  The astronomical observation of isopropyl cyanide further stresses the link
between the chemical composition of the ISM and molecular composition of the
meteorites in which there is a dominance of branched chain amino acids as
compared to the straight. However, observations of more branched chain
molecules in ISM will firmly establish this link. In the light of this, we have
considered C5H9N isomeric group in which the next higher member of the alkyl
cyanide and other branched chain isomers belong. High-level quantum chemical
calculations have been employed in estimating accurate energies of these
isomers. From the results, the only isomer of the group that has been
astronomically searched, n-butyl cyanide is not the most stable isomer and
therefore, which might explain why its search could only yield upper limits of
its column density without a successful detection. Rather, the two most stable
isomers of the group are the branched chain isomers, tert-butylnitrile and
isobutyl cyanide. Based on the rotational constants of these isomers, it is
found that the expected intensity of tert-butylnitrile is the maximum among
this isomeric group. Thus, this is proposed as the most probable candidate for
astronomical observation. A simple LTE (Local thermodynamic equilibrium)
modelling has also been carried out to check the possibility of detecting
tert-butyl cyanide in the millimetre-wave region.
