The potential energy surfaces of acetyl cyanide and its
functional isomer acetyl isocyanide in their electronic
ground state have been investigated using ab initio molecular orbital
MP2(FU)/6-311G** and QCISD(T)/6-311G** calculations. The molecular eliminations of
CH3COX (X = CN,NC) to CH3X + CO,
HX + CH2CO,
and CH2 + HCOX, the unimolecular rearrangements to
CH3C−OX and CH3O−CX carbenes,
1,3-hydrogen
migration to CH2C(OH)X, 1,3-methyl migration to
CH3NCCO, and radical decompositions to
X• + CH3CO
and CH3
• + COX have been examined.
Also the secondary decomposition processes of COCN and
CONC
radicals in their ground states and the secondary elimination of HX
from CH2C(OH)X have been investigated.
Cyanide−isocyanide isomerization is found to be the predominant
channel in the unimolecular reactions of
acetyl cyanide. Despite being exothermic, the decomposition of
acetyl cyanide into CH3CN + CO is found
to be kinetically unfavorable. Decomposition into HCN +
CH2CO, the reverse reaction of
CH3COCN synthesis,
is found to be a preferred dissociation pathway and is expected to
compete at high temperatures with the
1,3-hydrogen migration, yielding cyanovinyl alcohol. The barriers
for the rearrangement processes to carbenes
in acetyl isocyanide are quite high in contrast to those of
acetaldehyde and fluoroformaldehyde. The large
magnitude of the reverse barrier from the carbenes to acetyl cyanide
suggests the stability of the carbenes.
In the ground state of CH3COX, the
CH3−C bond cleavage is found to be more facile compared
to the
C(O)−X bond cleavage. The secondary dissociation from CONC
requires a rather small barrier, and both
cyanide and isocyanide forms are found to have very similar potential
energy surface.