- Email: [email protected]
Crystal and molecular structure of 1,2-dihydro-1-methyl-2-nitriminopyridine: X-ray and infrared studies
Journal of Molecular Structure 513 (1999) 69–77 www.elsevier.nl/locate/molstruc
Crystal and molecular structure of 1,2-dihydro-1-methyl-2nitriminopyridine: X-ray and infrared studies Z. Daszkiewicz, J.B. Kyziol⁄, J. Zaleski* Institute of Chemistry, University of Opole, 45-052 Opole, Oleska 48, Poland Received 4 January 1999; received in revised form 22 February 1999; accepted 2 March 1999
Abstract The crystal and molecular structure of 1,2 dihydro-1-methyl-2-nitriminopyridine (1) at 90.0(1) K have been determined. It ˚ , Z 4, R(F) 0.0259 crystallises in an orthorhombic Pna21 space group with a 7.753(2), b 13.829(3) and c 6.070(1) A 2 for 1856 unique reflections. The pyridine ring is planar, the N(1) nitrogen atom remains sp hybridised. The NNO2 group is twisted 268 along C–N bond and 158 along N–N bond. The twist is caused by a steric hindrance and/or weak C–H…O hydrogen bonds. IR spectra of (1), N-(2-pyridyl)-nitramine (4) and N-methyl-N-(2-pyridyl)-nitramine (9) were recorded in solution and in the solid state. The frequencies characteristic of the nitrimino group (1260 and 1436 cm 21) and the isomeric nitramine (1236 and 1536) differ markedly in the region of asymmetric stretching vibration. The spectra of (4) indicate that it exists in the nitrimino form in solution as well as in the solid state. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Nitramines; Nitrimines; X-ray structure; IR spectra
1. Introduction Pyridylnitramines are readily accessible intermediates and syntones, which can be prepared by the simple nitration of aminopyridines with mixed acids. Their methylation in an alkaline, aqueous solution provides only traces of the corresponding secondary nitramines; the main products are invariably nitrimines, such as 1.
The rearrangement of 1 in concentrated sulphuric acid gives 2-(N-methylamino)-5-nitro-pyridine (2) [1]. The final product may result either from the migration of N-methyl group or the Dimroth rearrangement involving ring opening and ring closure. Thermal rearrangement of 1 gives 1-methyl-2-pyridone (3) in moderate yield [2]. Chemical properties of the nitramines and nitrimines of the pyridine series differ in some aspects and do not resemble those of
* Corresponding author. 0022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00112-X
70
Z. Daszkiewicz et al. / Journal of Molecular Structure 513 (1999) 69–77
primary and secondary phenylnitramines. Several studies on the molecular structure of pyridylnitramines, carried out by the spectroscopic and rentgenostructural methods, could not give a clear structure– property relationship as yet. Kolehmainen et al. made multinuclear magnetic resonance studies and found that different types of nitrogen atoms exhibit very clearly separated ranges of 15N resonances [3,4]. More precisely, a broad region characteristic of the pyridine ring nitrogen can be divided into three sub-regions: 63–83 ppm, characteristic of secondary nitramines, 95–110 ppm, typical for primary nitramines and 112–125 ppm, in which the signals of 1-methyl substituted nitrimines appear. The rule is not applicable to the derivatives of N-(3-pyridyl)-nitramine which require a separate consideration. The products of methylation of primary pyridylnitramines can be distinguished by the chemical shift of the N-nitro nitrogen atom. The signal appears within 9–15 ppm in nitrimines and in the region of 30–35 ppm in secondary nitramines. In the spectra of primary N-pyridylnitramines, the resonance of N-nitro group appears within 15–35 ppm due to the prototropic tautomerism, as suggested by Kolehmainen [3]. The same process may be responsible for the differences in the resonance of the nitramide nitrogen. The signal appears within 190–200 ppm in the spectra of secondary nitramines and is shifted to ca. 212 ppm in the case of nitrimines. The resonances of primary nitramines vary within a broad range of 143–207 ppm. The proton transfer from the exo to endocyclic nitrogen must cause a significant change in the charge distribution in a molecule and, consequently, in its spectral and chemical properties. The studies in solutions indicate that the relative rates of proton migration and tautomeric equilibria are strongly structure dependent. Rentgenostructural studies lead to the same conclusion. Krygowski et al. demonstrated that simple Npyridylnitramines exist in the solid state as the
nitrimino tautomeric forms [5]. The acidity of a primary nitramino group is nearly the same as that of a carboxyl substituent, so in the presence of a basic centre in the aromatic ring, formation of the nitrimino or zwitterionic form is very probable. On the contrary, 2-nitramino-3-nitropyridine (5) exists in the crystal lattice in the nitramino tautomeric form, i.e. proton is bound to the exocyclic nitrogen atom [4].
In a typical, secondary aromatic nitramine, the NNO2 group is significantly twisted (e.g. for 668 in 6) along the Ar–N bond [6]. The nitramino group in 5 is also twisted for ca. 548 along C–N bond. The nitramine 5 and nitrimine 4 have a common feature: in the crystal lattice they are arranged in dimers connected by a N–H…N hydrogen bond. In the nitrimine 7, the NNO2 group diverges only 88 from the plane of the pyridine ring, probably due to the conjugation between two p-electron systems of the molecule 7 [7]. It should be mentioned, that in all cases the arrangement of the N–N and N–O bonds around the N-nitro nitrogen atom is planar. Our goal was to determine the molecular structure of 1 and particularly the conformation of the NNO2 group.
