*3.6. Complexes with Amino Acids–IR Spectra*

The simulated IR spectra of complexes of both calixarenes with selected amino acids can be found in the Supplementary Information.

The examination of vibrational modes of complexes of selected amino acids with calixarenes reveals many interesting features, with a general conclusion that the IR spectra in many cases can serve as a fingerprint of a complex. Especially useful in this aspect is the high-energy part of the spectrum, where the X-H stretch modes (either within one isolated bond or combined from several simultaneous motions within such bonds) undergo various modifications depending on their involvement in noncovalent interactions. In the following, as an exemplary case, we present only a short discussion of the IR spectra for the *wc* conformer.

A characteristic feature of all complexes with the *wc* conformer is a strong line at about 2500–2700 cm−1, which corresponds to the O-H stretching motion for the hydroxy group involved in a strong H-bond with the nitrogen atom from the amino acid group. One can see that the elongation of this bond is well correlated with the red shift of the frequency, as can be expected.

When one compares the region of 3000 cm−<sup>1</sup> and higher, significant differences in intensities and/or emergence or line shifting can be noticed, which represent a combined result of additional motions of the N-H stretching and–in some cases–O-H stretching from the amino acid side. In particular, there are instances where the N-H stretch becomes the highest energetic line, such as, e.g., in *wc*-CX with Asn, *wc*-CX with Leu, or *wc*-CX with Lys.

The stretch within the COOH group appears for the *wc* case as a well-separated line at about 1700 cm−1. In some cases, there is a second line in this region, such as for the Asn guest, which corresponds to the stretch of the CONH2 group. In both cases, the main stretching involves the double C-O bond, but this motion is concerted with the neighboring OH or NH2 groups.
