Subject: Thermochemistry and Point Group

From: [email-masked] (Frederic BOUYER)
Date: Fri, 2 Jun 1995 09:26:24 +0100

Hello Netters,

Is anyone aware of thermochemistry based on structural and vibrational HF
or DFT calculations to determine thermochemical functions?

My problem is the following :

I have two molecules (of the same stoechiometry) of which structural and
energetic differences are very (around 5 10-5 Ha, DMol BLYP functional)
very small. Only the point group is different, one is C3v, the other Cs.

The rotational contribution on thermochemical functions is not the same,
because of the symmetry number (3 or 1, depending on the point group) and
the consequence is drastic on the free enthalpy of formation ! (about 12
kJ/mol, and around 3 for the order of magnitude on the reaction constant at
1300K) I don't know how to decide which symmetry I am to take into account
(5 10-5 Ha is too small) (-by vibrational calculations only ? If I can get
frequencies at this level of accuracy-). I have verified these
thermo-quantities with a simple run with Gamess, and I have the same
difference (12 kJ/mol) on the free enthalpie of reaction (with the same
structures).

Why is there a so big difference between these thermo-quantities for these
two symmetries? I expected that thermochemical functions would be
continuous vs. the structure.

Any comments, suggestions ?

Frederic Bouyer
Lab.Electrochimie-ENSCP-Paris-France
[email-masked]
http://alcyone.enscp.jussieu.fr/
http://alcyone.enscp.jussieu.fr/Pages/LECA/GP/FB/