Crystal structure, vibrational and NMR studies and chemical quantum calculations of 2-phenylazo-5-nitro-6-methyl-pyridine (C12H10N4O2)

Journal of Molecular Structure 744–747 (2005) 377–392 www.elsevier.com/locate/molstruc

Crystal structure, vibrational and NMR studies and chemical quantum calculations of 2-phenylazo-5-nitro-6-methyl-pyridine (C12H10N4O2) J. Michalskia, E. Kucharskaa, M. Wandasa, J. Hanuzaa,b,*, A. Was´kowskab, M. Ma˛czkab, Z. Talika, S. Olejniczakc, M.J. Potrzebowskic a

Department of Bioorganic Chemistry, Faculty of Engineering and Economics, Institute of Chemistry and Food Technology, University of Economics, Wrocław, Poland b Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, Wrocław 50-950, Poland c Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, Ło´dz´, Poland Received 6 September 2004; revised 15 November 2004; accepted 16 November 2004 Available online 3 February 2005

Abstract Synthesis of 2-phenylazo-5-nitro-6-methyl-pyridine is described. Its X-ray structure is reported and discussed in terms of the molecular conformation of this compound. The crystal is triclinic, space group P-1, with the unit cell parameters aZ6.372(1), bZ7.522(2), ˚ , and aZ6.372(1), bZ89.62(3)8 and gZ101.57(3)8. The pyridine and phenyl rings are planar deflected by torsional angle cZ12.495(2) A ˚, JZ4.8(3)8. The crystal structure is stabilised by non-classical hydrogen interaction of the C–H/O type with C/O distance 3.307(5) A ˚ and C–H/O angle equal to 147.8(3)8. These interactions in the crystal structure couple pairs of the molecules H/O distance 2.481(3) A related by an inversion centre. FT-IR, Raman and NMR spectra of this compound have also been measured. The 6-31G(d,p) basis set with the B3LYP functional has been used to discuss the structure and dynamics of the compound studied. q 2004 Elsevier B.V. All rights reserved.

1. Introduction The pyridine derivatives have an important position among the heterocyclic compounds because they can be used as nonlinear materials and photochemicals. In particular, some of these crystals have been reported as frequency converters from NIR to the visible wavelength region. These crystals require a noncentrosymmetric structure of the unit cell what can be achieved by connecting two bulky groups in the molecule which is a component of the unit cell. The phenylazo- and phenylhydrazonitropyridines could constitute such NLO system. The nitropyridine derivatives are particularly interesting because they form an acceptor fragment of 2-adamantylamino-5-nitropyridine (AANP), which showed a very large nonlinearity among * Corresponding author. Address: Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, Wrocław 50-950, Poland. Tel.: C48 71 343 5021; fax: C48 71 441029. E-mail address: [email protected] (J. Hanuza). 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.11.072

the materials reported earlier [1]. Azobenzene and its derivatives have been also considered as model photochromic systems. These molecules undergo a reversible photochemical transformation in which the cycle consists of the isomerization between the trans and cis forms of the molecule. The trans–cis reaction is photochemically active. This effect is also present in other systems and azo- and hydrazo-pyridine derivatives are excellent candidates for such studies. The cis–trans isomerization and the acid base equilibria of azo compounds have been studied previously [2–4]. The importance of such systems follows from the fact that azobenzene derivatives could be applied as dyes for fibers [5,6], pH indicators [7] and colorimetric tests for protein studies [8]. The vibrational studies of trans-azobenzenes have been performed both in solution and in the solid state using infrared [9–12], resonance Raman [13–18] and Raman [19,20] spectroscopy. The assignment of the azo bands has been made with use of isotopically substituted derivatives

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[14,21–24] and resonance Raman profiles [25,26]. The vibrational analysis of trans-azobenzenes has been performed by Klima et al. [27] and Armstrong et al. [28]. The IR, Raman, resonance Raman spectra and molecular orbital calculations allow to assign the bands near 1400 cmK1 to the NaN bond stretch and other vibrations in the region 1100–1200 cmK1 to the Ph–NaN symmetric bend. It was stated that the coupling of the azo vibration with the phenyl modes, especially the C–H bending modes, contribute to the difficulty in assigning the azo vibrations [29–31]. Although extensive studies have been performed on azobenzenes, the phenyloazopyridine and its derivatives have not been systematically studied by means of X-ray, vibrational and chemical quantum methods, and the present knowledge on this class of compounds is still in progress. In the present paper, 2-phenylazo-5-nitro-6-methyl-pyridine of possible application of such systems as SHG systems or Raman lasers is studied. A number of such new compounds has been obtained by us. The present paper reports the results of our experimental (structural and vibrational) and theoretical (vibrational dynamics) studies of phenylazopyridine derivative. A special attention is devoted to molecular conformations of the –NaN– and C–H/O hydrogen bonds formed in these compounds and their influence on the molecular structure.

2. Experimental 2.1. Synthesis 2-Phenylazo-5-nitro-6-methyl-pyridine (PANP) was synthesised from phenylhydrazo-5-nitro-6-methyl-pyridine obtained previously from 2-fluoro-5-nitro-6-methylpyridine. 0.01 mole of 2-fluoro-5-nitro-6-methyl-pyridine was dissolved in 10 cm3 of methanol and 0.02 mole of 2-phenylohydrazine was added. The mixture was heated at 60 8C for 2 min and then stored at ambient conditions for 24 h. The solvent was distilled under the low pressure and the residue was extracted with the heated chloroform. Non-soluble phenylo-hydrazine fluorohydrat was filtered off and washed with the warm chloroform. The chloroformic extract was vaporized to the dryness and the residue was dissolved in a small amount of pharmaceutical petrol to remove the remnants of the phenylohydrazine. Non-soluble product, yellow small crystals, was filtered off, dried and re-crystallized from the mixture of benzene and pharmaceutical petrol. The yield of 2-phenylhydrazo-5-nitro-6-methyl-pyridine (C12H12N4O2) was 66.1% and its melting point was 146 8C. The chemical analysis: theoretical content: C 59.00, N 22.94%, H 4.95%; found: C 59.14, N 23.28% and H 4.81%. The synthesis of PANP was the continuation of the described above process. One gram of 2-phenylhydrazo-5nitro-6-methyl-pyridine was added to nitric acid of 1.4

