** 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.