CHARGE DENSITY ANALYSIS OF N-(3,
5 DINITROPHENYL)-N-METHYLNITRAMINE.
The nitramine rearrangement belongs to the class of aromatic
rearrangements involving migration of a substituent from heteroatom to the
aromatic ring. There are several derivatives of nitramide (NH2NO2),
which are more or less susceptible to the rearangement under the influence of
an acid or elevated temperature. Most of the primary and secondary aromatic
nitramines can be isomerised under proper conditions. On the other hand the
studies of spectral properties of the ring substituent
N-methyl-N-phenylnitramines have demonstrated that the interactions between the
nitramino group and the second substituent across the ring is negligible [1].
The rentgenostuctural analyses of
N-methyl-N-phenylnitramine and its ring substituted derivatives indicated that
nitramine p-electron
system is not conjugated with the aromatic sextet. That means that the formally
unshared electron pair on the amido nitrogen is delocalised towards the N-nitro
group and does not interact with the p-electrons of the ring. Consequently, we have observed that the dipole
moments of the series of nitramines obey the additivity rule, i.e. they are vector sums of the
contributions coming from the substituted ring and the nitramino group [2].
The nitramino group is planar, the atoms of the C2N-NO2
group are situated in the same plane. The N(7)N(8) bond is relatively long
(1.369(1)). The torsion angle along the Ar - NNO2 bond is equal to ca. 45o. Considering the
length of the Ar - NNO2 bond (1.416(1)), it can be concluded that the nitramino group is a free-rotating
substituent and the torsion angle is determined with the intermolecular
interactions.
The bond
lengths and angles within the Ar-nitro groups are typical for aromatic nitro
compounds. It should be mentioned, however, that the Ar-NO2 bond is
surprisingly long (1.466(1)); it is for ca. 0.05 longer than the analogous CN bond between the aromatic ring and the
nitramino group. Both nitro groups are twisted for 9o and 18o
in respect to the ring plane.
The high resolution single crystal
diffraction data set was collected with a KUMA diffractometer. After the
spherical refinement with SHELXL-97 the aspherical atom refinement was
performed with the program package XD using the Hansen Coppens formalism. The
hexadecapolar level of the multipolar expansion was used for carbon, nitrogen
and oxygen while dipolar was used for the hydrogen atoms. The residual map was
featureless (<0.1 e/3). Analysis of atomic charges show a
negative charge on NO2 group and a positive charge on NNO2
group. The N-N and N-O bonds show increased ionic character.
References:
1
Z.
Daszkiewicz, J. Zaleski, E. M. Nowakowska and J. B. Kyzioł Polish J. Chem.
76, 1113 (2002).
2
Z. Daszkiewicz, J. B. Kyzioł, W. W.
Preżdo and J. Zaleski J. Mol. Struct. 553, 9 (2000).