Subject: fluxious molecules

From: <[email-masked]>
Date: Tue, 10 Sep 96 17:02:24 +0200

Dear members of CCL!

I have two questions concerning vibrational spectroscopy (IR,Ra) of fluxious
molecules.
Problem 1:
two conformers are separated by a transition barrier that is so small that
the conformers are not distinguishable on the time scale of IR and Ra 
spectroscopy. One would then expect to observe average temperature dependent
signals. But what happens if the two conformers belong to different molecular
point groups with different selection rules?(eg. 1,2-disubstituted dislanes:
XH2SiSiH2X 
gauche conformer: C2 (all vibrations IR and Ra active)
anti conformer: C2h (Ag,Bg Ra active, Au,bu Ir active).
How would variable temperature IR and Ra spectra behave in this case?

Problem 2:
ab initio theory finds two conformers on the potential energy surface
of XH2SiSiH2X.
the energy of the two gauche conformer(C2) lies a bit above the energy of the
anti isomer(C2h).
The zero point energy of the torsional motion that interconverts
the two conformers is calculated to lie above the very small barrier of
interconversion of the conformers. So there is clearly one set of torsional
energy levels for this system. But what happens to the small amplitude
vibrations? On the assumption that small amplitude vibrations are calculated
to differ clearly for the two conformers- would one still see separated
individual small amplitude modes for both conformers in the vibrational
spectra or only those of the
anti conformer (C2h) as the gauche conformer represents something like a
`pseudominiumum`?

I hope you can help me (literature hints, books, quantum chemical
treatment of these problems)!
Best wishes to everyone!

Dipl.Ing. Robert Zink
Institut fuer Anorganische Chemie, Technische Universitaet Graz,
Stremayrgasse 16
present e-mail: [email-masked]