density heated up to 50 8C. The reaction is egzothermic with the secretion of nitrogen oxides giving the red color of the mixture. The mixture was heated at 50–60 8C for 5 min, stored at ambient conditions for 2 h and then diluted with 10 cm3 of water. The dry residue of orange color was filtered off and re-crystallized from acetonitrile. The synthesized orange crystals melt at 142 8C. The yield of the PANP (C12H10N4O2) was 74.4%. The results of the chemical analysis: theoretical content: C 59.49%, N 23.13%, H 4.16%; found: C 59.20%, N 23.40%, H 4.00%. 2.2. Chemical quantum calculations The molecular structure of PANP was optimised at the DFT level using the hybrid of Becke’s non-local three parameter exchange and correlation functional with the Lee–Yang–Parr correlation functional (B3LYP) [32–34]. The 6-31G(d,p) basis [35–41] has been used in the calculations. The X-ray data presented in this paper were the starting point of the structure optimisation. Raman and IR wavenumbers as well as their intensities were calculated at the same DFT level using the GAUSSIAN 98W program [42]. The calculated and experimental values were compared using the scaling factors method to correct the evaluated wavenumbers for vibrational anharmonicity and deficiencies inherent to the computational level used. The AniMol program [43] was used for visualisation of the vibrational normal modes of PANP molecule, and BALGA program [44] for estimation of the PED contributions of the internal coordinates to the normal modes. 2.3. Raman and IR measurements Room temperature FT-IR spectra (IR) in the 5000– 30 cmK1 region were measured on the BIORAD 575 spectrophotometer with a 2 cmK1 resolution. The Nujol and Fluorolube mulls as well as KBr pellets were used in the sampling procedure with the 1:300 ratio of the PANP sample and diluting agent. The wavenumbers of the IR bands were the same in all these measurements. Room temperature FT Raman spectra (RS) were measured using BRUKER 110/S spectrometer with the Nd:YAG laser excitation and 2 cmK1 resolution. These studies were made for the PANP powder sample. The number of collected scans was 100 and 32 for the IR and Raman spectra, respectively. The spectra of PANP, benzene and pyridine were measured for comparison. 2.4. NMR measurements High-resolution solid-state 13C NMR spectra were recorded at 74.67 MHz on a Bruker MSL 300 spectrometer, using cylindrical rotor. All spectra were recorded at 300 K for polycrystalline powder samples under conditions of

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magic angle spinning (MAS) with frequencies at 2 from 10 kHz. The 1H/13C cross-polarization (CP) pulse sequence incorporating the total suppression of sinning sidebands (TOSS) technique used. Solid-state 13C NMR spectra were recorded using the dipolar dephasing technique allowing the identification of carbon atom that are not directly bonded to hydrogen. Isotropic 13C chemical shifts are given relative to tetramethylsilane. The principal elements of the 13C chemical shift tensor and shielding parameters were calculated employing the WIN-MAS program [45]. The details describing the method and accuracy of calculations are exhaustively discussed elsewhere [46]. The resulting parameters are quoted using the convention that d11Od22Od33, together with the anisotropy, dCSA, defined as d33Kdiso, and the asymmetry parameter, h, defined as (d33Kd22)/d11, span U, defined as d11Kd33 and skew, k, defined as 3(d22Kdiso)/U where the isotropic chemical shift, diso is (d11Cd22Cd33)/3. 13 C NMR spectra of the sample dissolved in CDCl3 solution have been recorded on a Bruker 500 spectrometer Tetramethylsilane (TMS) was used as reference agent. 2D double quantum filtered 1H, 1H COSY experiments and 2D z-pulsed field gradient selected 1H, 13C heteronuclear multiple bond correlation (HMBC) experiments have been performed to assign reliably the 13C NMR spectra. Calculations of nuclear shielding of carbons have been performed using the GAUSSIAN 98 program with the B3LYP functional and different basis sets 6-311G, 6-311CCG, 6-311G**. These calculations were done for optimised geometry at the B3LYP/6-31G(d,p) level. The calculated isotropic shielding constants si were transformed to chemical shifts relative to TMS for 13C by diZsTMSKsi. These calculations were made on the same level of theory as the other DFT calculations. 2.5. X-ray diffraction of the PANP The crystal of dimensions 0.15!0.20!0.22 mm was chosen for the X-ray diffraction measurements on a fourcircle KM-4/CCD diffractometer (Kuma Diffraction). The intensity data were collected using graphite monochromated ˚ ) and u-scan technique Mo Ka radiation (lZ0.71073 A with DuZ0.808/one image. The exposure time for each frame was 20 s. The 960 images taken in nine different runs covered about 95% of the Evald sphere. One image selected as a standard was monitored after every 60 images, to control the crystal and the electronics stability. The unit cell parameters were determined and refined by a least-squares fitting of angular positions of 800 strongest reflections. The reflection intensities were integrated and corrected for Lorentz and polarization effects [47]. Absorption correction was not applied. Systematic extinctions and the intensity statistics indicated space group P-1. The structure was solved by direct methods using SHELXS program [48] and refined by full-matrix least squares procedure, with

379

Table 1 Crystal data, experimental conditions and the structure refinement parameters for 2-phenylazo-5-nitro-6-methyl-pyridine Identification code Empirical formula Formula weight ˚) Wavelength (A Crystal system, space group ˚ , deg): Unit cell dimensions (A a b c a b g ˚ 3) Volume (A Z, calculated density (Mg/m3) Absorption coefficient (mmK1) F(000) Crystal size (mm) q range for data collection (deg) Limiting indices h k l Reflections collected/unique R(int) Refinement method Data/parameters Goodness-of-fit on F2 Final R indices [IO2s(I)] ˚ K3) Largest diff. peak and hole (eA

PANP C12H10N4O2 242.24 0.71073 Triclinic, P-1 6.372(1) 7.522(2) 12.495(2) 99.06(3) 89.62(3) 101.57(3) 579.2(2) 2, 1.389 0.099 252 0.35!0.32!0.12 3.47–29.37 K8, 8 K7, 10 K16, 16 15631/2915 0.0736 Full-matrix least-squares on F2 2915/158 0.985 R1Z0.0808 wR2Z0.1689 0.554 and K0.385

anisotropic thermal displacement parameters for nonhydrogen atoms [49]. All H-atoms were located in the difference maps. Their positions were constrained assuming ‘ride-on’ model with isotropic temperature factors fixed to 1.2 Ueq of the atom to which the hydrogen was attached. The final difference map showed residual peaks, but they were of no physical meaning (Table 1). The crystal data, further details of the experimental conditions and the structure refinement parameters are given in Table 1.

