Vibrational spectra and conformations of bis(N-ethyl)nitramine molecule

MOLSTR 11038

Journal of Molecular Structure 509 (1999) 275–285 www.elsevier.nl/locate/molstruc

Vibrational spectra and conformations of bis(N-ethyl)nitramine molecule q L.V. Khristenko a, N.F. Pyatakov b, C. Van Alsenoy c, K. Franckaerts c, A.I. Fishman d, S.F. Mironov d, I.F. Shishkov a,1,*, L.V. Vilkov a, Yu.A. Pentin a a

Department of Chemistry, Moscow State University (MSU), Moscow 119899, Russia b Institute of Chemical Physics RAS, Moscow 117334, Russia c Department of Chemistry, University of Antwerp (UIA), Universietsplein 1, B-2610, Wilrijk, Belgium d Department of Physics, University of Kazan, Kazan, Russia Received 19 March 1999; accepted 19 April 1999

Abstract The Raman (50–3200 cm 21) and infrared (50–3200 cm 21) spectra of bis(N-ethyl)nitramine, (CH3CH2)2NNO2, in the liquid and crystal states have been recorded. Optimized geometries and conformational stabilities have been obtained from ab initio calculations utilizing the RHF/6-31G pp level. This compound was shown to have two stable conformations with a planar nitramine fragment and the CH3 groups orthogonal to it and located either on the same or on the different sides of it. The computed energy difference between two conformers is 0.57 kcal/mol. (CH3CH2)2NNO2 exists as a mixture of the two conformations in the liquid state, while only the most stable one, with the CH3 groups located on the different sides of the nitramine fragment, remains in crystal state. The vibrational frequencies have been calculated using ab initio scaled force fields, and the vibrational spectra have been interpreted in detail. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Vibrational spectra; Electron scattering; IR spectrum

1. Introduction The nitramines are widely used in applied chemistry. These compounds are interesting from a theoretical point of view as well. The problem of bond configuration of nitrogen amine atom and molecular conformations is important and discussed for the last years in detail (Refs. [1–3] (and references therein)). q In honour of Professor Peter Klæboe on the occasion of his 70th birthday. * Corresponding author. 1 He works since February till July 1999 at the University of Antwerp (UIA), Department of Chemistry, Universietsplein 1, B2610, Wilrijk, Belgium.

Recent gas electron diffraction studies of (N-chloromethyl-N-methyl)- [1] and bis(N-chloromethyl)- [2] nitramines showed that the most stable conformers have the C–Cl bonds which are practically orthogonal to the plane C2NNO2. In bis(N-chloromethyl)nitramine [2] two C–Cl bonds are placed on the opposite sides of the frame plane. It is possible to explain the found orthogonal conformers by the repulsive interactions of the negatively charged chlorine atoms and oxygen of the nitrogroup. Anomeric effect can be the alternative reason of the found conformation using the concept of the intramolecular interaction of the nitrogen lone pair electrons and antibonding s p orbital of C–Cl bonds.

0022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00227-6

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Fig. 1. IR spectra of (CH3CH2)2NNO2 in 400–3200 cm 21 region: (a) liquid; (b) crystal.

Fig. 2. Raman spectra (CH3CH2)2NNO2: (a) liquid; (b) crystal.

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277

Fig. 3. Raman spectra of liquid (a) and crystal (b) (CH3CH2)2NNO2 in 1100–1600 cm 21 region.

The purpose of this study was to clear the structure of bis(N-ethyl)nitramine (BENA). The chemical bonds of CH3CH2 have a very small polarity. One can expect some interactions of another kind as well. We used only the vibrational spectra and ab initio calculations. The gas electron diffractions method cannot give any certain answer about the molecular structure of (CH3CH2)2NNO2 because the electron scattering contribution from the interatomic distance between CH3 groups is very small.

