Astron. Astrophys. 333, 1007-1015 (1998)

5. Conclusion

The ab initio quantum chemical and dynamical calculations reported here provide a crucial test of the present astrochemical assumption that reaction (1) produces a significant fraction of product channel (1b), namely H₂ NC + H. This assumption is one route for models to reproduce the observed greater-than-unity HNC/HCN abundance ratio in cold dark clouds, because the metastable H₂ NC isomer subsequently leads to only the metastable neutral isomer HNC via dissociative recombination with electrons. Channel (1a), on the other hand, produces the linear ion HCNH + H, and the linear ion has long been thought to dissociatively recombine to both HNC and HCN with equal probability (Watson 1976), a position strongly reinforced by our recent calculations (Talbi & Ellinger 1998). Contrary to the presently accepted picture, however, we have found that reaction (1) leads mainly to product channel (1a), namely HCNH + H. There are two main reasons for our finding. First, when the reaction proceeds along the lowest potential energy surface, it leads directly to metastable H₂ NC in its lowest electronic state. Although there is a barrier for subsequent isomerization into the lowest electronic state of HCNH , our detailed dynamical calculations show that only 2-3 percent of the ground state H₂ NC product manages to avoid being converted into the linear ion. Secondly, the potential surface leading to the first excited (³ B₂) state of H₂ NC has a significant potential barrier so that low energy C + NH₃ collisions cannot possibly lead to this state as a product. Starting from the reactants C + NH , there is a second pathway to production of H₂ NC in its ³ B₂ electronic state; this involves a potential energy surface of quartet multiplicity. However, like the other pathway, this one also involves a potential energy barrier. Production of the H₂ NC ion in any electronic state thus seems to be unlikely for both the C + NH₃ and C + NH reactants. This theoretical result should be confirmed by laboratory reactivity studies on the ion product of reaction (1). Since laboratory studies are undertaken at higher densities than the interstellar environment, however, the results may have to be interpreted carefully. For example, it is possible, but not likely, that the H₂ NC ion produced in the laboratory can be stabilized via collisions before isomerization into the more stable linear form.

Are there other significant ion-molecule pathways leading to the metastable H₂ NC ion? In work yet to be reported (Talbi 1998), we have shown that another important reaction in interstellar clouds:

also leads to the linear HCNH ion. It would thus appear that an HNC/HCN abundance ratio greater than unity in cold interstellar clouds cannot be explained by ion-molecule formation reactions. It is also unlikely but not impossible that any destruction channels, be they ion-molecule or neutral-neutral, would be slower for the metastable HNC than for the stable HCN. Past research, including our own, has, on the contrary, looked at selective destruction reactions for HNC (processes (4) and (5)) to explain its lower abundance at higher temperatures.

To explain the low temperature HNC/HCN abundance ratio, we are left with the hypothesis of an additional neutral-neutral reaction pathway for HNC which must be more rapid than an analogous pathway for HCN. It would be even nicer if this hypothetical neutral-neutral pathway were only rapid at low temperature. Current reaction networks (Millar et al. 1997; Lee et al. 1996) do include neutral-neutral reactions such as:

although the rate coefficients are highly uncertain for a variety of reasons. If reaction (7) were very rapid at low temperatures, perhaps the low temperature HNC/HCN riddle would be at least partially solved.

But, even if neutral-neutral chemistry can explain the low temperature problem, we must still account for the high temperature problem by either finding additional depletion reactions for HNC or additional formation mechanisms for HCN. Work on the HNC + H reaction (process 4) has shown that this reaction cannot explain the decrease in HNC abundance in warm clouds below 300 K (Talbi et al. 1996). Work on HNC + O (process 5) is in progress and shows, to date, a rather high potential barrier, which would also indicate its inability to selectively deplete HNC at warm temperatures in the 100-300 K range.

In summary, despite many years of theoretical work and detailed interstellar models, it appears that we are still far removed from a complete understanding of the temperature dependence of the HNC/HCN abundance ratio in dense interstellar clouds.

© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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