3. Results and discussion 3.1. Crystal structure of the PANP compound The formula unit of PANP (C12H10N4O2) consists of two moieties, phenyl ring and 5-nitro-6-methyl-pyridine, linked through the –NaN– bond (Fig. 1). Atomic positional coordinates and equivalent isotropic displacement parameters U(eq) are given in Table 2 and the anisotropic displacement parameters are listed in Table 3. The average ˚ and of of the C–N distances in pyridine ring is 1.334(4) A ˚ . The latter value, being a little shorter C–C is 1.383(5) A ˚ reported for non-substituted pyridine, than 1.39–1.41 A may indicate some aromatic bond character [50]. The pyridine ring is planar and also the methyl carbons and N

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Fig. 1. Crystal structure of the PANP and the projection of the molecule parallel to the phenyl ring. The X-ray data (Tables 1–3) are related to the atom numbering presented in these pictures.

of the nitro groups are lying approximately in the plane (Fig. 1). Mean deviation from the least-squares planes ˚ . Also the phenyl through the pyridine ring is ca. 0.004 A ring is planar and preserves an aromatic character shown by alteration of long and short bonds. The pyridine ring with its substituents and phenyl ring take an extended configuration with the dihedral angle between two ring planes jZ 4.8(3)8. The projection of the molecule parallel to the phenyl ring is shown in Fig. 1.

The pC–NaN–Co azo-bridge in PANP takes transconformation with the following bond lengths and angles: ˚ ; NaN, 1.240; N–N–C, 114.5 C–N, 1.430 and 1.421 A and 116.18; C–NaN–C dihedral angle, 4.88. The dimensions and bond angles in the azo-bridge system aC–NaN–Ca show distinct short–long–short interchange sequence. The NaN distance fits well to the range from 1.22 to ˚ , reported for the extended conformations of the azo1.41 A bridge [51].

J. Michalski et al. / Journal of Molecular Structure 744–747 (2005) 377–392 Table 2 Atomic coordinates (!104) and equivalent isotropic displacement ˚ 2!103) for the PANP parameters (A

N(1) N(2) N(3) N(4) C(1) C(2) C(3) C(4) C(5) C(7) C(8) C(9) C(10) C(11) C(12) C(13) O(1) O(2) H(2) H(3) H(7a) H(7b) H(7c) H(9) H(10) H(11) H(12) H(13)

x

y

z

U(eq)

4690(4) 5685(5) 5122(5) K506(5) 4013(5) 1861(6) 416(6) 1110(5) 3274(6) 4199(6) 6741(5) 8867(6) 10305(6) 9672(7) 7585(6) 6133(6) K2107(5) K197(5) 1440 K1024 3578 3908 5717 9303 11720 10669 7159 4717

2790(4) 2842(4) 2389(4) 1334(5) 2419(4) 1681(5) 1356(5) 1744(5) 2472(5) 2893(6) 2789(4) 3620(5) 3917(5) 3378(5) 2568(5) 2272(5) 156(4) 2190(4) 1427 877 3820 1805 3316 3968 4488 3566 2221 1716

2926(2) 4697(3) 5584(3) 1370(3) 3884(3) 4089(3) 3237(3) 2247(3) 2088(3) 1025(3) 6412(3) 6273(3) 7129(3) 8104(3) 8244(3) 7404(3) 1433(2) 610(3) 4772 3332 792 497 1116 5612 7048 8672 8908 7498

53(1) 58(1) 59(1) 65(1) 48(1) 61(1) 60(1) 50(1) 53(1) 73(1) 50(1) 59(1) 65(1) 66(1) 67(1) 61(1) 91(1) 95(1) 73 72 109 109 109 70 78 79 80 73

U(eq) is defined as one-third of the trace of the orthogonalized Uij tensor.

˚ in the nitro The average N–O distance of 1.2215(4) A group is indication of clearly double bonds. The oxygen atoms are slightly shifted above and below the pyridine ring (Fig. 1). The enhanced principal axes of the thermal displacement ellipsoids evidence intense vibrational movements of the NO2 about the C–N bond (Table 3). Such behaviour of the NO2 groups is commonly observed in different nitrobenzenes [52] and nitropyridines [53,54]. It excludes formation of intermolecular strong hydrogen bonds. The crystal structure of PANP is stabilised by nonclassical hydrogen interaction of the C–H/O type [53] with ˚, the C(13)/OKx,Ky,1Kz distance equal to 3.307(5) A ˚ and the C–H(13)/OKx,Ky,1Kz H(13)/OZ2.481(3) A angle equal to 147.8(3)8. This interaction forms a pair of the molecules related by the inversion centre. The pairs are further linked about another inversion centre with the van der Waals contacts between them. The Cartesian coordinates of the hydrogen atoms are listed in Table 2. 3.2. Optimised structure of the PANP molecule The structure of this molecule was optimised in the chemical quantum calculations starting from the atomic positions established by X-ray diffraction studies. Table 3

381

Table 3 ˚ ) and angles (deg) of the PANP determined from X-ray Bond lengths (A diffraction studies and DFT calculations