2. Experiment IR and Raman spectra of BENA in liquid and crystal states, as well as in CCl4 and CH3CN solutions, were studied. IR spectra were recorded using IR-spectrometer Specord M-80 (250–3200 cm 21) and Bruker 113v Fourier transform spectrometer (50– 3200 cm 21). Raman spectra were recorded using Ramanor HG2S spectrometer with argon laser. Figs. 1–3 represent the obtained spectrograms, experimental frequencies are listed in Table 1. In the liquid state spectra a number of bands are broad and asymmetric, while in the crystal spectra the bands are narrowed, their maxima are displaced, some bands lose their asymmetry and some bands, lower than 400 cm 21, vanish. Fig. 3 exhibits Raman spectra of liquid and crystal BENA in the region of 1100– 1600 cm 21. The number of lines observed in the

crystal spectrum corresponds to one conformer. This points to the fact that the liquid BENA is a mixture, at least, of two conformers and that only one conformer remains in the crystal state. To make sure that two BENA conformers are present in the liquid state, the polarization CARS method [4–6] was used.

3. Analysis with the polarization CARS method The polarization CARS method can be fruitful in the case when normal vibration frequencies of various conformers do not differ considerably, offering a great promise for the analysis of this type of lines. In this method the dispersion of antistokes (or stokes) waves at the frequency v a ˆ 2v 1 2 v 2 (v s ˆ 2v 2 2 v 1) transmitted through a polarization analyzer is measured (v 1,2 are the frequencies of pump waves). When studying the Raman resonance with frequency V by CARS, the frequency v 2 is tuned so that the difference v 1 2 v 2 is close to V . The scattering wave consists of nonresonance and resonance coherent components. We can vary the interference conditions of these components by rotation of polarizer because there are definite distinctions between their polarization features. As mentioned above, in the Raman spectrum of BENA relevant lines either coincide or manifest unresolved structures. The polarization CARS spectra of liquid BENA state were investigated. The spectra

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Table 1 Experimental vibrational frequencies of (CH3CH2)2NNO2 (cm 21) (abbreviations used: s strong; m moderate; w weak; v very; sh shoulder; p polarized; dp depolarized; bd broad) IR spectra Liquid

Raman spectra CCl4 solution

CH3CN solution

Crystal

Liquid

60 w

64 vw

77 vw 127 w

78 vw 124 w 150 w

217 vvw

330 sh 354 vw 408 sh 420 w 432 sh 460 w 608 m

315 w 330 sh 350 vw 419 w 432 sh 460 w 606 m

333 m, p

608 m

764 m 774 sh 784 sh 822 m 946 w 970 vw 1040 m 1082 s

764 m 775 sh 785 sh 822 w 944 w

942 w 970 vw 1036 m 1080 m

1082 m

1200 vw 1230 vw

1205 vw 1230 vw

1196 vw 1225 vw

1286 vs

1286 vs

1288 vs

1346 m

1344 m

1322 vw 1348 w

432 m 470 m 612 s

760 s 774 s 788 m 818 s 952 m 972 s 1046 s 1078 vs 1088 vs 1144 w 1184 w 1200 sh 1225 sh 1232 vw 1286 s 1315 sh 1327 sh

419 s, p 432 sh, p? 461 w, dp 600 sh, dp 610 s, p 710 vw, p

1432 m

1458 m 1472 m

1376 s

1430 m

1460 s 1465 sh

1376 m 1382

Crystal

Approximate description

65 m 71 m 83 s 91 m 132 s 156 m 217 vvw 270 vvw – 339 m 366 vw – – 432 m 470 w 612 w 619 m

r w(CNC),

775 s, p

778 s

822 vs, p 946 w, dp 972 s, p 1042 vs, p 1081 s, p

821 s 952 w 974 s 1053 vs 1081 w 1085 w

1199 s, p 1224 vw, dp

1201 s 1224 sh 1234 w – 1286 m – 1330m – 1356 1359 m

1276 m, p 1286 sh 1320 w, dp 1348 w, p

1358 s 1378 s

Assignment

1376 vs 1384 m, p

1428 s 1448 s

1434 w, dp 1456 m, dp

1462 s 1480 s

1475 sh, dp

1382 m 1390 1395 w 1428 m – 1453 m 1460 w 1467 m 1483 w

t as(CN) t s(CN) t (NN) t (CC) d (NCC) (Cs) d (NCC) r r(NC2) r r(NC2) (Cs) d (CCN) (Cs) d (CNC) d (CCN) r r(NO2) d (NO2) r w(NO2) r r(CH2) r r(CH2) n s(C2N) n as(CC) n s(CC) n (NN) r r(CH3) r r(CH3) r r(CH3) n as(C2N) n s(NO2) r t(CH2) (Cs) r t(CH2) r t(CH2) (Cs) r w(CH2) r w(CH2) d s(CH3) d s(CH3) d as(CH2) d as(CH2) (Cs) d as(CH3) d as(CH3) d as(CH3) d as(CH3)

