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.