N(1)–C(2) N(1)–C(6) N(2)–C(2) N(2)–N(3) N(3)–C(8) N(4)–O(1) N(4)–O(2) N(4)–C(5) C(2)–C(3) C(3)–C(4) C(4)–C(5) C(5)–C(6) C(6)–C(7) C(8)–C(13) C(8)–C(9) C(9)–C(10) C(10)–C(11) C(11)–C(12) C(12)–C(13) C(2)–N(1)–C(6) N(1)–C(2)–N(2) C(3)–C(2)–N(2) C(3)–C(2)–N(1) C(3)–C(4)–C(5) C(4)–C(5)–C(6) C(4)–C(5)–N(4) C(6)–C(5)–N(4) C(5)–C(6)–N(1) C(5)–C(6)–C(7) C(7)–C(6)–N(1) C(2)–N(2)–N(3) N(2)–N(3)–C(8) C(9)–C(8)–C(13) N(3)–C(8)–C(9) N(3)–C(8)–C(13) C(8)–C(9)–C(10) C(9)–C(10)–C(11) C(10)–C(11)–C(12) C(11)–C(12)–C(13) C(12)–C(13)–C(8) O(1)–N(4)–O(2) O(1)–N(4)–C(5) O(2)–N(4)–C(5)

X-ray

DFT

1.321(4) 1.347(4) 1.430(4) 1.240(4) 1.421(4) 1.221(4) 1.222(4) 1.462(4) 1.410(5) 1.372(5) 1.365(5) 1.403(5) 1.504(5) 1.386(4) 1.396(5) 1.374(5) 1.377(5) 1.370(5) 1.366(5) 119.4(3) 113.2(3) 123.1(3) 123.7(3) 119.4(3) 121.1(3) 116.6(3) 122.3(3) 119.4(3) 125.0(3) 115.6(3) 114.5(3) 116.1(3) 119.4(3) 124.0(3) 116.6(3) 119.0(4) 120.7(3) 120.5(4) 119.6(4) 120.9(4) 123.0(4) 118.4(3) 118.6(3)

1.347 1.355 1.428 1.279 1.418 1.267 1.265 1.463 1.406 1.385 1.403 1.415 1.503 1.405 1.409 1.392 1.405 1.401 1.396

lists the chosen calculated bond lengths and bond angles together with the comparison of these results to the X-ray data discussed above. 3.3. IR and Raman spectra 3.3.1. Normal modes of the azo bridge Twenty-eight atoms of the PANP give rise to 78 vibrational degrees of freedom. Due to the low symmetry of the molecule (C1) and lack of any symmetry elements all modes should be observed both in the IR and Raman spectra. It means that every normal mode should have the counterparts in the IR and Raman spectra.

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Fig. 2. IR spectra of the PANP in solid state compared to the spectra of the unsubstituted pyridine and benzene as well as to the IR spectrum calculated.

The IR and Raman spectra recorded for the PANP are shown in Figs. 2 and 3. Having in mind the number of bands predicted for IR and Raman spectra we can verify these data with the theoretical DFT calculations. Table 6 lists the calculated wavenumbers together with the PED contributions of the internal coordinates into the normal modes. The atom numbering used in the DFT calculations is specified in Fig. 8. The scaling factors used to compare the calculated and observed wavenumbers are as follows: 0.95

for the bands in the 3000–3100 cmK1 range and 0.96 for the 1400–1700 cmK1 range. The values lower than 1400 cmK1 do not need the use of any scaling factor. The modes of the nitro group vibrations were also not scaled. The description of the modes is based on the commonly accepted nomenclature specified at the bottom of this table. The description of the modes involving the azo-bridge has been introduced by us in our previous papers on the azobipyridine derivatives [55,56]. The pC–NaN–Co bridge exhibits 12

Fig. 3. Raman spectra of the PANP in solid state compared with the spectra of the unsubstituted pyridine and benzene as well as to the Raman spectrum calculated.

J. Michalski et al. / Journal of Molecular Structure 744–747 (2005) 377–392

vibrational degrees of freedom which contain rotational and translational modes. In this set of vibrations there are inplane stretching and bending modes, out-of-plane bending modes, as well as some in-plane and out-of-plane torsional and twisting modes for which the bridge system behaves as a rigid body. In the later modes all atoms of the bridge move in the same direction and the vibration resembles the translation of the whole system. Fig. 4 shows the eigenvectors of all 12 modes involving motions of carbon and nitrogen atoms of the azo-bridge. The PED contributions are also presented in this picture. These 12 modes involve the most significant contribution of the carbon and nitrogen atoms displacement and significant PED contributions of the C–N and NaN coordinates into the normal mode. However, some other modes of the PANP

383

molecule also contain small contributions of these atomic displacements and respective internal coordinates. It is well known that the azo group usually exhibits a very weak band of the stretching n(NaN) vibration in the IR spectra but quite strong, characteristic band in the Raman spectra. This band appears at about 1450 cmK1 and is very sensitive to the environment and tautomeric form [57]. The present DFT calculations indicate that the n(NaN) vibration participate in two normal modes, n21 and n22, respectively. The contribution of the n(NaN) motion to these modes is 11 and 44% (Table 4, Fig. 4). Such result suggests that some resonance occurs between the pyridine and phenyl rings and the bridge system vibrations which couples these motions in the form of doublet transitions. The respective n21 mode is observed in the IR and Raman spectra as medium intensity

Fig. 4. Normal vibrations of the azo-bridge in the PANP molecule together with their wavenumbers and PED contributions.