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Table 1 (continued) IR spectra Liquid

Raman spectra CCl4 solution

1506 vs

1520 vs

2880

2880 w

2940 m

2940 m

2984 s

2984 s

CH3CN solution

Crystal

1496 vs 1520 sh 2880 vw 2918 w 2940 m 2956 w 2984 s 3004 m

were detected using CARS spectrometer [7] based on a Nd:YAG laser (second harmonic frequency v 1, pulse energy 70 mJ, Rhodanium 6G). The spectral widths of both lasers were not greater than 1 cm 21. The depolarization degree of pump waves was about 0.999. Table 2 Calculated geometrical parameters, relative energies and dipole momenta of possible conformers of (CH3CH2)2NNO2 molecule ˚ , angles in 8) (bond lengths are given in A Parameter

C2(I)

Cs(II)

C1(III)

N–N N–O C–N C–C C–H(9) C–H(10) C–H(11) C–H(12) C–H(13) O3 –N1 –N2 O3 –N1 –O4 N1 –N2 –C5 C5 –N2 –C7 N2 –C5 –C6 C6 –C5 –N2 –N1 a C8 –C7 –N2 –N1 a C5 –N2 –N1 –O3 C5 –N2 –N1 –O4 E, kcal/mol Dipole, Debye

1.3278 1.2004 1.4594 1.5207 1.0864 1.0828 1.0898 1.0868 1.0899 117.66 124.69 118.69 122.62 113.54 83.19 83.22 22.11 177.89 0.0 4.85

1.3293 1.2002 1.4611 1.5216 1.0856 1.0833 1.0899 1.0865 1.0890 117.67 124.64 118.16 123.63 114.35 279.03 79.03 21.16 179.16 0.57 4.83

1.3457 1.1969 1.4669 1.5224 1.0909 1.0801 1.0887 1.0889 1.0879 117.78 124.48 115.41 122.31 112.63 2167.13 275.83 12.74 2168.54 2.47 4.86

a /C6 –C5 –N2 –N1 ˆ /C8 –C7 –N2 –N1 ˆ 0 for the planar anti conformer of the (CH3CH2)2NNO2 frame.

Liquid

Assignment Crystal

Approximate description

1492 w 1517 w

d s(CH2) n as(NO2)

2884 w

n s(CH3)

2942 s, p

2942 s

n s(CH2)

2974 m, bd.

2975 m 2987 s 2991 m 3010 w

n as(CH3) n as(CH3)

1506 w, dp 1526 w, p 2884 w, p

n as(CH2)

The line with frequency 1042 cm 21 was chosen as analytical line. The lines observed in 950–1050 cm 21 region belong to the skeletal stretching vibrations and, as a rule, depend on the skeleton conformation. The line at 1042 cm 21 is very strong in the Raman spectrum. The complication (or splitting) of this line is possible if BENA has several conformers. This line has asymmetric contour in IR and Raman spectra (Fig. 4), the depolarization ratio being r ˆ 0.11. The widths

Fig. 4. Raman spectra of (CH3CH2)2NNO2: (a) Ik; (b) I ' × 4.82.

