Subject: conformer population

From: Ji Hai-tao <[email-masked]>
Date: Thu, 9 Nov 2000 22:10:21 +0800

Hi,CCLers!
I want to calculate the population of some conformer.I know that 
Boltzmann law is needed.As the conformers have been calculated by
ab initio Single Point Energy calculation.but I don't know if T=298K,
R=8.31.Whether I should make frequency calculation so that I can get 
some thermodynamic value?Can the single point energy be used directly?
then what are the values for R and T?

Song Yunlong
[email-masked]

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From: "Phillip D. Matz" <[email-masked]>
Date: Thu, 9 Nov 2000 09:13:27 -0800

Hello!

The single-point energy should not be used to calculate Boltzmann
distributions. The zero-point energy (ZPE) for each conformer must be
included along with the single-point energies to be used in a Boltzmann
population distribution model.  This would not be the case if the ZPE were
identical between conformers, unfortunately the ZPE is different between
conformers so it must be included.

Respectfully,
Phillip Matz
[email-masked]

----- Original Message -----
From: "Ji Hai-tao" <[email-masked]>
To: <[email-masked]>
Sent: Thursday, November 09, 2000 6:10 AM
Subject: CCL:conformer population

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From: "Bruce. Tattershall" <[email-masked]>
Date: Thu, 9 Nov 2000 18:20:39 +0000 (GMT)

If one has done a calculation for a gas-phase molecule, but one is interested
in conformer ratio in solution (and is prepared to ignore any differential
solvent effects), then should one take          
 'Sum of electronic and zero-point Energies' from the Gaussian freq output
 or 
 'Sum of electronic and thermal Energies' ?
The first is, I think, independent of temperature.

I had supposed that populations of vibrational levels might be similar
between gas phase and solution phase molecules, though I am not a 
Physical Chemist and have no grounds for this assumption.  Therefore using
differences in thermal energies between conformers should give a better
value of delta E.

Total entropy printed by Gaussian is dominated by translational and 
rotational terms, so I suppose the entropy calculation has no 
transferability to the solution phase, and one should be content to use
delta U rather than delta G in calculating K.

I should be pleased to receive authoritative advice from experts in this
field.

  Bruce Tattershall

Personal Email: [email-masked]
Postal address:
    Dr. B.W. Tattershall
    Department of Chemistry, University of Newcastle
    Newcastle upon Tyne, NE1 7RU, England