Computational investigations of gas and condensed phase systems
Abstract
Ab initio molecular orbital calculations have been used to determine energy profiles for the S$\sb{N}$2 reactions of OH$\sp-$ and OOH$\sp-$ with CH$\sb3$C$\ell$. Geometry optimizations were carried out at the Hartree-Fock level with the 6-31 + G(d) basis set, including second and third-order Moller-Plesset theory. Though the reactions are exothermic by 40-50 kcal/mol, both are found to have the double-well energy surfaces characteristic of gas-phase S$\sb{N}$2 reactions. Ab initio MO calculations have also been used to study the optimal reaction paths for the displacement reactions of chloride ion with formyl and acetyl chloride. Geometry optimizations were carried out primarily with the 3-21 + G basis set, while energies were computed at levels as high as MP3/MP2/6-31 + G(d). For formyl chloride, the tetrahedral adduct and a fully planar alternative with C$\sb{2v}$ symmetry are both transition states at the 3-21 + G level. A minimum energy reaction path and free energy of activation profile have been computed for the addition of dichlorocarbene to ethylene by ab initio MO methods. At the MP2(FULL)/6-31G(d) level, a 2 kcal/mol deep energy minima is found for a $\pi$-complex and the overall barrier to reaction is only 0.6 kcal/mol. However, the free energy barrier for the reaction is ca. 11.7 kcal/mol, and the $\pi$-complex is not a free energy minimum. Analysis of the Cambridge Structural Database and results of ab initio MO calculations provide insights into the bond angle widening at oxygen and lowered basicity observed for silyl ethers in comparison to alkyl ethers. An explanation that considers the detailed nature of the HOMO and concepts from Walsh diagrams accounts for the structural and chemical observations in a comprehensive manner. Monte Carlo statistical mechanics simulations and analyses of intrinsic interaction energies have been used to elucidate the observed strong binding of adenine derivatives to Zimmerman's molecular tweezer in chloroform. Hydrogen bonding and $\pi$-stacking are found to contribute about equally to the remarkably strong interaction of the tweezer and 9-methyladenine. The absolute free energy of binding for the acid and 9-methyladenine in chloroform was also computed.
Degree
Ph.D.
Advisors
Jorgensen, Purdue University.
Subject Area
Chemistry|Organic chemistry
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