Département de chimie moléculaire

Benzylpyridinium salts (BPs) have often been used as thermometer ions to obtain an energy calibration of mass spectrometric expts. (in particular to det. internal energy distributions of ions after the ionization process). Fragmentation of BP⁺ mol. ions is characterized by specific Bond Dissocn. Energies (BDE) which depend on the substituent group and its location on the benzyl ring. Although those BDE values are regularly re-evaluated by quantum chem. calcns., their exptl. detn. is still missing from the literature. In this paper, a modified Quadrupole-hexapole-Quadrupole (QhQ) mass spectrometer is used to obtain such values on 4 BP⁺ mol. ions (characterized by a wide range of C-N bond strengths) using Threshold Collision-Induced Dissocns. (TCID) and Rice-Ramsperger-Kassel-Marcus (RRKM) kinetic modeling. It is found that exptl. values are systematically 0.5 eV lower than their most recent theor. evaluations. Despite this shift, the abs. crit. energy values are maintained in the same order (pOMe < pMe < pCl < pCN), and the relative energy differences are in very good agreement. We argue that the obsd. 0.5 eV shift relates to the energy dependence of the Transition State's no. of states that is typical for barrier-less fragmentation processes and, relates to a kinetic rather than an energetic bottleneck. Notably, by taking into account the bond elongations characterizing the transition states and their corresponding calcd. crit. energies (E₀), close agreement is found with exptl. obtained E0 values. We thus conclude that much care should be taken when describing the transition state during an internal energy calibration procedure where the involved energy is lower than 3-4 eV.