Electrical and optical properties of the nitramine group and molecular structure of some N-nitramines

Article abstract of DOI:10.1023/A:1012383418868

The electrical and optical properties (dipole moment, electrooptical and optical Kerr constants, molecular polarizability anisotropy, polarizability tensor, molecular refraction) of N,N-dimethylnitramine as the simplest N-NO₂ derivative were studied by a set of experimental and theoretical methods with the aim to determine the components of the polarizability ellipsoid of the N-NO₂ group and develop a valence-optical scheme for calculating the optical and electrooptical parameters of N-NO₂ compounds. Conjugation of the p electrons of the imide nitrogen atom with the π electrons of the nitro group results in deviation of the properties of N,N-dimethylnitramine from the additive values. Comparative evaluation of the parameters of the C_sp3-NO₂, C_sp2-NO₂, and N-NO₂ groups was made.

Full text of DOI:10.1023/A:1012383418868

Russian Journal of General Chemistry, Vol. 71, No. 6, 2001, pp. 907 916. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 6, 2001,
pp. 966 976.
Original Russian Text Copyright
2001 by V. Prezhdo, Bykova, Daszkiewicz, Halas, Iwaszkiewicz-Kostka, O. Prezhdo, Kyziol, Blaszczak.
Electrical and Optical Properties of the Nitramine Group
and Molecular Structure of Some N-Nitramines
V. V. Prezhdo, A. S. Bykova, Z. Daszkiewicz, M. Halas, I. Iwaszkiewicz-Kostka,
O. V. Prezhdo, J. Kyziol, and Z. Blaszczak
Kharkov State Polytechnic University, Kharkov, Ukraine
Pedagogical Institute, Kielce, Poland
Opole University, Opole, Poland
Mickiewicz University, Poznan, Poland
Washington University, Seattle, the United States
Received November 30, 1999
Abstract The electrical and optical properties (dipole moment, electrooptical and optical Kerr constants,
molecular polarizability anisotropy, polarizability tensor, molecular refraction) of N,N-dimethylnitramine as
the simplest N NO derivative were studied by a set of experimental and theoretical methods with the aim to
2
determine the components of the polarizability ellipsoid of the N NO group and develop a valence-optical
2
scheme for calculating the optical and electrooptical parameters of N NO compounds. Conjugation of the p
2
electrons of the imide nitrogen atom with the electrons of the nitro group results in deviation of the proper-
ties of N,N-dimethylnitramine from the additive values. Comparative evaluation of the parameters of the
Csp
3
NO , C
sp2
NO , and N NO groups was made.
2
2 2
N-Nitramines are used as propellants and explo-
sives [1], which makes urgent studies of their physico-
chemical properties and molecular structure. Thanks
to specific features of the N N bond, these com-
pounds are active in photochemical reactions [2]. The
mechanism of the nitramine rearrangement is not yet
understood [3]. N,N-Dimethylnitramine is the sim-
plest nitramine; under standard conditions (T 298 K,
P 1 atm) it is a solid (mp 331 K) and is not explosive,
in contrast, e.g., to hexahydro-1,3,5-trinitro-1,3,5-tri-
azine (hexogen) or octahydro-1,3,5,7-tetranitro-1,3,5,7-
tetrazocine (octogen) [4]; therefore, the properties of
this compound are a subject of numerous theoretical
of N,N-dimethylnitramine [20], the N N bond length
is 0.126 nm. Later more accurate measurements gave
the bond length of 0.1331 nm, which is average be-
tween the length of the single N N (0.1425 nm) and
double N=N (0.1240 nm) bonds [13].
According to X-ray and neutron diffraction data
4, 13], the phase transition at 107 K is accompanied
[
by minor changes in the molecular structure of N,N-
dimethylnitramine. In the high-temperature crystalline
phase, the N,N-dimethylnitramine molecule has a
planar core with approximate C2v symmetry, but its
geometric parameters somewhat differ from those
determined for the gas phase. On transition to the low-
temperature crystal modification, the symmetry de-
[
5 9] and experimental [10, 11] studies. Also interest-
ing are aromatic nitramines, which rearrange into
creases from C2v to C [4], the angle between the
s
C-nitro compounds under the action of acids or other
N N bond and the C N C plane slightly changes,
becoming 11.30 , and the nitro group turns around the
N N bond by an angle of 1.60 . The barrier to internal
rotation around the N N bond in the N,N-dimethyl-
external factors. Migration of the NO group is intra-
2
molecular [1].
Experimental study of the molecular structure of
N,N-dimethylnitramine showed that its core is planar nitramine molecule was estimated experimentally at
1
1
both in the crystal [12, 13] and in the gas phase [14],
in contrast to the nonplanar nitramine [15 17]. This
9 kcal mol [21] and theoretically at 4 to 13 kcal mol
[22, 23]. The turn of the NO group by 90 , i.e., per-
2
difference is explained in [18, 19] by the different pendicular to the C N C plane, increases (from
nature of the N N bond in these molecules: The mole-
cular structure correlates with the bond length, becom-
ing more planar as the N N bond length decreases [5].
