Binding energies of chromium cations with fluorobenzenes from radiative association kinetics

The addition of electron-withdrawing substituents to an aromatic ring is ex pected to weaken the cation/pi-face binding via electron withdrawal from th e pi system. In order to characterize this effect quantitatively in a well behaved model system, rates of radiative association of Cr+ with the comple te set of fluorobenzenes, C6H6-nFn, where n = 0 - 6, were measured in the F ourier transform ion cyclotron resonance mass spectrometer. Radiative assoc iation rates are very sensitive to the ion-neutral bonding strength, and we ll established kinetic modeling methods permit the extraction of binding en ergy values from association observations. Modeling was based on variationa l transition state theory, using Becke-3 Lee-Yang-Parr (B3LYP) density func tional theory-calculated vibrational frequencies and infrared emission inte nsities of the ion-neutral complexes. The resulting "experimental" binding energies were compared with B3LYP density functional theory calculations fo r these systems. The calculations indicate that an in-plane fluorine bridgi ng binding site is favored for those neutrals having two adjacent fluorine substituents, while otherwise rr-facial binding is favored. The "experiment al" binding energies and the B3LYP density functional theory-calculated bin ding energies of the most favorable sites agree within about 10%. The pi-fa ce binding strength decreases by roughly 5 kcal mol(-1) with each added flu orine substituent on the ring. (Int J Mass Spectrom 185/186/187 (1999) 913- 923) (C) 1999 Elsevier Science B.V.