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Table 3 Scale factors for force field of C2 conformer of (CH3CH2)2NNO2 molecule Number

Type of coordinate

Scale factor

1 2 3 4 5 6 7 8 9 10 11

n (NO) r w(NO2) n (NN) d (NC2), r r(NC2) d ,r (CH2) n (CN), n (CH2), d ,r (CH3) n (CH3) n (CC) d (NCC) t (CC) t (NN), t (CN), r w(CNC)

0.66 0.72 0.74 0.78 0.81 0.82 0.83 0.86 0.89 0.96 1.00

of polarized and depolarized components of complex contour are equal to 14.4 ^ 0.2 and 14.0 ^ 0.4 cm 21, respectively. The experimental polarization CARS spectra in the spectral range 1020–1080 cm 21 for different angles of polarizer orientation e [4] are shown in Fig. 5. The resonance component has to be suppressed at e about 2218, if the line with frequency 1042 cm 21 is a singlet. In this case the CARS spectrum will contain only nondispersing background. From Fig. 5, we can see that there is no such polarizer orientation at which the total suppression of resonant component is observed. At e ˆ 221.58 the contour which is characteristic for interference between two resonances with opposite phases and equal amplitudes takes place. The results of the mathematical simulation of the spectra consisting of two Lorentzian components are shown in Fig. 5 by dotted lines. Small differences between the calculated and experimental spectra are connected with a considerable contribution from the closest lines 970 and 1080 cm 21 at these angles. The spectral parameters of the line 1042 cm 21 components are the following: the normalized susceptibilities: NR xR1 1111 =x1111 ˆ 1:1 ^ 0:1; NR xR2 1111 =x1111 ˆ 0:35 ^ 0:05;

where the depolarization ratios are: r 1 ˆ 0.08 ^ 0.01; r 2 ˆ 0.13 ^ 0.01; the halfwidths: G1 ˆ 6.3 ^ 0.2 cm 21;

G2 ˆ 10.0 ^ 0.5 cm 21; and the distance between maxima is V 1 – V 2 ˆ 4.5 ^ 0.5 cm 21. It is obvious that these components cannot be resolved in IR and spontaneous Raman spectra. The present results allow us to conclude that the liquid BENA is a mixture of two conformers. 4. Ab initio calculation By analogy with other nitramines [8,9] it is possible to suppose that BENA can exist as conformations with the methyl groups located either on the same side or on the different sides of the C2NNO2 fragment, which can be planar or nonplanar. To solve the problem, an ab initio calculation at RHF/6-31G pp level was carried out [10]. Three stable conformations were found, their geometrical parameters are listed in Table 2. One can notice from the table that the C2NNO2 fragment is planar in both conformations I and II, the CH3 groups being localized either on the different sides of the C2NNO2 fragment or on the same side. The first one is the most stable. In conformation III the C2NNO2 fragment is nonplanar, the energy difference between the two most stable and the third conformation is large, which makes the existence of BENA in the form III practically impossible. This conclusion is in good agreement with the results obtained by the polarization CARS method. Fig. 6 shows the two possible conformations of the (CH3CH2)2NNO2 molecule. The most stable conformation has a C2 symmetry, the other one has a Cs symmetry. 5. Vibrational assignment and normal coordinate analysis The interpretation of vibrational spectra of BENA and the identification of the crystal conformer was based on the spectral data of other nitramines studied by us [8,9,11–14], band intensities in IR and Raman spectra, depolarization ratios in Raman spectra, as well as on normal coordinate analysis using ab initio calculations. The following assignment is obvious: the n (CH2) and n (CH3) modes have frequencies in the 3000– 2850 cm 21 region. The frequencies in the 1480– 1350 cm 21 region are assigned to the d (CH3) and

L.V. Khristenko et al. / Journal of Molecular Structure 509 (1999) 275–285 Table 4 Observed and calculated frequencies (cm 21) (CH3CH2)2NNO2 molecule Number

Observed

Calculated (C2)

Calculated (Cs)

Assignment (PED, %) a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

3004 – 2991 2987 2975 2956 2940 2918 – 2884 1520 1472 1467 1460 1453 – 1428 1395 1390 1382 1358 1330 – 1286 1224 1201 1088 – 1078 1042 972 942 822 764 775 788 610 600 460

3003 3001 2988 2988 2959 2958 2940 2928 2895 2895 1518 1465 1458 1454 1444 1443 1429 1402 1397 1374 1350 1346 1314 1286 1230 1207 1093 1092 1079 1047 960 938 825 771 759 754 609 606 452