According to early studies of the molecular structure
0.1341 to 0.1467 nm) the N N bond length, with the
arrangement of the C N and N N bonds around the
imide nitrogen atom becoming tetrahedral [22]. There-
1
fore, the experimental rotation barrier of 9 kcal mol
1
070-3632/01/7106-0907$25.00 2001 MAIK Nauka/Interperiodica

908
PREZHDO et al.
can be regarded as the resonance energy of the nitra-
mine group.
Procedure for determining the main semiaxes of
the molecular polarizability ellipsoid. Usually the
components of the molecular polarizability ellipsoid
are determined from data on the molecular refraction,
electrooptical Kerr effect, and degree of depolarization
of the light scattering (or optical Kerr effect) [29 31].
These physical methods give three basic dependences
between the measured parameters and the semiaxes of
the molecular polarizability ellipsoids:
The electronic structure of the N,N-dimethylnitra-
mine molecule can be considered, in terms of the
resonance theory, as a hybrid of the following canoni-
cal structures:
4
N
3
b + b + b
1
2
3
MR =
,
(1)
b ) ]/2, (2)
The lone electron pair of the imide nitrogen atom
3
is shifted to the multicenter of the
orbital of the
2
2
2
2
=
[(b1
b ) + (b2
b ) + (b3
2
3
1
NO group but is not fully delocalized. CNDO calcu-
2
45k2T2
=
2
1
(2b1
b2
b3) +
b2
2
2
(2b2
b3
b )
lations with the minimal basis give for the N N
bond an order of 0.394 [24]. However, some results
complicate the simple resonance pattern. Ab initio
calculations of the Mulliken charge distribution on
atoms show that the nitrogen atom of the nitro group
bears a large positive charge (+0.30), whereas the
imide nitrogen atom is charged negatively ( 0.20)
2
1
2
3
+
(2b3
b ),
(3)
1
where MR is the molecular refraction; N is the Avo-
gadro number; b , b , and b are the main semiaxes
1
2
3
2
of the molecular polarizability ellipsoid;
molecular polarizability anisotropy; is the dipole
term of the electrooptical Kerr constant; 1, 2, and
are the projections of the molecular dipole moment
onto the axes of the chosen coordinate system; k is the
Boltzmann constant; and T is the temperature, K.
is the
2
[
24]. The very low electron density on the nitrogen
atom of the nitro group is also confirmed by the X-ray
photoelectron spectra [25]. The specific features of
charge distribution in the molecule should be reflected
in its dipole moment ; ab initio calculations, depend-
ing on the basis used, give the values ranging from
3
The system of Eqs. (1) (3) can be solved as a sys-
tem of three equations with three unknowns (b , b ,
4
.10 [24] to 5.727 D [22], which are consistent with
1
2
b3):
b1 + b2 + b3
A,
(4a)
2
2
2
(
b1
b ) + (b2
b ) + (b3
b )
B, (4b)
2
3
1
2
i
(2bi
bj
b )
C.
(4c)
k
Combined analysis of data on the light scattering
and electrooptical Kerr effect allows the dipole term
to be separated out from the molecular Kerr constant
mK [30]. Then,
Thus, the experimental and theoretical studies of
N,N-dimethylnitramine performed by now gave in-
sight into the structural features of this molecule.
However, studies of more complex molecules contain-
ing the N NO group require consideration of the
2 2
2
2kPd
Pe
4
05k T mK
2
C =
2,
whole set of properties of these compounds, including
polarizability and its anisotropy, which, along with
the geometric and energy parameters and electrical
dipole moments, determine the structure and reactivity
of substances. From the molecular anisotropies and
Kerr constants, the bond polarizability anisotropies are
calculated, which are used for calculating the additive
parameters in elucidation of the steric structure of the
molecules and analysis of interaction of nonbonded
atoms. The additive scheme allowing for the effect of
the bond surrounding can be principally constructed
only if appropriate model compounds can be selected.
In particular, for compounds containing the N NO2
group the best model compound is N,N-dimethylni-
tramine.
(5)
N
where P and P are the molar electronic and deforma-
tion polarizabilities of a substance, respectively.
e
d
The most general case accessible for total analysis
is a molecule with a three-axis polarizability ellipsoid
and the dipole moment directed along one of the axes
b1); this is just the case for N,N-dimethylnitramine.
Combination of Eqs. (4) gives the system (6a) (6c):
(
b1 = (A + C/ 2)3,
C/(6 2) + (6B
(6a)
3C2/ 4)1/2/6, (6b)
b2 = A/3
b3 = A/3
C/(6 2
)
(6B
3C2/
4 1/2
)
/6. (6c)
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 6 2001

ELECTRICAL AND OPTICAL PROPERTIES OF THE NITRAMINE GROUP
909
This method allows rigorous calculation of the
main semiaxes of the molecular polarizability ellip-
soids from the data of gas-phase measurements. All
determinations in solutions or neat liquids give only
apparent quantities [30]. Since a large set of data on
the optical and electrical parameters of solids were
obtained from measurements in benzene solutions
Table 1. Refractive indices of benzene solutions of N,N-
dimethylnitramine
T, C 656.3 nm 587.6 nm 546.1 nm 486.1 nm 435.8 nm
1.4% solution
22.8
24.5
25.7
27.7
1.4965 1.5013 1.5052 1.5129
1.4953 1.4994 1.5040 1.5118
1.5228
1.5216
1.5207
1.5194
1.5189
1.5177
1.5169
1.5160
1.5150
1.5140
1.5131
1.5121
1.5109
1.5098
[30, 32], in this work we performed all experimental
studies in benzene at 25 C, taking also into account
the solubility of the substances.
