Parity alternation of interstellar molecules cyanopolyynes HCnN (n = 1-17)

In this paper, we report the design of models for interstellar molecules HCnN (n = 1-17) by means of the B3LYP density functional method. We performed geometry optimization and calculation on vibrational frequency. We find that the ground-state (G-S) isomers of HCnN (n = 1-17) are with the N atom located at one end and the H atom at the other end of a Cn chain; they are all linear except for HC2N which is bent. When n is odd, the Cn chain is polyacetylene-like whereas when n is even, the Cn chain displays a structure that is cumulenic-like in the middle of the Cn chain. It is found that the G-S isomers of odd-n HCnN (n = 1-17) are more stable than those of even-n ones. The finding is in accord with the relative intensities of HCnN recorded in laboratory investigations, and in consistent with the results of objects observed in interstellar media. We provide explanations for such a trend of even/odd alternation based on concepts of the highest vibrational frequency, bonding character, electronic configuration, incremental binding energy, nucleus-independent chemical shift, and dissociation channels.