# Thermal Stability of Ionic Liquids: Effect of Metals
## 3 Results and Discussion  
Figure 1 shows that, after 4 h, all the samples displayed a brown color, indicating that the thermal treatment degrades [TBuMA][NTF\({}_{2}\)]. The color was more intense in the presence of metals, particularly steel and copper, and became darker with longer heating times. To understand the decomposition pathway(s), FTIR and HRMAS NMR experiments were performed on all the samples. Figure 2 shows the ATR-FTIR spectra of [TBuMA][NTF\({}_{2}\)] after 4, 24 and 168 h (samples 4B4, 4B24 and 4B168) of thermal treatment without metal plates. The spectrum of untreated IL (sample B) is also reported for comparison. As far as the IL anion is concerned, the strong absorptions at 1347 and 1177 cm\({}^{-1}\) are attributed to asymmetric and symmetric S=O stretching vibrations, respectively, the band at 1052 cm\({}^{-1}\) to asymmetric C-F stretching, the band at 739 cm\({}^{-1}\) to asymmetric S-N stretching, and the band at 614 cm\({}^{-1}\) to S=O scissoring. Specific TBuMA cation signals are expected at 1134 cm\({}^{-1}\), ascribable to symmetric C-N stretching, around 1470 cm\({}^{-1}\) due to methyl and methylene C-H bending, and at 1465 and 1378 cm\({}^{-1}\) due to C-H scissoring and methyl rocking, respectively. The spectra of all the samples are basically identical, except for slight differences in the 3250-3000 cm\({}^{-1}\) region (Figure 3), suggesting that the anion is not affected by the thermal treatment, and indicating a major involvement of the cation in the thermal degradation. Inspection of this region highlights that the original structure of the TBuMA cation, characterized by the large band at 3348 cm\({}^{-1}\) and the shoulder at 3040 cm\({}^{-1}\), due to ammonium absorptions, is modified after thermal treatment. The most significant changes are the decrease in the band at 3348 cm\({}^{-1}\) and the increase in the peak in the region between 3200 and 3100 cm\({}^{-1}\), both in the presence and absence of metals. This peak has been assigned to the medium intensity band of unsaturated hydrogen stretches (C=C-H) [22], and suggests the formation of alkenes. Figure 2: Representative ATR-FTIR spectra of B, 4B4, 4B24 and 4B168 samples in the 3400–2650 and 1740\(-\)600 cm\({}^{-1}\) regions. Figure 4 shows the trend of the area of the band at 3153 cm\({}^{-1}\) (3208-3103 cm\({}^{-1}\) baseline points) of the IL spectra with or without metal plates, as a function of the duration of the thermal treatment. Figure 4: Area of the peak at 3153 cm\({}^{-1}\) in the IL spectra after 4, 24 and 168 h of thermal treatment with or without metal plates as a function of incubation time. Figure 3: ATR-FTIR spectra of all samples in the 3400–2650 cm\({}^{-1}\) region. Further insight on the degradation process was gained from HRMAS NMR spectroscopy. The comparison of the \({}^{1}\)H NMR spectrum of the original compound [TBuMA][NTF\({}_{2}\)] with the spectra recorded on [TBuMA][NTF\({}_{2}\)] after heating for 4, 24, and 168 h (samples 4B4, 4B24, and 4B168) reveals that the signals of the original cation remain dominant, even at the longest heating time. Low-intensity peaks, due to the degradation products, appear in the samples subjected to thermal treatment, and their intensity tends to increase with the heating time. Figure 5 shows the NMR spectra of samples B (traces a and b) and 4B168 (traces c and d). Four new signals, resonating at 9.16, 8.31, 2.75 and 2.57 ppm, appear in the latter. Additional signals of lower intensity appear in the region between 5 and 6 ppm upon heating. Complete characterization of the degradation compounds was accomplished by the analysis of \({}^{1}\)H, 1D selective \({}^{1}\)H TOCSY, \({}^{13}\)C and 2D \({}^{1}\)H-\({}^{13}\)C HSQC spectra of 4B168 (Figures S1 and S2). The assignments of \({}^{1}\)H and \({}^{13}\)C NMR signals are reported in Tables S1 and S2.