1.4946
1.5032 1.5110
1.4933 1.4981 1.5020 1.5097
1.4929 1.4975 1.5015 1.5091
1.4917 1.4964 1.5003 1.5080
1.4909 1.4957 1.4995 1.5072
1.4901 1.4948 1.4987 1.5063
1.4891 1.4938 1.4978 1.5054
1.4882 1.4929 1.4967 1.5045
1.4875 1.4920 1.4960 1.5035
1.4864 1.4911 1.4948 1.5026
1.4854 1.4899 1.4937 1.5014
1.4843 1.4889 1.4927 1.5002
2.6% solution
2
3
8.7
0.2
Refractometric measurements and measurements
of the optical Kerr effect. The molecular polarizability
2
31.5
anisotropy
was calculated from the experimental
32.7
34.2
35.5
36.9
data on the optical Kerr effect in solutions, using the
equation [30]
2
2
2
2
6
nN1
n + 2
n + 2
i
a
2,
B0 =
38.6
0.2
41.9
(7)
4
5kT i a
3
3
4
where B is the optical Kerr constant; n is the refrac-
0
tive index of the medium; N is the number of mole-
1
cules in the unit volume; k is the Boltzmann constant;
2
0.6
1.4976 1.5021 1.5065 1.5138
1.4962 1.5010 1.5052 1.5127
1.4953 1.5002 1.5038 1.5118
1.4945 1.4991 1.5032 1.5109
1.4938 1.4985 1.5024 1.5102
1.4926 1.4974 1.5013 1.5089
1.4917 1.4965 1.5003 1.5080
1.4908 1.4953 1.4992 1.5070
1.4895 1.4942 1.4982 1.5058
1.4886 1.4933 1.4971 1.5048
1.4876 1.4923 1.4960 1.5037
1.4867 1.4913 1.4950 1.5028
1.4856 1.4902 1.4940 1.5016
1.4846 1.4893 1.4930 1.5005
1.4835 1.4881 1.4919 1.4995
1.4823 1.4870 1.4907 1.4984
1.5237
1.5227
1.5215
1.5207
1.5199
1.5188
1.5176
1.5167
1.5155
1.5144
1.5132
1.5123
1.5110
1.5100
1.5090
1.5079
T is the temperature, K;
light beam inducing the optical Kerr effect;
is the wavelength of the
i
22.5
23.8
25.2
2
28.1
29.7
3
3
3
3
3
3
4
is the
a
wavelength of the analyzing beam; n is refractive
i
index for the light beam inducing birefringence; and
na is the refractive index of the medium for the ana-
6.4
lyzing beam. The quantities n and n are difficult to
i
a
determine, since the inducing and analyzing beams are
generated with a pulse laser (see Experimental); there-
fore, they are determined by measurement of the re-
fractive index at experimentally accessible wave-
lengths (Table 1). The experimental refractive indices
determined at various temperatures were used to ob-
tain the dependences n = f(T) in the form n = a + bT
1.2
2.8
4.4
6.0
7.7
9.1
0.8
for = 1064 and
= 532 nm. The following depen-
i
a
42.6
4.2
dences were obtained for various concentrations of the
substance in solution c:
4
4
.5% solution
b 104
22.2
1.4961 1.5007 1.5046 1.5125
1.4946 1.4995 1.5032 1.5105
1.4936 1.4983 1.5023 1.5100
1.4925 1.4973 1.5011 1.5088
1.5223
1.5208
1.5200
1.5185
c, %
, nm
532
064
532
064
532
064
a
r
24.6
26.2
27.9
1
2
4
.4
.6
.5
1.5201276
1.4773596
1.5206487
1.4767863
1.5199812
1.4775478
6.2948983
5.7072952
6.5944395
5.6412409
6.4838437
5.9250106
0.9970
0.9959
0.9997
0.9938
0.9999
0.9971
1
1
1
2
9.5
1.4916 1.4961 1.5002 1.5078
1.4906 1.4953 1.4991 1.5068
1.4895 1.4942 1.4980 1.5058
1.4886 1.4933 1.4973 1.5049
1.4877 1.4924 1.4962 1.5038
1.4868 1.4913 1.4952 1.5028
1.4859 1.4906 1.4943 1.5019
1.4848 1.4894 1.4932 1.5009
1.4839 1.4885 1.4923 1.4998
1.4829 1.4874 1.4913 1.4988
1.5175
1.5164
1.5154
1.5145
1.5133
1.5123
1.5114
1.5103
1.5094
1.5082
31.0
2.5
33.9
3
The values of n and n determined from these depen-
i
a
dences are listed in Table 2.
