Chiralitätsanalyse mittels Femtosekunden Laserionisation Massenspektrometrie

Für die vorliegende Doktorarbeit wurden enantiosensitive Untersuchungen mit ultrakurzen Laserpulsen im Bereich einiger Femtosekunden (fs) durchgeführt. Dazu wurde die Chiralitätsanalyse mit der Laserionisation-Massenspektrometrie (LIMS) kombiniert. Die unterschiedliche Drehrichtung von links- (LCP)...

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Bibliographic Details
Main Author: Horsch, Philipp
Contributors: Weitzel, Karl-Michael (Dr. Prof.) (Thesis advisor)
Format: Dissertation
Published: Philipps-Universität Marburg 2013
Online Access:PDF Full Text
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Table of Contents: For this PhD thesis chirality analysis via circularly polarized fs-laser pulses is combined with laser ionization mass spectrometry (LIMS). The ion yields for left- and right circularly polarized laser pulses were measured and the circular dichroism (CD) in ion yields was determined. Special interest was given to the influence of laser pulse parameters on the chiral distinction expressed by the CD in ion yield. For the correction of experimental imperfections an achiral reference was used. Since any determined CD-effect for this reference is artificial, systematic errors, such as deviations of ideal circular polarization, and fluctuations e. g. in laser pulse energy can be corrected. In the first part of this PdD thesis chirality analysis in (1+2)-resonance enhanced multiphoton ionization (REMPI) of 3-Methylcyclopentanone (3-MCP) was investigated. The wavelength dependence of the CD-Effect between 300 nm and 330 nm was measured during my diploma thesis. By comparison of the CD in ion yield in fs-LIMS with the CD-effect measured with ns-laser pulses a difference of absolute values was observed. To further investigate how the CD in ion yield is influenced by the laser pulse duration a pulse shaper for ultraviolet light was used. A linear chirp was introduced, which influenced the laser pulse duration. The observed CD in ion yield for both enantiomers of 3-MCP increases with increasing linear chirp and pulse duration. Interestingly, the sign of the linear chirp parameter has no effect on the chiral distinction, indicating that the electric field strength is responsible for the observed increase of the CD in ion yield. In the second part of this thesis non-linear chiroptical ionization is studied. For this purpose several chiral epoxides are investigated using different ionization and or excitation conditions. For (R)-propylene oxide a CD in ion yield of approximately 2% was observed using laser pulses with central wavelengths of 810 nm and 878 nm. This CD-effect is one order of magnitude larger than the anisotropy factor in one-photon absorption. Since in (1+n)-REMPI the observed CD in ion yield and the CD-effect in one-photon absorption had the same value, this clearly shows the non-linear nature of the CD in ion yield for (R)‑propylene oxide at 810 nm and 878 nm. By studying the ion yields as a function of laser pulse energy, one can assume a five-photon process for 810 nm and a seven‑photon process for 878 nm. For ionization with 810 nm a five‑photon resonant excitation can be reasoned, since a excited state would be reached. The seven-photon process at 878 nm would lead to a direct multiphoton ionization of propylene oxide without resonant excitation of intermediate excited states. This would the first demonstration of a chiroptical method based on direct multiphoton ionization. It is interesting to note, that the CD-effects for 810 nm and 878 nm are quite similar, indicating they may result for the same chiroptical process. By taken into account the CD in ion yield for fragment ions at 810 nm a difference in sign for the parent ion and the fragment ions was observed. This holds true for both enantiomeric forms of propylene oxide and was indeed also observed for ionization with 878 nm. Apparently, the reaction pathways leading to the parent ion and the fragment ions exhibit different chirality. Due to the short pulse duration of fs-laser pulses, fragmentation is expected to occur after the ionization. Furthermore, it could be reasoned that the chirality of the parent ion is different from the chirality of the neutral molecule. Consequently, the fragment ions would resemble the chirality of the parent ion, while the parent ion resembles the chirality of the neutral molecule. Additional possible explanations are discussed in this thesis. Also, the difference in signs of parent ion and fragment ions is in contrast to (1+n)‑REMPI, where the same sign was observed for parent and fragment ions. Due to this difference it is interesting to analyze the CD-effect of all investigated ions. Furthermore, measurements regarding the influence of the laser pulse energy on this CD of the total ion yield (for all ions investigated) and on the CD-effect of distinct ions are performed. The CD-effects of distinct ions is decreasing with increasing laser pulse energy, indicating that high laser intensities ionize all molecules in the focus volume, disregarding the chriality of the molecules. This holds true for the CD of all ions analyzed. Despite the fact, that the parent ion is the most intense signal in the mass spectrum the CD of all ions analyzed is dominated by the CD-effect of the fragment ions, because both CD values agree in sign.