384

Table 4 Experimental (IR and Raman) and theoretical (B3LYP/6-31G(d,p)) wavenumbers for the PANP molecule Theoretical wave numbers

Experimental data

Normal vibrations with PED values greater than 10.0%

Assignment of the normal modes

Experimental wavenumbers of pyridine

Experimental wavenumbers of benzene

IR

RS

1 2 3

3107 3095 3086

3096 m 3084 m 3077 sh

IR

RS

IR

3095 sh 3076 vs 3064 s

56Kn(C3–H3)C44Kn(C4–H4) 56Kn(C4–H4)C44Kn(C3–H3) 92Kn(C9KH9)

n(C–H)fP n(C–H)fP n(C–H)fB

n1 n2 3081

3091 3070

4 5 6 7 8 9 10

3076 3064 3053 3042 3021 2978 2918

3061 m 3037 w 3025 m 3009 s 2977 m 2934 m 2911 w 2854 m

3057 sh 3046 sh 3038 sh 3009 m 2976 m 2933 s 2892 w

65Kn(C13–H13)C25Kn(C12–H12) 42Kn(C11–H11)C24Kn(C13–H13)C21Kn(C10–H10) 45Kn(C12–H12)C44Kn(C10–H10) 50Kn(C11–H11)C28Kn(C10–H10)C19Kn(C12–H12) 79Kn(C7–H7c)C11Kn(C7–H7a)C11Kn(C7–H7b) 50Kn(C7–H7a)C50Kn(C7–H7b) 39Kn(C7–H7a)C39Kn(C7–H7b)C21Kn(C7–H7c)

1590 1572 1568 1539

1599 s 1583 m 1567 s 1540 sh

1590 m 1583 m 1565 m

21Kn(C13–C12)C21Kn(C10–C9)C15Kn(C8–C13)C10Kn(C11–C10) 23Kn(C12–C11)C17Kn(C9–C8)C11Kn(C8–C13) 22Kn(C5–C4)C22Kn(C4–C3)C13Kn(N1–C2) 20Kn(C3–C2)C19Kn(C4–C3)C19Kn(C6–C5)C11Kd(C3–H3)

n(C–H)fB n(C–H)fB n(C–H)fB n(C–H)fB nas(CH3) nas(CH3) ns(CH3) Fermi resonance with 2d(CH3) n(fB)Cd(CH)B n(fB)Cd(CH)B n(fP)Cd(CH)P n(fP)Cd(CH)P

11 12 13 14 15

1480

1495 sh

16 17

1461 1458

18 19 20

1453 1424 1464

1447 sh 1421 m 1527 vs

1424 m 1518 w

18Kd[(C9–C10–H10)C(C11–C10–H10)]C17Kd[(C11–C12–H12) C(C13–C12–H12)]C16Kd[(C8–C9–H9)C(C10–C9–H9]) C12Kd[(C12–C13–H13)C(C8–C13–H13)]C10Kn(C11–C10) 70Kdas(CH3)C23Kd 0 as(CH3) 11Kd[(C10–C11–H11)C26Kd(C12–C11–H11)]C11Kn(C13–C12) C11Kd[(C9–H10–C10)C(C11–C10–H10)]C10Kn(C10–C9) C10Kd[(C11–C12–H12)C(C13–C12–H12)] 55Kd 0 as(CH3)C18Kdas(CH3) 19Kd[(C5–C4–H4)C(C3–C4–H4)]C14Kn(N2–N3)C11Kn(C3–C2) 20Kn(N4–O2)C20Kn(N4–O1)C15Kds(CH3)C13Kn(C2–N1)

21 22

1462 1457

1484 s 1469 w

1487 s 1469 vs

58Kds(CH3)C11Kn(N2–N3) 44Kn(N2–N3)C19Kds(CH3)

23

1408

1443 m

1449 s

16Kn(N4–O2)C12Kn(C2–N1)C12Kn(N4–O1)C10Kn(N1–C6)

24

1387

1387 m

1391 w

25

1376

1371 m

1371 m

26 27 28

1333 1293 1272

1315 m 1305 m 1345 vs

1316 m

19Kn(C10–C9)C16Kn(C12–C11)C14Kn(C13–C12) C13Kn(C8–C13)C13Kn(C11–C10)C11Kn(C9–C8) 28Kd[(C12–C13–H13)C(C8–C13–H13)]C20Kd[(C8–C9–H9) C(C10–C9–H9)]C13Kd[(C11–C12–H12)C(C13–C12–H12)] C12Kd[(C9–C10–H10)C(C11–C10–H10)] 34Kn(N1–C6)C13Kn(C2–N1)C12Kn(C6–C5) 23Kn(C2–N2)C12Kd(fP)C11Kn(C5–C4) 34Kn(N4–O1)C23Kn(N4–O2)C17Kn(C5–N4)C12Kd(NO2)

1345 vs

d(CH)BCn(fB)

n20 3091 3036 n2 3057

d(CH)B

n(fP) n(fP) ns(NO2)Cn(fP)Cd(CH)P

n2 3063 n7 3049

n8 1587 n4 1589 n21 1582 n5 1483

1582 1574

n22 1440

1434

1482

das(CH3) d(CH)BCn(fB)

das(CH3) das(CH3)Cn(fB) nas(NO2)Cds(CH3) Cn(fP)Cd(CH)P ds(CH3)Cn(NaN) n(NaN)Cds(CH3) Cn(fP)Cd(CH)P nas(NO2)Cn(fP) Cds(CH3) n(fB)

RS

n19 1480

J. Michalski et al. / Journal of Molecular Structure 744–747 (2005) 377–392

No.

d(CH)PCn(fP)Cn(C–NN) Cn(C–CH3) d(CH)BCd(CH)P d(CH)B

29

1255

1266 m

30 31

1234 1222

1247 m 1204 s

1249 s 1204 s

32

1198

1188 m

1187 s

33 34

1186 1122

1159 s 1150 s

1157 m 1148 vs

35

1102

1141 m

1141 vs

16Kd[(C5–C4–H4)C(C3–C4–H4)]C14Kn(C5–C4)C14Kn(N3–C8) C12Kn(C6–C7) 21Kd[(C5–C4–H4)C(C3–C4–H4)] 40Kd[(C10–C11–H11)C(C12–C11–H11)]C24Kd[(C11–C12–H12)C (C13–C12–H12)]C14Kd[(C9–C10–H10)C(C11–C10–H10)] 40Kd[(C10–C11–H11)C(C12–C11–H11)]C24Kd[(C11–C12–H12)C (C13–C12–H12)]C14Kd[(C9–C10–H10)C(C11–C10–H10)] 45Kd[(C4–C3–H3)C(C2–C3–H3)]C17Kn(C4–C3) 20Kn(C13–C12)C15Kn(C10–C9)C14Kd[(C12–C13–H13)C(C8– C13–H13)]C14Kd[(C8–C9–H9)C(C10–C9–H9])C11 Kd[(C10–C11–H11)C(C12–C11–H11)] 20Kd(fP)C17Kn(C5–N4)C17Kr 0 (CH3)

