Master of Science (MS)
Semester of Degree Completion
Sean A. Peebles
Weakly bound CH/π interactions, due to the abundance of C-H bonds and π systems that exist in larger organic molecules, are an important driving force in the construction of biomacromolecules, supramolecular assemblies and crystal packing of organic compounds. To assist in the characterization of CH/π interactions, a density functional theory (DFT) study has been conducted to determine a quick and cheap method for accurate determination of the binding energy and rotational constants for CH/π interactions. Seven complexes were used in this study: five benzene-HY complexes (where Y= C≡CH, Cl, F, Br and C≡N) and two fluorobenzene-HY complexes (where Y= C≡CH and Cl). The two DFT levels, ωB97XD and M06-2X, were found to precisely replicate the binding energies for the benzene and fluorobenzene complexes compared to the "gold standard" theory level, coupled cluster theory (CCSD(T)) with a complete basis set (CBS) extrapolation. ωB97XD is the best level for calculating binding energies when the optimization is BSSE corrected, and M06-2X is best when the optimization is BSSE uncorrected. For prediction of rotational constants, ωB97XD was the best overall DF T level, performing better for the benzene complexes when BSSE uncorrected, but was more accurate for the fluorobenzene complexes when BSSE corrected. Overall ωB97XD is recommended as the best DFT level for predicting binding energies and rotational constants for CH/π interactions.
The concentration of CO₂ in the atmosphere has increased dramatically over the years as a result of burning of coal, natural gas and oil. To reduce the excess CO₂ in the atmosphere, CO₂ capturing devices (that rely on non-covalent interactions to bind the CO₂) have been created. One way to recycle the captured CO₂ is to use it as a "green" supercritical solvent. Supercritical CO₂ has unique properties that can allow it to dissolve molecules containing fluorine atoms by mechanisms that are not fully understood. With the aim of understanding interactions between fluorinated species and CO₂, a study of 1,1-difluoroethylene-CO₂ was done. The experimental structure of 1,1-difluoroethylene-CO₂ was determined from the experimental rotational constants A= 5696.6440(9) MHz, B= 1121.85748(24) MHz and C= 939.4186(4) MHz. The structure of 1,1-difluoroethylene-CO₂ followed the same trend as other fluorinated ethylene-CO₂ complexes, with the CO₂ molecule interacting in the same plane as the fluoroethylene. The fluorinated ethylene-CO₂ complexes have similar parameters such as a C--F interaction distance ranging from 2.90 Å to 2.95 Å, and an F--C=O angle that ranges from 83° to 89°.
Lastly, a modification was made to the chirped-pulse Fourier-transform microwave (CP-FTMW) spectrometer by the addition of a mixing controller. The mixing controller was constructed to improve the intensity of the transitions for dimers, radicals, and ionic species by allowing variation of sample concentration and pressure in "real time". Using the mixing controller, the radical, C₄H∙, and the dimer, fluorobenzene-acetylene, were observed for the first time on the CP-FTMW spectrometer. The mixing controller has made it possible to observe other radical/ionic species on the CP-FTMW spectrometer (such as C5H∙ radical).
Anderton, Ashley M., "Computational and Experimental Studies of Non-Covalent Interactions: Characterization of CH...π Interactions and Structure of 1,1-Difluoroethylene...Carbon Dioxide" (2014). Masters Theses. 1318.