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Title: A Spectroscopic Study ofthe Effects of Substituent Choice on the Structure and Dynamics of Aromatic Molecular Complexes
Author: Tonge, Nicola M.
ISNI:       0000 0001 3535 0455
Awarding Body: University of York
Current Institution: University of York
Date of Award: 2007
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This thesis combines experimental gas phase spectroscopy with ab initio computational chemistry to study the effects of substituent choice on the structure and dynamics of a number of aromatic van der Waals complexes. In the first example presented, the effect of an unsubstituted vinyl group in styrene-Arn (n=1,2) provides not only a more extended Jr-system with which potential solvents can interact but also a non-planar ground state, which effectively doubles the number of potential binding sites. REMPI and ZEKE spectroscopies 'have been used to probe the first excited and cation ground states of both dimer and trimer species whilst ab initio calculations of the electronic ground states ofboth neutral and cation have been used to identify potential stable conformations. Evidence is presented for the exist~nce of more than one structural conformer in both styrene-Ar and styrene-Ar2. The remaining experimental chapters all examine halosubstitued benzenes solvated by ammonia, representing the prototypical. amine. In fluorobenzene, the effect of the highly electronegative susbtituent is to destabilise the weak Jr-hydrogen bond that characterised the binding of ammoma in benzene, in favour of an in-plane double-hydrogen bound complex. Assignment of the REMPI spectrum has been achieved from a combination of comparisons of intramolecular vibrational wavenumber shifts, computed ab initio intermolecular vibrational frequencies and calculations of ground state binding energies. The assignment to an in-plane conformer represents a departure from the existing assignments in the literature. Replacing the fluorine atoms with a halogen atom of lower electronegativity might help to better understand the electronic factors that influence where the solvent binds. In the next example, the bromobenzene-ammonia complex is studied, using a combination' of ab initio calculations and comparison of the REMPI spectrum with that of fluorobenzene-ammonia. In this case it appears that the lower electronegativity of bromine is offset by its poorer ability to back donate Jr-electrons to the ring and once again it is the in-plane conformer which is observed experimentally. The next chapter extends this work by looking at multiply substituted amine solvated polyhalobenzenes complexes. In addition to possible hydrogen bonds, such complexes can in principle also form so called halogen bonds. In the first part of this chapter, a number of different complexes are studied using computational methods in order to identify potentially suitable systems to study experimentally. In the second part, a detailed study of the tetrafluorobenzene-ammoniacomplex is presented. Two colour REMPI spectra show significant vibrational activity along the butterfly coordinate, with several quanta in this vibration being observed in combination with intermolecular vibrations. Evidence of strong coupling between the interand intramolecular modes is presented. The final part of the thesis presents an investigation into the post-ionisation chemistry within halobenzene+-ammonia clusters. Although there have been several previous investigations into the chemistry of these cation species, there has been no thorough spectroscopic investigation involving the determination of the reactive cluster structures. In addition to known products of the ion-molecule chemistry of the fluorobenzene- and bromobenzene-NH3 complexes, a number of new product channels have been identified. Reaction mechanisms for these have been suggested. Simultaneous to the spectroscopic investigation of these species, it has been possible to probe the cationic reactivity of these clusters, and begin to identify reactivity trends. Results indicate that the 1:2 (aromatic:ammonia) cluster reacts to form aniline, while the 1:1 complex reacts to form anilium+ and NH3+, possibly in competing processes.
Supervisor: Not available Sponsor: Not available
Qualification Name: University of York, 2007 Qualification Level: Doctoral
EThOS ID:  DOI: Not available