BOND-ENERGY OSCILLATION IN THE CLUSTER ION NO+(NO)(N)

The gas-phase equilibria of the clustering reaction of NO+ with NO and F- with NO were measured with a pulsed electron-beam high-pressure ma ss spectrometer. Van't Hoff plots of equilibrium constants lead to the determination of the thermochemical stabilities for NO+(NO)(n) with n =1-10 and F-(NO)(n) with n=1-3. The equilibrium constants K-n-1,K-n fo r the former reaction with n=4, 6, 8, and 10 were found to be larger t han K-n-2,K-n-1 with (n-1)=3, 5, 7, and 9, respectively. That is, the cluster ions NO+(NO)(n) with even n are thermochemically more stable t han the smaller ones, NO+(NO)(n-1), under the present experimental con ditions. The measured enthalpy (-Delta H-n-1,n(0)) and entropy changes (-Delta S-n-1,n(0)) show odd-even oscillation. This is due to the ele ctron-spin pairing effect, i.e., dimer pair formation in the cluster i ons. The sudden decrease in the bond energies for the cluster NO+(NO)( n) between n=2 and 3 suggests that the core in the cluster NO+(NO)(2) is NO+(NO)(2). The bond energy oscillation is also likely for the nega tive cluster ion F-(NO)(n). (C) 1996 American Institute of Physics.