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tokens: region. This demonstrates that additional chemical changes occur at temperatures higher than 120 K during the warm-up, when molecules and radicals become more mobile and could react each other more readily. Both Sample #2 and Sample #3 contain hydrogen, carbon, nitrogen, and oxygen. However, their corresponding organic residues (Res #2 and Res #3) do not display the same functional groups that give rise to the 2200 cm −1 feature. This is evident looking at the infrared spectra of the two samples in the region 2300–2000 cm −1 : both show two multi-peaked bands, but with profiles and components different one another. Thus Res #2 exhibits a 2200 cm −1 feature comparable to those observed for Res #1, Res #4, and Res #5 (see the bottom panel of Fig. 5 ), whereas Res #3 displays a quite sharp feature centred at 2158 cm −1 . We think that the presence of water as component of Sample #3 can affect the chemical rearrangements during proton bombardment at 17 K and especially during the annealing to room temperature, preventing or reducing the formation of molecules containing nitrile and iso-nitrile groups and encouraging the formation of molecules containing amide groups, as already stressed by commenting the XPS spectra in the previous section. Looking at the IR spectrum of Res #3, we are not sure about the chemical groups responsible of the band centred at 2158 cm −1 , even if we hypothesize a contribution of OCN − still trapped in the organic residue (Res #3) at room temperature. OCN − ion is one of the new species formed at 17 K after energetic processing of a frozen icy mixture containing oxygen, carbon and nitrogen bearing species (e.g. van Broekhuizen et al. 2005 ). Indeed it has been detected both in Sample #2 and in Sample #3 after proton bombardment (see the top panel of Fig. 5 ). During the annealing of Sample #3, we have recorded the OCN − peak position as a function of the sample temperature: we recognize a gradual shift towards lower wavenumbers as the sample temperature increases, from 2167 cm −1 at 17 K–2160 cm −1 at 260 K. Therefore the frequency of the peak observed for the refractory residue at room temperature (2158 cm −1 ) could be compatible with the trend of the recorded peak position of the OCN − ion into the ice, as shown in Fig. 6 . In this section, we are mainly focusing on the spectral region 2300 – 2000 cm −1 due to its link with astrobiological studies. In fact the triple CN bond stretching mode of nitriles considered relevant to pre-biotic chemistry falls in this region. The role of nitriles in formation of various pre-biotic compounds has been illustrated by Matthews Minard (2006) . Our experimental results demonstrate that the presence of oxygen in the icy samples is not necessary for the formation, at room temperature, of the multi-component band at 2200 cm −1 that we identify with nitrile (–C≡N) and isonitrile (–N≡C) functional groups. The refractory residues containing nitrogen, such as those analysed in this