What might the peak around 1500 cm^-1 in IR be attributed to?

25 February 2016 1 8K Report

The spectra were taken after a continuous flow of toluene (mixed up with nitrogen) passing through Co3O4.

The peaks around 2900 cm-1 and the peaks at 1450, 1407 cm-1 belong to methyl groups. The peak at 1601 cm-1 and one peak around 1500 cm-1 belong to aromatic ring.

The question is what does the other peak around 1500 cm-1 belong to? And which one belongs to aromatic ring?

Thank you!

Leonid V Vladimirov

Dear Kezhi Li:

Co3O4 - is very sensitive to presence of toluene and its traces and often used as sensor for toluene. The sensitivity depends on porosity of Co3O4. The so-called "mesoporous" Co3O4 (which contains pores of 5-50nm size) is most sensitive.

From the series of your spectra - seems that the ratio of intensities between 1508 and 1496 bands is changing during the process of flow. Namely the band at 1508 becomes stronger. This suggests that 1508 band has a strong contribution of interaction of toluene with Co3O4. Another band (1496) remains almost unchanged - which suggests that like 1601 band it belongs to C=C stretching of toluene. {By the way the band at 1450cm-1 also has rather large contribution in Potential Energy Distribution (PED) of C=C stretching in 1450cm-1 band as follows normal coordinates treatment ]

Best Regards, Leonid

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Kezhi Li

Institute of Engineering Technology, Sinopec Catalyst Co., Ltd.

Thank you Prof. Vladimirov.

But I still got confused for your answer. You mentioned that the 1508 peak has a strong contribution of interaction of toluene with Co3O4. What do you mean by "interaction"? Do you imply that toluene has a distortion in shape when adsorbed? If so, why is the 1496 (which might belong to the undistorted aromatic ring if I got you right) still exists?

Much appreciated for your help!

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Leonid V Vladimirov

Semenov Institute of Chemical Physics

You gave no information about the porosity of Co3O4. It is important because the number/amount of molecular interactions of toluene with Co3O4 is proportional to the total surface accessible for toluene. If the amount of surface is not enough to bind all the toluene molecules - then , quite naturally you will have 2 peaks - one of which arises from pure/neat toluene and another one - from toluene molecules bound to Co3O4.

Best Regards, Leonid

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Kezhi Li

Institute of Engineering Technology, Sinopec Catalyst Co., Ltd.

Thank you again!

The Co3O4 sample barely has any mesopores, which is a collection of cube-like crystals with average size about ~ 100 nm.

Is it common for in-situ gas adsorption experiment to detect both gaseous molecule and the adsorbed molecure at the same time? (The flow consists of 1000 ppm toluene balanced in nitrogen. By the way, the signal is collected by DRIFT.) It raises up some worry to me, because I did not really consider the existence of gaseous signal in IR before. As a result, those signals I thought belong to adsorbed molecure might actually belong to gaseous molecule.

My upmost appreciation!

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Duaa Alotaibi

Imam Abdul Rahman bin Faisal University

In primary amines, the N-H bending mode (scissoring) appears as a medium- to strong-intensity (broad) band in the range from 1640 to 1560 cm−1. In aromatic secondary amines, the band shifts to a lower frequency and appears near 1500 cm−1.

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