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New Conceptual Understanding of Lewis Acidity, Coordinate Covalent Bonding, and Catalysis Joshua A

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New Conceptual Understanding of Lewis Acidity, Coordinate Covalent Bonding, and Catalysis Joshua A Duquesne University Duquesne Scholarship Collection Electronic Theses and Dissertations Summer 2009 New Conceptual Understanding of Lewis Acidity, Coordinate Covalent Bonding, and Catalysis Joshua A. Plumley Follow this and additional works at: https://dsc.duq.edu/etd Recommended Citation Plumley, J. (2009). New Conceptual Understanding of Lewis Acidity, Coordinate Covalent Bonding, and Catalysis (Doctoral dissertation, Duquesne University). Retrieved from https://dsc.duq.edu/etd/1052 This Immediate Access is brought to you for free and open access by Duquesne Scholarship Collection. It has been accepted for inclusion in Electronic Theses and Dissertations by an authorized administrator of Duquesne Scholarship Collection. For more information, please contact [email protected]. NEW CONCEPTUAL UNDERSTANDING OF LEWIS ACIDITY, COORDINATE COVALENT BONDING, AND CATALYSIS A Dissertation Submitted to the Bayer School of Natural and Environmental Sciences Duquesne University In partial fulfillment of the requirements for the degree of Doctor of Philosophy By Joshua A. Plumley August 2009 NEW CONCEPTUAL UNDERSTANDING OF LEWIS ACIDITY, COORDINATE COVALENT BONDING, AND CATALYSIS By Joshua A. Plumley Approved August 2009 __________________________________ __________________________________ Jeffrey D. Evanseck Ellen Gawalt Professor of Chemistry and Biochemistry Assistant Professor of Chemistry and Dissertation Director Biochemistry Committee Member Committee Member __________________________________ __________________________________ Douglas J. Fox Jeffry D. Madura Director, Gaussian Inc. Chair and Professor of Chemistry and External Reader Biochemistry Committee Member __________________________________ David W. Seybert Dean, Bayer School of Natural & Environmental Sciences Professor of Chemistry and Biochemistry iii ABSTRACT Abstract NEW CONCEPTUAL UNDERSTANDING OF LEWIS ACIDITY, COORDINATE COVALENT BONDING, AND CATALYSIS By Joshua A. Plumley August 2009 Dissertation Supervised by Jeffrey D. Evanseck The focus of this dissertation is to correct misconceptions about Lewis acidity, uncover the physical nature of the coordinate covalent bond, and discusses how Lewis acid catalysts influence the rate enhancement of the Diels-Alder reaction. Large-scale quantum computations have been employed to explore many of Lewis’ original ideas concerning valency and acid/base behavior. An efficient and practical level of theory able to model Lewis acid adducts accurately was determined by systematic comparison of computed coordinate covalent bond lengths and binding enthalpies of ammonia borane and methyl substituted ammonia trimethylboranes with high-resolution gas-phase experimental work. Of all the levels of theory explored, M06-2X/6-311++G(3df,2p) provided molecular accuracy consistent with more resource intensive QCISD(T)/6-311++G(3df,2p) computations. iv Coordinate covalent bond strength has traditionally been used to judge the strength of Lewis acidity; however, inconsistencies between predictions from theory and computation, and observations from experiment have arisen, which has resulted in consternation within the scientific community. Consequently, the electronic origin of Lewis acidity was investigated. It has been determined that the coordinate covalent bond dissociation energy is an inadequate index of intrinsic Lewis acid strength, because the strength of the bond is governed not only by the strength of the acid, but also by unique orbital interactions dependent upon the substituents of the acid and base. Boron Lewis acidity is found to depend upon both substituent electronegativity and atomic size. Originally deduced from Pauling’s electronegativities, boron’s substituents determine acidity by influencing the population of its valence by withdrawing electron density. However, size effects manifest differently than previously considered, where greater σ-bond orbital overlap, rather than π-bond orbital overlap, between boron and larger substituents increase the electron density available to boron, thereby decreasing Lewis acidity. The computed electronegativity and size effects of substituents establish unique periodic trends that provide a novel and clearer understanding of boron Lewis acidity, consistent with first principle predictions. Lastly, it is discovered that the energetics associated with the transition structure converge much slower than what was observed for coordinate covalent bonded ground states. Consequently, it is harder to model activation barriers, as compared to binding energies, to within experimental accuracy, because larger basis sets must be employed. Hyperconjugation within dienophile ground states, initiated by geminal