Exploring molecular complexity with ALMA (EMoCA): Simulations of
  branched carbon-chain chemistry in Sgr B2(N)
  Using mm-wavelength data from ALMA, the EMoCA spectral line survey revealed
the presence of both the straight-chain and branched forms of propyl cyanide
(C$_3$H$_7$CN) toward the Galactic Center star-forming source Sgr B2(N2). This
was the first interstellar detection of a branched aliphatic molecule. Through
computational methods, we seek to explain the observed $i$:$n$ ratio for propyl
cyanide, and to predict the abundances of the four different forms of the
homologous nitrile, butyl cyanide (C$_4$H$_9$CN). We also investigate whether
other molecules will show a similar degree of branching, by modeling alkanes up
to pentane (C$_5$H$_{12}$). We use a coupled three-phase chemical kinetics
model to simulate the chemistry of Sgr B2(N2), using an updated chemical
network that includes grain-surface/ice-mantle formation routes for branched
nitriles and alkanes. We use the EMoCA survey data to search for the
straight-chain form of butyl cyanide toward Sgr B2(N2). The observed $i$:$n$
ratio for propyl cyanide is reproduced by the models. Butyl cyanide is
predicted to show similar abundances to propyl cyanide, and to exhibit strong
branching, with the $sec$ form clearly dominant over all others. The addition
of CN to acetylene and ethene is found to be important to the production of
vinyl, ethyl, propyl, and butyl cyanide. We report a non-detection of
$n$-C$_4$H$_9$CN toward Sgr B2(N2), with an abundance at least 1.7 times lower
than that of $n$-C$_3$H$_7$CN. This value is within the range predicted by the
chemical models. The models indicate that the degree of branching rises with
increasing molecular size. The efficiency of CN addition to unsaturated
hydrocarbons boosts the abundances of nitriles in the model, and enhances the
ratio of straight-to-branched molecule production. The predicted abundance of
$s$-C$_4$H$_9$CN makes it a good candidate for future detection toward Sgr
B2(N2).