36 37

1093 1056

1106 w 1073 s

1077 s

61Kr(CH3)C20Kr 0 (CH3)C14Kt(C7–C5–C6–N1) 24Kn(C11–C10)C21Kn(C12–C11)C15Kd(f)

r(CH3)Cg(fP) n(fB)Cd(fB)Cn(fP)

38 39

1054 1050

1035 m

g(CH)P g(CH)B

40 41

1043 1035

1018 w 999 m

67Kt(H4–C5–C4–C3) 44Kt(H10–C11–C10–C9)C36 Kt(H9–C8–C9–C10) 20K 0 (CH3)C16Kd(fB)C13Kn(N1–C6) 38Kd(fB)

42

1025

982 w

g(CH)B

43

986

969 sh

44

975

951 m

45

923

932 m

45Kt(H12–C11–C12–C13)C27Kt(H13–C12–C13–C8) C26Kt(H9–C8–C9–C10) 41Kt(H13–C12–C13–C8)C40Kt(H11–C10–C11–C12)C17Kt(H9– C8–C9–C10) 23Kn(C3–C2)C10Kn(C2–N2)C9Kn(C2–N1)C9Kn(C6–C7) C9Kd(fP)C6Kd(C8–N3–N2)C6Kr(CH3)C5Kd(fP) 19Kn(C9–C8)C19Kd(C2–N2–N3)C15Kd(C8–N3–N2)

46 47

915 887

898 m 858 m

899 m

48 49 50

821 819 797

838 s 784 s

841 vs

51

788

763 m

763 m

52 53

718 711

721 s 721 w

720 w

54 55 56

700 648 632

687 s 678 s

57

626

611 w

1002 m

951 m

679 vs

613 s

51Kt(H3–C4–C3–C2)C35Kt(H4–C5–C4–C3) 30Kt(H10–C11–C10–C9)C25Kt(H13–C12–C13–C8) C24Kt(H12–C11–C12–C13)C20Kt(H9–C8–C9–C10) 25Kt(fP)C23Kt(fB)C16Kt(H11–C10–C11–C12) 40Kd(NO2)C11Kd(fP) 52Kt(fP)C9Kt(H11–C10–C11–C12)C7Kt(H12–C11–C12–C13) C6Kt(N4–C6–C5–C4)C6Kt(H10–C11–C10–C9)C5 Ku(C5–O1–N4–O2) 13Kd(NO2)C11Kn(C6–C7)C10Kdas(fB)C7Kn(N3–C8) C6n(C6–C5)C6n(C8–C13)C6d(fP)C6das(fP)C5n(C3–C2) 48Kt(fB)C20Ku(NO2) 44Ku(NO2)C31Kt(fB)

d(CH)B

n9 1177

1178

d(CH)PCn(fP) n(fB)Cd(CH)B

d(fP)Cn(C–NN)Cr(CH3)

n25 1149

1148

n26 1069 n8 1032

1052 1031

r(CH3) d(fB)Cu(CH3)

n18 1036 n1 993

g(CH)B d(fP)Cn(C–CH3) Cds(CNNC) d(CNN)Cd(fB) Cd(fP)Cn(C–CH3) g(CH)P g(CH)B

n5 997

992

n15 943

g(CH)BCg(CH)P d(NO2)Cd(fP) g(CH)PCg(CH)B

d(NO2)Cn(C–CH3) Cd(fB)Cd(fP) g(CH)BCu(NO2) u(NO2)Cg(CH)B Cg(CH)PCd(fP)Cd(fB) d(fP)Cd(fB) d(fB) g(fP)Cu(NO2) d(fB)Cd(fP)Cd(CNO) Cd(CCH3)

n16 749

n17 704 n11 673 n27 657

653

n10 611

604

n6 608 (continued on next page)

385

25Kdas(fP)C15Kd 0 as(fP)C9Kd(NO2)C8Kn(C6–C5) 77Kd 0 as(fB) 27Kt(C7–C5–C6–N1)C22Ktas(fP)C20Kt(C6–N1–C2–N2) C11Ku(NO2) 23Kdas(fB)C8Kd(fP)C8Kd 0 as(fB)C7Kn(C5–N4) C7Kd[(N1–C2–N2)C(C3–C2–N2)]C6Kd[(N1–C6–C7) C(C5–C6–C7)]C6Kd(C2–N2–N3)C5Kn(C2–N2)

1218

J. Michalski et al. / Journal of Molecular Structure 744–747 (2005) 377–392

1036 vw

n6 1216

386

Table 4 (continued) No.