3
3
5.4
6.8
The molecular refraction of N,N-dimethylnitramine
was calculated by the equation [33]
38.3
40.0
41.4
43.2
n2 1 M1(2 N2) + M2N2
n2 + 2
1
N2
MR =
MR1(1 N2) ,
8)
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 6 2001
2
d
(

910
PREZHDO et al.
2
Table 2. Initial data for calculating the molecular refraction (MR ) and molecular polarizability anisotropy ( ) of N,N-
2
2
3
2
6
a
dimethylnitramine from solution data (benzene, 25 C, n1 1.49864, d1 0.87511 g cm ,
17.5
[32])
1
B0 108,
2,
6
2
2
,
Concentration,
wt %
d,
g cm 3
MR ,
2
N1 1023
N2
nD
ni
na
3
cm erg 1
2
6
cm
1
2
4
.4
.6
.5
0.01119 1.4992 0.87785
0.02244 1.4991 0.88063
0.03619 1.4990 0.88401
25.08
23.95
23.54
0.06766 1.4631 1.5044
0.06776 1.4628 1.5042
0.06788 1.4627 1.5038
3.90
4.64
5.83
17.414 11.5
14.596 16.1
14.807 25.4
a
(
N ) Mole fraction of the solute, (n ) refractive index of the solution at = 589.3 nm, (d) solution density, (n ) refractive index of
2 D i
the solution for the beam inducing the birefringence, (n ) refractive index of the medium for the analyzing beam, (N ) number of
a
1
3
2
molecules in 1 cm of the solution, (B ) optical Kerr constant of the solution, and ( ) molecular polarizability anisotropy of the
0
solution.
3
Table 3. Dipole moments ( , D), polarizability tensors (b ,
), anisotropic ( ) and dipole ( ) terms of the Kerr
ij
1
2
1
2
3
constant, molar Kerr constant (mK 10 ), mean polarizabilities ( ,
), and molecular polarizability anisotropies
2
6
(
,
) of N,N-dimethylnitramine, calculated by various quantum-chemical methods
Calculation
method
x
y
z
bxx
byy
bzz
bxy
RHF 6-31G*
RHF 6-311G
MP2 6-31G*
MP2 6-311G
0.0024
0.0025
0.0020
0.0020
0.0024
2.409
4.849
4.936
4.433
4.393
4.703
3.716
0.000
0.000
0.000
0.000
0.000
1.349
4.849
4.936
4.433
4.339
4.703
4.625
6.990
7.400
7.286
7.812
7.102
6.275
7.392
7.618
8.255
8.572
7.689
6.003
4.460
4.605
4.461
4.692
4.422
3.760
0.001
0.001
0.000
0.000
0.001
0.295
*
*
*
*
CISD 6-31G*
AM1
mK 1012
2
bxz
byz
1
2
RHF 6-31G*
0.000
0.000
0.000
0.000
0.000
0.875
0.000
0.000
0.000
0.000
0.000
1.430
0.901
1.007
1.386
1.508
1.082
1.713
103.0
103.4
122.9
114.7
111.9
436.8
438.9
522.8
488.8
475.3
187
6.280
7.576
8.469
11.656
12.683
9.100
*
*
*
RHF 6-311G
MP2 6-31G*
MP2 6-311G
6.541
6.667
7.025
6.404
5.346
*
CISD 6-31G*
AM1
46.37
14.410
where N is the mole fraction of the solute; n is the
tions was calculated by Eq. (7) from the optical Kerr
2
refractive index of the solution; M and M are the
constants B of the solutions. The resulting values of
1
2
0
2
molecular weights of the solvent and solute, respec-
are listed in Table 2. Assuming that the molecular
3
tively; d is the solution density, g cm ; MR =
2
anisotropy of benzene solutions of N,N-dimethylni-
1
3
2
2
2
2
6.155 cm is the molar refraction of benzene calcu-
tramine is an additive quantity [
=
N + (1
2 1
2
lated from the experimental data. All the quantities
N )], we calculated . This parameter increases with
2 2
required for calculating MR and the calculation re-
increasing concentration of N,N-dimethylnitramine,
which may be due to association of N,N-dimethylni-
tramine molecules via dipole dipole interactions
( 4.52 4.61 D [33]). Indeed, the calculated molecular
2
sults are listed in Table 2. The mean value of MR2
was compared with the values obtained by various
additive schemes [33]. Naturally, none of the schemes
2
contains the refraction of the N NO group; therefore,
polarizability anisotropy of the dimer with the anti-
2
3
6
the refraction exaltation is as high as 0.93 to 4.11 cm ,
parallel arrangement of the dipoles is 15.02
, and
6
2
depending on the atomic and group refractions used
that of the head-to-tail dimer is 147.7 . Only the
2
6
[33].
