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High-Level Ab Initio Quantum Chemical Studies of the Competition

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High-Level Ab Initio Quantum Chemical Studies of the Competition HIGH-LEVEL AB INITIO QUANTUM CHEMICAL STUDIES OF THE COMPETITION BETWEEN CUMULENES, CARBENES, AND CARBONES by SHIBLEE RATAN BARUA (Under the Direction of WESLEY D. ALLEN) ABSTRACT High-accuracy computations involving coupled-cluster methods in concert with series of correlation-consistent basis sets are utilized to explore the geometric structures, relative energetics, and vibrational spectra of some molecular systems with unusual properties, namely C(BH) 2, C(AlH) 2 and HCNO. Reliable focal point analyses (FPA) targeting the CCSDT(Q)/CBS limit for the ground electronic state of C(BH) 2 reveals a relative energy difference of only 0.02 kcal mol −1 between a linear and a bent (∠BCB ≈ 90°) structure, thus identifying an unusual case of an “angle-deformation” isomer. Highly accurate CCSD(T)/cc-pVTZ and composite c~CCSDT(Q)/cc-pCVQZ anahrmonic vibrational frequency computations precisely reproduced the experimental IR spectra for linear C(BH) 2, and made excellent predictions for the hitherto unobserved bent isomer. With the aid of elaborate bonding analyses, linear C(BH) 2 is described as a cumulene, while bent C(BH) 2 can be best characterized as a carbene with a little carbone character. A similar FPA treatment yields bent C(AlH) 2 (∠AlCAl ≈ 98°) as the ground electronic structure, comfortably placing it 9.60 kcal mol −1 below its linear counterpart, thus confirming the dominance of a carbene/carbone model for the Al analogue of C(BH)2. Confident predictions for the heretofore undetected bent C(AlH) 2 are made through anharmonic frequency computations at the CCSD(T)/cc-pV(T+d)Z level. Next, a highly accurate and computationally demanding AE-CCSDT(Q)/CBS treatment predicts a bent ground electronic structure for the classic quasilinear HCNO molecule (∠HCN ≈ 174°), lying a miniscule 0.22 cm −1 below the corresponding linear geometry, thus indicating an intermediate between a cumulene and a carbene model. Exhaustive investigation is carried out on the geometric structures and for the harmonic vibrational frequencies for both linear and bent HCNO, and a similarly elaborate benchmarking is pursued for the HCN molecule. Finally, a rigorous theoretical analysis of the topology of polytwistane is performed to reveal a non-repeating, helical, carbon nanotube. Utilizing homodesmotic equations and including explicit computations as high as CCSD(T)/cc- pVQZ, the FPA treatment of the enthalpy of formation ultimately yields (polytwistane) = ° ∆ +1.28 kcal (mol CH) −1 , thus demonstrating the thermodynamic and synthetic viability of this polymer when compared to acetylene. INDEX WORDS: Cumulene; Carbene; Carbone; Coupled-cluster theory; Correlation- consistent basis sets; Basis-set extrapolation; Focal point analysis; Angle- deformation isomer; Vibrational perturbation theory; Isotopic shifts; Intrinsic reaction path; Quantum tunneling; Quasilinear; Homodesmotic equations; Carbon nanotube; Saturated polymer; Double helix; Irrational periodicity. HIGH-LEVEL AB INITIO QUANTUM CHEMICAL STUDIES OF THE COMPETITION BETWEEN CUMULENES, CARBENES, AND CARBONES by SHIBLEE RATAN BARUA B.A., Berea College, Berea, KY, 2008 A Dissertation Submitted to the Graduate Faculty of The University of Georgia in Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY ATHENS, GEORGIA 2014 © 2014 Shiblee Ratan Barua All Rights Reserved HIGH-LEVEL AB INITIO QUANTUM CHEMICAL STUDIES OF THE COMPETITION BETWEEN CUMULENES, CARBENES, AND CARBONES by SHIBLEE RATAN BARUA Major Professor: Wesley D. Allen Committee: Henry F. Schaefer III Gary E. Douberly Electronic Version Approved: Maureen Grasso Dean of the Graduate School The University of Georgia May 2014 DEDICATION To my Dad, Mr. Mukul Ratan Barua , whose constant encouragement and support have brought me this far. iv ACKNOWLEDGEMENTS A lot of people have influenced me in positive ways, but the one person who stood a mile ahead of everyone else was my Dad. His constant encouragement, his support, and his compassion for the growth and success of his two children were unparalleled in my opinion. He was not only my father, but a great friend as well. Despite his limited income as a government official, he provided the best education available out there in Bangladesh for his two children, and I am ever so grateful to him for that. From thousands of miles away during our phone conversations, however brief they might be, he always mentioned how happy he would be to see me get my Doctorate. I could not fulfill his wish during his lifetime, nor could I be there when he breathed his last on May 3 rd 2013. As I inch closer to my degree, I realize that he would be the happiest person in the world right now; happier than me would probably not be an exaggeration. I