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Working
group for 
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Prof. Dr. Michael Schmitt |
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Working
group for
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Prof. Dr. Michael Schmitt |
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Molecular structures in the electronic
ground state can be
determined via a large variety of well established techniques like
microwave spectroscopy, X-ray diffraction, neutron diffraction, or NMR
spectroscopy. Geometries of electronically excited states are much more
difficult to obtain. In our group rotationally resolved fluorescence
spectroscopy is applied to extract the rotational constants of the
molecule under investigation. Further information can be gather from
the spectra, like centrifugal distortion constants, orientation of the
transition dipole moment, or life time of the excited state. Other
complementary techniques, that also yield structural information of
electronically excited states are the resonant ionization variant of
rotationally resolved vibronic spectroscopy, which has been pioneered
in the group of Neusser
in Munich, and rotational coherence spectroscopy, introduced by Felker,
and improved considerably in the group of Brutschy
and Riehn in Frankfurt. A resolution of 1 part in 108
over the whole UV range
is necessary to rotationally resolve the spectra of large
molecules.
The experimental setup for the
rotationally resolved laser induced
fluorescence (LIF) consists of a single frequency ring dye laser pumped
with either with 6 W of the 514 nm line of an Ar+ laser or
with 7 W of the 515 nm line of a diode pumped cw Yb:YAG laser
(ELS MonoDisk-515). The light is coupled into an external folded ring
cavity for second harmonic generation (SHG). The molecular beam
is formed by expanding a mostly heated sample seeded typically in 200 -
1000 mbar of argon through a 100 - 300 µm hole into the
vacuum. The molecular beam machine consists of three
differentially pumped vacuum chambers which are linearly connected by
two skimmers (1 mm and 3 mm, respectively) in order to reduce the
Doppler width. The molecular beam is crossed at right angles in
the third chamber with the laser beam 360 mm downstream of the
nozzle. The resulting fluorescence is collected perpendicularly
to the plane defined by laser and molecular beam by an imaging optics
setup consisting of a concave mirror and two plano-convex lenses.
The resulting Doppler width in this setup is 15 MHz (FWHM).
In some experiment the Doppler width was 25 MHz, because of a slightly
different arrangement of the optical system. The integrated
molecular fluorescence is detected by a photo multiplier tube whose
output is discriminated and digitized by a photon counter and
transmitted to a PC for data recording and processing. The
relative frequency is determined with a calibrated quasi conical
Fabry-Perot interferometer with a free spectral range (FSR) of about
150 MHz . The absolute frequency is determined by recording the
iodine absorption spectrum and comparing the transitions to the
tabulated lines.
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Tryptamine itself, but even more its analogs serotonin
(5-hydroxytryptamine) and melatonin (5-methoxy-N-acetyl-tryptamine) are
known as neurotransmitters. Their conformational stabilities and preferences are of great importance for quantitative structure-activity relationships in neurotransmitter receptor interactions. Since all biological processes take place in an aqueous environment, the interaction with a defined number of solvent molecules is of great interest. The question arises how many water molecules are necessary to lock the large variety of energetically accessible conformations to the biologically active one(s). The spectrum shown below is one of the most impressive example for the power of the genetic algorithm based automated assignment. Eight overlapping isotopomers of the tryptamine B conformer have been fit simultaneously using the genetic algorithm automated technique. Further details can be found in: Schmitt, M., Böhm, M., Ratzer, C., Vu, C., Kalkman, I. and Meerts, W. L.: Structural selection by microsolvation: conformational locking of tryptamine. J. Am. Chem. Soc. 127 (2005), 10356 |
