Enantioselective Spectroscopy and Photochemistry in the Gasphase
Resonance-enhanced multiphoton-ionization (REMPI) stands out due to extraordinary selectivity and sensitivity. Since REMPI is a ionisation process, the so-produced ions may be selected in a mass spectrometer, in addition. Time-of-flight mass spectrometers are here of particular advantage. REMPI with all its variations is an optimal means to perform electronic spectroscopy and to study chemical and photochemical processes in the rarefied gas phase. A very special variation of REMPI is enantio-sensitive ionization.
Circular dichroism in ion yield has promising new potentials for chiral analysis. One is the combination with mass spectrometry. Circular dichroism may be involved in REMPI by using circularly polarized laser light. This will result in slightly enhanced absorption (and thus ionization) of one enantiomer and slightly reduced absorption of the other. Unfortunately, the absorption cross sections for left and right circularly polarized light mostly differ by few tenths of a percent or even less. Laser fluctuations make it impossible to resolve such small differences in signal intensity.
It is now our idea to take two mass spectra which are slightly shifted against each other. These twin mass spectra originate from the same laser pulse which has been divided up into two partial beams. They can be measured in a time-of-flight mass spectrometer for every single laser pulse. Both spectra are subject to the same original laser pulse fluctuations. Relative differences between the two mass spectra are therefore dependent of these fluctuations in a far lower extent. If one mass spectrum is due to right circularly polarized light, the other to left circularly polarized light, the circular dichroism manifests itself in the relative intensity difference of two neighbouring peaks in the twin mass spectra. This relative intensity can now be resolved with much higher accuracy.
In addition, multiphoton-ionisation offers the possibility of including double resonances and nonlinear photoeffects for which a considerably stronger enantio-selective discrimination is hoped to occur. First IR UV-induced emission spectra and nonlinear spectroscopy observed by other groups indicate such an enhancement. There is still basic research to do what also opens the possibility to discover new effects.
Another approach to combine chirality-probe and mass spectrometry is based on the different features of diastereomeric weakly bound complexes of two chiral molecules. These may be formed in supersonic molecular beams as homo- and hetero-complexes with equal or opposite handedness of the partner molecules. They show noticeable differences in spectroscopy and are subject of research for several groups. These differences may even allow for enantio-selective preparation of molecular species. Other group try to exploit the different fragmentation behaviour of homo and hetero-complexes which are formed in chemical reactions prior to mass spectrometric investigation.
The methods of "enantio-sensitive laser mass spectrometry" developed in our group will help
- to find and to study enantio-selective heterogeneous catalytic processes
- to study the catalyticc activity of chiral metal and bimetal clusters deposited on substrates, in particular
- to probe the chirality of product molecules formed by these catalytic processes on a nanoscale
but our vision also is, to apply these methods
- to molecular ions,
- to laser-desorbed biomolecules and maybe even
- to matrix assisted laser desorption/ionisation (MALDI).
- Deutsche Forschungsgemeinschaft (DFG)
- European Research Council (ERC) - Advanced Grant