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Title:The photochemical and mechanochemical ring opening of cyclobutene from first principles
Author(s):Ong, Mitchell T.
Director of Research:Martinez, Todd J.
Doctoral Committee Chair(s):Martinez, Todd J.
Doctoral Committee Member(s):Makri, Nancy; Lisy, James M.; Moore, Jeffrey S.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Theoretical Chemistry
Woodward-Hoffmann Rules
Ab Initio Molecular Dynamics
Abstract:A textbook example of a pericyclic reaction is the electrocyclic ring opening of cyclobutene. The stereochemical outcome of this reaction can typically be predicted using the Woodward-Hoffmann rules, which are based on conservation of orbital symmetry. According to these rules, the thermal ring opening reaction will proceed in conrotatory fashion, while the photochemical ring opening reaction will proceed in disrotatory fashion. Over the years, there have been very few exceptions to the Woodward-Hoffmann rules. However, two apparent exceptions to these rules occur when the ring opening of cyclobutene is initiated by exciting the molecule with light or by applying a mechanical force. In this work, we use first principles quantum mechanics and molecular dynamics to examine the photochemical and mechanochemical ring opening of cyclobutene. We use the ab initio multiple spawning (AIMS) method in order to examine the short time dynamics of cyclobutene. We have integrated the COLUMBUS software package into AIMS, which contains analytical gradients and nonadiabatic couplings for multi-reference configuration interaction (MRCI). This provides us with a more accurate potential energy surface in which to run our dynamics on. We find that the stereochemistry for ring opening is determined on the ground state. We have also identified two S1/S0 intersections that could be relevant to the photochemical ring opening reaction. One of these intersections contains a sizable C=C torsional angle, which could explain the mixture of conrotatory and disrotatory products seen in experiment. In addition, we model the mechanical response of cyclobutene within the context of a force-modified potential energy surface (FMPES). Using this model, we perform ab initio steered molecular dynamics (AISMD) to predict possible reaction outcomes when an external force is applied. Furthermore, we locate minimum energy pathways directly on the FMPES using the nudged elastic band (NEB) algorithm. Application of an external force can bias the reaction to produce a specific product by reshaping the potential energy surface and effectively blocking off access to other competing pathways. Barrier heights and stationary point geometries along the pathways can also change as a function of the force. Finally, we apply AISMD and NEB to other new mechanophores that are currently being studied including gem-difluorocyclopropane and dicyano-cyclobutane. Both of these methods can be used to screen new mechanophores, which will aid in designing new polymeric materials that respond favorably to mechanical stress.
Issue Date:2010-05-14
Rights Information:Adapted with permission from M. T. Ong, J. Leiding, H. Tao, A. M. Virshup and T. J. Martínez, J. Am. Chem. Soc., 131 (18), 6377 (2009). Copyright 2009 American Chemical Society. Adapted with permission from M. J. Kryger, M. T. Ong, S. A. Odom, N. R. Sottos, S. R. White, T. J. Martínez and J. S. Moore, J. Am. Chem. Soc., 132 (13), 4558 (2010). Copyright 2010 American Chemical Society.
Date Available in IDEALS:2012-05-15
Date Deposited:May 2010

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