We know R-COO- in aqueous has a band around 1540cm-1, but in dry samples it should shift to around 1560cm-1 like R-COONa.
Dear Zhao Yao many thanks for your interesting technical question. IR band for the asymmetric C–O stretching frequency can vary between ca. 1540 and 1560 cm–1 depending on the the nature of the organic substituent R in R–COO–. In this context, please have a look at the following relevant research article in which the adsorption of monocarboxylates (acetate, benzoate, and cyclohexanecarboxylate) at the water/goethite interface has been studied by FTIR spectroscopy. The respective IR frequencies are listed in Table 1 (see attached):
Adsorption of monocarboxylates at the water/goethiteinterface: The importance of hydrogen bonding
Article Adsorption of monocarboxylates at the water/goethite interfa...
Good luck with your research! 👍
P.S. In IR spectroscopy we normally speak about "bands", not "peaks" as in NMR spectra. You might also wish to correct the phrase "shit to around 1560cm-1" in your original question. To edit the question just go to the question page and click on the down arrow next to your name. Then you can easily edit the question. 😎
Hi Frank T. Edelmann , thanks a lot. I'm still a little confused. R-COOH has a band at around 1710 cm-1 and R-COONa in dry samples has a band around 1560 cm-1. R-COO- on the water and goethite surface will have a band at around 1540 cm-1. My question is that in dry samples R-COO- should chelate with some cations and the band at 1540cm-1 shift to higher values. Why the band at 1540cm-1 still appear in dry samples?
Dear Zhao Yao thank you for response and for asking. In this context, please go through the following interesting review article:
Attenuated Total Reflection – Infrared Spectroscopy Applied to the Study of Mineral – Aqueous Electrolyte Solution Interfaces: A General Overview and a Case Study
This paper can be freely downloaded as public full text from the general internet (see attached pdf file). Marked in yellow on page 104 you will find a statement about the IR frequencies of formate (HCOO–) anions adsorbed on a titanium dioxide (TiO2) surface. A band at 1540 cm–1 was assigned to the νas(COO) vibration of the carboxylate anion interacting with the surface. Adsoprtion of carboxylates on goethite is also mentioned on page 105. For example, for adsorbed laurate anions a band at 1545 cm–1 was found. As shown in the attached schematic formula drawing this is consistent with a bidentate complex in which a carboxylate anion bridges two surface Fe atoms.
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