The comet 67P/Churyumov-Gerasimenko investigated by the Rosetta Mission

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Valeric acid (pentanoïc acid)

The spectrum of solid valeric acid deposited at 25 K displays a broad band between 2400 and 3400 cm-1 onto which are superimposed three strong peaks at 2875 cm-1 as CH3 symmetric stretching, 2938 cm-1 as CH2 asymmetric stretching and 2961 cm-1 as CH3 symmetric stretching (Fig. 3.10; Lin-Vien et al., 1991). Structure-less shoulders are observed on both sides of these aliphatic features. By increasing the temperature, sharp peaks appear on this broad band. As reported before, we have here overtones and combination bands that involve the OH bond, as well as the –OH stretching mode above 3000 cm-1. The detailed assignment is presented in Table 3.5.

Fumaric acid (trans-butenedioic acid)

Fumaric acid is a dicarboxylic acid that contains carbon double bonds. This compound is solid at room temperature, accordingly spectra have been collected on a pellet at room temperature. The spectrum shares similarities with the spectra of acids presented before. We observe a broad band between 2500 cm-1 and 3300 cm-1, due to the –OH stretching mode, C-H stretching, possibly overtones of CH bending and combination modes involving the OH end (Fig. 3.11). The intense and sharp peak at 3083 cm-1 is due to the CH stretching in the =C-H group. The C=C bond absorbs at 1676 cm-1 and C=O at 1687 cm-1, on the whole this band is sharp and intense. Table 3.6 presents the comparison of the measurement fumaric acid and reference from Spectral Database for Organic Compounds (SDBS) by AIST Japan and proposition of band assignment based on infrared handbook of organic molecules (Lin-Vien et al., 1991).
Figure 3.11. Spectrum of fumaric acid as pellet at room temperature and its structure.

Glycolic acid (hydroxyacetic acid)

Glycolic acid is the simplest of the hydroxylated carboxylic acid. It has two C atom and one -OH group in its side (alcoholic) chain. We have collected one spectrum on a pellet, which fits very well with the spectrum provided by the NIST database.
With respect to other acids investigated before, an intense band at 3258 cm-1 appears, which is due to the alcoholic group –OH (Lin-Vien et al., 1991). Overall, the broad structure extending from 2500 to 3500 contains many peaks and overall a complex structure, controlled by the fundamental –OH and CHx stretching modes, as well as combination modes for the low wavenumber features.
Figure 3.12. Molecule structure of glycolic acid (NIST).
The highest and sharp peak at 1731 cm-1 and 1255 cm-1 are C=O stretching and C-O stretching. Figure 3.13 shows the comparison spectra of solid glycolic acid to the NIST standard spectrum: the fingerprint region has complicated and massive features due to bending mode and its combination.
Figure 3.13. Comparison of solid glycolic acid spectrum and its NIST reference at room temperature.

Lactic acid

Lactic acid is also a member of the hydroxylated carboxylic acids group. It has two –OH groups in its structure, one being attached to central carbon atom. The broad feature in the range 2500-3550 cm-1 looks broader and less structured than in other acids. The small peak at 2946 cm-1 and 2996 cm-1 are due to CH stretching modes, while the rest of the band is controlled by –OH stretching modes and combination modes. The sharp peaks at 1729 cm-1 and 1126 cm-1 point to the C=O bond and C-O bond vibrations. Figure 3.15 shows the spectrum of lactic acid in pellet, which is similar to liquid lactic acid (SDBS, Japan database). This is question of the crystallinity in the solid phase.
Figure 3.15. Spectrum of lactic acid at room temperature as pellet and liquid (SDBS database).

