formula N–O. Such diversity of bonding, represented by different SSDs of ELF
field, can be quantitively distinguished and described using topological concepts
and the catastrophe theory.
Finally, interesting question arises: what is the driving force behind the N–O
bond length variation. The answer is not straightforward as the studies on FONO
and HOONO show that the topology of ELF depends on the quality of wave
function used for analysis. Furthermore, different conformers can exhibit different
number and type of attractors in the N–O bond as has been shown for example for
HOONO, (CF 3 ) 2 NO–NO and (CH 3 ) 2 NO–NO. Nevertheless some generalisations
are possible.
Analysis of the V(N) and V(O1) covariances for the protocovalent bond shows
that electron density exchange with the nearby large basins of the lone pairs V 1 (N)
and V i=1,2 (O1) is larger than mutual delocalisation between the V(N) and V(O1)
basins. Thus the nature of the N–O bond can result from dispersive interaction
through instantaneous atomic dipole-dipole interactions –N+ –O+, +N– +O–
yielding energetic stabilisation. Formation of atomic dipoles would arise from large
delocalisation of electron density with the lone pairs, V 1 (N) and V i=1,2 (O1). In that
case, polarizability of the central O1 atom, dependent on electronegativity/
polarizability of the M, X and R groups bonded to O1 is essential.
Based on the concept of hard and soft acids and bases [54], the electronic
structure of the N–O bond in alkaline nitrites (bonding type I) is a consequence of
bonding between the O1 atom and a hard acid type atom M: Li
+
, Na
+
, K
+ , Rb
+
.
Similar bonding type can be expected for the MO–NO bond, formed by soft acid
type M atoms (Ag
+
). The bonding types II, III and IV with two V(N), V(O1) basins,
single V(N) basin and those without valence basins in the N–O1 bond, respectively
are effect of the interaction of the H, X, OH, R, (CF 3 ) 2 N and (CH 3 ) 2 N groups with
O1 atom. Those groups are characterised as either hard or soft bases. The protocovalent N–O1 bond (type II) is observed mainly for large group of the organic
nitrites with the alkyl group R (including cyclohexyl group), (CH 3 ) 2 N, BrONO and
IONO. The alkyl and Br
− , I
− groups are highly polarisable, therefore they do not
significantly perturb the O1 valence electrons. This enables delocalisation between
V(O1) and V i=1,2 (O1) basins. When weakly polarisable groups, F
− , Cl
− and OH
−
i.e. hard bases are bound to O1 atom, the polarising effect on O1 atom is intensified
withdrawing electron density from the N–O1 bond and weakening it. This can be
associated with the V(O1) basin disappearance (type III). Finally, a very strong
polarising effect is expected for the (CF 3 ) 2 N group due to the lack of valence basins
(type IV) caused by large withdrawal of electron density from the N–O1 bond,
diminishing further the O1 atom delocalisation in the valence shell and decreasing
the stabilising dispersion-like interaction.
Acknowledgments The authors are grateful to the Wroclaw Centre for Networking and
Supercomputing for generous allocation of computer time.
19 Diversity of the Nature of the Nitrogen-Oxygen Bond …
549
field, can be quantitively distinguished and described using topological concepts
and the catastrophe theory.
Finally, interesting question arises: what is the driving force behind the N–O
bond length variation. The answer is not straightforward as the studies on FONO
and HOONO show that the topology of ELF depends on the quality of wave
function used for analysis. Furthermore, different conformers can exhibit different
number and type of attractors in the N–O bond as has been shown for example for
HOONO, (CF 3 ) 2 NO–NO and (CH 3 ) 2 NO–NO. Nevertheless some generalisations
are possible.
Analysis of the V(N) and V(O1) covariances for the protocovalent bond shows
that electron density exchange with the nearby large basins of the lone pairs V 1 (N)
and V i=1,2 (O1) is larger than mutual delocalisation between the V(N) and V(O1)
basins. Thus the nature of the N–O bond can result from dispersive interaction
through instantaneous atomic dipole-dipole interactions –N+ –O+, +N– +O–
yielding energetic stabilisation. Formation of atomic dipoles would arise from large
delocalisation of electron density with the lone pairs, V 1 (N) and V i=1,2 (O1). In that
case, polarizability of the central O1 atom, dependent on electronegativity/
polarizability of the M, X and R groups bonded to O1 is essential.
Based on the concept of hard and soft acids and bases [54], the electronic
structure of the N–O bond in alkaline nitrites (bonding type I) is a consequence of
bonding between the O1 atom and a hard acid type atom M: Li
+
, Na
+
, K
+ , Rb
+
.
Similar bonding type can be expected for the MO–NO bond, formed by soft acid
type M atoms (Ag
+
). The bonding types II, III and IV with two V(N), V(O1) basins,
single V(N) basin and those without valence basins in the N–O1 bond, respectively
are effect of the interaction of the H, X, OH, R, (CF 3 ) 2 N and (CH 3 ) 2 N groups with
O1 atom. Those groups are characterised as either hard or soft bases. The protocovalent N–O1 bond (type II) is observed mainly for large group of the organic
nitrites with the alkyl group R (including cyclohexyl group), (CH 3 ) 2 N, BrONO and
IONO. The alkyl and Br
− , I
− groups are highly polarisable, therefore they do not
significantly perturb the O1 valence electrons. This enables delocalisation between
V(O1) and V i=1,2 (O1) basins. When weakly polarisable groups, F
− , Cl
− and OH
−
i.e. hard bases are bound to O1 atom, the polarising effect on O1 atom is intensified
withdrawing electron density from the N–O1 bond and weakening it. This can be
associated with the V(O1) basin disappearance (type III). Finally, a very strong
polarising effect is expected for the (CF 3 ) 2 N group due to the lack of valence basins
(type IV) caused by large withdrawal of electron density from the N–O1 bond,
diminishing further the O1 atom delocalisation in the valence shell and decreasing
the stabilising dispersion-like interaction.
Acknowledgments The authors are grateful to the Wroclaw Centre for Networking and
Supercomputing for generous allocation of computer time.
19 Diversity of the Nature of the Nitrogen-Oxygen Bond …
549