  Titan harbors a dense, organic-rich atmosphere primarily composed of N$_2$
and CH$_4$, with lesser amounts of hydrocarbons and nitrogen-bearing species.
As a result of high sensitivity observations by the Atacama Large
Millimeter/submillimeter Array (ALMA) in Band 6 ($\sim$230-272 GHz), we
obtained the first spectroscopic detection of CH$_3$C$_3$N
(methylcyanoacetylene or cyanopropyne) in Titan's atmosphere through the
observation of seven transitions in the $J = 64\rightarrow63$ and $J =
62\rightarrow61$ rotational bands. The presence of CH$_3$C$_3$N on Titan was
suggested by the Cassini Ion and Neutral Mass Spectrometer detection of its
protonated form: C$_4$H$_3$NH$^+$, but the atmospheric abundance of the
associated (deprotonated) neutral product is not well constrained due to the
lack of appropriate laboratory reaction data. Here, we derive the column
density of CH$_3$C$_3$N to be (3.8-5.7)$\times10^{12}$ cm$^{-2}$ based on
radiative transfer models sensitive to altitudes above 400 km Titan's middle
atmosphere. When compared with laboratory and photochemical model results, the
detection of methylcyanoacetylene provides important constraints for the
determination of the associated production pathways (such as those involving
CN, CCN, and hydrocarbons), and reaction rate coefficients. These results also
further demonstrate the importance of ALMA and (sub)millimeter spectroscopy for
future investigations of Titan's organic inventory and atmospheric chemistry,
as CH$_3$C$_3$N marks the heaviest polar molecule detected spectroscopically in
Titan's atmosphere to date.
