March 2017


The detection of the linear molecule HC4N toward IRC +10216 was reported in 2004 by Cernicharo et al., another of the numerous detections made with the IRAM 30m telescope. A dozen rotatational transitions were observed for linear HC4N; parallel searches for two other isomers with branched and cyclic structures were unsucessful. The initial detection of linear HC4N has not been confirmed to date.

Researcher Links
J Cernicharo
M Guélin
JR Pardo

HCnN species with even n are triplet radicals. The first member of the series, HC2N, was detected in 1991. It is bent rather than linear. No compounds with n>4 have been identified to date in the ISM. HC4N has singly occupied orbitals in both the πx and πy systems, as shown below:

x2 x2 x1

The 2πx2 orbital shows that there's pi bonding between C1 and C2, but there is also a small amount of pi bonding between C2 and C3. The bonding in HC4N can thus be viewed as a resonance between the two configurations shown to the left. The top configration dominates, however. The singly occupied 3π orbitals are consistent with this: the largest amplitude is on C3, but there's some amplitude on C1 as well.

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