value of 11.5
obtained from the experimental data
for the 1.4% solution is nicely consistent with the
results of the quantum-chemical calculations (Table 3)
2
The molecular polarizability anisotropy
of solu-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 6 2001

ELECTRICAL AND OPTICAL PROPERTIES OF THE NITRAMINE GROUP
911
1
2
a
Table 4. Experimental dipole moments ( , D) and molar Kerr constants (mK2 10 , esu) of N,N-dimethylnitramine
Solvent
P2
,
P ,
cm
(sK2)
1014
(mK2)
1012
H
L
H
L
d
L
L
cm3
3
Benzene
13.71
0.281
0.244
0.428
473.5
436.6
24.43
24.43
4.69
4.50
342.8
509
1
1.90
0.0221 84.23
Tetrachloromethane
10.90
4
.61 [27]
535 [27]
546 [26]
Dioxane
4.52 [26]
a
(
,
) Parameters of the Hedestrand extrapolation equation [34], (
,
,
,
) parameters of the Le Fevre extrapolation
L
H
H
L
L
L
equation, (P ) solute polarizability at infinite dilution, (P ) deformation polarizability, and (sK ) specific Kerr constant of the
2
d
2
solute.
and can be therefore taken as the molecular anisotropy
N,N-dimethylnitramine are within 3.174 5.237 D.
The experiment gives = 4.50 4.69 D (Table 4).
of N,N-dimethylnitramine. Attempts to measure B in
0
more dilute solutions failed because of small differ-
ences between the values of the solvent and solute.
2
The fairly high values of the electrooptical molar
Kerr constants of N,N-dimethylnitramine, both calcu-
lated (Table 3) and experimental (Table 4), are due to
the large dipole moment of the molecule, so that the
Dipole moment and molar electrooptical Kerr
constant. To determine the main semiaxes of the
polarizability ellipsoid of the N,N-dimethylnitramine
molecule by Eqs. (1) (6), the dipole moment and
the molar Kerr constant mK of this compound should
be known. Their experimentally determined values are
listed in Table 4. Comparison with published data
obtained with other solvents shows that and mK are
solvent-dependent.
dipole term of the Kerr constant
is considerably
2
greater than the anisotropic term ( ) (Table 3). The
1
2
calculated values of mK, , , and (mean molecu-
lar polarizability) reasonably agree with the experi-
ment.
From the experimental values of MR, mK, , and
, using Eqs. (4) (6), we calculated the main semi-
2
To compare with the experimental values, we cal-
culated the Mulliken charge distribution on atoms of
N,N-dimethylnitramine, its dipole moment, and the
molar Kerr constant by various quantum-chemical
methods. Since there is no unambiguous theoretical
definition for the atomic charge in a molecule, only
comparison of the calculated and experimental dipole
axes of the polarizability ellipsoid of the N,N-dimeth-
ylnitramine molecule: b = 10.42, b = 11.07, b =
1
2
3
3
7.29
9.59
and the mean molecular polarizability
[ = (b + b + b )/3]. The obtained values of
=
3
1
2
3
b , b , and b are somewhat higher than the values
1
2
3
calculated quantum-chemically (see b , b , and b
xx
yy
zz
moments can be a criterion for checking the adequacy in Table 3), which is also indicative of the solvent
of the calculated charge distribution. The results along
with data of [5, 8] are listed in Tables 3 and 5. Com-
parison of the data calculated for the geometric param-
eters that were optimized in different approximations
shows that, although the nitrogen atom of the nitro
group bears a higher positive charge (from 0.831 in
MP2/6 31G** to 0.216 in QCISD/STO-3G), in all
effect and the apparent character of the experimental
values; however, they are consistent with the results
obtained for dichloromethane solution (b = 9.26, b =
1
2
3
8.15, b = 6.00
) [27]. From the b , b , and b
3
1 2 3
values obtained for the N,N-dimethylnitramine mole-
cule, using the tensor-additive scheme, we subtracted
the contributions from six C H bonds believed to be
the calculations this charge is compensated with an isotropic [30] and obtained the values of the main
excess by the negative charges of the two oxygen
atoms, so that the total charge of the nitro group is
negative. The calculated negative charge on the imide
nitrogen atom is insufficient for neutralization of the
positive charges of the two methyl groups. Inclusion
into the polarization basis of the p functions on hydro-
gen atoms in MP2 calculation appreciably decreases
the charges of the methyl group, but its total charge
is weakly sensitive to the basis set and geometric
parameters [8]. The calculated dipole moments of
semiaxes of the polarizability ellipsoid of the C N
2
NO moiety: b = 6.52, b = 7.17, and b = 3.39 3.
2
1
2
3
These apparent values can be used for constructing the
valence-optical scheme for calculating the electroopti-
cal and optical properties of various N-nitramines
from data for benzene solutions. The nitramine group
proper, N NO , has the following axes of the polari-
2
zability ellipsoid: b = 5.32, b = 5.79, and b =
1
2
3
3
2.01
. The somewhat different values obtained in
[27] (b = 4.04, b = 3.05, and b = 0.72 ) are pro-
2
1
2
3
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 6 2001

912
PREZHDO et al.