2. Experimental 2.1. N-(2-pyridyl)-nitramine (4) 2-Aminopyridine (9.40 g, 0.1 mol) was added to cold sulphuric acid (20 ml). The viscous solution was stirred on an ice-bath and the solution of absolute nitric acid (12.3 ml, 0.3 mol) in sulphuric acid (10 ml) was added dropwise. The mixture was maintained at 58C for 15 min and poured into 500 ml water then warmed to the boiling point. The clear solution was cooled and the product was collected by filtration and
Z. Daszkiewicz et al. / Journal of Molecular Structure 513 (1999) 69–77
71
Table 1 Crystal data and structure refinement for 2-NNO2C5H4NCH3 Empirical formula Formula weight Temperature Wavelength Crystal system, space group Unit cell dimensions
Volume Z, Calculated density Absorption coefficient F(000) Crystal size u range for data collection Index ranges Reflections collected/unique Completeness to 2u 30.07 Refinement method Data/restraints/parameters Goodness-of-fit on F 2 Final R(F) indices [I . 2s (I)] R indices (all data) Absolute structure parameter Largest diff. peak and hole D/s max
C6 H7 N3 O2 153.15 90.0(1) K ˚ 0.71073 A Orthorhombic, Pna21 ˚ a 7.753(2) A ˚ b 13.829(3) A ˚ c 6.070(1) A ˚ 650.8(2) A 4, 1.563 Mg/m 3 0.121 mm 21 320 0.4 × 0.4 × 0.6 mm 3 2.95–30.078 210 # h # 10, 0 # k # 19, 28 # l # 8 3561/1856 [R(int) 0.0204] 97.5% Full-matrix least-squares on F 2 1855/1/129 1.088 R1 0.0259, wR2 0.0696 R1 0.0275, wR2 0.0714 0.6(8) 0.28 and 2 0.16 e A 23 0.001
Table 2 Atomic coordinates ( × 10 4), isotropic (hydrogen) and equivalent ˚ × 10 3) for non-hydrogen isotropic displacement parameters (A atoms for 2-NNO2C5H4NCH3. U(eq) is defined as one-third of the trace of the orthogonalised Uij tensor
N(1) C(2) C(3) C(4) C(5) C(6) N(7) N(8) O(9) O(10) C(11) H(3) H(4) H(5) H(6) H(11A) H(11B) H(11C)
x
y
z
U(eq)
2096(1) 2447(1) 1862(1) 926(1) 572(1) 1193(1) 3473(1) 3398(1) 2184(1) 4558(1) 2694(1) 2179(22) 560(19) 267(24) 1022(22) 2238(20) 3910(22) 2337(23)
7312(1) 8267(1) 8937(1) 8633(1) 7648(1) 7006(1) 8419(1) 9307(1) 9881(1) 9505(1) 6581(1) 9594(12) 9113(11) 7396(13) 6338(12) 6769(10) 6607(11) 5943(13)
4560(1) 4201(2) 5786(2) 7573(2) 7871(2) 6352(2) 2408(1) 1511(2) 1871(1) 169(1) 2967(2) 5537(29) 8660(26) 9042(31) 6506(29) 1520(28) 2862(30) 3502(34)
11(1) 11(1) 14(1) 15(1) 15(1) 13(1) 13(1) 13(1) 18(1) 17(1) 14(1) 22(4) 16(4) 27(4) 22(4) 14(3) 20(4) 30(4)
Fig. 1. Molecular structure of 1,2-dihydro-1-methyl-2-nitriminopyridine.
recrystallised from water (600 ml). N-(2-pyridyl)nitramine (11.00 g, 79%) was obtained. It melts with the decomposition at 196–2008C; lit. [2] m.p. 198– 2038C. MS, m/z (int.): 139 (11, M 1), 94 (14), 93 (100), 66 (39), 39 (27). 2.2. 1,2-Dihydro-1-methyl-2-nitriminopyridine (1) N-(2-pyridyl)-nitramine (2.80 g, 0.02 mol) and potassium carbonate (3.60 g, 0.2 mol) were dissolved in water (25 ml). Dimethyl-sulphate (1.8 ml, 0.02 mol) was added and the mixture was shaken at room temperature until homogeneous. The solution was cooled to 08C. A precipitate was collected by filtration and crystallised from water. 1,2-Dihydro-1methyl-2-nitriminopyridine (2.21 g, 72%) was obtained as white needles, m.p. 158–1598C; lit. [2] m.p. 160–1618C. MS, m/z (int.): 153 (63, M 1), 107 (100), 92 (24), 80 (46), 78 (46), 66 (18), 53 (21).
72
Z. Daszkiewicz et al. / Journal of Molecular Structure 513 (1999) 69–77
Crystals suitable for X-ray diffraction studies were grown by slow evaporation of the methanol solution. X-ray data collection was carried out at 90 K on KM4 KUMA diffractometer with MoKa radiation and Oxford Cryosystem cooler (dry nitrogen gas stream, temperature, stability 0.1 K). Lattice parameters were refined from setting angles of 35 reflections in the 188 , 2u , 358 range. During data collection v – u scan technique was used (scan speed 0.02–0.18 s 21, scan width 1.38). Two control reflections measured after an interval of 50 reflections show that intensity variation was negligible. A list of calculated and observed structure factors may be obtained from authors on request. The structure was solved by direct methods and refined by a full-matrix least-squares method using shelx97 program [8]. Lorentz and polarisation corrections were applied. All hydrogen atoms were
3. Results and discussion
located from difference Fourier synthesis. Nonhydrogen atoms were refined anisotropically, temperature parameters for hydrogen atoms were freely refined. The details of the data collection and processing are listed in Table 1. In Table 2 are shown the atomic co-ordinates and isotropic displacement parameters for non-hydrogen atoms. The structure drawings were prepared using shelxtl program [9]. The infrared spectra were recorded on a Philips PU9804 FTIR spectrometer in KBr pellets or in saturated solutions in deuteriated solvents which were transparent within the 1200–1600 cm 21 region. The composite bands were resolved by the differentiation.
The differences between the C–C bond length which are formally single and double in 1 do not ˚ and are nearly the same as observed exceed 0.05 A in the naphthalene rings which are aromatic beyond any doubt. All the bonds around N(1) are arranged in plane, indicating trigonal hybridisation of the endocyclic nitrogen atom. There may only be one conclusion: despite the commonly used schematic notation, the pyridine ring in 1 is aromatic. In the next scheme, the geometry of the NNO2 group in 1, 1,4-dihydro-1-methyl-4-nitriminopyridine (7) [7] and 2-nitramino-3-nitropyridine (5) [4] is presented.
To avoid any complication which may emerge from the hydrogen bonding, we have examined the molecular structure of 1,2-dihydro-1-methyl-2-nitriminopyridine (1) at 300 K. The results are rather confusing. The bond lengths and valence angles do not differ from the typical values but the torsion angles along the C(2)–N(7) and N(7)–N(8) bonds are much higher than those found in the nitrimine 7. The measurement was repeated at 90 K but there was no significant difference in the geometry of the molecule at the ambient and liquid nitrogen temperature. The geometry of 1 molecule is presented in Fig. 1, the relevant numerical data are collected in Table 3. Let us first consider the structure of the pyridine ring. The bond lengths of 1 are given on the scheme below together with two structures taken from [5].