3005 2999 2989 2987 2966 2960 2946 2933 2901 2898 1519 1466 1457 1452 1448 1441 1429 1402 1394 1375 1346 1345 1319 1289 1227 1207 1095 1088 1080 1048 966 931 825 770 755 754 607 606

(34) n as(CH2) , 1 (33) n as(CH2) (32) n as(CH2) 1 (31) n as(CH2) , (36) n as(CH3) 0 , 1 (33) n as(CH3) 0 (36) n as(CH3) 0 1 (33) n as(CH3) 0 , (47) n as(CH3) 1 (47) n as(CH2) , (47) n as(CH3) , 1 (47) n as(CH3) (43) n s(CH2) , 1 (43) n s(CH2) (41) n s(CH2) 1 (41) n s(CH2) , (48) n s(CH3) 1 (47) n s(CH3) , (47) n s(CH3) , 1 (47) n s(CH3) (58) n as(NO2) 1 (14) r r(NO2) (24) d (CH2) , 1 (24) d (CH2) (42) d as(CH)3 0 , 1 (41) d as(CH3) 0 (28) d as(CH)3 0 1 (28) d as(CH3) 0 , (44) d as(CH)3 , 1 (44) d as(CH3) (40) d as(CH)3 1 (40) d as(CH3) , (43) d (CH2) 1 (43) d (CH2) , (20) d s(CH3) 1 (20) d s(CH3) , 1 (20) r w(CH2) 1 (20) r w(CH2) , (33) d s(CH3) 1 (33) d s(CH3) , (23) d s(CH3) 1 (23) d s(CH3) , (30) r t(CH2) , 1 (30) r t(CH2) (31) r w(CH2) 1 (31) r w(CH2) , (29) r t(CH2) , 1 (29) r t(CH2) (38) n s(NO2) 1 (17) d (NO2) 1 13 n (NN) 32 n as(NC2) 1 (13) r t(CH2) , 1 (13) r t(CH2) (14) r r(CH2) 1 (14) r r(CH2) , 1 (13) r r(CH3) , 1 (13) r r(CH3) (17) r r(CH3) 0 , 1 (17) r r(CH3) 0 1 (20) n (CC) (24) r r(CH3) 0 , 1 (24) r r(CH3) 0 1 (18) n (CC) (13) r r(CH2) 1 (13) r r(CH2) , 1 (10) r r(CH3) , 1 (10) r r(CH3) 16 n (NN) 1 20 n s(NC2) (21) n (CC) , 1 (21) n (CC) (30) n (CC) 1 (30) n (CC) , (39) d (NO2) 1 (8) r r(CH3) 1 (8) r r(CH3) (77) r w(NO2) (21) r r(CH2) 1 (21) r r(CH2) , (23) r w(NO2) 1 (21) r r(CH2) 1 (21) r r(CH2) , (38) r r(NO2) 1 (11) r r(NC2) (26) d (NO2) 1 (26) n (NN) (32) d (CCN) 1 (32) d (CCN) ,

421 40

432 419 b 408 b 350 330 315 b

41 42 43 44 45 46 47 48

217 150 127 77 60 a b

420 419 405 357 328 236 230 135 128 90 47

(63) d (CNC) 1 (6) n (NN) (48) r r(NC2) 1 (13) r r(NO2) (35) d (CCN) 1 (35) d (CCN) ,

312 228 226 144 145 80 78

(38) t (CC) 1 (38) t (CC) , (41) t (CC) 1 (41) t (CC) , (84) t (NN) (35) r w(CNC) 1 (28) t (CN) 1 (28) t (CN) , (41) t (CN) 1 (41) t (CN) , (67) r w(CNC) 1 (17) t (CN) 1 (17) t (CN) ,

P.E.D. are given for the C2-conformer, the , sign is used for the description of H3C(8)H2C(7)-group. Frequencies are vanished in Raman spectrum of crystal BENA.