Lewis acid interactions, is found to govern the strength of the coordinate covalent bond between the v Lewis acid and the dienophile. A novel interpretation is presented where the strength of the coordinate covalent bond within the Lewis acid activated dienophile is governed by donor-acceptor orbital interactions between the π-density present on the carbonyl group to the σ* orbitals on the Lewis acid, rather than the main donor-acceptor motif between the oxygen lone pair and the empty 2p orbital on the Lewis acid. Moreover, the same hyperconjugation within the dienophile controls the rate enhancement of the Lewis acid catalyzed Diels-Alder reaction, by modulating the energy of the dienophile’s lowest unoccupied molecular orbital. A new understanding of Lewis acidity and coordinate covalent bonding has been achieved to better describe and predict the structure and electronic mechanism of organic reactions. vi ACKNOWLEDGMENT Acknowledgement I thank Jeffrey D. Evanseck for his encouragement, guidance, and humor, which have kept me motivated over the last five years. Jeff has always been there for me when his insight was needed, whether it was on a professional or personal level. His uncanny attention to detail and clarity has inspired me to become a better teacher and scientist. I feel lucky to have had such an excellent advisor and I am proud to call him my friend. I thank Jeffry D. Madura for his guidance and advice. His presence on my committee is appreciated, as well as his unique sense of humor that could put a smile on my face, regardless of the situation. I thank Douglas J. Fox for his expertise and discussions pertaining to my research and the proper execution of Gaussian. His expertise in electronic structure methods and computer science has been a valuable asset to myself and the Evanseck research group (ERG). I thank the current and previous members of the ERG, whom I consider a second dysfunctional family away from home. I will cherish all of our memories together; especially the nights spent in the Southside, a wonderful land filled with Mike and Tony’s gyros and Dee’s stale Iron City. I would especially like to thank Venkata Pakkala, Lauren Matosziuk, Ed Franklin, Ryan Newton, Steve Arnstein, and Sarah Mueller Stein. I gratefully acknowledge the Department of Chemistry and Biochemistry at Duquesne University, as well as SGI, NSF, DOE, and DOD for their financial support during my Ph.D. experience. I especially thank my mother, grandmother, and grandfather who have supported me in the pursuit of all my life goals. Without my family’s loving guidance, I would not vii have been as successful as I am today. I would like to thank specifically my grandfather, for he is directly responsible for my excellent work ethic. He taught me that nothing is impossible and to never give up. He would have been proud of my accomplishments. viii TABLE OF CONTENTS Page Abstract .............................................................................................................................. iv Acknowledgement ............................................................................................................ vii List of Figures .................................................................................................................. xiii List of Tables .................................................................................................................... xx List of Abbreviations ...................................................................................................... xxii 1 Introduction .................................................................................................................... 1 1.1 The Chemical Bond ....................................................................................... 1 1.1.1 The Covalent Versus Coordinate Covalent Bond ............................ 6 1.2 Acid and Base Theory .................................................................................. 11 1.3 The Diels-Alder Reaction ............................................................................ 17 1.3.1 Frontier Molecular Orbitals and Their Influence upon Reactivity and Selectivity. ............................................................................... 18 1.4 Objective of Thesis ...................................................................................... 23 2 Electronic Structure Modeling ..................................................................................... 26 2.1 Quantum Mechanical Calculations .............................................................. 26 2.2 Schrödinger’s equation ................................................................................ 27 2.3 The Variational Theorem ............................................................................. 30 2.4 Hartree-Fock ...............................................................................................
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