** The rotational barrier could not be estimated owing to the fact that
the fragments across C1-C2 bond are already orthogonal to each other.
The most representative of all the cationic oximes considered in this
study is O-5 . It shows BDE of the order of 106.72 kcal/mol. The
C1-C2 rotational barrier is 4.64 kcal/mol. The discussion on all other
species in Table 1 can be done in comparison to that of O-5 .
The cationic oximes containing 3-membered carbenes (O-1 )
exhibits relatively larger BDE, however, O-2 exhibit comparable
BDE and also both O-1 & O-2 exhibit larger C1-C2
rotational barrier in comparison to that of O-5 . On the other
hand, the cationic oximes with 4-membered carbenes (O-3 andO-4 ) exhibit relatively comparable BDE and relatively large
C1-C2 rotational barriers. The cationic oximes with 5-membered carbene
are all quite comparable in terms of rotational barrier as well as BDE.
The system O-10 containing a mesoionic carbene is exhibiting
slightly different characteristic features in comparison to other
5-membered ring containing systems. It exhibits relatively large BDE
value and very small C1-C2 rotational barrier. It is important to note
that O-13 shows maximum C1-C2 rotational barrier and the
greater bond dissociation energy among all the species considered in
Figure 3. Also, the rotational barrier and bond dissociation energy
values are significantly different from that of O-14 . This can
be attributed to the fact that carbene like character is weak in N-4
alkyl pyridyl-2-ylidene in comparison to the same in N-2 alkyl
pyridyl-2-ylidene [35]. The molecule O-12 is slightly
different from all others as it is a ketoxime (an aryl derivative ofO-5 at C2 position). This molecule adopts a torsional angle of
N-C1-C2-N up to 47º. Because of this reason, it is not possible to
estimate the rotational barrier. Also the bond dissociation energy value
has been estimated to be very small in comparison to other species in
Figure 3.
Table 2 lists the important geometric, electronic and energy parameters
of the NNHOs O′-1 to O′-15 . The C1-C2 bond length
values in O′-1 to O′-15 are in the range of 1.35 Å to
1.44 Å. The rotational barrier values around C1 and C2 in O′-1to O′-15 are in the range of 8-35 kcal/mol. The Bond
dissociation energy (BDE) values of O′-1 to O′-15 are
in the range of 20-50 kcal/mol (very low), thus making these species
highly unstable.
Table 2 Geometric and energy parameters designed compoundsO′-1 to O′-15 obtained at M06/6-311++G(d,p) level of
theory. The bond lengths are in Å units and the energy values are in
kcal/mol.