September 2011

Butadiynyl radical

The discovery of the C4H (butadiynyl) radical toward IRC +10216 was reported in 1978 by Guélin et al., based upon 4 observed doublet rotational transitions that were predicted with molecular structure calculations. The species was subsequently detected in the Taurus Molecular Cloud TMC-1 by Irvine et al. in 1981 and by Bell et al. in 1982. Both of these papers identified new spectroscopic features of C4H. The rotation spectrum was measured in the lab by Gottlieb et al. in 1983, confirming the earlier theoretical predictions. The spectrum of C4D was measured by Woodward et al. and then identified in TMC-1 in 1989 by Turner. The C4H- anion was identified in 2007.

The butadiynyl radical can be formed by removing one of the H atoms from diacetylene (butadiyne). This suggests a ground state (2Σ+) where the singly-occupied orbital is parallel to the molecular axis. While this is indeed the most stable state, there is a 2Π state that lies only about 200 cm-1 higher in energy (as measured by Zhou et al.). The singly occupied orbitals of each state are shown below.
2Σ+ 2Π


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