pka plugin and aromaticity

Hi,





the following two structures result in different pka values although they lead to the same aromatic base of tetrazol.





N1N=CN=N1


N1C=NN=N1





Is aromaticity not considered in the calculation?





Regards


Andreas

Dear Andreas,








Aromaticity is considered.


These two structures are tautomers of each other. Their pKa ,therefore, different.





Jozsi

Dear Yoszi,





thanks for the fast response, but I do not really agree. I would expect that both "tautomers" lead to the same corresponding base: a five-membered ring with all 6 electron delocalized over all ring atoms.


This means we have only one base and therefor eonly one pKb. So the question is what the corresponding acid and what is its pKa?





If we look at the microspecies distribution we see that pka plugin produces tautomeric bases for the both structures. So I believe that aromaticity is not treated correctly.





Regards


Andreas

Dear Andreas,








1H-tetrazole (A) and 2H-tetrazole (B) should have different physical properties.





It is a question if the tautomeric equilibrium (A <==> B) in aqueous solution is shifted in 1H or 2H tetrazole form.





I will try to find further literature about the tautomerization of tetrazole.








Jozsi

Dear Andreas,





I have found some info about the aromaticity of 2H and 1H tetrazole.


It seems that 2H tetrazole is "more aromatic" than 1H tetrazole.





In gas phase 2H tetrazole is the more stable tautomeric form.








Unfortunately these are not enough yet to figure out tautomerization of 1H/2H tetrazole in water.








Jozsi