Temperature Dependence of Cluster Ion Decomposition in a Quadrupole Ion Trap

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Rate coefficients for the thermal decomposition of H+(H2O)3,4, H+(CH3OH)3, H+(C2H5OH)2, H+(CH3CN)2, H+((CH3)2CO)2, NO3-(HNO3)1,2, and Cl-H2O are measured as a function of temperature in a quadrupole ion trap over the pressure range 0.2−2 mTorr of He. The kinetics are in the low pressure limit and the decomposition activation energies are significantly less than the bond energies. The difference between the bond energy and the activation energy is reproduced by theory. The vibrational frequencies of the cluster ions necessary for the theoretical treatment of the dissociation rate constants are calculated ab initio at the HF/6-31G* level. This work demonstrates that cluster ion bond energies may be determined accurately from activation energies for dissociation at the low-pressure limit. The measurements also yield fundamental information about the intermolecular energy transfer between the helium buffer gas and the cluster ions.