Abstract. IR spectroscopy provides a valuable tool for the characterisation and identification of interstellar molecular species.Here, we present 6-9 µm spectra of a sample of reflection nebulae, HII regions, YSOs, evolved stars and galaxies that show strong unidentified infrared bands, obtained with the SWS spectrograph on board ISO. The IR emission features in this wavelength region show pronounced variations. 1) The 6.2 µm feature shifts from 6.22 to 6.3 µm and clearly shows profile variations.2) The 7.7 µm complex is comprised of at least two subpeaks peaking at 7.6 and one longwards of 7.7 µm. In some cases the main peak can apparently shift up to 8 µm. Two sources do not exhibit a 7.7 µm complex but instead show a broad emission feature at 8.22 µm.3) The 8.6 µm feature has a symmetric profile in all sources and some sources exhibit this band at slightly longer wavelengths. For the 6.2, 7.7 and 8.6 µm features, the sources have been classified independently based on their profile and peak position. The classes derived for these features are directly linked with each other. Sources with a 6.2 µm feature peaking at ∼6.22 µm exhibit a 7.7 µm complex dominated by the 7.6 µm component. In contrast, sources with a 6.2 µm profile peaking longwards of 6.24 µm show a 7.7 µm complex with a dominant peak longwards of 7.7 µm and a 8.6 µm feature shifted toward the red. Furthermore, the observed 6-9 µm spectrum depends on the type of object. All ISM-like sources and a few PNe and Post-AGB stars belong to the first group while isolated Herbig AeBe stars, a few Post-AGB stars and most PNe belong to the second group. We summarise existing laboratory data and theoretical quantum chemical calculations of the modes emitting in this wavelength region of PAH molecules. We discuss the variations in peak position and profile in view of the exact nature of the carrier. We attribute the observed 6.2 µm profile and peak position to the combined effect of a PAH family and anharmonicity with pure PAHs representing the 6.3 µm component and substituted/complexed PAHs representing the 6.2 µm component. The 7.6 µm component is well reproduced by both pure and substituted/complexed PAHs but the 7.8 µm component remains an enigma. In addition, the exact identification of the 8.22 µm feature remains unknown. The observed variations in the characteristics of the IR emission bands are linked to the local physical conditions. Possible formation and evolution processes that may influence the interstellar PAH class are highlighted.

Figure 18 presents the C-O stretching vibrational frequencies of the first-row transition-metal monocarbonyl cations, neutrals, and anions in solid neon; similar diagrams have been reported for neutral MCO species in solid argon, but three of the early assignments have been changed by recent work and one new assignment added. The laser-ablation method produces mostly neutral atoms with a few percent cations and electrons for capture to make anions; in contrast, thermal evaporation gives only neutral species. Hence, the very recent neon matrix investigations in our laboratory provide carbonyl cations and anions for comparison to neutrals on a level playing field. Several trends are very interesting. First, for all metals, the C-O stretching frequencies follow the order cations > neutrals > anions with large diagnostic 100-200 cm-1 separations, which is consistent with the magnitude of the metal d to CO pi * donation. Second, for a given charge, there is a general increase in C-O stretching vibrational frequencies with increasing metal atomic number, which demonstrates the expected decrease in the metal to CO pi * donation with increasing metal ionization potential. Some of the structure in this plot arises from the extra stability of the filled and half-filled d shell and from the electron pairing that occurs at the middle of the TM row; the plot resembles the "double-humped" graph found for the variation in properties across a row of transition metals. For the anions, the variation with metal atom is the smallest since all of the metals can easily donate charge to the CO ligand. Third, for the early transition-metal Ti, V, and Cr families, the C-O stretching frequencies decrease when going down the family, but the reverse relationship is observed for the late transition-metal Fe, Co, and Ni families. In most of the present discussion, we have referred to neon matrix frequencies; however, the argon matrix frequencies are complementary, and useful information can be obtained from comparison of the two matrix hosts. In most cases, the neon-to-argon red shift for neutral carbonyls is from 11 to 26 cm-1, but a few (CrCO) lie outside of this range. In the case of FeCO and Fe(CO)2, it appears that neon and argon trap different low-lying electronic states. In general, the carbonyl neutrals and anions have similar shifts but carbonyl cations have larger matrix shifts. For example, the FeCO+ fundamental is at 2123.0 cm-1 in neon and 2081.5 cm-1 in argon, a 42.5 cm-1 shift, which is larger than those found for FeCO- (11.7 cm-1) and FeCO (11.7 cm-1). It is unusual for different low-lying electronic states to be trapped in different matrices, but CUO provides another example. The linear singlet state (1047.3, 872.2 cm-1) is trapped in solid neon, and a calculated 1.2 kcal/mol higher triplet state is trapped in solid argon (852.5, 804.3 cm-1) and stabilized by a specific interaction with argon. The bonding trends are well described by theoretical calculations of vibrational frequencies. Table 5 compares the scale factors (obse...

