2.2.1. FT-IR Spectroscopy

Infrared spectroscopy can provide a quick insight into the formation of inclusion complexes, in particular for guests that contain, like efavirenz, oscillators sensitive to the hydrophobic environment of the host cavity and located in a spectral area free from host bands (which could eventually superimpose with it). The carbonyl (C=O) group of efavirenz is thus an excellent probe for inclusion. The carbonyl stretching frequency occurs at 1741 cm−<sup>1</sup> in the spectrum of pure efavirenz, appearing blueshifted and with maxima centred at 1751 and 1746 cm−<sup>1</sup> in the spectra of the 3:2 and the 1:1 products, respectively (Figure 3). A blueshift, i.e., an increase in the stretching energy of an oscillator, can be the result of the lower polarisation of the C=O group, with increased electron density in the double bond. It is frequently the result of an apolar environment around the affected oscillator, such as that caused by inclusion into the cavity of γ-CD. For the 1:1 product, the smaller blueshift can be interpreted as the combined result of two contributions, one from the inclusion complex and a second one originating from a small contamination with non-included efavirenz.

**Figure 3.** Fourier-transform infrared spectra of efavirenz (EFV), γ-CD and the two freeze-dried solid products starting from mixed solutions with γ-CD:EFV proportions of 3:2 and 1:1 (FD).
