Millimeter and submillimeter spectroscopy of methylallene, CH$_3$CHCCH$_2$

TL;DR

This work extends the rotational spectroscopy of methylallene to 720 GHz by recording transitions up to high $J$ and $K_a$, and by deriving a comprehensive set of distortion and internal-rotation parameters. Using Gaussian 16 with DFT, the authors seed the analysis with quartic and sextic distortion terms and refine the Hamiltonian through extensive spectral fitting (961 lines, 33 parameters) across 36–501 GHz and into the submillimeter. The resulting parameter set yields accurate predictions for line positions up to 720 GHz, supporting searches for methylallene in cold and warm interstellar environments, even though no unambiguous detection has yet been reported. Overall, the work provides a robust spectroscopic model and a valuable tool for astrochemical investigations of methylallene as a potential building block in interstellar hydrocarbon formation.

Abstract

Small polycyclic aromatic hydrocarbons and somewhat larger cyano derivatives were detected in the cold dark cloud TMC-1 recently. Their formation from smaller hydrocarbons is not well understood, in part because abundances of many species are not known. Methylallene, CH$_3$CHCCH$_2$, may be one of the building blocks, but its rotational spectrum was characterized only to a very limited extent. We recorded rotational transitions in the 36$-$501 GHz region to extend the existing line list of methylallene and thus enable searches for the molecule in space. Quantum-chemical calculations were carried out to evaluate initial spectroscopic parameters. We obtained transition frequencies with $J \le 61$ and $K_a \le 21$ and resolved the internal rotation splitting of the CH$_3$ group at least partially. As a result, a full set of distortion parameters up to sixth order along with two octic ones were determined, as well as parameters describing the internal rotation of the methyl group. The spectroscopic parameters are accurate enough to identify methylallene up to 720 GHz, sufficient for searches even in the warm interstellar medium.

Figures (4)

• Figure 1: Sketch of the methylallene molecule. Carbon atoms are symbolized by gray spheres while hydrogen atoms are indicated by small, light gray spheres. The $a$-axis is in the paper plane and the $b$-axis is rotated slightly toward the viewer.
• Figure 2: Section of the millimeter wave spectrum of methylallene in the region of the $J = 19 - 18$ higher $K_a$$a$-type transitions. The $K_a$ quantum numbers as well as the methyl internal rotor symmetry labels A and E are indicated. The asymmetry splitting of the two transitions in each higher $K_a$ is unresolved in the A components, while the E components occur in different locations. Several unassigned lines, indicated by asterisks, probably originate from excited vibrational states of methylallene.
• Figure 3: Quantum number coverage of the $a$-type transitions in the final fit. Blue squares indicate $R$-branch transitions and green diamonds $Q$-branch transitions. The $K_a = J - K_c$ levels appear as integer values whereas the $K_a = J - K_c +1$ levels appear lowered by 0.5.
• Figure 4: Same as Figure \ref{['a-type-coverage']}, but for $b$-type transitions. Red stars mark $P$-branch transitions.