I'm trying to simulate the sulfate anion structure in Gaussian, but my double bonds disappeared after the calculations. I had the same problem when I was trying to simulate the acetate anion.
For the sulfate, I put two double bounds in my structure and I decreased their length compared to the two remaining single length.
The following are my file details.
- Job tipe: Opt + freq (minimum)
- Method: Ground state. DFT. Default spin. B3LYP. 3-24G. Singlet.
- General: Ignore symmetry.
- Solvation: IEFPCM. Water
Gaussian calculations do not provide directly bond orders. Bond orders can be calculated after a geometry optimization by means of Wiberg bond indexes (NBO) or Mayer bond indexes. The question suggests anyway that Nelson Colman refers to the graphical representation of bonds in GaussView or some similar visualisation software. This is only a graphical artefact based on the comparison of bond distances with the sum of relevant radii: the graphical representation of bonds (single, triple, …) has no QM meaning.
Hello Nelson Colman
The Gaussian software has default values for different types of bonds, because of which it may or may not show whether the bond is a double bond, triple, or aromatic.
For visualization purposes, you can change the bond order after optimization. Prof Massimiliano Arca has provided an excellent answer.
As for SO42-, the optimized structure may show that all the bond lengths are equal unless you have put geometrical constraints.
I hope this helps.
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