3 The IR spectrum

As a typical example, Fig. 2 (adapted from Robin & Rouxhet 1976) displays the IR absorbance spectrum of a young kerogen of type II (H/C=1.32, O/C=0.104). From Robin et al. (1975), their peak wavenumber (cm^-1) and wavelength ($\mu$m), integrated intensity (cm/mg) and assignment are

1.   $\sim$3400 ($\sim$2.95); 46.2; OH stretch; due partly to chemically bonded OH groups and partly to adsorbed or trapped, not chemically bonded, H₂O molecules; the latter is responsible for the long redward tail, through H-bondings with other parts of the skeleton; peak position and band profile change considerably with H₂O content (depending on evolutionary stage or heat treatment); see Robin & Rouxhet (1976) and Unsworth et al. (1988). This feature is clearly distinct from the water ice band, which peaks near 3.1 $\mu$m and has a much steeper red wing (associated to that observed in the sky towards young stellar objects (Gibb et al. 2000).

2.   3060 (3.27); C-H aromatic or olefinic stretch, barely visible here, and only measurable for very small H/C and O/C ratios.

3.   2920 (3.42); 48.2; blend of anti-symmetric and symmetric CH₃ stretch at 2962 and 2872, asymmetric and symmetric CH2 stretch at 2926 and 2853; CH stretch at
     2890 cm^-1, respectively.

4.   1710 (5.85); 14; C=O (ketone) stretch.

5.   1630 (6.15); 11.4; disputed (but perhaps concurrent) assignments to H₂O deformation, quinonic C=O with H bond and C=C olefinic and aromatic stretch (also see Painter       et al. 1981).

6.   1455 (6.87); 6.1; asymmetric CH₂ and CH₃ deformation.

7.   1375 (7.28); 1.2; symmetric CH₃ deformation.

8.   1800 to 900 (5.5 to 11); 67.3; massif, or plateau (underlying broad band) peaking near 8 $\mu$m, due to C...C and C-O stretch, C-H in-plane bend and OH deformation       (see Durand 1980).

9.   930 to 700 (11 to 14); aromatic out-of-plane bending, depending on the number of adjacent protons; barely visible here, and never exceeds an intensity of 5.

The intensity, K, given here (third number) is related to the absorbance, $\alpha$, by

$$\alpha \ ({\rm cm}^{-1})=2.3 \times 10^{3}\ \rho K/\Delta\nu,$$ (1)

where $\rho$ is the material density in g cm^-3 ($\sim$1) and $\Delta \nu$, the bandwidth in cm^-1. K (cm/mg) is 10^-3 times the intensity usually defined in the astrophysical literature; it is also related to the integrated cross-section per C atom, $\Sigma$, by

$$\Sigma \ ({\rm cm^{2}\,cm^{-1}\,C\,atom}^{-1})=4.7 \times 10^{-20... ...C}+\frac{16 O}{12 C}\right){K} \approx4.7\ {\rm to} \ 7\times 10^{-20}~{\rm K}.$$ (2)

As expected, when referred to the corresponding functional group, $\Sigma$ turns out to be of the same order of magnitude as the corresponding average values, when available, given by Wexler (1968, p. 88-89), based on measurements of a large number of small molecules.

Not unexpectedly, a strong relationship holds between the bands at 2920 and 1455  cm^-1 for all kerogen types and evolutionary stages (Robin & Rouxhet 1976): the ratio of their integrated intensities is $\sim$8. Also, K (3.4 $\mu$m) increases linearly with H/C, from 0 at $\leq$0.3 at least up to 80 cm/mg at H/C=1.3.

Figure 3: Spectral evolution of kerogens from the same basin but different depths, from 700 to 2500 m. Adapted from Espitalie et al. (1973).

Figure 4: Chemical representation of a type II kerogen at the start of diagenesis (adapted from Behar & Vandenbroucke 1986). Following common practice, C-H bonds are not represented. Carbon chain skeletons are shown as broken, undulating, lines. The aromatic clusters of benzenic rings are shaded. Various functional groups and oxygen bridges are labeled.

Figure 5: The C-H stretch band of a) GC/IRS6E, in absorption (Pendleton et al. 1994); b-d) 3 post-AGB stars in emission: IRAS 08341+0852 (Joblin et al. 1996), IRAS 04296+3429 (Geballe et al. 1992) and CRL 2688 (Geballe et al. 1992); e) reflexion nebula NGC 2023 in emission (Sellgren 1984).

Figure 6: The C-H stretch absorption band of coal as a function of evolutionary stage: a) Vouters mine (O/C=0.06, H/C=0.75), b) Mericourt mine (O/C=0.028, H/C=0.59), c) Escarpelle mine (O/C=0.018, H/C=0.46), d) and e) Escarpelle sample annealed at 525 and 600 K, respectively. Adapted from Guillois (1996).