GAS-PHASE THERMOCHEMICAL STABILITIES OF CLUSTER IONS [(N-2)(M)(AR)(N)](-5() WITH (M+N)=1)

Thermochemical stabilities of nitrogen-argon cluster ions [(N-2)(m)(Ar )(n)](+) were measured using a pulsed-electron beam mass spectrometer. The thermochemical data obtained for the exchange reactions N-4(+) Ar = N2Ar+ + N-2 (Delta H-0 = -1.0 kcal/mol) and Ar2+N2=N2Ar+ + Ar (De lta H-0 = -2. 1 kcal/mol) lead to the determination of the bond dissoc iation energies (D), D(N2Ar+-->N2+Ar) =26.8, D(N-2 Ar+ --> Ar+ + N-2) = 30.5, and D (Ar-2(+) --> Ar+ + Ar) = 18.4 kcal/mol. For the mixed cl uster ions [(N-2)(m)(Ar)(n)](+) the irregular decreases in bond dissoc iation energies are found with (m+n) =3-->4. This fall-off indicates t hat the core ions in the mixed cluster ions are trimer cations, [(N-2) (m)(Ar)(n)](+) with (m+n)=3 in agreement with the experimental results by Magnera and co-workers [Chem. Phys. Lett. 192, 99 (1992); J. Chem. Sec. Faraday Trans. 86, 2427 (1990)]. The most stable cluster ions of [(N-2)(m)(Ar)(n)](+) are found to be those composed of the core ion N 2Ar+ N-2 solvated by further N-2 ligands. The rate of exchange reactio n (N-2)(m)(+) + Ar = N2Ar+N2(N-2)(m-3)+N-2 was found to become slower with decrease of temperature due to the existence of energy barrier. A b initio [G2(MP2)] calculations on some cluster ions [(N-2)(m)(Ar)(n)] (+) were performed. Trimer cations have linear geometries, and the fou rth neutral molecule is coordinated in the bridged (bidentate) form to the trimer cation. (C) 1997 American Institute of Physics.