Reaction of laser-ablated Fe atoms with oxygen molecules in a condensing argon stream produced FeO, FeO2, FeO3, FeO4, Fe2O, Fe2O2, and Fe2O4 molecules, which are identified from oxygen isotopic shifts and multiplets in matrix infrared spectra. The Fe + O2 reaction gives symmetrical bent, symmetrical cyclic, and asymmetrical bent FeO2 isomeric products with triplet, triplet, and quartet isotopic absorptions, respectively, using statistically mixed 16,18O2 as the reagent. The major reaction product symmetrical bent OFeO iron dioxide molecule (150 ± 10°) is characterized by stretching fundamentals at 945.8 and 797.1 cm-1, and the asymmetric bent FeOO form exhibits a 1204.5 cm-1 absorption. The cyclic isomer Fe(O2) produced spontaneously during annealing in solid argon absorbs at 956.0 cm-1. Oxygen and iron isotopic absorptions show that FeOFe is a symmetrical bent (140 ± 10°) molecule. Rhombic Fe2O2 absorbs at 517.4 cm-1. Evidence is presented for isomers of FeO3, FeO4, and Fe2O4. Density functional theory was used to calculate energies, structures, and frequencies for product molecules to support their identification.

This paper presents the results of an investigation of the molecular characteristics that underlie the observed peak position and profile of the nominal 6.2 m interstellar emission band generally attributed to the CC stretching vibrations of polycyclic aromatic hydrocarbons (PAHs). It begins with a summary of recent experimental and theoretical studies of the spectroscopic properties of large (>30 carbon atoms) PAH cations as they relate to this aspect of the astrophysical problem. It then continues with an examination of the spectroscopic properties of a number of PAH variants within the context of the interstellar 6.2 m emission, beginning with a class of compounds known as polycyclic aromatic nitrogen heterocycles (PANHs; PAHs with one or more nitrogen atoms substituted into their carbon skeleton). In this regard, we summarize the results of recent relevant experimental studies involving a limited set of small PANHs and their cations and then report the results of a comprehensive computational study that extends that work to larger PANH cations including many nitrogen-substituted variants of coronene + (C 24 H þ 12 ), ovalene + (C 32 H þ 14 ), circumcoronene + (C 54 H þ 18 ), and circum-circumcoronene + (C 96 H þ 24 ). Finally, we report the results of more focused computational studies of selected representatives from a number of other classes of PAH variants that share one or more of the key attributes of the PANH species studied. These alternative classes of PAH variants include (1) oxygen-and silicon-substituted PAH cations; (2) PAH-metal ion complexes (metallocenes) involving the cosmically abundant elements magnesium and iron; and (3) large, asymmetric PAH cations.Overall, the studies reported here demonstrate that increasing PAH size alone is insufficient to account for the position of the shortest wavelength interstellar 6.2 m emission bands, as had been suggested by earlier studies. On the other hand, this work reveals that substitution of one or more nitrogen atoms within the interior of the carbon skeleton of a PAH cation induces a significant blueshift in the position of the dominant CC stretching feature of these compounds that is sufficient to account for the position of the interstellar bands. Subsequent studies of the effects of substitution by other heteroatoms (O and Si), metal ion complexation ( Fe + , Mg + , and Mg 2+ ), and molecular symmetry variation-all of which fail to reproduce the blueshift observed in the PANH cations-indicate that N appears to be unique in its ability to accommodate the position of the interstellar 6.2 m bands while simultaneously satisfying the other constraints of the astrophysical problem. This result implies that the peak position of the interstellar feature near 6.2 m traces the degree of nitrogen substitution in the population, that most of the PAHs responsible for the interstellar IR emission features incorporate nitrogen within their aromatic networks, and that a lower limit of 1%-2% of the cosmic nitrogen is sequestered within the interstellar PAH populati...

A new analysis of the intra-unit charge polarization and inter-unit donation for the interaction of ligands with metals is presented. The analysis is based on the calculation of SCF wave functions with constraints imposed on the variational space used for the molecular orbitals of metal–ligand clusters. With this approach, various kinds of ligand and metal charge rearrangement can be studied separately and their energetic contribution to the bonding can be determined. For the first time, this contribution for metal to ligand and ligand to metal donation is established. The constrained space orbital variation CSOV approach also clearly shows the similarities and the differences between the bonding of CO and NH3. The results are obtained for Al4CO and Al4NH3 clusters chosen to simulate the adsorption of the ligands at an on-top site of the Al(111) surface. However, the features found for these model systems have significance for the analysis of ligand–metal bonding in general. The analysis gives new estimates for the importance of charge donations which are different from those commonly believed. It also shows new important features of the interaction. One unexpected feature of the metal–NH3 interaction is that the electrostatic attraction of the effective dipole moments of the metal and ligand units makes an important contribution to the bond. Inter-unit donation of charge does not seem to be important for this system.