Temperature dependence of cluster ion decomposition in a quadrupole ion trap

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