Figure 3. Temperature- and CO₂ concentration-dependent FTIR spectra of 1a filled with ¹²CO₂.

(a) Spectra in the bending region of ¹²CO₂. Δ_{103 K} (5.49 cm⁻¹), Δ_{293 K} (4.92 cm⁻¹), and Δ_{423 K} (4.67 cm⁻¹) represent the separation between the two peak maxima at 103, 293, and 423 K, respectively. Similar spectra are reported for 1a filled with ¹³CO₂ in Fig. S11. (b) Spectra in the asymmetric stretch region of ¹²CO₂. The peaks enclosed with rounded rectangles in the spectral region of ~2323 and ~2327 cm⁻¹ represent a vibrational hot band (starting from the first excited state of the bending mode) and a peak originating from the existence of 2 CO₂ molecules in a single cavity, respectively. (c) Spectra in the spectral region of v₂ = 1 → v₂ = 2 transition (hot band). (d) Spectra of 1a with low CO₂ content in the spectral region of the asymmetric stretch. Note that the spectra in (a,b and c) were obtained from the same sample (high content of CO₂), whereas the spectra in (d) were obtained from a different sample (low content of CO₂).