larger b y factors o f 105-107 than collision rates for the activating collisions w i t h methane molecules.
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