A Raman Spectroscopic and Computational Study on the Effects of Electron Withdrawing Groups on Halogen Bonding

Allen, Katelyn (2017) A Raman Spectroscopic and Computational Study on the Effects of Electron Withdrawing Groups on Halogen Bonding. Undergraduate thesis, under the direction of Nathan Hammer from Chemistry and Biochemistry, University of Mississippi.

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Abstract

Here, the effects of individual electron withdrawing groups on halogen bond strength are investigated using Raman spectroscopy and electronic structure calculations. Halogen bonding is a unique and relatively unexplored noncovalent interaction that has recently garnered considerable attention due to its applications in materials development. When a halogen, such as chlorine, bromine, or iodine, accepts electron density from a Lewis base such as the nitrogen of pyridine a halogen bond arises. The anisotropic distribution of electron density about a covalently bound halogen, also known as the σ-hole, plays a significant role in halogen bond formation. The magnitude of the electropositive region increases from chlorine < bromine < iodine. Using a combination of Raman spectroscopy and computational chemistry, eight halogen bond donors with four unique electron withdrawing moieties were evaluated for use as potential molecular building blocks in organic optoelectronic devices. First, the C-I stretches of the ten monomers were assigned using experimental Raman spectra in junction with computed Raman frequencies and activities. The C-I stretch was tracked as a function of electron withdrawing moiety, producing trends that directly connected donor strength to molecular structure. The observation of a red shift in the C-I stretch of the halogen bond donor suggested an electrostatically stronger σ-hole and thus a greater binding energy with halogen bond acceptors. Additionally the C-I stretch, in twelve co-crystals synthesized from promising donors and acceptors, was assigned and tracked. The change in the C-I stretch for the co-crystals was similar to that of the monomers. With this, the change in the C-I stretch as a function of complexation was determined by comparing the C-I stretch of the monomeric halogen bond donor with the C-I stretch of the halogen bond complex. The trend observed agreed with that of the C-I stretch within both the monomer and co-crystal systems. Interestingly, the results of this project suggest that although the σ-hole present on the halogen plays a large role in halogen bond formation, the concentrated belt of electron density surrounding the halogen also significantly contributes to halogen bond strength as well as the vibrational characterization of the molecule.

Item Type: Thesis (Undergraduate)
Creators: Allen, Katelyn
Student's Degree Program(s): B.S. Chemistry (Biochemistry)
Thesis Advisor: Nathan Hammer
Thesis Advisor's Department: Chemistry and Biochemistry
Institution: University of Mississippi
Subjects: Q Science > QD Chemistry
Depositing User: Katelyn Allen
Date Deposited: 19 May 2017 18:21
Last Modified: 19 May 2017 18:21
URI: http://thesis.honors.olemiss.edu/id/eprint/943

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