Subject: Symmetry number

From: [email-masked] (Frederic BOUYER)
Date: Fri, 8 Sep 1995 17:39:45 +0100

Hi everybody,

Is there anybody out there who can explain to me the exact definition (and
physical/chemical meanings) of the symmetry number, the one that occurs in
the rotational contributions of thermochemistry functions ?

Thank you in advance.

Frederic Bouyer
Lab. Electrochimie et de Chimie Analytique
http://alcyone.enscp.jussieu.fr/Pages/LECA/GP/FB/

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From: "John-M. Sichel" <[email-masked]>
Date: Fri, 8 Sep 1995 14:36:05 -0300 (ADT)

> the rotational contributions of thermochemistry functions ?

The symmetry number is defined as the order of the rotational subgroup of
the molecular symmetry group. This equals the number of indistinguishable
orientations of the molecule which can be interconverted by rotation
alone.  
Exs. NH3: 3,  CH4: 12,  C2H4: 4,  C6H6: 12,  XeF4(square planar): 8

The thermodynamic significance (in NH3 for example) is that rotation leads
to sets of 3 identical "states" which are really only one state according
to the Pauli principle.  The partition function (sum-over-states) is
therefore smaller by a factor of 3, and the entropy is reduced by R ln 3. 

John Sichel, Universite de Moncton, Canada