Degree Name

Master of Science (MS)

Semester of Degree Completion

2014

Thesis Director

Sean A. Peebles

Abstract

CH...π interactions are non-covalent interactions which occur in organic compounds that are important in biological and chemical sciences in many ways. Microwave spectroscopy is an important technique that can be used to detect CH...π interactions in the gas phase molecules. Previously microwave spectroscopic studies of complexes containing CH...π interactions, benzene...HCCH (Ulrich, N.W.; Seifert, N.A.; Dorris, R.E.; Peebles, R.A.; Pate, B.H.; Peebles, S.A., Phys. Chem. Chem. Phys. 2014, 16, 8886-8894) and fluorobenzene...HCCH (Ulrich, N.W.; Songer, T.S.; Peebles, R.A.; Peebles, S.A.; Seifert, N.A.; Perez, C.; Pate, B.H. Phys. Chem. Chem. Phys., 2013, 15, 18148-18154) were done at Eastern Illinois University and the CH...π distance in the two complexes was found to be almost identical. The next approach was to study how the degree of fluorination affects the CH...π interaction of the dimer. Therefore experimental studies of difluorobenzene...HCCH complexes were initiated. In this project, the dimers 1,2-difluorobenzene...HCCH, 1,3-difluorobenzene...HCCH, and 1,4-difluorobenzene...HCCH were studied theoretically although a spectroscopic analysis was carried out only for 1,2-difluorobenzene...HCCH. In addition to the difluorobenzene...HCCH complexes, theoretical calculations have been performed for another complex with CH...π interactions, fluorobenzene...(HCCH)2, in an attempt to identify this trimer, which may be present in an earlier scan of fluorobenzene...HCCH.

Spectral analysis of the dimer 1,2-difluorobenzene...HCCH has shown that the lowest energy structure of the dimer undergoes internal rotation which complicates the rotational spectrum, doubling all spectral transitions of the complex. The rotational constants A, B and C obtained by spectral fitting for the lowest energy structure (where the acetylene is tilted towards the fluorine atoms of the 1,2-difluorobenzene monomer) of 1,2-difluorobenzene...HCCH dimer agree with ab initio rotational constants with a percent relative error of 7.7 %, 9.3 % and 15.2 % respectively. Theoretical calculations performed for the difluorobenzene...HCCH complexes show that the CH...π distances are higher in difluorobenzene...HCCH dimers than the benzene...HCCH or fluorobenzene...HCCH.

From the theoretical studies performed for fluorobenzene...(HCCH)2 it was found that the lowest energy trimer structure gives rise to c-type transitions and the CH...π distance of the trimer (perpendicular distance from acetylene molecule to the ring plane of fluorobenzene monomer) is nearly the same as the theoretical CH...π distance of fluorobenzene...HCCH dimer and the distance between two acetylene monomers in the trimer is nearly the same as the theoretical CH...π distance of the acetylene dimer.

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