Cyanopolyynes as Organic Molecular Wires in the Interstellar Medium

Abstract

Cyanopolyynes (H[C≡C]<sub>n</sub>-CN or HC<sub>2n+1</sub>N, where n = 1, 2, 3, …, n) are commonly observed in the interstellar medium (ISM) as well as in the envelopes of carbon-rich stars. These linear molecular structures can be well described with a one-dimensional conduction model, which considers the scattering processes of electrons through the charge transfer conduction bridge of the H[C≡C]<sub>n</sub>-molecular wire containing the CN group as an electron-acceptor terminal unit. Therefore, our results using this model enable a better understanding of the longest molecules observed in interstellar space and provide new insight into why these particular cyanopolyynes reach a maximum length, such as is observed from astronomical experimental spectral data and cosmological chemical models. Dipole moments and geometrical parameters of these cyanopolyynes were obtained from ab initio molecular orbital calculations using the restricted Hartree-Fock approach and 6-311G* basis set, in order to obtain the inner resistance as a new parameter of chemical reaction feasibility for this molecular series. Using this last molecular parameter, we have been able to analyze the possibility of identifying long molecular species that can be found under local thermodynamic equilibrium in some ISM such us HC<sub>25</sub>H, HC<sub>27</sub>H, and HC<sub>29</sub>N, which have not been observed at present

Expected Radial Column Density of Methylcyanopolyynes in CW Leonis (IRC+10216)

“…Thus, by means of this γ conduction constant, we have determined the molecular resistances as a function of the wire length according to Equation (2). Table 3 depicts the molecular resistance of these methylcyanopolyynes and we have determined a linear resistivity of 74.9 µΩ/cm, similar to cyanopolyynes (72.6 kΩ/cm) and other molecular wires previously reported [19].…”

“…In this Figure, the observed trend is better than r 2 > 0.999 and the final parameters described by Equation 1in Figure 1 are presented in Table 2. Furthermore, from Figure 1 we can observe how this molecular series converge to a maximum dipole moment as function of the polyynic wire in a similar behavior than the cyanopolyynes [19]. Thus, according to Equation (1), the observed limit is determined by (µ o + µ ∝ ), where the dipole moment in this oligomeric series converge to 8.21 ± 0.01 (Debyes).…”

“…On the other hand, the γ molecular wire conduction constant of methylcyanopolyynes exhibits a similar behavior to the cyanopolyynes [19]. Certainly, the methyl group induces a slight perturbation to the molecular wire nevertheless the observed change is lower than 3%.…”

“…In spite of the scarce studies on this subject, our results permit to project the interstellar chemistry of these particular methylcyanopolyynes [9] [19]. Hence, L(n) as the molecular wire lengths and µ n as the dipole moments of these methylcyanopolyynes were calculated using this theoretical approach ( Table 1).…”

“…In the present work, we have extended our cyanopolyynes study [19] to the methylcyanopolyynes, in order to find out the expected radial column density of new species to be present in CW Leonis (IRC +10216). Geometrical and dipole moment parameters calculated from Ab initio molecular orbital calculations, permitted us to determine the molecular resistances, necessary data to correlate the radial column densities in a similar trend previously observed in cyanopolyynes species [19].…”