Dear Partha,

Since the molecule has a symmetry, the tautomerism occur in both sides, thus two proton are transferred. 

You may see the work in:

Russian Journal of Organic Chemistry

November 2006, Volume 42, Issue 11, pp 1662-1667

First online:

Tautomerism of anthraquinones: V. 1,5-Dihydroxy-9,10-anthraquinone and its substituted derivatives

V. Ya. Fain, , B. E. Zaitsev , M. A. Ryabov

http://link.springer.com/article/10.1134/S1070428006110108

Please note that the experimental work is conducted in EtOH, whereas DFT calculations in the gas phase. Further you should calculate the activation energy for the proton transfer from both sides of the molecule. This can give a clear picture if the two proton are transferred or not.

Hoping this will be helpful,

Rafik

Sir, Thank you for your advise. I  already read the paper and tried to DFT calculation in aqueous phase for all the tautomers. Here tautomer with two proton transfer i.e. 9,10 dihydroxy 1,5 anthraquinone from 1,5 dihydroxy 9,10 anthraquinone  is highly unstable and optimization of this results the initial 1,5 dihydroxy anthraquinone.  I found only one proton transfer in B3LYP protocol with 6-31g(d) basis set.  Again the C2h symmetry has been lost during proton transfer.

More over in another paper i.e. Journal of molecular structure, 542,63-77,2001 ab initio (HF/3-21G) calculation has been done, they show 2 proton transfer but the reliability of long range proton transfer in SCRF method by ab-initio method is not suitable one .

Waiting for your help.

I suggest that you run your calculations with Truhlar's method such as Hybrid GGA: MPW1K, MPW3LYP, MPWLYP1M, SOGGA11-X or you can select other keywords from the following link:

http://comp.chem.umn.edu/info/dft.htm

I should emphasize that DFT is not good enough for predicting ion structures, therefore, use the hybrid method mentioned above.

Good luck,

Rafik

Mr. Sengupta,

Where you have read about H-transfer (intramolecular) of 1,5 dihydroxy anthraquinone in GS?

My preliminary, IRC and DFT-MD data as well as GS frequency analysis and optimization in gas-phase have shown that the isolated tautomeric form 1,5 dihydroxy anthraquinone is more stable from a thermodynamic standpoint (D(DG) = 26.16 kcal.mol-1) comparing with corresponding 9,10-Dihydroxy-anthracene-1,5-dione tautomer (DG (1,5 dihydroxy anthraquinone) = 0.03338 hartree/particle); DG (9,10-Dihydroxy-anthracene-1,5-dione) = 0.075899 hartree/particle). And there is not found intramolecular H-transfer.

If we could speak about H-transfer in GS, somewhere, this would be in the system 1,5 dihydroxy anthraquinone.2H2O (attachment). Not the isolated molecule as written above. The preliminary static DFT/IRC data have shown H-transfer but as one H-reaction (attachment). Nevertheless that IRC analysis of 1,5 dihydroxy anthraquinone.3H2O excludes again H-transfer (attachment).

A more comprehensive analysis should confirm, I am almost sure, that in this system we could talk about intramolecular H-bonding in GS, rather than about intramolecular H-transfer, furthermore not only in GS. There is very large D(DG) between both he thautomers. Usually for such as systems there is not confirmed both theoretically and experimentally a H-transfer process.

You could try, for example, to study the different effects of the environment, like acidity, polarity of the solvent, UV-irradian and more, in orger to obtain conditions which can induce a H-transfer. But this should be induced H-transfer.

Respected,  humbly I  present the the references

1. M.L. Ferrerio et al Journal of Molecular Structure( tTheochem) 542,2001, 63-77

2. V. Ya. Fain, , B. E. Zaitsev , M. A. RyabovRussian Journal of Organic Chemistry42,2006,1662-1667

Please don't take it otherwise, I get some evidence for the proton transfer in B3LYP/-31g(d) protocol in the ground state in SCRF medium. Please  send me your mail id so that  I can share the data obtained in my calculation.