  Straight-chain (normal-propyl cyanide, n - C3H7CN) and branched-chain
(iso-propyl cyanide, i - C3H7CN) alkyl cyanides are recently identified in the
massive star-forming regions (Sgr B2(N) and Orion). These branched-chain
molecules indicate that the key amino acids (side-chain structures) may also be
present in a similar region. The process by which this branching could
propagate towards the higher-order (butyl cyanide, C4H9CN) is an active field
of research. Since the grain catalysis process could have formed a major
portion of these species, considering a realistic set of binding energies are
indeed essential. We employ quantum chemical calculations to estimate the
binding energy of these species considering water as a substrate because water
is the principal constituent of this interstellar ice. We find significantly
lower binding energy values for these species than were previously used. It is
noticed that the use of realistic binding energy values can significantly
change the abundance of these species. The branching is more favorable for the
higher-order alkyl cyanides with the new binding energies. With the inclusion
of our new binding energy values and one essential destruction reaction (i -
C3H7CN + H -> CH3C(CH3)CN + H2, having an activation barrier of 947 K),
abundances of t - C4H9CN dramatically increased.