Z. Daszkiewicz et al. / Journal of Molecular Structure 513 (1999) 69–77 Table 3 ˚ ), angles (8) and selected torsion angles for 2Bond lengths (A NNO2C5H4NCH3 N(1)–C(6) N(1)–C(2) N(1)–C(11) C(2)–N(7) C(2)–C(3) C(3)–C(4) C(4)–C(5) C(5)–C(6) N(7)–N(8) N(8)–O(10) N(8)–O(9) C(6)–N(1)–C(2) C(6)–N(1)–C(11) C(2)–N(1)–C(11) N(7)–C(2)–N(1) N(7)–C(2)–C(3) N(1)–C(2)–C(3) C(4)–C(3)–C(2) C(3)–C(4)–C(5) C(6)–C(5)–C(4) N(1)–C(6)–C(5) N(8)–N(7)–C(2) O(10)–N(8)–O(9) O(10)–N(8)–N(7) O(9)–N(8)–N(7) N(8)–N(7)–C(2)–C(3) N(8)–N(7)–C(2)–N(1) C(2)–N(7)–N(8)–O(9) C(2)–N(7)–N(8)–O(10)
1.361(1) 1.367(1) 1.473(1) 1.364(1) 1.411(1) 1.372(1) 1.402(1) 1.368(1) 1.344(1) 1.244(1) 1.251(1) 122.0(1) 118.3(1) 119.7(1) 113.1(1) 129.1(1) 117.6(1) 120.5(1) 120.3(1) 118.3(1) 121.3(1) 116.0(1) 121.2(1) 115.8(1) 122.8(1) 2 28.3(2) 158.0(2) 2 18.2(2) 166.5(2)
The C–N and N–N bonds in 5 are slightly longer than in 1 and 7. It can be rationalised in a simple way: in the 5 molecule, the same number of valence electrons has to bind one more atom viz. hydrogen on N(7). The NNO2 groups in nitrimines 7 and 1 look nearly the same: in the 7 molecule, the C–N bond is ˚ and the N–N bond is shorter for longer for 0.004 A ˚ 0.015 A but these differences are negligible. The most important difference is the N–N–C valence angle which is smaller in 1. It should be noted that there is a large difference between N–C–C and N–C–N angles between the nitrimino group and the ring, probably resulting from repulsion between NNO2 and the aromatic ring. If the pyridine ring and NNO2 group were arranged in plane, it must have reduced the distance ˚ , while between O(9) and H(3) to the value of ca. 2.1 A ˚ . In the sum of Van der Waals radii amounts ca. 2.60 A fact, the conformation of NNO2 group is not planar,
73
the nitrimino group is twisted ca. 168 along the N–N bond and ca. 258 along the C–N bond (cf. Table 3). Such an arrangement seems to be strange for a first sight as it must decrease the effectiveness of p bonding. The mean deviation of the 1 molecule from the plane, calculated by the least-squares method, for all ˚. the pyridine ring atoms does not exceed 0.006 A From this point of view (Fig. 1, lower part) it can be seen that the N(7) nitrogen atom is shifted up to ˚ , N(8) is shifted down to 0.38 A ˚ , while the 0.08 A oxygen O(10) atom is situated nearly in the plane. The second oxygen O(9) is pushed out of the plane ˚ . Deviations from the plane of the aromatic for 1.13 A ˚ for H(5) to 0.05 A ˚ hydrogen atoms vary from 0.02 A for H(3). Such geometry may result from the intermolecular C–H…O hydrogen bond, from the steric hindrance or intermolecular interactions in the crystal lattice which disturbs coplanarity of the 1 molecule. In the IR spectrum of N-methyl-N-(2-pyridyl)nitramine (9), the most intense bands at 1276 and 1534 cm 21 can be assigned to the symmetric and asymmetric stretch of the N-nitro group Fig. 2(c). The analogous bands in the spectra of ring substituted N-methyl-N-phenyl-nitramines appear in the regions of 1285–1299 and 1517–1536 cm 21 [10]. The spectrum of 1 differs from those of secondary nitramines in some aspects. A broad band with the maximum at 1236 cm 21 is an envelope of two strong (1263, 1235 cm 21) and three weak (1294, 1313 and 1334 cm 21) peaks. In the spectrum registered in the chloroform-d1 solution, only one strong and sharp band (1258 cm 21) is observed in the region characteristic of the NNO2 symmetric stretch. Consequently, we must conclude that the absorption at ca. 1260 cm 21 is characteristic of the nitrimino group of that kind as in 1 molecule. Its asymmetric counterpart is not a good group frequency due to the medium intensity and that the skeletal vibrations give some bands in the same region. Considering the frequencies characteristic of the 2-substituted pyridine ring [11, p. 223], the only signal which can be assigned to the asymmetric stretching vibrations of the N-nitro group is that at 1436 cm 21 (1450 cm 21 in the CDCl3 solution). Such a difference (ca. 100 cm 21), in comparison with 9, may help to distinguish between the structure of the true secondary nitramine and the nitrimine as 1. The spectrum of the primary nitramine
74
Z. Daszkiewicz et al. / Journal of Molecular Structure 513 (1999) 69–77
Fig. 2. Infrared spectra in KBr of N-(2-pyridyl)-nitramine (4) and its N-methyl derivatives 1 and 9.
Z. Daszkiewicz et al. / Journal of Molecular Structure 513 (1999) 69–77
75
Fig. 2. (continued)
4 is more complex; the bands corresponding to the NNO2 stretching vibrations appear at 1265 and 1428 cm 21 (1263 and 1432 cm 21 in solution). There are, however, some very strong bands (in KBr: 1543, 1444, 1375, 1317 and 1229 cm 21) which are absent in the spectrum registered in the diluted solution in acetonitrile-d6. They must come from the intermolecular interactions; such a behaviour is characteristic of hydrogen-bonded compounds, e.g. alcohols [11, p. 273]. The spectra of primary nitramine 4, in the region of NNO2 stretch, are similar to those of 1, indicating prevalence of the nitrimino tautomeric form of 4. There are no significant differences between the NNO2 absorption in the solid state and in solutions, the molecules 1 and 4 exist in the same form in both states. It should be mentioned here that isotopic [ 15N– NO2] labelling of N-(4-pirydyl)-nitramine and the nitrimine 7 revealed that the nitrimino group stretching vibrations appear in the 1228–1260 cm 21 and 1409–1448 cm 21 regions [12].
