Note from the author: I would like to thank the triphenylamine community for citing this old paper whenever the structure of triphenylamine is mentioned. Many thanks!
Structure of Triphenylamine: rotational, vibrational and electronic spectroscopy
Introduction
If the molecule consists of parts with a well-known structure, the
rotational constants of the entire molecule contain enough information
to determine its structure in the gas phase. An example is
triphenylamine (TPA), a
molecule which consists of a nitrogen atom with three phenyl groups
attached to it. Since the structure of each phenyl group is known,
there are only a few unknown parameters left, which are related to the
relative orientation of the phenyl groups. Therefore, it is possible to
determine the structure of the entire molecule. Additionally, the shape of the rotational resolved spectrum directly provides the symmetry of the molecule: TPA has a 3 fold rotational symmetry!
Both vibrationally and rotationally resolved spectra of the S1 <-- S0
transition in jet-cooled triphenylamine (TPA) around 340-320 nm are
reported. Medium resolution spectra (0.5-1.0 cm-1 resolution) are
recorded using (1+1)-Resonance Enhanced Multi Photon Ionization (REMPI)
with mass selective Time-Of-Flight (TOF) detection in a pulsed molecular
beam apparatus. The origin of the S1 <-- S0 transition is at 29520.7
cm-1, higher than halfway to the ionization potential (IP) found at 6.89
eV. A vibrational progression in the symmetric torsion mode (114 cm-1)
as well as in the symmetric C--N stretching mode (280 cm-1) is observed
in the electronic spectra. The spectrum of the most abundant isomer of
the TPA--Ar (TPA--Kr) complexes is blue-shifted by 211 cm-1 (216 cm-1)
with respect to the spectrum of the free TPA molecule. High resolution
spectra are recorded using Laser Induced Fluorescence (LIF) in a cw
molecular beam apparatus. Individual rotational transitions are
resolved and the spectrum shows unambiguously that TPA is a symmetric
top molecule. The spectrum of the blue-shifted TPA--Ar isomer is the
spectrum of a symmetric top molecule as well, and therefore the Ar atom
has to be located on the C3 symmetry axis, either on top of or
underneath the umbrella formed by the phenyl rings. It appears that
when Ar or Kr forms a complex with TPA, the first Ar, Kr, atom goes
preferentially in a position on the C3 symmetry axis of TPA, a position
which causes an abnormal blue-shift of the spectrum. With the first
rare gas atom located in this special position, the second rare gas atom
is forced into a `normal' position, i.e. above one of the phenyl-rings,
causing a normal red-shift with respect to the TPA--Ar complex.
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prior permission of the author and the publisher.
G. Meijer, G. Berden, W.L. Meerts, H.E. Hunziker, M.S. de Vries,
and H.R. Wendt.
Spectroscopy on triphenylamine and its van der Waals complexes.
Chem. Phys. 163 (1992) 209-222.