Why does the FTIR-ATR spectrum for hexamethylenediammonium adipate not have a peak for the N-H bond?

I have synthesized pure hexamethylenediammonium adipate by reacting equimolar quantities of adipic acid and hexamethylenediamine in ethanol. The diammonium salt, being insoluble in ethanol, was crystallized out and the mixture filtered. Cold ethanol was used to wash the residue to remove any unreacted acid/amine. The salt was then dried in a vacuum oven to remove any ethanol.


This is the structure of the salt:


[![enter image description here](https://i.stack.imgur.com/Ycrqc.png)](https://i.stack.imgur.com/Ycrqc.png)


The FTIR-ATR spectrum of the salt after oven-drying does not have a peak for the N-H bonds (at around 3500-3300 cm^-1). Is this due to the positive charge on the nitrogen?


[![enter image description here](https://i.stack.imgur.com/zRLHS.jpg)](https://i.stack.imgur.com/zRLHS.jpg)




Your anticipation of absorption bands of $\ce{-NH2}$ close $3300\ldots \pu{3500 cm^-1}$ is about free and neutral amine. Your intended product is about an (organic) ammonium, $\ce{C-NH^+\_3}$. Both the additional hydrogen as well as the additional positive charge, *formally* on nitrogen alters the bond strength $\ce{N-H}$. By consequence, the position of the absorption band will change. (Assuming the vibrations still were the same *as a movement*, maybe the additional charge equally altered the change of the dipole moment during the vibration; if this were the case, the intensity of the absorption band could change, too).


The first literature reference I found is by Vouyiouka *et al.* Note, their setup used differs from yours; instead of a neat sample (your experiment), it is in nujol; instead of ATR (your experiment), it is by transmission (KBr crystal). It still is indicative to read their report which states



> 
> the characteristic peaks of the PA-6,6 salt at $\pu{2210 cm^-1}$ corresponding to the strength vibration of the ionized amine groups ($\ce{-NH^+\_3}$), at $\pu{1521 cm^-1}$ corresponding to the deformation vibration of $\ce{-NH^+\_3}$, and at $1648$ and $\pu{1451 cm^-1}$ corresponding to the vibration of the $\ce{COO-}$ group.
> 
> 
> 


The absorption bands seem present, the one reported at $\approx \pu{2210 cm^-1}$ partly covered by a broader signal around $\approx \pu{(2230 \pm 20) cm^-1}$. On occasion, the intensity of an absorption band for the same material is weaker for the solid (here: crystalline) sample, than for sample in the liquid state. Do you have complementary literature references for recorded IR spectra (since your setup uses the ATR technique, *preferably* literature references *equally* using the ATR technique)? Can you ascertain the diamine used was in good shape, as well as the sample of adipic acid? Since ATR-IR spectroscopy does use only a small sample, and does not require long time of preparation/experimentation, recording two ATR-IR spectra of the starting materials *for comparison* with literature as well as the product obtained would be an additional option.


Vouyiouka, S. N.; Koumantarakis, G. E.; Papaspyrides C. D. Preparation and Solid-State Polyamidation of Hexamethylenediammonium Adipate: The Effect of Sodium 5-Sulfoisophthalic Acid. *J. Appl. Pol. Sci.* **2007**, *104*, 1609–1619 [doi 10.1002/app.25762](https://doi.org/10.1002/app.25762).




---


A few hints about improving the record / the presentation of the record with ATR-IR:


* Contrasting to pressing a recording an IR spectrum in transmission mode, in the ATR mode, infrared radiation shortly has contact with your sample. The penetration depth of the radiation (before returning into the spectrometer) depends on the refractive indices of the sample and the window material (Ge more often seen for routine instruments, coated/non-coated diamond more often for research instruments), and the varied frequency of the incident infrared radiation. Both influence the intensity of the absorption bands eventually plot in the spectrum; ascertain that this *transmission* correction is applied prior to the background correction.
* The *preferred* representation of the IR spectra depends on the audience. Chemists are used to decreasing wavenumbers from left to right (as you did) on the abscissa, yet to plot the percentage of (remaining) transmittance as ordinate. This yields «hanging absorption bands». By appearance of your plot, you possibly engage ThermoFisher Nicolet's software Omnic; there, the adjusting swap only requires a few additional mouse clicks.
* In your representation, the abscissa continuously decreases linearly in wavenumbers. This unnecessarily extends a region *less* interesting for (organic) chemistry on expense of the *more* interesting region for (organic) chemistry of the *chemical fingerprint*, i.e., $\approx 1200\ldots \pu{700 cm^-1}$. To ease *visual interpretation* it is better to split the abscissa into two, or three regions. (In case of Thermo's Omnic, either a) at $\pu{2000 cm^-1}$, or b) at $2200$ and $\pu{1000 cm^-1}$; in each case marked by a vertical dashed line.) Again, this will not add, nor remove data to your spectrum recorded. However, it will ease to recognize individual absorption bands. Annotations on peaks will be easier to read, too.


[![enter image description here](https://i.stack.imgur.com/yIjqm.gif)](https://i.stack.imgur.com/yIjqm.gif)
* As usual, retain a copy of the raw, unprocessed data, as well of the data after application of transmission correction, background correction, peak pic, etc. To prevent a vendor lock-in by Thermo's binary data `.sp` format, consider [.jcamp-dx](https://en.wikipedia.org/wiki/JCAMP-DX) and .csv in plain text. This allows you (and others past you) to replicate your work, as well as the re-use of the data in subsequent experiments.


 7