Another interesting region of the infrared absorption is that near 3000 cm 21, where the protons stretching vibrations are observed. Unfortunately, 1 and 4 are insoluble in non-polar solvents as carbon
Fig. 3. Packing diagram of 1,2-dihydro-1-methyl-2-nitriminopyridine showing C–H…O intermolecular interactions.
76
Z. Daszkiewicz et al. / Journal of Molecular Structure 513 (1999) 69–77
disulphide, hence only the spectra of the solid compounds can be compared. There are two separate Ar–H peaks (3065, 3011 cm 21) in the spectrum of 9 and a sharp band (3085 cm 21) with some weak shoulders (3068, 3048, 3030 cm 21) in the 1 spectrum. Their appearance in the spectrum may be attributed to the intermolecular interactions causing the aforementioned deviations of some C–H bonds from the plane of aromatic ring. The corresponding bands in the spectrum of 4 cannot be recognised. There is a strong and broad absorption (2500–3220 cm 21) with several sub-maxima; the most intense ones are observed at 2864, 2808 and 2719 cm 21. The shift and enhancement of the N–H absorption indicates that the N–
1263 cm 21 is accompanied with some shoulders. Conclusion is that the peculiar conformation of the nitrimino group is determined with the p -electron system. However, the crystal structure of 1 (Fig. 3) indicates the close intermolecular distances between the pushed out oxygen O(9) and H(4), H(6) and H(11) atoms (Table 4). These C–H…O interactions contribute, to some extent to the resultant conformation of the nitrimino group (twist along the C–N and N–N bonds). Considering the structure of the pyridine ring and NNO2 group, we must look at 1 as the resonance hybrid of the mesomeric forms depicted below.
H…N hydrogen bond may be a decisive factor in the arrangement of 4 molecules in the crystal lattice. The shift of the frequency characteristic of the Ar–H bonds from 3011–3065 cm 21 in 9 to higher wave numbers (3030–3085 cm 21) in 1 suggests that there is no any attractive interaction between the nitrimino group and the adjacent proton H(3). The intramolecular hydrogen bond can be excluded also due to the geometrical reasons: the O(9)–H(3) line is nearly perpendicular to the C(3)–H(3) bond. The ˚ ) distance between O(9) and H(3) close (2.13 A should be considered as the sterical hindrance responsible for the deformation of the nitrimino group. However, the intermolecular interactions in the crystal lattice may also play some role in resultant conformation. The stretching frequency of the NNO2 group is the same in the solid state and in solution, but in the first case the maximum at
The formulae 1a and 1b have no physical meaning; they represent the electronic structure of molecule 1 when considered together. The canonical form 1a allows rotations along the N–N bond; the second one (1b) makes possible deviation from the coplanarity by rotation along the C–N bond. Each of these deformations suffices to enlarge the H…O distance to the sum of the Van der Waals radii. The actual conformation is a compromise between these possibilities. As shown in Fig. 1, it leads to the arrangement as planar as possible considering repulsive interaction between O(11) and H(3) atoms. Some explanations can be given if we assume an analogy in the electronic structure of N, N-dimethylnitramine and the nitrimine 1. The electron structure of the nitramino group was thoroughly studied by the photoelectron spectroscopy supported by computational methods. The results were consistent and convincing hence interpretation
Table 4 Close interatomic contacts involving hydrogen atoms
C(4)–H(4)…O9 C(6)–H(6)…O(9) C(11)–H(11C)…O(9)
˚) C–H (A
˚) H…O (A
˚) C…O (A
C–H…A (8)
0.98(2) 0.94(2) 0.98(2)
2.55(2) 2.45(2) 2.55(2)
3.197(1) 3.213(1) 3.340(1)
131(1) 138(1) 138(1)
Z. Daszkiewicz et al. / Journal of Molecular Structure 513 (1999) 69–77
of the resonance interaction in the NNO2 group is straightforward.
The formulae drawn above in a conventional way, inform us that the outer valence shell in the molecule of N,N-dimethylnitramine consists of four multicentre p -orbitals of non-bonding character [13].
The resonance in pyridylnitrimines may be interpreted in an analogous way, despite of some differences in the structure of N,N-dimethylnitramine and 1. Rotation along the C(2)–N(7) and N(7)–N(8) bonds do not influence the energies of the s-orbitals forming the inner valence shell. If the p -orbitals have nonbonding character, their energies will also remain unaffected with small deformation observed in the NNO2 group. Hence, rotation along the C–N and N–N bonds may result from the steric hindrance. This explanation must be considered as a provisional one until the studies on the charge distribution in nitrimines will be accomplished.
77
References [1] A. Thomas, P. Tomasik, G. Herman-Matusiak, Bull. Acad. Sci. Pol. Chem. 23 (1975) 311. [2] Z. Daszkiewicz, A. Domanski, J.B. Kyziol⁄, Chem. Papers 51 (1997) 22. [3] E. Kolehmainen, K. Laihia, R. Kauppinen, R. Gawinecki, D. Rasal⁄a, Magn. Reson. Chem. 31 (1993) 659. [4] E. Kolehmainen, K. Laihia, R. Kauppinen, R. Gawinecki, D. Rasal⁄a, Magn. Reson. Chem. 30 (1992) 527. [5] T.M. Krygowski, D. Pawlak, R. Anulewicz, D. Rasal⁄a, R. Gawinecki, G. Ha¨felinger, M.N. Homsi, F.K.H. Kuske, Acta Chem. Scand. 50 (1996) 808. [6] K. Ejsmont, J.B. Kyziol⁄, Z. Daszkiewicz, M. Bujak, Acta Crystallogr. C54 (1998) 672. [7] M. Bujak, K. Ejsmont, J.B. Kyziol⁄, Z. Daszkiewicz, J. Zaleski, Acta Crystallogr. C54 (1998) 1945. [8] G.M. Sheldrick, SHELX97, Program for Solution and Refinement of Crystal Structure, University of Go¨ttingen, Germany, 1997. [9] G.M. Sheldrick, SHELXTL, Siemens Analytical X-ray Instruments Inc., Madison, WI, USA, 1990. [10] Z. Daszkiewicz, E. Nowakowska, W.W. Prez˙do, J.B. Kyziol⁄, Pol. J. Chem. 69 (1995) 1437. [11] N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press, New York, 1964. [12] Unpublished results. [13] M.G. White, R.J. Colton, T.H. Lee, J.W. Rabalais, J. Chem. Phys. 8 (1975) 391.
