NO1 bond than for the NO2 bond. For the syn isomer of MONO, with two equal
NO1 and NO2 bonds, the transient values of
N have been calculated within a very
narrow range i.e. 1.51e (Na)–1.56e (Li, Rb).
The highly polarised structure of M
δ+
[ONO]
δ− yields the question about the
nature of the bonding in isolated [ONO]
1− anion and its similarity to the [ONO]
δ−
group in the alkaline nitrites. Topological analysis of ELF performed for the C 2v
optimised structure (r(N–O) = 1.263 Å, <(O–N–O) = 117°) has shown the same
core and valence attractors as those observed in MONO. The basin populations for
V(N,O1) and V(N,O2) equal 1.58e thus they are very similar to the values obtained
for MONO (C 2v ). It supports the observation that alkaline nitrites consist of a metal
cation and nitrite anion.
The alkaline nitrites are examples of molecules with the nitrite group characterised by a single disynaptic V(N,O1) and V(N,O2) basins, typical for the ‘normal’
covalent bonds. It is worth noting that the bond lengths are smaller than 1.32 Å thus
these bonds are essentially shorter than a formally single N–O1 bond in the nitrous
acid molecule, where the protocovalent bond has been identified.
19.3.3 Halogen Nitrites and Peroxynitrous Acid
The electronic structure has been studied for four halogen nitrites XONO
(XO1NO2, X = F, Cl, Br, I) and peroxynitrous acid, HOONO (HOO1NO2). For the
XONO molecules, two conformers with syn and anti orientation of the X–O bond
in respect to the terminal N=O bond have been found. For the HOONO molecule,
three conformers have been investigated: syn-syn, syn-perp and anti-perp.
The B2PLYP/aug-cc-pVTZ optimised geometrical structure of the syn conformer of XONO is 2.99 kcal/mol (F), 3.29 kcal/mol (Cl), 3.89 (Br) and
4.66 kcal/mol (I) more stable than the anti conformer. The results for the HOONO
molecule show the syn-syn isomer as the most stable, but the syn-perp and anti-perp
conformers are only 0.92 and 3.15 kcal/mol less stable, respectively.
Comparison of the N–O1 bond lengths, collected in Table 19.3, shows longer
N–O1 bond in the less stable anti conformer of all XONO molecules. The difference
between the N–O1 bond length in the syn and anti isomers is 0.04 Å (F), 0.06 Å
(Cl), 0.15 Å (Br) and 0.16 Å (I), respectively. Similarly, the N–O1 bond length for
the less stable syn-perp and anti-perp conformers of HOONO are 0.11 and 0.10 Å
longer than in the most stable syn-syn conformer. In the case of the syn conformers
of XONO, the shortest N–O1 bond is observed for IONO (1.348 Å) and the longest
for ClONO (1.496 Å). For the anti conformers, the shortest N–O1 bond is obtained
for IONO (1.504 Å) and the longest for ClONO (1.554 Å). With such differences in
the bond lengths, significant differences in the topological structure of ELF-field
can be expected. It needs to be stressed, however, that the geometrical structure of
IONO has been optimised using pseudopotential approximation for the core electrons of iodine, while for other halogen atoms the all-electron basis set has been
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S. Berski and A.J. Gordon
NO1 and NO2 bonds, the transient values of
N have been calculated within a very
narrow range i.e. 1.51e (Na)–1.56e (Li, Rb).
The highly polarised structure of M
δ+
[ONO]
δ− yields the question about the
nature of the bonding in isolated [ONO]
1− anion and its similarity to the [ONO]
δ−
group in the alkaline nitrites. Topological analysis of ELF performed for the C 2v
optimised structure (r(N–O) = 1.263 Å, <(O–N–O) = 117°) has shown the same
core and valence attractors as those observed in MONO. The basin populations for
V(N,O1) and V(N,O2) equal 1.58e thus they are very similar to the values obtained
for MONO (C 2v ). It supports the observation that alkaline nitrites consist of a metal
cation and nitrite anion.
The alkaline nitrites are examples of molecules with the nitrite group characterised by a single disynaptic V(N,O1) and V(N,O2) basins, typical for the ‘normal’
covalent bonds. It is worth noting that the bond lengths are smaller than 1.32 Å thus
these bonds are essentially shorter than a formally single N–O1 bond in the nitrous
acid molecule, where the protocovalent bond has been identified.
19.3.3 Halogen Nitrites and Peroxynitrous Acid
The electronic structure has been studied for four halogen nitrites XONO
(XO1NO2, X = F, Cl, Br, I) and peroxynitrous acid, HOONO (HOO1NO2). For the
XONO molecules, two conformers with syn and anti orientation of the X–O bond
in respect to the terminal N=O bond have been found. For the HOONO molecule,
three conformers have been investigated: syn-syn, syn-perp and anti-perp.
The B2PLYP/aug-cc-pVTZ optimised geometrical structure of the syn conformer of XONO is 2.99 kcal/mol (F), 3.29 kcal/mol (Cl), 3.89 (Br) and
4.66 kcal/mol (I) more stable than the anti conformer. The results for the HOONO
molecule show the syn-syn isomer as the most stable, but the syn-perp and anti-perp
conformers are only 0.92 and 3.15 kcal/mol less stable, respectively.
Comparison of the N–O1 bond lengths, collected in Table 19.3, shows longer
N–O1 bond in the less stable anti conformer of all XONO molecules. The difference
between the N–O1 bond length in the syn and anti isomers is 0.04 Å (F), 0.06 Å
(Cl), 0.15 Å (Br) and 0.16 Å (I), respectively. Similarly, the N–O1 bond length for
the less stable syn-perp and anti-perp conformers of HOONO are 0.11 and 0.10 Å
longer than in the most stable syn-syn conformer. In the case of the syn conformers
of XONO, the shortest N–O1 bond is observed for IONO (1.348 Å) and the longest
for ClONO (1.496 Å). For the anti conformers, the shortest N–O1 bond is obtained
for IONO (1.504 Å) and the longest for ClONO (1.554 Å). With such differences in
the bond lengths, significant differences in the topological structure of ELF-field
can be expected. It needs to be stressed, however, that the geometrical structure of
IONO has been optimised using pseudopotential approximation for the core electrons of iodine, while for other halogen atoms the all-electron basis set has been
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S. Berski and A.J. Gordon