Alkali chloride cluster ion dissociation examined by the kinetic method: Heterolytic bond dissociation energies, effective temperatures, and entropic effects

Abstract

Branching ratios have been measured as a function of collision energy for the dissociation of mass-selected chloride-bound salt cluster ions, Cl-M i ] ϩ , where M i ϭ Na, K, Cs. The extended version of the kinetic method was used to determine the heterolytic bond dissociation energy (HBDE) of Rb-Cl. The measured value of 480.8 Ϯ 8.5 kJ/mol, obtained under single collision conditions, agrees with the HBDE value (482.0 Ϯ 8.0 kJ/mol), calculated from a thermochemical cycle. The observed effective temperature of the collisionally activated salt clusters increases with laboratory-frame collision energy under both single-and multiplecollision conditions. Remarkably, the effective temperatures under multiple collision conditions are lower than those recorded under single-collision conditions at the same collision energy, a consequence of the inability of the triatomic ions to store significant amounts of internal energy. Laboratory-frame kinetic energy to internal energy transfer (T3 V) efficiencies range from 3.8 to 13.5%. For a given cluster ion, the T3 V efficiency decreases with increasing collision energy. Many features of the experimental results are accounted for using . Not only is this information useful in the interpretation and prediction of ionic reactivities and ion/molecule mechanisms, but the paucity of alternative sources of this information has made its acquisition an important application of ion/molecule reactions [2,3]. Methods for determining absolute values of thermochemical quantities are rarely applicable, so relative values are typically measured by kinetic and equilibrium measurements on gas-phase ions. Theoretical calculations serve a special reference function [4]. Under appropriate conditions, the kinetics of the dissociation of molecular cluster ions can yield relative, but quantitative, thermochemical information on the constituent species [5]. The kinetic method, based on this concept, has been widely used for estimating thermochemical properties [5,6]. Cluster ions bound via protons (eq 1), as well as other atomic or polyatomic anions or cations can be isolated and their dissociations followed in a tandem mass spectrometry experiment [7]. In eq 1, k 1 and k 2 represent the rate constants for two competitive dissociation channels of the proton-bound dimer to yield B 1 H ϩ and B 2 H ϩ , respectively. The ratio of rate constants, viz. the branching ratio of two fragment ion abundances, is related logarithmically to the gas-phase basicities of B 1 and B 2 , in the standard form of the kinetic method:In eq 2, ⌬(⌬G) is the difference in gas-phase basicities of B 1 and B 2 , R is the gas constant, and T eff is the effective temperature (the parameter in the kinetic method which connects the degree of fragmentation of

Measuring internal energy deposition in collisional activation using hydrated ion nanocalorimetry to obtain peptide dissociation energies and entropies

“…tic energy of 20 eV, 4.9%-11% for a lab frame kinetic energy of 30 eV, and 3.8%-8% for a lab frame kinetic energy of 50 eV) [59]. This trend is consistent with energy-transfer in low collision energy (1-100 eV) CAD occurring via a combination of two limiting mechanisms [24,60,61].…”

A study of the cesium cation bonding to carboxylate anions by the kinetic method and quantum chemical calculations

“…This original approach, using negative clusters, is complementary to the method originally used by Cooks and coworkers. [19,20] Our approach is likely applicable to mixed clusters formed from other alkali metals and counterions or acids similar to those studied here. The limitation for a more general application is the lack of experimental data for calibration of the kinetic method, which currently relies on DFT-calculated data.…”

“…Actually, Cooks and + generated by fast atom bombardment. [19,20] In this work, we show that CID of the negatively charged clusters [(NO 3 − )Cs + (RCOO − )] − may be a means of estimating the relative cesium cation affinity (CsCA) or basicity (CsCB) of carboxylate ions, through the use of the kinetic method combined with quantum chemical calculations. CsCA and CsCB correspond to the enthalpy and Gibbs free energy of reaction 1, respectively:…”

The fragmentation dynamics of small Cs(CsI)+ cluster ions under low-energy multiple collision conditions

“…Recent investigations include ion mobility studies for the differentiation of cluster structures [4], as well as the use of trapped ion diffraction to obtain information about the cluster lattice [5]. The fragmentation dynamics has also been a focal point of several recent studies [1][2][3]6].…”