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Methyl valeric and aromatic hydroxylated carboxylic acids

These acids are hydroxylated and contain aliphatic and aromatic side-chains, with different branching position of the hydroxyl group (Fig. 3.16). 2-Hydroxy-4-methylvaleric acid contains a –OH group on C1 in the aliphatic side chain. Three isomers of the hydroxy-2-naphtoic acid, containing a naphtoic group as the side chain have been investigated to test the effect of the -1.0 position of the –OH group: 2-Hydroxy-4-methyl-naphtoic acid , 3-hydroxy-4- naphtoic acid and 5-Hydroxy-4-methyl- naphtoic acid.
The spectra are displayed on Figure 3.17. 2-Hydroxy-4-methylvaleric shows a strong and narrow band at ~3500 cm-1, which is due to the stretching mode of the hydroxyl branched on the aliphatic group. The –OH bond in the carboxylic group generates a large band between 2400 cm-1 and 3200 cm-1, which includes several peaks and has a complex structure, and is controlled by the combination modes and overtones of the –OH group, the stretching of the carboxylic –OH and the CH stretching bands.
Interestingly, the three isomers of the naphtoic acid show different spectra. For the 2-hydroxy-2-naphtoic acid, a broad band is present from 2300 cm-1 up to 3300 cm-1, mostly due to combination and overtones bands of the carboxylic –OH group. There is no specific signature of the hydroxyl group branched on the naphtoic group. In contrast, 3-hydroxy and 6-hydroxy displays a strong peak at 3285 cm-1 and 3600 cm-1, respectively, due to the –OH stretching mode of the naphtoic hydroxyl group. The width of the band is also very different. Here, the distance between the hydroxyl and carboxylic groups influences the interaction of –OH with C=O, through hydrogen bonds. If the –OH hydroxyl is far from the carboxylic group, the interaction becomes weaker and –OH appears as detached single peak at higher wavenumber.
Table 3.7 shows the comparison of 1-hydroxy and 2-hydroxy naphtoic carboxylic with SDBS Japan database and band assignment from handbook of Lien-Vin et al., 1991.

Table of contents :

1. The comet 67P/Churyumov-Gerasimenko investigated by the Rosetta Mission
1.1 Reservoir and internal structure and composition
1.2 Mission to comets: from 1986 to the Rosetta mission
1.3 The VIRTIS instrument
1.4 The reflected spectra collected by VIRTIS-M
1.5 The preliminary results on comet 67P/CG composition: 2014-2016
2. Materials and methods
2.1 Sample and sample preparation
2.1.1 Sample description
2.1.2 Milling process
2.1.3 Sample dehydration
2.1.4 Opaques and semi-volatiles mixing
A. Mechanical mixing
B. Sublimating ice-dust mixtures
2.2 Reflectance measurements
2.2.1 Spectro-gonio radiometer
2.2.2 Enviromental cells
2.3 Transmittance measurements
2.3.1 Pellet preparation
2.3.2 Thin film preparation by spin coating
2.3.3 Thin film preparation in cryogenic cell
2.3.4 Thickness estimation of thin films
2.3.5 Microscopic measurements
3. Transmission spectra of pure carboxylic acids and ammonium salts
3.1 Transmission measurements
3.1.1 Formic acid
3.1.2 Acetic acid
3.1.3 Propionic acid
3.1.4 Butyric acid
3.1.5 Valeric acid (pentanoïc acid)
3.1.6 Fumaric acid (trans-butenedioic acid)
3.1.7 Glycolic acid (hydroxyacetic acid)
3.1.8 Lactic acid
3.1.9 Methyl valeric and aromatic hydroxylated carboxylic acids
3.1.10 Ammonium salts
3.2 Carboxylic acids and ammonium salts in comet 67P/CG
3.2.1 Carboxylic acids
3.2.2 Ammonium salts
4. Reflection measurements of cometary analogs
4.1 Mechanical mixture
4.1.1 Mixture of carboxylic acids and graphite
4.1.2 Mixture of carboxylic acids and basalt
4.1.3 Mixture of ammonium salts and olivine
4.1.4 Mixture of ammonium salts and dark refractory
4.2 Sublimation residues produced in the laboratory
4.2.1 Sublimation residue of pyrrhotite-fg-pl -ammonium salts
4.2.2 Effects of homogenous sub-micrometric pyrrhotite in sublimation residue
4.2.3 Effects of brighter refractory in sublimation residue
4.2.4 Sublimation residue with different refractories
4.2.5 The band depth of sublimation residues
4.3 Implication for the analysis of VIRTIS reflectance spectra of comet 67P/CG

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