would like to take this opportunity to show my utmost gratefulness to the best Dad in the world for always being there for me and believing in me. Thank You. Thank You from the depth of my heart. My family members have always played a huge supporting role in my life. My elder brother Parag and my mom have backed me up and urged me forward. It’s sad how the three of us are so far apart from each other right now; my mom in Bangladesh, my brother in Sydney, and me being here in the States. Hopefully some day we could all live close together as a family once again. Special thanks to all my cousins (Muna, Nina, Turna, Upama, Pallab, Kallol, Hillol), who still remain the closest people in my life no matter how much time passes between our phone conversations. I have a lot of friends here in the States and back in Bangladesh that I need to v mention right now, but would soon run out of pages if I do so. However, I would especially like to thank my friend Kelsey Turner for providing me the support that I needed during my father’s death, and for always being there for me when I needed somebody to rely on. I often jokingly call my group-mate Jowa as my “semi-mentor”, and rightly so. He has helped me enormously throughout my research work. Being completely unaware of how computational quantum chemistry actually works, I had to learn everything from scratch. Jowa was patient with me and taught me well. I was very lucky to have someone like him to guide me through. He is an excellent co-worker and a very good friend, and I am still trying to find a way to beat him in badminton. I would also like to thank my friend Kedan for keeping me entertained with her lively smile in the office; I’ll definitely miss our fun and random conversations. Last, but not least, I would like to show my sincere gratefulness to the faculty members at the CCQC, Dr. Schaefer and Dr. Allen. Dr. Schaefer has provided us with a wonderful facility conducive for state-of-the-art research work. I admire his friendly personality and great stories during his lectures. My mentor, Dr. Allen, has been very supportive, caring, and accommodating towards all his students. Our group has a good number of international students, and he has never said no to any of our requests for visiting our respective homelands. The only thing he would ask for is whether we have internet access back home so that we could continue our research work away from the office. His Advanced Quantum course gifted me many sleepless nights and hundreds of pages of homework assignments. In hindsight, the struggle was totally worth it since I learned an outrageous amount in that class. It has surely been a pleasure working under Dr. Allen for the last six years or so. Now, off to some quantum chemistry... vi TABLE OF CONTENTS Page ACKNOWLEDGEMENTS .............................................................................................................v CHAPTER 1 HIGH-LEVEL AB INITIO QUANTUM CHEMICAL THEORY AND ITS APPLICATION IN CARBON CHEMISTRY 1 1.1 INTRODUCTION .............................................................................................1 1.2 THEORETICAL METHODS ............................................................................3 1.3 BASIS-SET EXTRAPOLATION ...................................................................10 1.4 FOCAL POINT ANALYSIS ...........................................................................11 1.5 AUXILIARY CORRECTIONS.......................................................................15 1.6 ANHARMONIC VIBRATIONAL FREQUENCIES .....................................16 1.7 RESEARCH OVERVIEW ..............................................................................19 REFERENCES ......................................................................................................20 2 NEARLY DEGENERATE ISOMERS OF C(BH) 2: CUMULENE, CARBENE, OR CARBONE? 24 2.1 ABSTRACT .....................................................................................................25 2.2 INTRODUCTION ...........................................................................................26 2.3 COMPUTATIONAL METHODS ...................................................................28 2.4 RESULTS AND DISCUSSION ......................................................................31 2.5 THE ALUMINUM ANALOGUE, C(AlH) 2....................................................59 vii 2.6 CONCLUSIONS..............................................................................................71 2.7 ACKNOWLEDGEMENTS .............................................................................73 REFERENCES ......................................................................................................74 3 QUASILINEARITY IN FULMINIC ACID (HCNO) MOLECULE 84 3.1 ABSTRACT .....................................................................................................84
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