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Fig. 5. Polarization CARS spectra (solid lines) and simulated spectra (dotted lines) of (CH3CH2)2NNO2 for different orientations of polarizer (angle e ). Dashed lines show the nonresonant backgrounds.

d (CH2) bends, while the r (CH2) and r (CH3) vibrations are observed below 1400 cm 21. Frequencies relation is as follows: r w(CH2) . r t(CH2) . r r(CH3) . r r(CH2). The r r(CH3) and r r(CH2) vibrations are mixed, as well as the r w(CH2) and r t(CH2) modes are mixed with the n s(NO2) and n as(NC2) stretches. The strong IR band at 1520 cm 21 was assigned to n as(NO2), whereas n s(NO2) correlates to the 1286 cm 21 band, which is strong in the IR spectrum and moderate and polarized in the Raman spectrum. According to the results of vibrational calculations, a lower frequency should be assigned to n as(NC2). The proper band must be depolarized in the Raman spectrum. Only the band at 1224 cm 21, which is

very weak both in IR and Raman spectra, fits these requirements. The bands at 1200 and 1081 cm 21, strong and polarized in the Raman spectrum, are caused by r r(CH3), while the frequencies between 800 and 750 cm 21 correspond to r r(CH2). In the IR spectrum of the liquid 784 and 774 cm 21 shoulders are observed on the strong band at 764 cm 21, that correspond to the r w(NO2) vibration. The frequencies at 784 and 774 cm 21 are assigned to the r r(CH2) vibrations. In the IR spectrum of crystal these shoulders become strong bands. In the Raman spectrum one strong polarized band at 775 cm 21 is present. The 820 cm 21 band is assigned to n s(NC2), it is

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283 21

Fig. 6. Two models of (CH3CH2)2NNO2 molecule.

moderate in the IR spectrum and strong and polarized in the Raman spectrum. Stretching vibrations n (NN) and n (CC) should correlate with frequencies in the 900–1050 cm 21 region. The following bands are observed there: a very strong and polarized one in the Raman spectrum at 1042 cm 21, a strong and polarized one in the Raman spectrum at 970 cm 21, as well as a weak and depolarized one at 942 cm 21. The 942 cm 21 band can be assigned only to the n as(CC) vibration for the reason symmetry of both stable conformers, whereas the other two, either to n s(CC), or to n (NN). From ab initio calculation it is obtained that n (NN) . n (CC) and the difference between n s(CC) and n as(CC) is found to be smaller than between n (NN) and n (CC). The difference between the values of frequencies for

n s(CC) and n as(CC) does not exceed 40 cm in diethyl ether [15]. On this basis, we assign the 1042 cm 21 band to the n (NN) vibration, and the 970 cm 21 band to n s(CC). Bending vibrations d (NO2) and r r(NO2) have close frequencies and correlate with a band situated at 610 cm 21, with a shoulder at 600 cm 21, which is strong and polarized in Raman and moderate in IR spectra. Bands in the 500–300 cm 21 region are due to bending skeletal vibrations d (CCN), d (CNC) and r r(NC2). The r w(NC2) vibration frequency is less than 100 cm 21, thus a weak band both in IR and Raman spectra at 60 cm 21 may correspond to it. The 217 cm 21 band, very weak in IR and Raman spectra, originates from one of the torsional vibrations about the C–C bonds. The bands at 77 and 156 cm 21 can be assigned to the t (CN) torsional vibrations, while the band at 127 cm 21 can be assigned to the t (NN) torsional vibration. As mentioned above, there are no cases in which any band in the region higher than 500 cm 21 in the IR and Raman spectra of crystal BENA vanishes. Ab initio calculation shows almost the same values of modal frequencies in this region for C2 and Cs conformers, whereas they are distinguished below 500 cm 21 and the bands at 419, 408 and 315 cm 21 are disappeared in the Raman spectrum of crystal of BENA. According to the results of ab initio calculation, the C2 conformer remains in the crystal of BENA. Force fields for the C2 and Cs conformers were calculated at the HF/6-31G pp level of theory. The force constant matrices, obtained in Cartesian coordinates, were transformed to the local symmetry (valence) coordinates [16]. First of all the vibrational frequencies for both conformers were computed with unscaled force field. Proceeding from the interpretation of vibrational spectra of BENA, mentioned above, and the method described in Ref. [17], scale factors for ab initio force field of the most stable conformer C2 were obtained. The scale factors for the r w(NC2), t (CN) and t (NN) torsions were set equal to unity, as frequencies calculated from an unscaled force field were lower than the corresponding experimental values. The computed scale factors are listed in Table 3. The same scale factors were adopted for Cs conformer. Frequencies and modes, calculated using the scaled