Table 5. Mulliken atomic charge distribution on atoms and dipole moments ( , D) of N,N-dimethylnitramine, calculated
by various quantum-chemical methods
H8
1
H7
O5
C 3
9
4
H
H
N N (
1
0
2
O6
C
H11
H12
RHF
6-31G**
MP2
6-31G
[8]
Atom
no.
RHF
STO-3G 6-31G
RHF
RHF
6-311G
MP2
MP2
QCISD MC-SCF
**
AM1
CNDO
*
**
STO-3G 6-31G*
STO-3G
[5]
[8]
1
2
3
4
5
6
7
8
9
0.125
0.123
0.266
0.603
0.358
0.358
0.109
0.077
0.129
0.108
0.127
0.078
4.63
0.082
0.082
0.123
0.566
0.339
0.339
0.011
0.021
0.002
0.011
0.002
0.022
4.47
0.079
0.061
0.204
0.265
0.231
0.221
0.079
0.121
0.075
0.078
0.064
0.093
3.85
0.306
0.286
0.428
0.809
0.526
0.511
0.187
0.268
0.185
0.196
0.188
0.223
5.14
0.139
0.138
0.438
0.768
0.483
0.483
0.182
0.138
0.137
0.137
0.138
0.182
4.85
0.061
0.034
0.406
0.618
0.443
0.428
0.109
0.171
0.122
0.121
0.102
0.140
5.24
0.088
0.070
0.183
0.219
0.184
0.181
0.074
0.109
0.069
0.070
0.080
0.085
3.17
0.354
0.334
0.296
0.618
0.433
0.420
0.187
0.252
0.185
0.194
0.185
0.216
4.74
0.286
0.286
0.443
0.831
0.508
0.508
0.227
0.185
0.188
0.188
0.185
0.227
4.82
0.085
0.067
0.179
0.216
0.181
0.175
0.071
0.106
0.067
0.068
0.078
0.081
3.08
0.408
0.408
0.170
0.529
0.377
0.377
0.173
0.173
0.173
0.173
0.173
0.173
1
1
1
,
0
1
2
D
1
2
a
Table 6. Dipole moments ( , D) and molar Kerr constants (mK2 10 , esu) of some substituted N-nitramines
Comp.
no.
P ,
cm
P2
,
(sK2)
1014
(mK )
(mK )
2 calc
H
L
H
L
L
L
d
2 exp
1012
3
cm3
1012
459
525
730
240
I
14.04
0.562
0.035
0.198
0.029
0.934
0.047
0.354
0.030
24.28
27.52
59.79
44.51
485.6
499.0
497.3
429.1
4.75
4.80
4.63
4.34
393
402
413
161
456
533
719
245
7
.93
13.96
.42
13.58
.10
11.89
.65
0.010
0.020
0.034
0.096
40.63
61.34
79.61
28.09
II
8
III
IV
6
6
a
For the sense of
,
,
,
,
,
, P , P , and (sK ), see the note to Table 4.
L 2 d 2
H
H
L
L
L
bably due to the fact that calculations in [27] were
(i-C H ) NNO (III), and C H (CH )NNO (IV). The
4 9 2 2 6 5 3 2
based on different values of the molecular polariza- calculated characteristics were compared with the
2
6
bility anisotropy
polarizability
(
(28.37
) and mean molecular
experimental data (Table 6). The core of I was as-
(7.80 ) of N,N-dimethylnitramine
the source of these values was not given).
sumed to be in the half-chair (C ) conformation, as
2
suggested by the existing theoretical views (the pla-
2
narity of the C N NO fragment suggests the sp
The thus determined polarizability parameters of
the N NO group were used to calculate the Kerr
constants of a series of nitramines: N-nitropyrrolidine
C H NNO (I), N-nitropiperidine C H NNO (II),
2
2
hybridization of nitrogen, and on introduction of an
2
2
sp -hybridized atom into a five-membered ring the
half-chair C becomes the equilibrium conformation
4
8
2
5
10
2
2
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 6 2001

ELECTRICAL AND OPTICAL PROPERTIES OF THE NITRAMINE GROUP
913
Table 7. Comparison of the electrical and optical properties of the Csp
from the properties of the corresponding nitro compounds (benzene, 25 C)
3
NO , C
sp2
NO , and N NO groups, evaluated
2
2
2
Parameter
N NO2 [(CH ) NNO ]
Csp
2
NO2 (C H NO ) Csp
3
NO (CH NO )
3 2
2
6
5
2
2
3
2
Dipole moment ( , D)
Mean polarizability ( , 3)
4.69; 4.50;
.52 [26]; 4.61 [27]
4.41
4.01 [36]
3.10 [36]
4
2.97
1332 [32]
2.84
54 [32]
1
2
Molar Kerr constant (mK 10 )
509, 535, 546 [32]
Components of the polarizability tensor ( 3):
bL (b1)
bT (b2)
5.26
5.87
2.09
5.23 [30]
2.15 [30]
1.52 [30]
11.82
3.56 [30]
3.63 [30]
1.33 [30]
5.13
bV (b )
3
Molecular polarizability anisotropy ( , 6)
2
12.35
[
35]). The interconversion between the two conforma-
parameters [ as(NO2), s(NO2), (ONO), (NO2),
(NO2), (NO )] of the N NO group appreciably
tions of I occurs most probably via the planar form
whose energy is higher by only 10 kJ mol 1 [35].