Crystal and molecular structure of 1,2-dihydro-1-methyl-2nitriminopyridine: X-ray and infrared studies Z. Daszkiewicz, J.B. Kyziol⁄, J. Zaleski* Institute of Chemistry, University of Opole, 45-052 Opole, Oleska 48, Poland Received 4 January 1999; received in revised form 22 February 1999; accepted 2 March 1999
Abstract The crystal and molecular structure of 1,2 dihydro-1-methyl-2-nitriminopyridine (1) at 90.0(1) K have been determined. It ˚ , Z 4, R(F) 0.0259 crystallises in an orthorhombic Pna21 space group with a 7.753(2), b 13.829(3) and c 6.070(1) A 2 for 1856 unique reflections. The pyridine ring is planar, the N(1) nitrogen atom remains sp hybridised. The NNO2 group is twisted 268 along C–N bond and 158 along N–N bond. The twist is caused by a steric hindrance and/or weak C–H…O hydrogen bonds. IR spectra of (1), N-(2-pyridyl)-nitramine (4) and N-methyl-N-(2-pyridyl)-nitramine (9) were recorded in solution and in the solid state. The frequencies characteristic of the nitrimino group (1260 and 1436 cm 21) and the isomeric nitramine (1236 and 1536) differ markedly in the region of asymmetric stretching vibration. The spectra of (4) indicate that it exists in the nitrimino form in solution as well as in the solid state. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Nitramines; Nitrimines; X-ray structure; IR spectra
1. Introduction Pyridylnitramines are readily accessible intermediates and syntones, which can be prepared by the simple nitration of aminopyridines with mixed acids. Their methylation in an alkaline, aqueous solution provides only traces of the corresponding secondary nitramines; the main products are invariably nitrimines, such as 1.
The rearrangement of 1 in concentrated sulphuric acid gives 2-(N-methylamino)-5-nitro-pyridine (2) [1]. The final product may result either from the migration of N-methyl group or the Dimroth rearrangement involving ring opening and ring closure. Thermal rearrangement of 1 gives 1-methyl-2-pyridone (3) in moderate yield [2]. Chemical properties of the nitramines and nitrimines of the pyridine series differ in some aspects and do not resemble those of
* Corresponding author. 0022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00112-X
70
Z. Daszkiewicz et al. / Journal of Molecular Structure 513 (1999) 69–77
primary and secondary phenylnitramines. Several studies on the molecular structure of pyridylnitramines, carried out by the spectroscopic and rentgenostructural methods, could not give a clear structure– property relationship as yet. Kolehmainen et al. made multinuclear magnetic resonance studies and found that different types of nitrogen atoms exhibit very clearly separated ranges of 15N resonances [3,4]. More precisely, a broad region characteristic of the pyridine ring nitrogen can be divided into three sub-regions: 63–83 ppm, characteristic of secondary nitramines, 95–110 ppm, typical for primary nitramines and 112–125 ppm, in which the signals of 1-methyl substituted nitrimines appear. The rule is not applicable to the derivatives of N-(3-pyridyl)-nitramine which require a separate consideration. The products of methylation of primary pyridylnitramines can be distinguished by the chemical shift of the N-nitro nitrogen atom. The signal appears within 9–15 ppm in nitrimines and in the region of 30–35 ppm in secondary nitramines. In the spectra of primary N-pyridylnitramines, the resonance of N-nitro group appears within 15–35 ppm due to the prototropic tautomerism, as suggested by Kolehmainen [3]. The same process may be responsible for the differences in the resonance of the nitramide nitrogen. The signal appears within 190–200 ppm in the spectra of secondary nitramines and is shifted to ca. 212 ppm in the case of nitrimines. The resonances of primary nitramines vary within a broad range of 143–207 ppm. The proton transfer from the exo to endocyclic nitrogen must cause a significant change in the charge distribution in a molecule and, consequently, in its spectral and chemical properties. The studies in solutions indicate that the relative rates of proton migration and tautomeric equilibria are strongly structure dependent. Rentgenostructural studies lead to the same conclusion. Krygowski et al. demonstrated that simple Npyridylnitramines exist in the solid state as the
nitrimino tautomeric forms [5]. The acidity of a primary nitramino group is nearly the same as that of a carboxyl substituent, so in the presence of a basic centre in the aromatic ring, formation of the nitrimino or zwitterionic form is very probable. On the contrary, 2-nitramino-3-nitropyridine (5) exists in the crystal lattice in the nitramino tautomeric form, i.e. proton is bound to the exocyclic nitrogen atom [4].
In a typical, secondary aromatic nitramine, the NNO2 group is significantly twisted (e.g. for 668 in 6) along the Ar–N bond [6]. The nitramino group in 5 is also twisted for ca. 548 along C–N bond. The nitramine 5 and nitrimine 4 have a common feature: in the crystal lattice they are arranged in dimers connected by a N–H…N hydrogen bond. In the nitrimine 7, the NNO2 group diverges only 88 from the plane of the pyridine ring, probably due to the conjugation between two p-electron systems of the molecule 7 [7]. It should be mentioned, that in all cases the arrangement of the N–N and N–O bonds around the N-nitro nitrogen atom is planar. Our goal was to determine the molecular structure of 1 and particularly the conformation of the NNO2 group.
