Kinetically stable high-energy isomers of C14H12 and C12H10N2 derived from cis-stilbene and cis-azobenzene
Jon Baker
Krzysztof Wolinski
0
) Department of Chemistry, Maria Curie-Sklodowska University
, pl. Maria Curie-Sklodowska 3, 20-031 Lublin,
Poland
Following on from our recent enforced geometry optimization (EGO) investigation of isomerization in cisstilbene (J Comput Chem, in press) we report the discovery of two interesting new, symmetrical fused sandwich isomers of both cis-stilbene and the related cis-azobenzene. The isomers were obtained by applying external forces to pairs of carbon atoms from each of the benzene rings in cis-stilbene and cis-azobenzene simultaneously, and are all at least 100 kcal mol-1 higher in energy than the starting material. Each new structure was characterized as a minimum by vibrational analysis. Despite their high energy, all of the new isomers appear to be kinetically stable with respect to rearrangement back to cis-stilbene or cis-azobenzene, respectively.
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In previous publications we reported the discovery of
several potentially kinetically stable new isomers of
C14H12 and C12H10N2 derived from cis-stilbene (1) [1]
and cis-azobenzene (2) [2] (see Fig. 1), respectively. They
were obtained using enforced geometry optimization
(EGO), in which pairs of carbon atoms, one from each of
the two phenyl rings, were pushed together along the line
joining the two atomic centers, by means of an applied
external force [3]. This force is added to the normal
gradient vector computed at that geometry at each cycle
of a geometry optimization and typically results in bond
formation between the atoms involved. All atoms in the
system adjust their relative positions so as to counter the
external force and the final result of the optimization is
usually a new structure, albeit one that is highly strained.
Whether the new structure is stable, i.e., represents a local
minimum on the potential energy surface, can be
determined by carefully reoptimizing with the external force
removed. If the new structure is stable, it will relax but
remain substantially intact; if not it usually reverts back to
the starting structure.
All the kinetically stable isomers found in our earlier
work were obtained by forcing together only a single pair
of carbon atoms and all lay energetically no higher than 90
kcal mol-1 from the starting material for the stilbene
isomers, and under 30 kcal mol-1 in the case of
cisazobenzene. In this communication we report the discovery
of some interesting high-energy symmetric fused
sandwich isomers of both stilbene and azobenzene obtained by
forcing together multiple pairs of carbon atoms (in this case
all six) from each of the two phenyl rings.
Results and discussion
The standard methodology used in this work is density
functional theory (DFT) [4, 5] using the B3LYP hybrid
Fig. 1 Schematic showing structure and atom labeling of cis-stilbene (1) and cis-azobenzene (2)
exchange-correlation functional [6, 7] with the 6-31G*
basis set [8]. All calculations were carried out using the
PQS program package incorporating the PQSMol graphical
user interface for post-job visualization and display [9]. All
(a) C14H12-Cs
(b) C14H12-C2v
stationary points found at B3LYP/6-31G* were reoptimized at
MP2/6-311G** to ensure that the energetics remained stable.
The new isomers, which we call C14H12-Cs, C14H12-C2v,
C12H10N2-Cs and C12H10N2-C2v are shown in Fig. 2. They
were all obtained by pushing together all symmetry
equivalent pairs of carbon atoms from the two phenyl rings
in cis-stilbene and cis-azobenzene, respectively. As denoted
by the names, the new isomers have either Cs or C2v
symmetry. The optimization history for this procedure
starting from cis-stilbene using an applied force of 0.1 au
is depicted in Scheme 1, below.
As can be seen, the energy initially rises to a maximum,
then falls and then starts to oscillate. The calculation did not
in fact converge, but simply stopped after reaching the
maximum allowed number of optimization cycles.
Apparently there is simply too much internal strain in the system
for it to settle down. If the geometry corresponding to the
highest energy oscillation is taken and allowed to relax (i.e.,
is reoptimized after removing the external force) then the
C2v minimum results; if the lower energy structure is
relaxed then the result is the Cs minimum. The situation is
similar for cis-azobenzene.
As shown in Fig. 2 the new isomers are symmetrical
fused sandwich compounds in which the former phenyl
rings lay one on top of the other (or side-by-side depending
on ones point of view). In the C2v isomers, every carbon
atom in each of the phenyl rings forms a bond to its
(c) C12H10N2-Cs
(d) C12H10N2-C2v
Fig. 2 High energy isomers of C14H12 and C12H10N2 obtained via an
enforced geometry optimization by pushing together all symmetry
related pairs of carbon atoms in the phenyl rings of cis-stilbene and
cis-azobenzene, respectively
Scheme 1 EGO optimization history
Fig. 3 Transition states for
decomposition of the new Cs
and C2v isomers of C14H12 and
C12H10N2, (...truncated)