The calculated molar Kerr constant (mK 459
2
2
differ from those of the Csp
3
NO and C NO2
sp2
2
groups. The same conclution can be made from a
comparison of the electrical and optical characteristics.
1
2
1
(
0
esu) reasonably agrees with the experiment
Table 6). For II, based on data of [35], we assumed
the chair boat equilibrium and obtained mKcalc
25 10 12 esu, which is close to the experimental
However, the parameters of the N NO group are
2
=
closer to those of the Csp
2
NO group than to those
2
5
of the Csp NO group (Table 7). This fact suggests
3
2
1
2
value (533 10
esu). Rotation around the C N
that the planar (or nearly planar [8]) structure of the
N,N-dimethylnitramine molecule is the most favorable
bonds in III does not affect the dipole moment but
affects the constituents of the molecular polarizability
tensor on which the Kerr constant is dependent. The
calculations were performed for equal angles of the
turn of the (CH ) CH group relative to the initial
planar structure with the trans orientation of the C C
bonds relative to the C N NO plane. The experimen-
tal value corresponds to = 120 , i.e., to the usual
gauche gauche conformation of the C C N C C
moiety. Single crystal X-ray diffraction study of IV
showed that the dihedral angle between the Me N
NO plane and the aromatic ring of the molecule for
different p-substituted derivatives varies from 65.6
for the p
interaction between the imide nitrogen
atom and the nitro group. The calculated values of
both the mean polarizability and the semiaxes of the
3
2
polarizability ellipsoids of the Csp NO , C NO2,
3
sp2
2
and N NO bonds differ considerably. As in analysis
2
2
2
of the refraction exaltation, it is impossible to distin-
guish the contributions from the conjugation and bond
nature; the semiaxes considered separately (b , b , b )
1
2
3
vary to a greater extent than the mean polarizabilities
Therefore, we cannot conclude that the exaltation
.
2
is directed along the X NO axis. Along with increase
2
in b (b ), the aryl derivatives are, as a rule, character-
L
1
[
36] to 72.3 [37]. Assuming
= 70 , from the ob-
d
ized by depression along the b (b ) and b (b ) axes,
T 2 V 3
tained semiaxes of the molecular polarizability ellip-
so that the whole polarizability ellipsoid significantly
rearranges. The main semiaxes of the polarizability
soid of the N NO group and the corresponding pa-
rameters of the other fragments [30] we obtained
mKcalc = 245 10
the experimental and calculated mK values are reason-
ably consistent, and the obtained semiaxes of the
polarizability ellipsoid of the N NO group can be
used for calculating the optical and electrooptical
properties of molecules.
2
ellipsoid of the N NO group calculated by the ten-
2
1
2
2
esu. As seen from Table 6, all
sor-additive scheme (b 5.84, b 3.94, b 1.43
)
1
2
3
substantially differ from the experimental values (b
1
3
5
.38, b 5.79, b 2.01
) and the values calculated
2
3
2
quantum-chemically using data in Table 5 (b 4.00
1
3
3
.17, b 4.76 3.58, b 0.88 0.82
), which also
2
3
shows that interaction of the imide nitrogen atom with
the nitro group causes rearrangement of the whole
Comparison of the electrical and optical charac-
polarizability ellipsoid of the N NO group.
2
teristics of the Csp
groups. Comparison of the spectral characteristics of
the Csp NO , C NO , and N NO groups deter-
mined from studies of CH NO , C H NO , and
3
NO , C NO , and N NO2
sp2
2 2
EXPERIMENTAL
3
sp2
2
2
2
Synthesis of N,N-dimethylnitramine. Several
3
2
6
5
2
(
CH ) NNO , respectively, showed [6] that all the
routes to secondary nitramines are known. For example,
3
2
2
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 6 2001

914
PREZHDO et al.
3
2
2
5
1
3
4
9
1
7
3
8
1
0
2
4
6
1
1
2
Scheme of a unit for observing the optical Kerr effect: (1) Nd:YAG laser, (2) dichroic mirror, (3) filter, (4) polarizer,
5) lense with the focal length f = 30 cm, (6) lense with the focal length f = 50 cm, (7) Kerr cell, (8) analyzer, (9) mono-
chromator, (10) photomultiplier, (11) photodiode, and (12) digital oscilloscope.
(
N,N-dimethylnitramine can be prepared by nitration
of dimethylamine with acetone cyanohydrin nitrate
Determination of the dipole moments and elec-
trooptical molar Kerr constants. Dielectrometric
studies were performed with a Dipol’ apparatus
(OKBA, Angarsk). The dielectric permittivity and
density of dilute (up to 10 3 M) solutions of N,N-
dimethylnitramine in benzene were measured. The
molar polarizability of the solute extrapolated to in-
finite dilution P2 was calculated by the Hedestrand
[38]. However, the use of gaseous dimethylamine and
formation of hydrogen cyanide as a by-product require
serious precautions in the course of the synthesis.