2. Experimental 2.1. N-(2-pyridyl)-nitramine (4) 2-Aminopyridine (9.40 g, 0.1 mol) was added to cold sulphuric acid (20 ml). The viscous solution was stirred on an ice-bath and the solution of absolute nitric acid (12.3 ml, 0.3 mol) in sulphuric acid (10 ml) was added dropwise. The mixture was maintained at 58C for 15 min and poured into 500 ml water then warmed to the boiling point. The clear solution was cooled and the product was collected by filtration and
Z. Daszkiewicz et al. / Journal of Molecular Structure 513 (1999) 69–77
71
Table 1 Crystal data and structure refinement for 2-NNO2C5H4NCH3 Empirical formula Formula weight Temperature Wavelength Crystal system, space group Unit cell dimensions
Volume Z, Calculated density Absorption coefficient F(000) Crystal size u range for data collection Index ranges Reflections collected/unique Completeness to 2u 30.07 Refinement method Data/restraints/parameters Goodness-of-fit on F 2 Final R(F) indices [I . 2s (I)] R indices (all data) Absolute structure parameter Largest diff. peak and hole D/s max
C6 H7 N3 O2 153.15 90.0(1) K ˚ 0.71073 A Orthorhombic, Pna21 ˚ a 7.753(2) A ˚ b 13.829(3) A ˚ c 6.070(1) A ˚ 650.8(2) A 4, 1.563 Mg/m 3 0.121 mm 21 320 0.4 × 0.4 × 0.6 mm 3 2.95–30.078 210 # h # 10, 0 # k # 19, 28 # l # 8 3561/1856 [R(int) 0.0204] 97.5% Full-matrix least-squares on F 2 1855/1/129 1.088 R1 0.0259, wR2 0.0696 R1 0.0275, wR2 0.0714 0.6(8) 0.28 and 2 0.16 e A 23 0.001
Table 2 Atomic coordinates ( × 10 4), isotropic (hydrogen) and equivalent ˚ × 10 3) for non-hydrogen isotropic displacement parameters (A atoms for 2-NNO2C5H4NCH3. U(eq) is defined as one-third of the trace of the orthogonalised Uij tensor
N(1) C(2) C(3) C(4) C(5) C(6) N(7) N(8) O(9) O(10) C(11) H(3) H(4) H(5) H(6) H(11A) H(11B) H(11C)
x
y
z
U(eq)
2096(1) 2447(1) 1862(1) 926(1) 572(1) 1193(1) 3473(1) 3398(1) 2184(1) 4558(1) 2694(1) 2179(22) 560(19) 267(24) 1022(22) 2238(20) 3910(22) 2337(23)
7312(1) 8267(1) 8937(1) 8633(1) 7648(1) 7006(1) 8419(1) 9307(1) 9881(1) 9505(1) 6581(1) 9594(12) 9113(11) 7396(13) 6338(12) 6769(10) 6607(11) 5943(13)
4560(1) 4201(2) 5786(2) 7573(2) 7871(2) 6352(2) 2408(1) 1511(2) 1871(1) 169(1) 2967(2) 5537(29) 8660(26) 9042(31) 6506(29) 1520(28) 2862(30) 3502(34)
11(1) 11(1) 14(1) 15(1) 15(1) 13(1) 13(1) 13(1) 18(1) 17(1) 14(1) 22(4) 16(4) 27(4) 22(4) 14(3) 20(4) 30(4)
Fig. 1. Molecular structure of 1,2-dihydro-1-methyl-2-nitriminopyridine.
recrystallised from water (600 ml). N-(2-pyridyl)nitramine (11.00 g, 79%) was obtained. It melts with the decomposition at 196–2008C; lit. [2] m.p. 198– 2038C. MS, m/z (int.): 139 (11, M 1), 94 (14), 93 (100), 66 (39), 39 (27). 2.2. 1,2-Dihydro-1-methyl-2-nitriminopyridine (1) N-(2-pyridyl)-nitramine (2.80 g, 0.02 mol) and potassium carbonate (3.60 g, 0.2 mol) were dissolved in water (25 ml). Dimethyl-sulphate (1.8 ml, 0.02 mol) was added and the mixture was shaken at room temperature until homogeneous. The solution was cooled to 08C. A precipitate was collected by filtration and crystallised from water. 1,2-Dihydro-1methyl-2-nitriminopyridine (2.21 g, 72%) was obtained as white needles, m.p. 158–1598C; lit. [2] m.p. 160–1618C. MS, m/z (int.): 153 (63, M 1), 107 (100), 92 (24), 80 (46), 78 (46), 66 (18), 53 (21).
72
Z. Daszkiewicz et al. / Journal of Molecular Structure 513 (1999) 69–77
Crystals suitable for X-ray diffraction studies were grown by slow evaporation of the methanol solution. X-ray data collection was carried out at 90 K on KM4 KUMA diffractometer with MoKa radiation and Oxford Cryosystem cooler (dry nitrogen gas stream, temperature, stability 0.1 K). Lattice parameters were refined from setting angles of 35 reflections in the 188 , 2u , 358 range. During data collection v – u scan technique was used (scan speed 0.02–0.18 s 21, scan width 1.38). Two control reflections measured after an interval of 50 reflections show that intensity variation was negligible. A list of calculated and observed structure factors may be obtained from authors on request. The structure was solved by direct methods and refined by a full-matrix least-squares method using shelx97 program [8]. Lorentz and polarisation corrections were applied. All hydrogen atoms were
3. Results and discussion
located from difference Fourier synthesis. Nonhydrogen atoms were refined anisotropically, temperature parameters for hydrogen atoms were freely refined. The details of the data collection and processing are listed in Table 1. In Table 2 are shown the atomic co-ordinates and isotropic displacement parameters for non-hydrogen atoms. The structure drawings were prepared using shelxtl program [9]. The infrared spectra were recorded on a Philips PU9804 FTIR spectrometer in KBr pellets or in saturated solutions in deuteriated solvents which were transparent within the 1200–1600 cm 21 region. The composite bands were resolved by the differentiation.
The differences between the C–C bond length which are formally single and double in 1 do not ˚ and are nearly the same as observed exceed 0.05 A in the naphthalene rings which are aromatic beyond any doubt. All the bonds around N(1) are arranged in plane, indicating trigonal hybridisation of the endocyclic nitrogen atom. There may only be one conclusion: despite the commonly used schematic notation, the pyridine ring in 1 is aromatic. In the next scheme, the geometry of the NNO2 group in 1, 1,4-dihydro-1-methyl-4-nitriminopyridine (7) [7] and 2-nitramino-3-nitropyridine (5) [4] is presented.
To avoid any complication which may emerge from the hydrogen bonding, we have examined the molecular structure of 1,2-dihydro-1-methyl-2-nitriminopyridine (1) at 300 K. The results are rather confusing. The bond lengths and valence angles do not differ from the typical values but the torsion angles along the C(2)–N(7) and N(7)–N(8) bonds are much higher than those found in the nitrimine 7. The measurement was repeated at 90 K but there was no significant difference in the geometry of the molecule at the ambient and liquid nitrogen temperature. The geometry of 1 molecule is presented in Fig. 1, the relevant numerical data are collected in Table 3. Let us first consider the structure of the pyridine ring. The bond lengths of 1 are given on the scheme below together with two structures taken from [5].
