[43] from the Turbomole programme library. IONO molecule has been optimized
using the aug-cc-pVTZ-PP pseudopotential [44] for the core electrons of the iodine
atom (ecp-28). Other atoms have been described using the aug-cc-pVTZ basis set.
In order to obtain the values of iodine molecular orbitals, the single-point calculations have been carried out using TZVPPall basis set for I atom from the
Turbomole programme library. Other atoms have been described with the
aug-cc-pVTZ basis set. An energy difference between the conformational isomers
of the studied molecules (ΔE) has been corrected by ΔZPVE.
Topological analysis of the Electron Localization Function (ELF) has been
carried out using TopMod09 suite [45, 46]. The wave function has been obtained
using approximation for the DFT(B2PLYP) method proposed by Feixas et al. [47],
including the natural orbitals and their occupancies only. The ELF functions have
been calculated over a rectangular parallelepipedic grid with a step of 0.05 bohr.
Graphical representations have been obtained by ChemCraft [48], the UCSF
Chimera package [49] and VMD [50] programs.
In the plot showing the relationship between basin populations,
N and N–O1
bond lengths for all HONO, MONO, XONO and RONO nitrites, the values for
IONO have been omitted. In case of the alkaline nitrites both N–O1 and N–O2
bonds have been taken into account.
19.3 Results and Discussion
19.3.1 Nitrous Acid
All compounds chosen for the study are derivatives of the simplest molecule
containing the nitrite group, nitrous acid, HONO (HO1NO2). Two conformers of
HONO are known (C s symmetry), one syn (cis) and one anti (trans) characterising
the H–O1 bond position in relation to the terminal N=O2 bond. The DFT
(B2PLYP)/aug-cc-pVTZ optimised geometrical structure of the anti conformer is
0.63 kcal/mol more stable than the syn conformer. The single N–O1 bond in the syn
conformer (1.397 Å) is 0.043 Å shorter than the length of the equivalent bond in the
anti conformer. The terminal N=O2 bond, described as formally double bond in the
Lewis formula, is 0.013 Å shorter in the syn conformer than in the anti conformer.
Since the double N=O2 bond is shorter than the single N–O1 bond, the difference in
their electronic structures can be anticipated.
The results of the N–O1 bond electronic structure, studied in the real space using
topological analysis of ELF, will vary depending on the computational method
used. At the DFT(B2PLYP)/aug-cc-pVTZ computational level, the N–O1 bonds in
both conformers are described by two monosynaptic attractors, V(O1) and V(N).
Such ELF-topology is characteristic for the protocovalent bond. The core and
valence attractors in the syn and anti conformer of the nitrous acid are shown in
Fig. 19.3. Similar analysis, carried out using the B3LYP hybrid electron density
19 Diversity of the Nature of the Nitrogen-Oxygen Bond …
533
using the aug-cc-pVTZ-PP pseudopotential [44] for the core electrons of the iodine
atom (ecp-28). Other atoms have been described using the aug-cc-pVTZ basis set.
In order to obtain the values of iodine molecular orbitals, the single-point calculations have been carried out using TZVPPall basis set for I atom from the
Turbomole programme library. Other atoms have been described with the
aug-cc-pVTZ basis set. An energy difference between the conformational isomers
of the studied molecules (ΔE) has been corrected by ΔZPVE.
Topological analysis of the Electron Localization Function (ELF) has been
carried out using TopMod09 suite [45, 46]. The wave function has been obtained
using approximation for the DFT(B2PLYP) method proposed by Feixas et al. [47],
including the natural orbitals and their occupancies only. The ELF functions have
been calculated over a rectangular parallelepipedic grid with a step of 0.05 bohr.
Graphical representations have been obtained by ChemCraft [48], the UCSF
Chimera package [49] and VMD [50] programs.
In the plot showing the relationship between basin populations,
N and N–O1
bond lengths for all HONO, MONO, XONO and RONO nitrites, the values for
IONO have been omitted. In case of the alkaline nitrites both N–O1 and N–O2
bonds have been taken into account.
19.3 Results and Discussion
19.3.1 Nitrous Acid
All compounds chosen for the study are derivatives of the simplest molecule
containing the nitrite group, nitrous acid, HONO (HO1NO2). Two conformers of
HONO are known (C s symmetry), one syn (cis) and one anti (trans) characterising
the H–O1 bond position in relation to the terminal N=O2 bond. The DFT
(B2PLYP)/aug-cc-pVTZ optimised geometrical structure of the anti conformer is
0.63 kcal/mol more stable than the syn conformer. The single N–O1 bond in the syn
conformer (1.397 Å) is 0.043 Å shorter than the length of the equivalent bond in the
anti conformer. The terminal N=O2 bond, described as formally double bond in the
Lewis formula, is 0.013 Å shorter in the syn conformer than in the anti conformer.
Since the double N=O2 bond is shorter than the single N–O1 bond, the difference in
their electronic structures can be anticipated.
The results of the N–O1 bond electronic structure, studied in the real space using
topological analysis of ELF, will vary depending on the computational method
used. At the DFT(B2PLYP)/aug-cc-pVTZ computational level, the N–O1 bonds in
both conformers are described by two monosynaptic attractors, V(O1) and V(N).
Such ELF-topology is characteristic for the protocovalent bond. The core and
valence attractors in the syn and anti conformer of the nitrous acid are shown in
Fig. 19.3. Similar analysis, carried out using the B3LYP hybrid electron density
19 Diversity of the Nature of the Nitrogen-Oxygen Bond …
533