approximated from DFT(B2PLYP)/aug-cc-pVTZ calculations, are 0.60e, 0.51e for
syn conformer and 0.51e, 0.40e for anti conformer. In both cases the basin
describing the less electronegative atom yields a larger value of
N. This can be
related to the annihilation of the V(O1) attractor at the first stage of the dissociation
of the N–O1 bond (as explained above). Furthermore, larger values of
N obtained
for the V(O1) and V(N) basins for the syn conformer correspond to its shorter
N–O1 bond. This can be a result of the larger value of electron density associated
with the shorter N–O1 bond. It is worth emphasizing, that the amount of electron
density found in the N–O1 region describing the N–O1 chemical bond is much
smaller than a formal value of 2e. In the light of topological analysis of ELF the
N–O1 bond does not have a typical covalent character when a single electron pair is
shared between N and O1 atoms. The bond nature is determined by highly delocalised electron density, represented by Lewis structures with the N
+
O1
− and
N
−
O1
+ bonds. Delocalisation of the electron density among ELF-basins is quantitatively described by the values of covariance matrix [51]. For the V(O1) and V
(N) basins of the protocovalent bond, the electron density is delocalised mainly
with large valence basins that are located in close proximity, namely non-bonding
basins V i=1,2 (O1) and V 1 (N). The magnitude of the covariance for V(O1) with two
V i=1,2 (O1) lone pairs is 0.10 (each lone pair) and this value is larger than the value
of 0.07 for delocalisation with the V(N) basin (within protocovalent bond). In the
case of the V(N) basin the largest covariance value has been obtained for delocalisation with the non-bonding basin V 1 (N) and is 0.11.
At the CCSD level of calculations [25] similar values of the basin population
have been obtained. The value of
N for the V(N,O1) basin is 1.15e (syn) and 0.58e,
0.44e (anti) for the V(N), V(O1) basins, respectively. These results confirm less
covalent character of the longer N–O1 bond and more covalent character of the
N–O1 bond for the syn conformer than predicted by DFT calculations.
Nevertheless, the comparison of the CCSD, DFT(B2PLYP) and DFT(B3LYP)
results (see Table 19.1) shows that classification of the nature of the N–O1 bond is
strongly method dependent. The double bond, N=O2, in both molecules and using
all computational methods, is described by the single disynaptic bonding basin
V(N,O2) with the basin population varying from 2.02e to 2.13e. This value does
not correspond to that characteristic of a classical double N=O2 bond as predicted
by the Lewis formula (4e). Similarly, formally single H–O1 bond exhibits basin
populations in the range between 1.78e and 1.82e, smaller than formal value of 2e.
19.3.2 Alkaline Nitrites
The nature of bonding in inorganic nitrites, MONO (alkaline nitrites, MO1NO2),
has been studied for four molecules: LiONO, NaONO, KONO and RbONO. Two
stable conformers have been identified with the syn and anti configuration of the
formal M–O1 bond in respect to the terminal NO2 bond (see Fig. 19.4). The NO1
19 Diversity of the Nature of the Nitrogen-Oxygen Bond …
535
syn conformer and 0.51e, 0.40e for anti conformer. In both cases the basin
describing the less electronegative atom yields a larger value of
N. This can be
related to the annihilation of the V(O1) attractor at the first stage of the dissociation
of the N–O1 bond (as explained above). Furthermore, larger values of
N obtained
for the V(O1) and V(N) basins for the syn conformer correspond to its shorter
N–O1 bond. This can be a result of the larger value of electron density associated
with the shorter N–O1 bond. It is worth emphasizing, that the amount of electron
density found in the N–O1 region describing the N–O1 chemical bond is much
smaller than a formal value of 2e. In the light of topological analysis of ELF the
N–O1 bond does not have a typical covalent character when a single electron pair is
shared between N and O1 atoms. The bond nature is determined by highly delocalised electron density, represented by Lewis structures with the N
+
O1
− and
N
−
O1
+ bonds. Delocalisation of the electron density among ELF-basins is quantitatively described by the values of covariance matrix [51]. For the V(O1) and V
(N) basins of the protocovalent bond, the electron density is delocalised mainly
with large valence basins that are located in close proximity, namely non-bonding
basins V i=1,2 (O1) and V 1 (N). The magnitude of the covariance for V(O1) with two
V i=1,2 (O1) lone pairs is 0.10 (each lone pair) and this value is larger than the value
of 0.07 for delocalisation with the V(N) basin (within protocovalent bond). In the
case of the V(N) basin the largest covariance value has been obtained for delocalisation with the non-bonding basin V 1 (N) and is 0.11.
At the CCSD level of calculations [25] similar values of the basin population
have been obtained. The value of
N for the V(N,O1) basin is 1.15e (syn) and 0.58e,
0.44e (anti) for the V(N), V(O1) basins, respectively. These results confirm less
covalent character of the longer N–O1 bond and more covalent character of the
N–O1 bond for the syn conformer than predicted by DFT calculations.
Nevertheless, the comparison of the CCSD, DFT(B2PLYP) and DFT(B3LYP)
results (see Table 19.1) shows that classification of the nature of the N–O1 bond is
strongly method dependent. The double bond, N=O2, in both molecules and using
all computational methods, is described by the single disynaptic bonding basin
V(N,O2) with the basin population varying from 2.02e to 2.13e. This value does
not correspond to that characteristic of a classical double N=O2 bond as predicted
by the Lewis formula (4e). Similarly, formally single H–O1 bond exhibits basin
populations in the range between 1.78e and 1.82e, smaller than formal value of 2e.
19.3.2 Alkaline Nitrites
The nature of bonding in inorganic nitrites, MONO (alkaline nitrites, MO1NO2),
has been studied for four molecules: LiONO, NaONO, KONO and RbONO. Two
stable conformers have been identified with the syn and anti configuration of the
formal M–O1 bond in respect to the terminal NO2 bond (see Fig. 19.4). The NO1
19 Diversity of the Nature of the Nitrogen-Oxygen Bond …
535