Kinetics and dynamics of isolated clusters
Abstract
Experimental and theoretical studies of isolated molecular clusters are presented. Experiments demonstrating anomalous inverse heavy-atom KIEs in the unimolecular dissociation of chloroalkane-chloride complex ions are described. Single-stage and tandem mass spectra were collected under various ion source conditions in various instruments. KIEs were sensitive to the numbers, types and energies of the collisions inducing dissociation but not to the collision partner. The observed KIEs are classified and a plausible mechanism is presented. It is suggested that a threshold, centrifugal phenomenon present under a narrow range of conditions (i.e., energy and angular momentum distributions) is responsible for the results. Manipulation of energetic processes in various mass spectrometric environments allows crude control of this anomalous kinetic phenomenon and the observed isotopic abundances. The data presented demonstrate the tractable production and manipulation of sizeable inverse heavy-atom isotope effects in these ion/molecule complexes. Theoretically, kinetics and dynamics of conformational change and water shuttling are studied for a lattice model of hydrated biomolecular clusters. Clusters are represented as a two-color bead chain on a two-dimensional square lattice. Bead chains are composed of H and P units (H, hydrophobic, P, polar). Water binding sites available to the single water (W) are nearest-neighbors of P units. The potential energy landscape of HPPHP and HPPHP+W are mapped by exact enumeration where HH, PW and PWP interactions, and combinations thereof, are taken as favorable. A conformational move set is defined with only single monomer end and corner motions of the chain. Water shuttling moves are defined between nearest-neighbor and next-nearest-neighbor binding sites. The energy model for this move set allowed a disconnectivity graph analysis which showed that the potential energy landscape of the dry chain is rough. With the addition of water, more funnel-like features are apparent. Time evolution of the occupancy of each potential energy minimum is modeled with a master equation approach in the microcanonical and canonical ensembles.
Degree
Ph.D.
Advisors
Szleifer, Purdue University.
Subject Area
Chemistry
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