Z. Daszkiewicz et al. / Journal of Molecular Structure 513 (1999) 69–77 Table 3 ˚ ), angles (8) and selected torsion angles for 2Bond lengths (A NNO2C5H4NCH3 N(1)–C(6) N(1)–C(2) N(1)–C(11) C(2)–N(7) C(2)–C(3) C(3)–C(4) C(4)–C(5) C(5)–C(6) N(7)–N(8) N(8)–O(10) N(8)–O(9) C(6)–N(1)–C(2) C(6)–N(1)–C(11) C(2)–N(1)–C(11) N(7)–C(2)–N(1) N(7)–C(2)–C(3) N(1)–C(2)–C(3) C(4)–C(3)–C(2) C(3)–C(4)–C(5) C(6)–C(5)–C(4) N(1)–C(6)–C(5) N(8)–N(7)–C(2) O(10)–N(8)–O(9) O(10)–N(8)–N(7) O(9)–N(8)–N(7) N(8)–N(7)–C(2)–C(3) N(8)–N(7)–C(2)–N(1) C(2)–N(7)–N(8)–O(9) C(2)–N(7)–N(8)–O(10)
1.361(1) 1.367(1) 1.473(1) 1.364(1) 1.411(1) 1.372(1) 1.402(1) 1.368(1) 1.344(1) 1.244(1) 1.251(1) 122.0(1) 118.3(1) 119.7(1) 113.1(1) 129.1(1) 117.6(1) 120.5(1) 120.3(1) 118.3(1) 121.3(1) 116.0(1) 121.2(1) 115.8(1) 122.8(1) 2 28.3(2) 158.0(2) 2 18.2(2) 166.5(2)
The C–N and N–N bonds in 5 are slightly longer than in 1 and 7. It can be rationalised in a simple way: in the 5 molecule, the same number of valence electrons has to bind one more atom viz. hydrogen on N(7). The NNO2 groups in nitrimines 7 and 1 look nearly the same: in the 7 molecule, the C–N bond is ˚ and the N–N bond is shorter for longer for 0.004 A ˚ 0.015 A but these differences are negligible. The most important difference is the N–N–C valence angle which is smaller in 1. It should be noted that there is a large difference between N–C–C and N–C–N angles between the nitrimino group and the ring, probably resulting from repulsion between NNO2 and the aromatic ring. If the pyridine ring and NNO2 group were arranged in plane, it must have reduced the distance ˚ , while between O(9) and H(3) to the value of ca. 2.1 A ˚ . In the sum of Van der Waals radii amounts ca. 2.60 A fact, the conformation of NNO2 group is not planar,
73
the nitrimino group is twisted ca. 168 along the N–N bond and ca. 258 along the C–N bond (cf. Table 3). Such an arrangement seems to be strange for a first sight as it must decrease the effectiveness of p bonding. The mean deviation of the 1 molecule from the plane, calculated by the least-squares method, for all ˚. the pyridine ring atoms does not exceed 0.006 A From this point of view (Fig. 1, lower part) it can be seen that the N(7) nitrogen atom is shifted up to ˚ , N(8) is shifted down to 0.38 A ˚ , while the 0.08 A oxygen O(10) atom is situated nearly in the plane. The second oxygen O(9) is pushed out of the plane ˚ . Deviations from the plane of the aromatic for 1.13 A ˚ for H(5) to 0.05 A ˚ hydrogen atoms vary from 0.02 A for H(3). Such geometry may result from the intermolecular C–H…O hydrogen bond, from the steric hindrance or intermolecular interactions in the crystal lattice which disturbs coplanarity of the 1 molecule. In the IR spectrum of N-methyl-N-(2-pyridyl)nitramine (9), the most intense bands at 1276 and 1534 cm 21 can be assigned to the symmetric and asymmetric stretch of the N-nitro group Fig. 2(c). The analogous bands in the spectra of ring substituted N-methyl-N-phenyl-nitramines appear in the regions of 1285–1299 and 1517–1536 cm 21 [10]. The spectrum of 1 differs from those of secondary nitramines in some aspects. A broad band with the maximum at 1236 cm 21 is an envelope of two strong (1263, 1235 cm 21) and three weak (1294, 1313 and 1334 cm 21) peaks. In the spectrum registered in the chloroform-d1 solution, only one strong and sharp band (1258 cm 21) is observed in the region characteristic of the NNO2 symmetric stretch. Consequently, we must conclude that the absorption at ca. 1260 cm 21 is characteristic of the nitrimino group of that kind as in 1 molecule. Its asymmetric counterpart is not a good group frequency due to the medium intensity and that the skeletal vibrations give some bands in the same region. Considering the frequencies characteristic of the 2-substituted pyridine ring [11, p. 223], the only signal which can be assigned to the asymmetric stretching vibrations of the N-nitro group is that at 1436 cm 21 (1450 cm 21 in the CDCl3 solution). Such a difference (ca. 100 cm 21), in comparison with 9, may help to distinguish between the structure of the true secondary nitramine and the nitrimine as 1. The spectrum of the primary nitramine
74
Z. Daszkiewicz et al. / Journal of Molecular Structure 513 (1999) 69–77
Fig. 2. Infrared spectra in KBr of N-(2-pyridyl)-nitramine (4) and its N-methyl derivatives 1 and 9.
Z. Daszkiewicz et al. / Journal of Molecular Structure 513 (1999) 69–77
75
Fig. 2. (continued)
4 is more complex; the bands corresponding to the NNO2 stretching vibrations appear at 1265 and 1428 cm 21 (1263 and 1432 cm 21 in solution). There are, however, some very strong bands (in KBr: 1543, 1444, 1375, 1317 and 1229 cm 21) which are absent in the spectrum registered in the diluted solution in acetonitrile-d6. They must come from the intermolecular interactions; such a behaviour is characteristic of hydrogen-bonded compounds, e.g. alcohols [11, p. 273]. The spectra of primary nitramine 4, in the region of NNO2 stretch, are similar to those of 1, indicating prevalence of the nitrimino tautomeric form of 4. There are no significant differences between the NNO2 absorption in the solid state and in solutions, the molecules 1 and 4 exist in the same form in both states. It should be mentioned here that isotopic [ 15N– NO2] labelling of N-(4-pirydyl)-nitramine and the nitrimine 7 revealed that the nitrimino group stretching vibrations appear in the 1228–1260 cm 21 and 1409–1448 cm 21 regions [12].
