Cumulenic and heterocumulenic anions: potential interstellar species?

Abstract

A recent theoretical investigation by Terzieva & Herbst of linear carbon chains, Cn where n ≥ 6, in the interstellar medium has shown that these species can undergo efficient radiative association to form the corresponding anions. An experimental study by Barckholtz, Snow & Bierbaum of these anions has demonstrated that they do not react efficiently with molecular hydrogen, leading to the possibility of detectable abundances of cumulene‐type anions in dense interstellar and circumstellar environments. Here we present a series of electronic structure calculations which examine possible anionic candidates for detection in these media, namely the anion analogues of the previously identified interstellar cumulenes CnH and Cn−1CH2 and heterocumulenes CnO (where n = 2–10). The extraordinary electron affinities calculated for these molecules suggest that efficient radiative electron attachment could occur, and the large dipole moments of these simple (generally) linear molecules point to the possibility of detection by radio astronomy.

Astronomical detection of C$\mathsf{_{4}H^-}$, the second interstellar anion

“…The strong similarity between the averaged line profiles of both molecules indicates that C 4 H and its anion are spatially coexistent. Adopting for C 4 H − a permanent dipole moment µ = 6.2 D (Botschwina, quoted by Blanksby et al 2001), we derive a rotational temperature T rot (C 4 H − ) = 23 ± 2 K, somewhat lower than that of C 4 H, and a column density N(C 4 H − ) = 7.1 ± 2.0 × 10 11 cm −2 (see Fig. 3), which yields N(C 4 H − )/N(C 4 H) = 1/4200.…”

“…Thus, k ratt is high for large molecules with a high EA. Among those, the polyacetylenic chain families C 2n H, which have exceptionally large electron affinities (EA ≥ 3 eV, Blanksby et al 2001) and whose smallest members had just been observed in IRC +10216 (Guélin et al 1978) and in the dark cloud TMC-1 (Guélin et al 1982), stuck out as the most likely progenitors of large interstellar anions. It is remarkable that with this simple reasoning, Herbst (1981) predicted 26 years ago that C 4 H − and, more generally, C 2n H − (n ≥ 2) may be abundant in interstellar and circumstellar clouds ([C 2 nH − ]/[C 2 nH] = 0.1−0.01).…”

“…The rms noise on this spectrum is 0.7 mK per 1-MHz channel. From this and other spectra we derive a 3-sigma upper limit of [C 8 H − ]/[C 8 H] ≤ 1/5, taking into account the difference in permanent dipole moments (6.5 D for C 8 H versus 10.4 D for C 8 H − , Blanksby et al 2001) and the effect of fine structure splitting in C 8 H. Our limit is not low enough to set significant constraints on the electron radiative attachment rate on C 8 H.…”

“…Dalgarno & McCray 1973;Herbst 1981;Petrie 1996;Millar et al 2000;Blanksby et al 2001). It was pointed out that a high electron affinity and a large number of vibrational states increase greatly the sticking coefficient of electrons, so that large, negatively charged carbon chains of the form C n H − may be abundant.…”

Rotational spectroscopy for astrophysical investigations

“…The presence of negative ions in the interstellar medium was predicted many years ago on general grounds or on the basis of ion-molecule chemical models (see, for example, Dalgarno and McCray 1973;Herbst 1981;Millar et al 2000;Blanksby et al 2001). It was pointed out that a high electron affinity and a large number of vibrational states increase greatly the sticking coefficient of electrons; for instance, large, negatively charged carbon chains of the form C n H -were predicted to be quite abundant.…”