Drift Tube Ion Mobility Measurements for Thermochemistry, Kinetics and Polymerization of Cluster Ions

Abstract

DRIFT TUBE ION MOBILITY MEASUREMENTS FOR THERMOCHEMISTRY, KINETICS AND POLYMERIZATION OF CLUSTER IONS By RIDHA BEN MOHSEN MABROUKI, PHD A Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Virginia Commonwealth University. Virginia Commonwealth University, 2007 Major Director: M. Samy El-Shall PhD., Professor, Department of Chemistry In this work, the Drift Tube Ion Mobility technique is used to study the hydrophobic hydration and solvation of organic ions and measure the thermochemistry and kinetics of ion-molecule reactions. Furthermore, an exploratory study of the intracluster polymerization of isoprene will be presented and discussed. The ion hydration study is focused on the C3H3 cation and Pyridine radical cation. The chemistry of the cyclic C3H3 cation has received considerable attention and continues to be an active area of research. The binding energies of the first 5 H2O xix molecules to c-C3H3 were determined by equilibrium measurements. The measured binding energies of the hydrated clusters of 9-12 kcal/mol are typical of carbon-based CH···X hydrogen bonds. The ion solvation with the more polar CH3CN molecules results in stronger bonds consistent with the increased ion-dipole interaction. Ab initio calculations show that the lowest energy isomer of the c-C3H3(H2O)4 cluster consists of a cyclic water tetramer interacting with the c-C3H3 ion, which suggests the presence of orientational restraint of the water molecules consistent with the observed large entropy loss. The c-C3H3 ion is deprotonated by 3 or more H2O molecules, driven energetically by the association of the solvent molecules to form strongly hydrogen bonded (H2O)nH clusters. The kinetics of the associative proton transfer (APT) reaction C3H3 + nH2O (H2O)nH + C3H2 exhibits an unusually steep negative temperature coefficient of k = cT 63±4 (or activation energy of -32 ± 1 kcal mol). The behavior of the C3H3/water system is exactly analogous to the benzene /water system , suggesting that the mechanism, kinetics and large negative temperature coefficients may be general to multibody APT reactions. These reactions can become fast at low temperatures, allowing ionized polycyclic aromatics to initiate ice formation in cold astrochemical environments. The solvation energies of the pyridine radical cation by 14 H2O molecules are determined by equilibrium measurements in the drift cell. The binding energies of the pyridine(H2O)n clusters are similar to the binding energies of protonated pyridineH(H2O)n clusters that involve NH··OH2 bonds, and different from those of the solvated radical benzene(H2O)n ions that involve CH··OH2 bonds. These relations indicate that the observed pyridine ions have the distonic C5H4NH structure that can xx form NH··OH2 bonds. The observed thermochemistry and ab initio calculations show that these bonds are not affected significantly by an unpaired electron at another site of the ion. The distonic structure is also consistent with the reactivity of pyridine in H atom transfer, intra-cluster proton transfer and deprotonation reactions. The results present the first measured stepwise solvation energies of distonic ions, and demonstrate that cluster thermochemistry can identify distonic structures. The gas phase clustering of small molecules around the hydronium ion is of fundamental interest and is relevant to important atmospheric and astrophysical processes. In this work, the equilibrium constants for the formation of the H3O(X)n clusters with X = H2, N2 and CO and n = 1-3 at different temperatures are measured using the drift tube technique. The arrival time distributions (ATDs) of the injected H3O and the H3O(X)n clusters formed inside the cell are measured under equilibrium conditions. The resulting binding energies for the addition of one and two hydrogen molecules are similar [3.4 and 3.5 kcal/mol, respectively). For the N2 clustering with n = 1-3, the measured binding energies are 7.9, 6.9 and 5.4 kcal/mol, respectively. The clustering of CO on the H3O ion exhibits a relatively strong binding energy (11.5 kcal/mol) consistent with the dipole moment and polarizability of the CO molecule. Theoretical calculations of the lowest energy structures are correlated to the experimental results. Finally, intracluster polymerization leading to the formation of covalent bonded oligomer ions has been investigated following the ionization of neutral isoprene clusters. The results indicate that isoprene dimer cation has a structure similar to that of the limonene radical cation.