Theoretical wave numbers

Experimental data

IR 604

59 60 61

571 528 501

551 w 510 m 498 m

62 63 64 65

467 430 419 378

460 w 427 w 408 w 375 m

66

345

322 w

67

292

286 w

68

284

69

237

243 w

70 71 72

236 218 186

227 w 174 w

73 74

138 94

132 m 105 w

75

64

66 w

76

40

45 m

77 78

22 18

Assignment of the normal modes

15Kd 0 as(fP)C14Kd[(N1–C2–N2)C(C3–C2–N2)]C 13Kd[(C6–C5–N4)C(C4–C5–N4)]C11Kn(C5–C4) 41Kd 0 as(fP)C17Kdas(fB) 56Ktas(fB)C22Kt(N3–C8–136–C12) 27Kr(NO2)C16Kd[(C9–C8–N3)C (C13–C8–N3)] 66Kt 0 as(fP)C18Ktas(fP) 100Kt 0 as(fB) 17Kn(C5–N4)C14Kr(NO2)C11Kdas(fP) 32Kd[(N1–C6–C7)C(C5–C6–C7)]C15Kr(NO2)C12Kdas(fP) C10Kn(C5–N4)C10Kd[(C9–C8–N3)C(C13–C8–N3)] 24Kt(C6–N1–C2–N2)C21Kt(N4–C6–C5–C4)C18Ktas(fB) C18Kt(C2–N2–N3–C8)C11Kt(fP) 31Kd[(C6–C5–N4)C(C4–C5–N4)]C24Kd[(N1–C6–C7) C(C5–C6–C7)]C12Kd[(N1–C2–N2)C(C3–C2–N2)]C11 Kd[(C9–C8–N3)C(C13–C8–N3)] 28Ktas(fB)C22Kt(N3–C8–C13–C12)C16Kt(N2–N3–C8–C9)C 11Kt(C6–N1–C2–N2) 25Kt(N4–C6–C5–C4)C19Kt(C7–C5–C6–N1)C19Kt(CH3) C18Kt 0 as(fP) 24Kd[(C6–C5–N4)C(C4–C5–N4)] 76Kt(CH3)C13Kt(C7–C5–C6–N1) 18Kd[(C9–C8–N3)C(C13–C8–N3)]C14Kd(C8–N3–N2)C12 Kd[(N1–C2–N2)C(C3–C2–N2)]C11Kd[(C6–C5–N4)C(C4–C5–N4)] 36Kt(N2–N3–C8–C9)C21Ktas(fP)C10Kt(C3–C2–N2–N3) 37Kt(N3–C8–C13–C12)C22Ktas(fP)C18Kt(N2–N3–C8–C9)C 14Kt(C2–N2–N3–C8) 32Kd(C2–N2–N3)C29Kd(C8–N3–N2)C20Kd[(N1–C2–N2) C(C3–C2–N2)]C12Kd[(C9–C8–N3)C(C13–C8–N3)] 38Kt(C6–N1–C2–N2)C24Kt(C2–N2–N3–C8)C 11Kt(N3–C8–C13–C12) 75Kt(NO2) 65Kt(C3–C2–N2–N3)C18Kt(N2–N3–C8–C9)C13Kt(NO2)

d(fP)Cd(fB)Cd(CCH3)

RS 580 s 552 s 528 w 496 s

410 s

340 w, b

269 s

230 s 214 m 189 m 137 s

Experimental wavenumbers of pyridine

Experimental wavenumbers of benzene

IR

RS

IR

n18 406

407

RS

d(fP)Cd(fB)Cd(CNN) g(fB)Cg(C–NN)Ct(CH3) d(CNN)Cd(CCH3)Cd(C–NO2) g(fP) g(fB) d(CNO)Cu(NO2)Cg(fP) r(CH3)Cr(NO2) t(fB)Ct(fP) r(NO2)Cr(CH3)Cr(NN)

t(fB)Ct(fP) t(fP)Ct(CH3) r(NO2) t(CH3) g(NN)Cg(CNN)Cg(CNO2) g(NN)Cg(fP) g(fB) t(CNNC) g(CNNC) t(NO2)Ct(CH3) t(CNNC)Ct(NO2)

The atom numbering used in DFT calculations is specified in Fig. 8. The scaling factors 0.95 for the 3000–3100 cmK1 region; 0.96 for the 1400–1700 cmK1 region. The values lower than 1400 cmK1 are not scaled. n, stretching; d, bending; g, bending; t, torsion; u, wagging; r, rocking; fP, pyridine ring; fB, phenyl ring; fP, C(2,3,4,5,6) and N(19); phenyl hydrogens, H(9,10,11,12,13); fB, C(8,9,10,11,12,13); pyridine hydrogens, H(3,4); CH3, C(7)–H(7a,7b,7c); NO2, N(4)–O(1,2).

J. Michalski et al. / Journal of Molecular Structure 744–747 (2005) 377–392

58

Normal vibrations with PED values greater than 10.0%

J. Michalski et al. / Journal of Molecular Structure 744–747 (2005) 377–392

band at 1484 and 1487 cmK1, respectively. The n(NaN) vibration contributes 11% to this mode being coupled with ds(CH3). The n22 mode gives a strong Raman line at 1469 cmK1 and due to the strongest contribution of the NaN bond motion (44%), it should be assigned as the main n(NaN) vibration. Other characteristic vibrations of the fP– NaN–fB core appear in the regions specified in Fig. 4 where the contributions of the respective modes are also given. It should be pointed out that vibrations of the –CNNC– core are strongly coupled with the vibrations of the pyridine, phenyl and their substituents. This means that strong p electron aromatic systems influence the vibrations of the azo-bridging group. 3.3.2. Vibrations of the nitro-group The most complete vibrational analysis of the nitrogroup in nitrobenzene has been done by Shlyapochnikov et al. [58] and Clarkson and Smith [59]. Following their analysis seven modes of the C–NO2 moiety have been identified: three stretching modes: nas(NO2), ns(NO2), n(C–N) and four bending modes: d(NO2), t(NO2), u(NO2) and g(NO2). Some of these vibrations, especially the out-ofplane bending modes, are mixed with the benzene ring modes [58,59]. Besides, the application of the 13C and 15N isotopes allowed to explain the intensity anomalies and splitting of some bands in the region 1550–1620 cmK1. These bands have been assigned to the coupled nas(NO2)C n(f) vibrations. Eight modes of the C–NO2 moiety in nitrobenzene have been identified and assigned in the following manner: n1, nas(NO2)Cn(f) at about 1523 cmK1; n2, ns(NO2) at 1347 cmK1; n3, n(C–N)Cd(f) at 1108 cmK1; n4, ds(NO2)Cd(f) at 852 cmK1, n5 and n6, u(NO2)Cg(f) at 793 and 702 cmK1; n7, das(NO2)Cd(f) at 532; and n8, t(NO2) at about 50 cmK1. The DFT calculations show that the PANP exhibits a several modes with the greater or smaller contribution of the nitro group vibrations (Table 4). These are: † n20 at 1518 (RS) and 1527 (IR) cmK1 with the 40% contribution of nas(NO2), † n23 at 1443 (IR) and 1449 (RS) cmK1 with the 28% contribution of nas(NO2), † n28 at 1345 (IR and RS) cmK1 described by the participation of the following vibrations: 57% n(NO2)C17% n(C–NO2)C12% d(NO2). It should be assigned to the ns(NO2) mode, † n35 at 1073 (IR) and 1077 (RS) cmK1 with the contribution of the 17% n(C–NO2), † n49 at 838 (IR) and 841 (RS) cmK1 that should be described as d(NO2) due to 40% contribution of the bending ONO coordinates. This coordinate contributes also in 13% to the n51 mode observed at 763 cmK1 in IR and Raman spectra, † u(NO2) vibration contributes two modes, n52 and n53, in the 20 and 44%, respectively. These bands are observed at 721 (IR) and 720 (RS) cmK1,

