Is the QST2 method suitable to find the transition state of an enatiomeric R-S interconversion?

I am trying to calculate the energy barrier between the two possible enantiomeric components of a polymer with Gaussian. I wonder if I am able to locate a transition state between two of those structures with the keyword `opt=QST2`?




**Maybe, but more likely no.** Depending on the complexity of your molecule the guess from `QST2` is not good enough. Additionally you might run into some unforeseen coordinate system troubles.


As a small demonstration I'll use $\ce{CHFClBr}$ and the DF-M06L/def2-SVP level of theory and Gaussian 09 Rev. D.01. 


I'll optimise one conformation and then just exchange the $y$ with the $z$ column.



```

%chk=m06ldef2svp.chk
#p M06l/def2SVP/W06
gfinput gfoldprint iop(6/7=3)
symmetry(none)
opt

title: optimisation

0 1
C        0.000000000      0.000000000      0.000000000
H       -0.530433945      0.045629756     -0.953288460
F       -0.905627118      0.105968719      1.035862034
Cl       1.161565217      1.332282245      0.093221601
Br       0.942449903     -1.690284863      0.135370837


```

That gives the following file for the transition state search. It is important, that the ordering in the two input blocks is identical, otherwise the program will fail horribly.



```

%chk=m06ldef2svp.chk
#p M06l/def2SVP/W06
gfinput gfoldprint iop(6/7=3)
symmetry(none)
opt(QST2, maxcycle=50)

title: optimised conformation

0 1
 C     0.004249     0.005882     0.022885
 H    -0.510890     0.037500    -0.945427
 F    -0.880186     0.102687     1.016533
Cl     1.139127     1.361087     0.088671
Br     0.915653    -1.713561     0.128503

title: yz exchanged

0 1
 C     0.004249     0.022885     0.005882
 H    -0.510890    -0.945427     0.037500
 F    -0.880186     1.016533     0.102687
Cl     1.139127     0.088671     1.361087
Br     0.915653     0.128503    -1.713561


```

In this case the calculation will produce the following error:



```

 GradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGrad
 Berny optimization.
 Bend failed for angle     2 -     1 -     5
 Tors failed for dihedral     2 -     1 -     5 -     3
 Tors failed for dihedral     2 -     1 -     5 -     4
 FormBX had a problem.
 Error termination via Lnk1e in /path/to/g09/l103.exe at {a date}.

```

The problem is that in any case of switching stereo centres, you most likely have have an $\approx180^\circ$ angle at one point and then the automatic generation of redundant internal coordinates will fail.  

You can (maybe) prevent this at a much higher computational cost by optimising in Cartesian coordinates. Alternatively you specify the redundant coordinates by yourself, but even then it is probably best to use `QST3`.  

In some rare cases, such calculations can actually give you a starting structure for a real TS search.


In any case, a little chemical intuition is necessary and often a single step mechanism is much, much higher in energy than a step-wise path. Sometimes it is better to use your imagination and guess a transition state (which you can then optimise with very low level tools like force fields or pm6), and refine later on.




---


The example I chose is certainly an extreme one. In this case isomerisation will likely happen via $$\ce{^{$S$}CHFClBr <--> [CHFCl]+ + Br- <--> ^{$R$}CHFBrCl}$$ or similar, and a transition state won't be easy to find.


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