Subject: FLUXIONAL MOLECULES

From: "E. Lewars" <[email-masked]>
Date: Wed, 11 Sep 1996 15:29:49 -0400 (EDT)

1996 Sept 11

  [email-masked] asked about conformational isomerism
and IR/NMR.  Hdere's my $0.01 worth.

Hello,
Sorry if I can't contribute too much to the solution of your problems, but I
have two comments:

(1) Fluxional molecules are systems like cycloheptatriene/norcaradiene, or
    bullvalene.  The term fluxional is usually applied to  molecules that
    participate in *valence tautomerism*, not to conformationally mobile
    molecules (of course, almost all molecules are conformationally
    mobile).

(2)  You say "Two conformers are...not distinguishable on the time scale of
     IR and Ra spectroscopy."  In *NMR* spectroscopy it is common for
     conformationally mobile species to show at room temp. only an average set
     of NMR signals, but to display on cooling the peaks of the individual
     conformers.  However, so far as I know the frequency of IR radiation is
     so much higher than that of radio waves that IR/Raman spectroscopy always
     takes snapshots of the individual conformers.  The freq of rotation about
     single bonds is about 10**12 Hz (ca. 100 cm**-1), while IR radiation has a
     freq of ca. 10**13 Hz (ca. 1000 cm**-1).  So IR does not see the
     individual conformers as a blur.  For example, the IR of butane is a
     superimposition of the IR's of the gauche and anti conformers.

      Ich denke, Sie mu:ssen eine a:usserst niedrige Barrierre haben, wenn es
     nicht mo:glich ist, die individuelle Konformationen zu unterscheiden.
===
  Errol Lewars
====

>Subject: CCL:fluxious molecules
>Sender: Computational Chemistry List <[email-masked]>
>To: FULTRX"[email-masked]"@ccl.net

>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.
======================