It is a bit different. The exact statement should be that:

In the case the Fe- citrate complex formation takes place, then its both the asymmetric and symmetric –COO- stretching frequencies should be shifted to higher wavenumbers than those of the SODIUM SALT of the acid,i.e. sodium citrate” .In fact, this concept{called DIRECTION SHIFT criterion} was given by Parmar and Kaur. Dr S.S. Parmar happens to be my celebrated guide.

REF:

S.S. Parmar and H.Kaur, Trans. Metal Chem.,7(1982)79.

Dear Dr. Nasiri,

Can you please tell me what the original and the final wavenumbers are? Maybe then I shall be able to help.

Dear Prof. Alokmay Datta 

In a pure citric acid ftir results the asymmetric and symmetric stretching frequencies are 1725 and 1397 cm-1 respectively which has changed to 1623 and 1384 cm-1. So, Δν = 1623-1384=239. 

Based on literature Δν values greater than about 200 cm-1 are thought to indicate monodentate binding, whereas 180-150cm-1 indicates binuclear bidentate (bridging) complexes, and lower than 100cm-1 indicates mononuclear bidentate binding (chelation)  (Gao, Metge, Ray, Harvey, & Chorover, 2009). So can I conclude here that citric acid bind to iron oxide by monodentate binding?

On the other hand, do you think that I can determine the outer or inner sphere Fe-carboxylate complex formation from this ftir results?

Thanks a billion 

Schematic diagram of Fe-carboxylate complexes. How I can analysis type of binding? 

Firstly, check that the wave numbers you have given are indeed for the carboxylate (COO-) and not the acid form (COOH);  these will have different stretches because one has a C=O and the other does not. Only when you have the COO- can you have a symmetric and antisymmetric stretch for the carboxylate. Secondly, if you have the COONa form of citrate then you can look at the wave number difference for this salt as your comparison assuming you still have a carboxylate when you complex to the iron oxide. Finally, after adsorbing the citrate to the iron oxide surface, compare to both the COOH and the COO- versions, its possible in mono dentate complexation that the C=O stretch will be visible and no antisymmetric or symmetric stretch is visible. For bi-dentate complexation the change in the wave numbers can be used to determine bridging between two irons or to one iron centre. In other words there is no easy answer, you have to check all the possibilities…..

[A] I, humbly, bring to the kind information of my young friends that the DIRECTION SHIFT(d.s) criterion as given by Parmar et al is an improvement over the Currtis criterian* of deciding the bonding modes of the transition metal ions with carboxylate ion from the values of v(Cooasym) and ∆ v [difference between v(ooCasym)and v(OOCsym)].

[B] Further, in our case the carboxylate ion could coordinate with the first transition metal ions in 7 different coordinating modes- ionic(1) , Unidentate (1) Symmetric bridging (3) ,Symmetric chelation(1) and Asymmetry chelation (1 ). I , hereby, attach the results for your kind perusal.

{C} Our group consisting of five scholars ( over a span of 13-14 years) have prepared over 140 complexes of these metal ions (excepting Ti/C ions) and the d.s. criterian never failed us.

* N.F.Curtis, J. Chem. Soc.A,(1968)1579.

For Mdm. Rozita Nasiri

May I , very humbly ,suggest to pls. concentrate on three points- v(COONa) of sodium citrate, v(COOasym) and v(Coosym) of the complex? If both these value are lower than that of v(COONa) of sodium citrate, then the complex has undergone asymmetric chelalation with Fe(II). But if the v(COOasym) is of lower and v(COOsym) is of higher value than the sodium citrate, then F(II) is symmetrically coordinated.

I send your self the pictures of both the Symmetric and the the Asymmetric modes for your perusal. You should change them as suggested in the attached file.

Dear Dr. Nasiri,

You have calculated the shift,  considering peaks representing different vibrational modes and therefore the value is so high.  For conclusions as you seek should be treated with the same vibrational mode

Thanks a lot for the helpful comments. So, it could not be concluded and the only data interpretation is determination of assignable peaks to certain functional groups. 

I am sorry, Rozita, for my delayed response but I would definitely not say that your data is inconclusive. I would first like to know whether the values you have got for citric acid (NOT any citrate) is that of an aqueous solution and if yes, what is its pH? This is important because,  as Franca says, the separation between symm and asymm stretches depends on the formation of carboxylate ion.  Then please get the value checked with standard literature data. However, the correlation between metal-carboxylate coordination and the separation is really not very much dependent on pH because of the partial covalent nature of this bonding and your estimate of a monodentate coordination is quite safe. Also, if the original peaks did correspond to carboxylate group in the citric acid, the fact that both peaks are red-shifted (the shift in the symm peak is large) show that they are attached to a heavy element such as iron. What is interesting is that the separation is quite large which may mean a large 'strain' between the two CO bonds. I would think that if there were formation of Fe-citrate at the surface and inside there would have been two pairs of symm-asymm peaks.

Dear Prof. Alokmay

The values that I got for citric acid is not in aqueous solution and its belongs to the original pure citric acid powder analysed by  FTIR. Besides, the value of synthesized iron oxide nanoparticles and iron oxide nanoparticles coated with citric acid analysed by FTIR to compare and prove the coating of iron oxide by citric acid. 

Thanks a lot 

Citrate band positions shift after interaction with metal oxides.

Some further assignments for wavenumber positions of citric acid/citrate before and after interaction with (in this case) Al2O3 (in the wet state) can be found in this reference.

C. Hidber et al., J. Am. Ceram. Soc. 79(7), 1857 (1996)

Best regards,

Martin Müller