Results and Discussion
NHCs are highly nucleophilic, they can donate electron density to main group elements like carbon, nitrogen etc. and examples are carbones and nitreones [43-52]. In the series of oximes studied in this work, the oxime center is electron deficient because the C=N-OH center carries a proton. Hence, the chances are very high to consider the electron donation from NHC to the oxime center. This distribution of electron density would be reflected in the interaction between the NHC unit and the oxime unit in these compounds. Consequently, the C1-C2 bond lengths, their bond dissociation characteristics are expected to be significantly different from the standard values. The C1-C2 double bond character is supposed to become much weaker and hence the rotational barriers are expected to be low. These factors are reflected in the quantum chemically estimated bond characteristics in the cationic oximes (O-1 to O-15 , Figure 3). Similarly, in the corresponding neutral species (O′-1 to O′-15, Figure 4), the C1-C2 bond characteristics are expected to be significantly influenced. Traditionally, O-14 was considered as a system carrying positively charged ring and neutral oxime center, as inI (Figure 2). However, the positive charge does not get localized at the site of alkylation; hence, it is more appropriate to consider O-14 as the right representation of I . In fact, I and I′ may be in resonance equilibrium and it is better to write I with overall positive charge as inO-14 as the right representation, rather than localized positive charge. The same was emphasized by Van Havere et al . who reported the crystal structure of pralidoxime [32]. This argument is applicable to all structures in Figure 3.
Fig. 3 The cationic oxime species explored in this work.O-5 , O-6 , O-13 to O-15 were experimentally explored earlier. All other species are the designed analogs.
Fig. 4 Nitroso N -heterocyclic olefins O′-1 toO′-15 are designed species and none are known experimentally.O′-14 was earlier studied using quantum chemical methods.
O-1 carries a 3-membered cyclic system and this compound has been designed based on the carbocyclic carbene (CCC) which was isolated in the past [57]. O-2 , O-3 and O-4 have been designed inspired from the carbodicyclopropenylidene system [58], the experimentally known 4π electron 4-memberedN -heterocyclic carbene [59] and the reported 6π electron 4–membered cyclic carbene [58], respectively. O-5 toO-8 carry well-known NHC systems based on imidazoles and thiazoles [60]. O-9 contains CAAC type NHC [61].O-10 and O-11 carry the mesoionic carbene systems [4]. O-12 is similar to O-5 except for the fact that the oxime center carries aromatic ring substitution. O-13and O-14 carry the N-alkyl pyridine ring systems, which were experimentally explored [25]. O-15 carries a quaternary N-center and considering this as a carbene using this kind of substituent is not an easy proposition. This is a known compound, which was tested for its therapeutic applications [30].
Table 1 shows the geometric, energetic and electronic characteristic features of the oximes (O-1 to O-15 ). The C1-C2 bond length values in O-1 to O-15 are in the range of 1.42 Å to 1.46 Å. These values are much smaller than the standard C-C single bond length (1.54 Å) and much longer than the standard C=C double bond length (1.34 Å). All these molecules adopt co-planarity of the fragments across C1-C2 bond (except O-12 ). This establishes that there is some additional interaction between C1 and C2 in these molecules in comparison to the single bond but not sufficiently strong to consider it as a double bond. The Wiberg bond index is only marginally larger than one. The C1-C2 rotational barrier values in O-1 toO-15 are in the range of 2.5-11 kcal/mol, (much smaller than that in alkenes) implying that the C-C interaction does not carry double bond character. The bond dissociation energy (BDE) values ofO-1 to O-15 are in the range of 95-154 kcal/mol (slightly high than C-C single bond dissociation in ethane that is ~73 kcal/mol and also smaller than the C=C BDE in ethylene 183 kcal/mol using the same basis set). These values are facilitated by the fact that upon the heterolytic dissociation, the two fragments can be shown to be independent species that is NHC and the protonated fulminic acid. O-12 is a ketoxime, its bond dissociation energy (BDE) value is very small because it carries non-coplanar arrangement between NHC and an oxime unit. The weaker is the C1-C2 BDE, the stronger is the opportunity to consider L→C interaction in these species. It is intriguing to note that inO-7 and O-8, the BDE values are small and quite comparable (95.50 and 95.90 kcal/mol, respectively). This implies that the carbenes containing sulfur in a heterocyclic ring carry stronger L→C character. All the data presented in Table 1 support the hypothesis that the NHC character for the heterocyclic ring in O-1 toO-15 is significantly high. The NHC being nucleophilic, it is donating electron density towards the electron deficient protonated fulminic acid component of this series of compounds. Hence, the L→C interaction is strongly supportable in the cationic oximes.
Table 1 Geometric and energy parameters of 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.