The linear carbon chain butadiynylformyl radical (HC₅O) was detected by McGuire and co-workers toward TMC-1 using the Green Bank Telescope, as reported in 2017. While the detection is based on a single rotational transition, it includes the four lines that arise from the hyperfine splitting associated with state of the molecule. As a ²Π state, the ground state of HC₅O splits into two levels, which is known as fine structure. The spectroscopic constants measured by Mohammed and co-workers were used for the detection of HC₅O. They were able to measure the constants for the ²Π_1/2 state without also generating the spectrum of the ²Π_3/2 state due to the very cold beam of molecules they were able to form. The detection of HC₅O toward TMC-1 was confirmed in 2021 by Cernicharo et al., who observed a total of 14 rotational transitions.