Hydrogen Isocyanide, HNC: A Key Species in the Chemistry of Titan's Ionosphere?

First detection of hydrogen isocyanide (HNC) in Titan’s atmosphere

“…This reaction, which produces HCN and HNC in approximately equal yields, was suggested as the origin of the high abundance of HNC (HNC ∼ HCN) in dense molecular clouds as early as 35 years ago (Watson 1976). Petrie (2001) further considered various loss processes for HNC, including (i) photon-induced isomerization (HNC + hν → HCN); (ii) photolysis (HNC + hν → H + CN); (iii) H-catalyzed isomerization (HNC + H → HCN + H); (iv) protonation with ionospheric cations (XH + + HNC → X + HCNH + ); and (v) reactions with neutral radicals (X + HNC → XCN + H or XH + CN). He dismissed (i) and (ii) as being insignificant compared to protonation and retained the other three loss mechanisms, focussing on process (v) with X = CH 3 .…”

“…The possible presence of HNC in Titan's atmosphere was first considered by Petrie (2001). Based on pre-Cassini photochemical models (e.g.…”

“…Based on pre-Cassini photochemical models (e.g. Ip 1990;Galand et al 1999;Banaszkiewicz et al 2000) which predicted that HCNH + is one important or even the dominant ion at the ionospheric peak, Petrie (2001) postulates the dissociative recombination of HCNH + (HCNH + + e − → HNC + H) as an important source of HNC. This reaction, which produces HCN and HNC in approximately equal yields, was suggested as the origin of the high abundance of HNC (HNC ∼ HCN) in dense molecular clouds as early as 35 years ago (Watson 1976).…”

“…As discussed by Petrie (2001), protonation is likely to involve virtually all hydrogen-bearing ions, so it is reasonable to adopt a total XH + concentration equal to the electron density. In particular, the HCNH + + HNC reaction has been investigated theoretically, and suggested to be a significant loss channel for HNC in molecular clouds (Pichierri 2002).…”

“…This will further increase its chemical lifetime, in relation to the decline of ion-molecule reactions. Petrie & Osamura (2004) find that in the neutral atmosphere, H-catalyzed isomerization (3) may be the main loss channel for HNC, with additional contributions from reactions with CN and C 3 N. Photolysis and photo-isomerization of HNC, albeit negligible at ionospheric levels, must also become significant there, especially the latter, which has a very low energy threshold (8510 Å, see Petrie 2001). Additional formation routes to HNC in the neutral atmosphere should also be investigated, in particular its production through N( 4 S) + 3 CH 2 .…”