Solvation of Copper Ions by Acetonitrile. Structures and Sequential Binding Energies of Cu+(CH3CN)x, x = 1−5, from Collision-Induced Dissociation and Theoretical Studies

Abstract

Collision-induced dissociation of Cu+(CH3CN)x, x = 1−5, with xenon is studied as a function of kinetic energy using guided ion beam mass spectrometry. In all cases, the primary and lowest energy dissociation channel observed is endothermic loss of one acetonitrile molecule. The cross section thresholds are interpreted to yield 0 and 298 K bond energies after accounting for the effects of multiple ion-molecule collisions, internal energy of the complexes, and dissociation lifetimes. Density functional calculations at the B3LYP/6-31G* level of theory are used to determine the structures of these complexes and provide molecular constants necessary for the thermodynamic analysis of the experimental data. Theoretical bond dissociation energies are determined from single point calculations at the B3LYP/6-311+G(2d,2p) level and also with an extended basis on Cu+ of 6-311+G(3df) using the B3LYP/6-31G* optimized geometries. The experimental bond energies determined here are in excellent agreement with previous experimental measurements made in a high-pressure mass spectrometer for the sum of the first and second bond energy (i.e., Cu+(CH3CN)2 → Cu+ + 2CH3CN) when these results are properly anchored. Excellent agreement between theory and experiment is also found for the Cu+(CH3CN)x, x = 1 and 2 clusters. Theory systematically underestimates the binding energy in the larger clusters such that higher levels of theory are necessary to adequately describe the very weak noncovalent interactions in these systems.