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Table 5 The stretching force constants of (CH3CH2)2NNO2 molecule ˚ 21) (mdyn A Force constant

C2 conformer

Cs conformer

F(NO) F(NN) F(CN) F(CC)

8.2478 5.8923 4.6593 4.2885

8.2611 5.8488 4.6391 4.2768

ab initio force fields of both conformers are shown in Table 4, together with experimental data. Calculated frequencies agree with the experimental data quite well. It was noted that the use of the same scale factor for all r (CH2) vibrations results in the calculated frequencies of r r(CH2) smaller than r w(NO2), although an inverse ratio is observed experimentally. Moreover, for the C2 conformer, the modes r r(CH2) and r w(NO2) (B symmetry) are mixed.

6. Discussion The spectroscopic features, the polarized CARS method as well as the results of ab initio calculations show that (CH3CH2)2NNO2 in liquid state exists as the equilibrium of two conformers, having a planar nitramine fragment and the CH3 groups practically orthogonal to it and located either on the same or on the opposite sides of frame. The latter conformer is the most stable and remains in the crystal. As shown in Table 2, the geometrical parameters for both conformers are quite close. The bond lengths of N–N, C–N and C–C in Cs conformer are slightly ˚ ) than those of C2 conformer. larger (,0.001–0.002 A The CCN and CNC angles in the Cs conformer are by

18 bigger than those in C2 conformer. It can be explained by the steric interactions, which are stronger in the Cs conformer than in the C2 one. It is very important to note that the Cs-model has some steric strain because of interaction of two CH3 groups (C6…C8 (C2) ˆ 4.689, C6…C8 (Cs) ˆ ˚ ). 3.883 A The orthogonal positions of the CH3 groups in (CH3CH2)2NNO2 are the peculiarities of the nitramine derivatives. For example, in CH3CH2ONO2 [18] there is an equilibrium of anti (more stable) and gauche conformers. Sterically, it is impossible to have the conformer of (CH3CH2)2NNO2 with the anti positions of both CH3 groups simultaneously. Therefore, we have a difference in the conformations of two molecules (CH3CH2)2NNO2 and CH3CH2ONO2. The streching force constants of the C2 and Cs conformers are listed in Table 5. The force constants of the N–N, C–N and C–C bonds for the Cs conformer are smaller than those for the C2 one, whereas the opposite is true for the N–O bond. The relatively low scale factor values from 0.66 to 0.74 (Table 3), probably, can be explained by the low level of the calculation, which did not include the effect of electron correlation. This effect is displayed in the decrease of calculated NO and NN bond lengths. The ab initio scaled force constants for bondstretching in the (CH3)2NNO2, (CH2ClCH2)2NNO2 and (CH3CH2)2NNO2 molecules are compared in Table 6. The nitramine fragments of (CH2ClCH2)2NNO2 [9] and (CH3CH2)2NNO2 are planar, in contrast to (CH3)2NNO2 molecule in which it is nonplanar because of small pyramidality of amine nitrogen. The difference in the force constants of N–N bond of (CH3)2NNO2 and (CH2ClCH2)2NNO2 and (CH3CH2)2NNO2 may be ascribed to this fact.

Table 6 ˚ 21) The stretching force constants of related molecules (mdyn A

F(NO) F(NN) F(CN) F(CC) a

(CH3)2NNO2 [3] a Cs

(CH3CH2)2NNO2 [this work] C2

(CH2ClCH2)2NO2 [9] C2(GG)

8.39 5.59 4.92

8.25 5.89 4.66 4.29

8.22 5.90 4.90 4.23

The force constant values were obtained by using the frequencies for IR spectrum in CCl4 solution.

L.V. Khristenko et al. / Journal of Molecular Structure 509 (1999) 275–285

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