# Thermal Stability of Ionic Liquids: Effect of Metals
## 3 Results and Discussion
The dominant thermal degradation products are N-dibutyl-N-methylammonium (1) and N-butyl-N-methylammonium (2), as outlined in Scheme 1; these compounds are compatible with the Hoffman elimination of one or two alkyl chains from the original cation [23]. For these compounds, the signals due to the hydrogen atoms labeled as H1 and H2 in Scheme 1 are clearly observable. Additional signals, characterized by lower  
Figure 5: \({}^{1}\)H HRMAS NMR spectra of [TBuMA][NTF\({}_{2}\)] **(a,b)** and 4B168 (**c,d)**; in **(b,d)**, the vertical scale used in **(a,c)** is expanded by the factors reported on each spectral region. Selected signals of the degradation compounds are labeled as \({}^{\prime}\)H\({}_{\text{ij}}\)”, with i representing the atom number and j indicating the degradation product (Scheme 1). Signals of water protons and residual protons of deuterated DMSO are marked with asterisks. intensities, were assigned to 1-butene (Scheme 1, compound 3), in agreement with the hypothesized degradation pathway. These assignments are indicated in Figure 5, where the signals are labeled as \"Hij\", with i representing the atom number and j the degradation product. The amount of compounds 1 and 2 was determined from the \({}^{1}\)H spectra of all the samples, with respect to the amount of the original cation B, using the integrals of the H2,1 and H2,2 signals. Figure 6 shows the values of these integrals as a function of the heating time, where the intensity of the corresponding signal of the non-degraded ionic liquid, occurring at 2.97 ppm, is arbitrarily set to 100. It was, thus, found that, even at the longest heating time, about six molecules of compound 1 and two molecules of compound 2 were present every 100 molecules of ionic liquid cation. This estimate agrees with that obtained using H1,1 and H1,2 signals. Interestingly, the concentration of compound 2 increased at a slower rate than compound 1, in agreement with the fact that the formation of compound 2 requires the preliminary formation of compound 1. Moreover, the amount of compound 3 was always much lower than that expected on the basis of the stoichiometry of the degradation pathways, as clearly evident from the large scaling factor necessary to visualize the signals of this compound (Figure 5d). This can be explained by the volatility of 1-butene or its further degradation/reactions. The presence of 1-butene is compatible with the alkene absorption band between 3200 and 3100 cm\({}^{-1}\) detected by FTIR (3090 cm\({}^{-1}\) in the FTIR spectrum of butene in the vapor gas phase). Unfortunately, the absence of specific absorption lines prevented the detection of compounds 1 and 2 in the FTIR spectra. The \({}^{1}\)H NMR spectra recorded after heating for up to 168 h in the presence of metal plates do not show significant differences with respect to the 4B168 spectrum (Figure 7). The main signals of the NMR spectrum of 4B168 are those of the original ionic liquid. For both steel and copper, the spectral lines are broadened, with steel inducing larger line broadening of the signals compared to copper. This broadening, not observed in the case of brass, is probably due to the presence of dissolved paramagnetic metal ions resulting from corrosion of the metal/alloy. Figure 8 shows that the signals of the degradation compounds 1, 2 and 3 are also present in the spectra of samples 1B168 and 3B168, whereas, in the case of 2B168, only sharp signals, due to compound 3, are observed. In the latter case, if compounds 1 and 2 are present, their concentration is below detection limits. However, the occurrence of 1-butene signals suggests that the elimination reactions sketched in Scheme 1 also take place in the presence of copper. The absence of the H1,1 and H1,2 signals could be explained by hypothesizing that the ammonium compounds 1 and 2 release H\({}^{+}\) and the amine formed coordinates to a copper ion.