The better route to N,N-dimethylnitramine is oxida-
tion of the corresponding nitroso derivative. As oxi-
dant was used trifluoroperoxyacetic acid prepared in
situ by the reaction of trifluoroacetic anhydride or
trifluoroacetic acid with 90% hydrogen peroxide [39].
In this study we oxidized dimethylnitrosamine with
dimethyldioxirane, which was used previously for
oxidation of amines to C-nitro compounds [40] and
of pyridine to pyridine N-oxide [41]. N,N-Dimethyl-
procedure [34]. The resulting values of
= d /dN2
H
(N is the mole fraction of the solute), = d /dN2,
2
and also P , P = 1.1R (R is the molar refraction),
2
D
D
D
and
are listed in Table 4. Calculation of the di-
exp
pole moment by the vector-additive scheme was based
on data in [43]. The electrooptical Kerr constants of
N,N-dimethylnitramine solutions B were determined
on the apparatus and by the procedure described in
[44]. To calculate the molar Kerr constants mK, the
measurement results were processed according to [45].
The concentration dependences of the measured solu-
nitramine was prepared as follows. To a solution of
3
7
.5 g of N,N-dimethyl-N-nitrosamine in 100 cm
3
of acetone we added 500 cm of a buffer solution
pH 7 8.5) containing 120 g of K HPO and 13.6 g
(
2
4
of KH PO and then, in portions over a period of
2
4
8
h, 61.4
g
of OXONE (2KHSO KHSO4
tion parameters were expressed as = 1(1 +
1(1 + ), n = n (1 + ), and B = B (1 + 2),
where
and 1, , n , and B are the corresponding solvent
2),
5
L
K SO ). Dimethyldioxirane formed in situ by the
reaction of acetone with the OXONE [41]. The mix-
ture was stirred for an additional 4 h. The product was
=
2
4
L
2
1
2
1
is the weight concentration of the solutions
2
1
1
1
3
extracted with methylene chloride (4 100 cm ). The
parameters. Extrapolation to infinite dilution was per-
extract was dried over anhydrous sodium sulfate, the
formed graphically. The values of L, L, , and mK
solvent was evaporated, and the residue was recrystal- are listed in Table 4. The molar Kerr constants were
lized from n-hexane. Yield of N,N-dimethylnitros-
amine 5.1 g (57%), mp 54 55 C. N-Nitropiperidine
and N-nitropyrrolidine were prepared similarly from
the corresponding N-nitroso derivatives. Yield of N-
nitropyrrolidine 88%; the product was crystallized
from n-hexane (mp 56 57.5 C). Yield of N-nitro-
piperidine 83%; the product was isolated by vacuum
distillation, bp 63 65 C (2 mm), and then crystallized
from n-hexane cooled with dry ice. The other N-nitr-
amines were prepared as described in [42].
calculated by the tensor-additive scheme using data
in [30].
Study of the optical Kerr effect and refractive
indices. In the measuring unit for determining the op-
tical Kerr constant (see figure), we used an Nd:YAG
pulse laser operating in the subnanosecond mode
( 0.4 ns) and generating the polarized light with a
wavelength of 1064 nm and the second harmonic with
a wavelength of 532 nm. The birefringence in the cell
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 6 2001

ELECTRICAL AND OPTICAL PROPERTIES OF THE NITRAMINE GROUP
915
was induced by the light with 1064 nm (E 40 mJ),
12. Allen, P.W. and Sutton, L.E., Acta Crystallogr., 1950,
i
and the wavelength
of the analyzing beam was
vol. 3, no. 4, pp. 679 684.
a
5
32 nm. The beams were focused with long-focus
1
3. Krebs, B., Mandt, J., Cabbedick, R.E., and
Small, R.W.H., Acta Crystallogr., Sect. B, 1979,
vol. 35, no. 3, pp. 402 410.
lenses to obtain the highest pulse energy and the best
geometry in the cell with a sample. At the analyzer
outlet, a set of filters was arranged to transmit the
analyzing light only; this light, after passing a mono-
chromator, was received by a photomultiplier, where-
as the beam inducing the birefringence was recorded
with a photodiode. The signals from both beams were
registered with an oscilloscope.
1
1
1
1
4. Stolevik, R. and Rademacher, P., Acta Chem. Scand.,
1
969, vol. 23, no. 4, pp. 672 678.
5. Beevers, C.A. and Trotman-Dickenson, A.F., Acta
Crystallogr., 1957, vol. 10, no. 1, pp. 34 39.
6. Tyler, J.K., J. Mol. Spectrosc., 1963, vol. 11, no. 1,
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The refractive indices of solutions were measured
with a Pulfrich Refraktometr-PR2 device (Carl Zeiss,
Jena) using various light sources (656.3, 587.6, 546.1,
and 435.8 nm).
7. Sadova, N.I., Slepnev, G.E., Tarasenko, N.A., Zai-
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1
1
2
2
2
8. Flournoy, J.M., J. Phys. Chem., 1962, vol. 36, no. 6,
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9. Lloyd, S.A., Umstead, M.E., and Lin, M.C., Energ.
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