Another interesting region of the infrared absorption is that near 3000 cm 21, where the protons stretching vibrations are observed. Unfortunately, 1 and 4 are insoluble in non-polar solvents as carbon
Fig. 3. Packing diagram of 1,2-dihydro-1-methyl-2-nitriminopyridine showing C–H…O intermolecular interactions.
76
Z. Daszkiewicz et al. / Journal of Molecular Structure 513 (1999) 69–77
disulphide, hence only the spectra of the solid compounds can be compared. There are two separate Ar–H peaks (3065, 3011 cm 21) in the spectrum of 9 and a sharp band (3085 cm 21) with some weak shoulders (3068, 3048, 3030 cm 21) in the 1 spectrum. Their appearance in the spectrum may be attributed to the intermolecular interactions causing the aforementioned deviations of some C–H bonds from the plane of aromatic ring. The corresponding bands in the spectrum of 4 cannot be recognised. There is a strong and broad absorption (2500–3220 cm 21) with several sub-maxima; the most intense ones are observed at 2864, 2808 and 2719 cm 21. The shift and enhancement of the N–H absorption indicates that the N–
1263 cm 21 is accompanied with some shoulders. Conclusion is that the peculiar conformation of the nitrimino group is determined with the p -electron system. However, the crystal structure of 1 (Fig. 3) indicates the close intermolecular distances between the pushed out oxygen O(9) and H(4), H(6) and H(11) atoms (Table 4). These C–H…O interactions contribute, to some extent to the resultant conformation of the nitrimino group (twist along the C–N and N–N bonds). Considering the structure of the pyridine ring and NNO2 group, we must look at 1 as the resonance hybrid of the mesomeric forms depicted below.
H…N hydrogen bond may be a decisive factor in the arrangement of 4 molecules in the crystal lattice. The shift of the frequency characteristic of the Ar–H bonds from 3011–3065 cm 21 in 9 to higher wave numbers (3030–3085 cm 21) in 1 suggests that there is no any attractive interaction between the nitrimino group and the adjacent proton H(3). The intramolecular hydrogen bond can be excluded also due to the geometrical reasons: the O(9)–H(3) line is nearly perpendicular to the C(3)–H(3) bond. The ˚ ) distance between O(9) and H(3) close (2.13 A should be considered as the sterical hindrance responsible for the deformation of the nitrimino group. However, the intermolecular interactions in the crystal lattice may also play some role in resultant conformation. The stretching frequency of the NNO2 group is the same in the solid state and in solution, but in the first case the maximum at
The formulae 1a and 1b have no physical meaning; they represent the electronic structure of molecule 1 when considered together. The canonical form 1a allows rotations along the N–N bond; the second one (1b) makes possible deviation from the coplanarity by rotation along the C–N bond. Each of these deformations suffices to enlarge the H…O distance to the sum of the Van der Waals radii. The actual conformation is a compromise between these possibilities. As shown in Fig. 1, it leads to the arrangement as planar as possible considering repulsive interaction between O(11) and H(3) atoms. Some explanations can be given if we assume an analogy in the electronic structure of N, N-dimethylnitramine and the nitrimine 1. The electron structure of the nitramino group was thoroughly studied by the photoelectron spectroscopy supported by computational methods. The results were consistent and convincing hence interpretation
Table 4 Close interatomic contacts involving hydrogen atoms
C(4)–H(4)…O9 C(6)–H(6)…O(9) C(11)–H(11C)…O(9)
˚) C–H (A
˚) H…O (A
˚) C…O (A
C–H…A (8)
0.98(2) 0.94(2) 0.98(2)
2.55(2) 2.45(2) 2.55(2)
3.197(1) 3.213(1) 3.340(1)
131(1) 138(1) 138(1)
Z. Daszkiewicz et al. / Journal of Molecular Structure 513 (1999) 69–77
of the resonance interaction in the NNO2 group is straightforward.
The formulae drawn above in a conventional way, inform us that the outer valence shell in the molecule of N,N-dimethylnitramine consists of four multicentre p -orbitals of non-bonding character [13].
The resonance in pyridylnitrimines may be interpreted in an analogous way, despite of some differences in the structure of N,N-dimethylnitramine and 1. Rotation along the C(2)–N(7) and N(7)–N(8) bonds do not influence the energies of the s-orbitals forming the inner valence shell. If the p -orbitals have nonbonding character, their energies will also remain unaffected with small deformation observed in the NNO2 group. Hence, rotation along the C–N and N–N bonds may result from the steric hindrance. This explanation must be considered as a provisional one until the studies on the charge distribution in nitrimines will be accomplished.
77
References [1] A. Thomas, P. Tomasik, G. Herman-Matusiak, Bull. Acad. Sci. Pol. Chem. 23 (1975) 311. [2] Z. Daszkiewicz, A. Domanski, J.B. Kyziol⁄, Chem. Papers 51 (1997) 22. [3] E. Kolehmainen, K. Laihia, R. Kauppinen, R. Gawinecki, D. Rasal⁄a, Magn. Reson. Chem. 31 (1993) 659. [4] E. Kolehmainen, K. Laihia, R. Kauppinen, R. Gawinecki, D. Rasal⁄a, Magn. Reson. Chem. 30 (1992) 527. [5] T.M. Krygowski, D. Pawlak, R. Anulewicz, D. Rasal⁄a, R. Gawinecki, G. Ha¨felinger, M.N. Homsi, F.K.H. Kuske, Acta Chem. Scand. 50 (1996) 808. [6] K. Ejsmont, J.B. Kyziol⁄, Z. Daszkiewicz, M. Bujak, Acta Crystallogr. C54 (1998) 672. [7] M. Bujak, K. Ejsmont, J.B. Kyziol⁄, Z. Daszkiewicz, J. Zaleski, Acta Crystallogr. C54 (1998) 1945. [8] G.M. Sheldrick, SHELX97, Program for Solution and Refinement of Crystal Structure, University of Go¨ttingen, Germany, 1997. [9] G.M. Sheldrick, SHELXTL, Siemens Analytical X-ray Instruments Inc., Madison, WI, USA, 1990. [10] Z. Daszkiewicz, E. Nowakowska, W.W. Prez˙do, J.B. Kyziol⁄, Pol. J. Chem. 69 (1995) 1437. [11] N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press, New York, 1964. [12] Unpublished results. [13] M.G. White, R.J. Colton, T.H. Lee, J.W. Rabalais, J. Chem. Phys. 8 (1975) 391.