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tokens: , which were obtained using a density functional theory level of theory (M05/6-31G(d)). An analysis of the observed intensities through a rotational diagram provides a rotational temperature of 9 ± 1 K. We performed a model fitting directly to the observed line profiles as described by Cernicharo et al. (2021a) , with the result that the best match between the computed synthetic spectrum and the observations was obtained for T r = 7 K and N(H 2 CCCHCCH) = (1.2 ± 0.2) × 10 13 cm −2 . Figure 1 shows the synthetic spectrum. Only the transition 9 19 − 8 1, 8 required a correction intensity by a factor of 0.8, while for all the other lines the model matches the observations perfectly. Using the H 2 column density derived by Cernicharo et al. (1987) , the abundance of H 2 CCCHCCH relative to H 2 towards TMC-1 is 1.2 × 10 −9 . This abundance is similar to that of vinyl acetylene ( Cernicharo et al. 2021a ), about ∼3 below that of propylene (CH 3 CHCH 2 ; Marcelino et al. 2007 ), and a factor of ten below that of methyl acetylene (CH 3 CCH; Cabezas et al. 2021 ). Hence, allenyl acetylene is one of the most abundant hydrocarbons in TMC-1, and probably, together with CH 3 C 4 H (see Sect. 3.2 ), the most abundant compound containing five carbon atoms. It is interesting to compare the abundance of allenyl acetylene with that of cyano allene (H 2 CCCHCN). This species has been recently analysed by Marcelino et al. (2021) , resulting in a rotational temperature of 5.5 ± 0.3 K and a column density of (2.7 ± 0.2) × 10 12 cm −2 . Consequently, the abundance ratio of H 2 CCCHCCH over H 2 CCCHCN is ∼4.5, that is to say the acetylenic derivative of allene is slightly more abundant than the cyanide one. A similar value (∼1.8) was obtained for the abundance ratio between CH 2 CHCCH and CH 2 CHCN ( Cernicharo et al. 2021a ). The measured frequencies of the lines observed in TMC-1 can be used to improve the rotational and distortion constants of H 2 CCCHCCH. We used the fitting code FITWAT described in Cernicharo et al. (2018) . Table 1 provides the results obtained by fitting the laboratory data of McCarthy et al. (2020) alone, and those obtained from a fit to the merged laboratory plus the TMC-1 frequencies. A significant improvement in the uncertainty in the rotational and distortion constants was obtained. The fit to the laboratory data alone results in exactly the same constants as those obtained by McCarthy et al. (2020) . The merged fit is recommended to predict the frequency of the rotational transitions of this species above 50 GHz. Predictions could be accurate enough up to 150 GHz to allow for a search of this species in the 3 mm domain. Table B.1 provides the observed and calculated frequencies and their differences. Table 1. Rotational and distortion constants of H 2 CCCHCCH. Notes. (a) Laboratory frequencies from McCarthy et al. (2020) . 3.2. Methyl diacetylene, CH<sub>3</sub>C<sub>4</sub>H This C 5 H 4 isomer was found in TMC-1 by MacLeod et al.