Prof. Roman Boulatov, University of Illinois at Urbana-Champaign

Understanding chemical reactivity with molecular force probes

Physical Chemistry Seminar Series

We design, synthesize and study medium-size molecules to understand quantitatively how reaction rates change when the motion of atoms that convert reactants into products is coupled to directional motion at larger scales, from sub-um to mm. Such multiscale coupling is ubiquitous in Nature, underlying phenomena as diverse as fragmentation of polymers that contributes to failure of mechanically loaded materials, reactions in shear flows and at evolving interfaces and operation of motor proteins. Our objectives are two-fold: (1) to extend the formalism of the transition state theory to multiscale reactions and (2) to develop a general predictive framework to guide the design of new materials that exploit such coupling.

Conventionally, multiscale reaction dynamics has been studied using macromolecules but quantitative molecular interpretations of such experiments have proven challenging. Our approach exploits the fact that restoring force quantifies both multiscale coupling and molecular strain resulting from such coupling. Restoring force develops whenever an object of any size is distorted from its optimal geometry. Unlike strain energy, the relationship between the rate of a chemical reaction and the restoring force of the strained reactant(s) is an intrinsic property of the reaction landscape and is largely independent of the size of the molecule within which the reaction occurs. This property allows the effects of multiscale coupling on localized reactions to be inferred by studying much more tractable small molecules instead of polymers. To do so, we have developed molecular architectures to vary restoring forces of diverse functional groups in <30 pN increments up to ~600 pN. By applying this technique to mechanistically distinct reactions we have succeeded in the last 2 years in validating the two key postulates of chemomechanics that have eluded experimental tests for the past 50 years: (1) the activation barrier changes linearly with restoring force up to ~500 pN and (2) its sensitivity to force is controlled by a single atomic pair whose nature is determined by the reaction mechanism. These findings mean that to predict accurately rates of localized chemical reactions in mechanically loaded polymers, shear flows and other anisotropically stressed environments one only needs to know the readily accessible transition-state geometries of the free functional group (or monomer) and the degree of multiscale coupling (e.g., forces).

Guided by these insights, we have designed molecules that fragment orthogonal to their restoring force axis and that become more stable to fragmentation in response to tensile force and demonstrated experimentally such counterintuitive responses. The availability of small simple molecules with diverse responses to tensile force and of general rules to guide the design of other such molecules opens up new opportunities for creating stress-responsive, actuating, energy-dissipating and "smart-delivery" polymers that we and others are currently pursuing.

For more information contact Prof. Mostafa El-Sayed (404-894-0292).