I optimized two diastereomers in GAUSSIAN and got two values of total energy. So, I need to decide are these diastereomers energetically equal or one of them is more preferable termodinamically. What difference in total energy (kcal/mol) is considered to be significant? My compounds have 2 kcal/mol difference. Can I consider one of them to be more preferable? Or such difference is trivial?
Hi,
usually 3-4 kcal/mol would give raio 99/1 if dynamic equilibrium is considered (at room temperature), one could assume that one is slightly more favoured with 2 kcal/mol, but with no experimental data backing it up it may reside within the margin of error of Gaussian calculations. Similarly basis sets and solvent effects may change the result.
Hope this helps,
Kaspars
First of all: use Joule, not calories! There are at least 3 different definitions for "cal". If you do not know which definition you are using, do not use calories at all.
An energy difference of "2 kcal/mol" at room temperature gives you a Boltzmann factor of about 0.034 . But you cannot simply equate this with the concentration ratio of the diastereomers, for there may also be entropic effects to consider.
Moreover: Did you do the GAUSSIAN calculations for isolated molecules or did you consider a solvent? Did you run a simple SCF calculation or did you use a post-SCF method (which would give you, among other effects, the contribution of intermolecular dispersion forces)? Do the "2 kcal/mol" remain unchanged when you use a different basis set?
Mrs/Miss Stepanova,
The value is 0.4 kcal.mol-1. Nevertheless, ideal chemical accuracy should be 0.1 kcal.mol-1. For example, the latter value is the difference of the energy of some important polymorphs of pharmaceutics, for example.
Mrs/Miss Stepanova,
In addition please pay attention to the following units' conversion:
1 a.u. =
= 2625.5 kJ.mol-1 =
= 627.51 kcal.mol-1 =
= 27.212 eV =
= 2.1947E5 cm-1.
It depends on the temperature. If it is within the kT range then I would state that the difference is within the kT range. This is what chemical engineers do.
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