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Title:A quantitative and predictive model for mechanochemical kinetics and its experimental validation
Author(s):Kucharski, Timothy J.
Director of Research:Boulatov, Roman
Doctoral Committee Chair(s):Boulatov, Roman
Doctoral Committee Member(s):Girolami, Gregory S.; Moore, Jeffrey S.; Suslick, Kenneth S.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
reaction kinetics
mechanoresponsive polymers
force-dependent kinetics
Abstract:The fact that the application of mechanical force to materials can change the reactivity of their constituent molecules (or ions) has been known for millennia, but the complexity of these multi-scale phenomena limited their partial explanation to the theoretical until the advances in micromanipulation techniques in recent decades allowed for the application of force to single molecules. Traditional theories of chemical reactivity have failed to explain the phenomenal rate enhancements observed (up to 10^15-fold), and though a body of empirical relationships between force and reactivity exists, a physically sound and quantitative model for mechanochemical kinetics did not. Truly taking advantage of all that mechanochemistry offers for the design of novel mechanoresponsive and actuating materials requires a clear conceptual framework describing how and why mechanical force affects chemical reactivity. This work describes the development of such a model for mechanochemical kinetics and its experimental validation using a series of paradigmatic electrocyclic and SN2 reactions. Pioneering a new technique for mechanochemical analysis complementary to single-molecule force experiments, we synthesized several series of macrocycles containing the reactive moieties of interest and a photoisomerizable molecular actuator, stiff stilbene (1,1'-Delta-biindan). In small enough macrocycles, irradiation with UV light generates a highly strained E isomer in which the reactive moiety experiences nearly uniaxial strain. The difference in reactivity between the strained and unstrained isomers is quantified experimentally, and the difference in strain between the two photoisomers is quantified as force (i.e., the gradient of energy with respect to position). Through theory and experiment, the local molecular degree of freedom which dictates a molecule’s response to force is identified, and the relationship between its elongation and stretching compliance is demonstrated. As expected by historical and contemporary thought, acceleration by tensile force is observed when the reactive moiety elongates to reach the transition state, as is observed in the electrocyclic ring opening of trans-3,4-dimethylcyclobutene and the hydrolysis of primary sulfonates. However, two other possibilities exist which arise from the cancellation or complete negation of scissile bond elongation: force insensitivity and inhibition by tensile force; our model predicts and explains both of these situations and also explains why the same reaction can be both accelerated by force in one direction and inhibited by force in another. In more complexreaction sequences, such as pre-equilibrium kinetics, the application of force can even change the overall kinetic profile of a sequence of reactions. As the relative energies of transition states and minima change as increasing amounts of force are applied, the identity of the rate-determining step can change at critical amounts of force, leading to kinetic crossover, a phenomenon whose richness and complexity is underappreciated. By clarifying the fundamental relationship between force and reactivity, our model provides a deeper understanding of the operational principles of phenomena at the interfaces of chemistry, biology, soft matter physics, and materials science. Along with the description of the experimental work to validate our model for mechanochemical kinetics, this work also provides speculation for the future directions of the field of mechanochemistry, both identifying old questions that may now be assailable and new questions that have not yet been asked. Because mechanochemistry bridges the disciplines of chemistry, materials science, soft-matter physics and molecular biology, the opportunities for a clearer understanding of mechanochemical kinetics to have a broad impact on science and technology are great.
Issue Date:2011-08-26
Rights Information:Copyright 2011 Timothy John Kucharski
Date Available in IDEALS:2013-08-27
Date Deposited:2011-08

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