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).