Beiträge zur Analytik von Ketonen und Estern : Strukturbestimmungen mittels MB-FTMW Spektroskopie und Quantenchemie

In this thesis the structure and dynamics of molecular systems were determined by a combination of quantum chemical calculations and microwave spectroscopic investigations. The microwave studies have been carried out using molecular jets and the spectral lines were assigned to the corresponding rotational transitions using the computer codes XIAM, ERHAM, and BELGI-Cs. Within this thesis six molecules, two esters and four ketones, have been studied. The tert-butyl acetate molecule can exist as a cis and a trans conformer, where the trans conformer as the energetically favored structure has been identified in the experiment. The barrier to internal rotation was found to be 112 cm-1 and is slightly higher than in most fruit esters. In the case of ethyl pivalate three trans conformers were predicted with quantum chemical calculations at the MP2/6-311++G(d,p) level of theory and observed in the experiment. Because the moment of inertia of the tert-butyl group (approx. 110 uÅ2) in ethyl pivalate is considerably bigger than that one of the acetyl methyl group (approx. 3.2 uÅ2) in the fruit esters internal rotation could not be observed. Therefore, the ethyl pivalate molecule can be described as an almost rigid top. Using a two-dimensional potential energy surface the existence of five conformers of diisopropyl ketone could be confirmed and the strong coupling between the isopropyl groups could be studied in detail. In the case of pinacolone a very strong coupling between the rotational motions of the acetyl methyl group and the tert-butyl group has been found on a two dimensional potential energy surface. However, only one conformer with an approximate Cs symmetry was confirmed by the experiment. Methyl neopentyl ketone and methyl isobutyl ketone have a similar structure. Using quantum chemical calculations at the B3LYP/6-311++G(d,p) and the MP2/6-311++G(d,p) level of theory all possible comformers for both molecules were predicted. With the analysis of the spectral lines the corresponding stable conformer and the internal rotation were determined. The hindering barrier of methyl neopentyl ketone was found to be approximately 174 cm-1, while the barrier to internal rotation of the acetyl methyl group in methyl isobutyl ketone was determined to be 250.5 cm-1. Therefore, an important result of this thesis is, that if the substituent in a ketone is only slightly changed, this will have a tremendous influence on the barrier to methyl internal rotation in the molecule