Entwicklung der Ansteuerung eines Molekülstrahl-FT-Mikrowellenspektrometers und mikrowellenspektroskopische Untersuchungen an einigen Fruchtestern

Within this work the control software of a molecular beam FT microwave spectrometer has been developed and some fruit esters have been studied by microwave spectroscopy. With the spectrometer broad band scans can be performed and highly resolved measurements at certain frequencies can be carried out. The program package developed has a modular structure in order to allow for easy changes within the individual modules. Moreover, a user-friendly program surface has been developed. The individual hardware components of the spectrometer were controlled via a timer/counter board. This board allowed for short allocation times (a few ns) and enabled phase synchronous repetition of the experiment. When the control software of the fast A/D converter (up to 100 MS/s) was developed it was found, that the start of the recording showed some jitter with respect to the trigger signal. This problem has been solved by a special hard and software. The biggest disadvantage of the spectrometer is its narrow bandwidth due to the big quality factor of the resonator and the long time needed for sampling a broad frequency range. Within the spectroscopic part of this work some fruit esters (ethyl isovalerate, ethyl butyrate, isoamyl acetate, and ethyl acetate) were studied by microwave spectroscopy. These molecules are relatively large for microwave spectroscopy, but their vapour pressure turned out to be sufficient to observe them in the gas phase. At the beginning of the investigations broad band scans were carried out and the lines found were remeasured at high resolution. As a result of such an analysis the rotational constants of the respective molecules were obtained and compared with the quantum chemical predictions. When methyl internal rotation was observed, also the angles of the internal rotor axis with respect to the principal axes could be compared. In the case of ethyl isovalerate the analysis of the broadband scan yielded the rotational constants of one conformer. Some c-type transitions could be observed, thus a significant dipole moment component in c direction is present. By ab initio calculations a conformer was predicted which was in agreement with the observed rotational constants, however it showed up a mirror plane perpendicular to the c axis. This would cause a zero dipole moment in c direction which is in contradiction to the experimental results. In this case future ab initio calculations are necessary to find a geometry which is in agreement with the experimentally observed conformer. In the case of ethyl butyrate the evaluation of the spectrum yielded two sets of rotational constants which allowed an unambiguous assignment of two different conformers. Both conformers (EBU-1 and EBU-2) are near prolate tops.Isoamyl acetate and ethyl acetate showed splittings due to low barrier internal rotation. In both cases the internal rotor is a methyl group. For each substance one conformer was found. The barrier to internal rotation was very close to 100 cm-1 for both molecules. Also these molecules are near prolate tops