Gas-phase thermochemical stabilities of cluster ions [(N2)m(Ar)n]+ with (m+n)=1–5

Abstract

Thermochemical stabilities of nitrogen-argon cluster ions [(N2)m(Ar)n]+ were measured using a pulsed-electron beam mass spectrometer. The thermochemical data obtained for the exchange reactions N4++Ar=N2Ar++N2 (ΔH0=−1.0 kcal/mol) and Ar2++N2=N2Ar++Ar (ΔH0=−2.1 kcal/mol) lead to the determination of the bond dissociation energies (D), D(N2Ar+→N2++Ar)=26.8, D(N2Ar+→Ar++N2)=30.5, and D(Ar2+→Ar++Ar)=28.4 kcal/mol. For the mixed cluster ions [(N2)m(Ar)n]+, the irregular decreases in bond dissociation energies are found with (m+n)=3→4. This fall-off indicates that the core ions in the mixed cluster ions are trimer cations, [(N2)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. Soc. Faraday Trans. 86, 2427 (1990)]. The most stable cluster ions of [(N2)m(Ar)n]+ are found to be those composed of the core ion N2Ar+N2 solvated by further N2 ligands. The rate of exchange reaction (N2)m++Ar=N2Ar+N2(N2)m−3+N2 was found to become slower with decrease of temperature due to the existence of energy barrier. Ab initio [G2(MP2)] calculations on some cluster ions [(N2)m(Ar)n]+ were performed. Trimer cations have linear geometries, and the fourth neutral molecule is coordinated in the bridged (bidentate) form to the trimer cation.