387

† r(NO2) vibrations give significant contribution into the following modes: n56 at 611 (IR) and 613 (RS) (11%), n58 at 580 (RS) (26%), n61 at 498 (IR) and 496 (RS) (27%), n64 at 408 (IR) and 410 (RS) (14%) and n65 at 375 (IR) cmK1 (15%). Due to the greatest contribution of this coordinate into the n58 and n61 modes those should be assigned to the rocking vibrations of this group in the PANP. † n70 mode at 230 (RS) cmK1 is the out-of-plane bending g(NO 2) with 48% contribution of this internal coordinate, † n72 mode calculated at 22 cmK1 corresponds to torsional t(NO2) vibration (75% contribution). The vibrations of the nitro group, especially those in the FIR region, are strongly mixed with the pyridine and phenyl rings vibrations. 3.3.3. Vibrations of the methyl group The vibrations of the methyl group in the PANP are observed in the typical for this group ranges [60,61]. The asymmetric and symmetric stretching vibrations are observed in the 2970–3010 and 2900–2940 cmK1 regions, respectively. The weak band at 2850 cmK1 arises probably from a Fermi resonance with the bending vibrations of this chromophore. The bending vibrations of the methyl group contributes into the das(CH3) modes (n16, n18, n19) which appear in the 1420–1500 cmK1 range and into the ds(CH3) modes (n20–n23) observed in the 1380–1500 cmK1 range. The symmetric bending vibrations of this group appear at higher wavenumbers than they are usually observed due to their strong coupling with the n(NaN) and nas(NO2) vibrations. The other bands of the PANP involving the methyl groups are observed in the following regions: n(C–CH3), 1247–1249; r(CH3), 1030–1080; u(CH3), 760–1040; d(C–CH3), 290– 400; and t(C–CH3), 230–240 cmK1 (Table 4). 3.3.4. Pyridine and phenyl rings as well as C–H vibrations The vibrations of the pyridine ring [62], phenyl ring [63] and hydrogen atoms bonded to the rings are well described in the literature. They appear in the measured here spectra of PANP in typical regions. The spectra of the pyridine and benzene are included in Table 4 and Figs. 2 and 3 for the comparison. The respective modes of these units are easily identified in the spectra of PANP. Very characteristic vibrations of the whole skeleton of the PANP molecule are observed in the region below 160 cmK1. Two rings and azo-bond participate in these normal modes which lead to the translational: n(fP,fB) and torsional r(fP,fB) vibrations of the fP–NaN–fB skeleton. t(CNNC) modes are observed in the region below 100 cmK1.

4. Discussion of NMR results Fig. 5 shows the 13C NMR spectra of PANP in the solid state and CDCl3 solution. The chemical shifts for aromatic

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Fig. 5.

13

C NMR spectra of the PANP in the solid state (a) and CDCl3 solution (b).

carbons are listed in Table 5. The atom numbering in the discussion of the NMR data is specified in Fig. 8. The chemical shifts are situated in the range of 107–161 ppm. They are typical for aromatic carbons, but strongly depend

on the type of substituent. 13C NMR spectrum recorded in CDCl3 solution exhibits large differences in chemical shifts for carbons C9, C13 as compared to solid-state spectrum (see Fig. 5). For another carbons, the shifts are comparable

J. Michalski et al. / Journal of Molecular Structure 744–747 (2005) 377–392 Table 5 13 C NMR chemical shifts for the PANP in the solid state and CDCl3 solution Carbon

Solid-state

Solution

C(2) C(3) C(4) C(5) C(6) C(8) C(9) C(10) C(11) C(12) C(13)

161.53 107.13 133.68 144.9 153.89 150.16 111.92 129.86 132.23 129.86 131.98

163.25 110.89 135.21 145.56 154.13 151.93 123.83 129.13 133.21 129.13 123.83

The atom numbering is the same as in Fig. 8.

(correlation coefficient for the whole of aromatic carbons R2Z0.9217). The large difference for C13 may be explained by a strong deshielding resulting from the non-classical hydrogen bond in the crystal. This is confirmed by results of X-ray analysis (different C13/C8 and C9/C8 bond lengths; Table 3). Interesting information about the molecular structure of PANP can be obtained from an analysis of chemical shift tensor elements (dii) [64] and chemical shift tensor parameters: the anisotropy (dCSA), the asymmetry parameter (h), span (U) and skew (k). The static 13C NMR solid Table 6 Principal components of the

389

spectrum of PANP always contains the sidebands if sample spinning is low. With sample spinning higher than 2 kHz the broad lines can be narrowed to isotropic signals. Under slow MAS (magic angle spinning) condition (ca. 2 kHz), the isotropic lines are symmetrically flanked by spinning sidebands. This can be further used to calculate the chemical shift anisotropy and other NMR parameters. The calculated values of 13C principal components of the chemical shift tensor dii, the anisotropy dCSA, the asymmetry parameter h, span U, skew k, are given in Table 6. The span parameter reflects the distortion of the geometry from ideal tetrahedral or cubic symmetry. The values found for PANP are placed in the range 147– 222 ppm. For the majority of aromatic carbons, the span values are similar (190–220 ppm) but for the carbon bounded with the nitro group (C17) the value 147 ppm has been found. This relatively low value is probably a result of the steric hindrance of the nitro group. The skew parameter reflects the distribution of electron density around the nuclei under investigation. This parameter generally is in the range from K1 (for d22Zd33) to C1 (for d11Zd33), as results from equation kZ3(d22K diso)/(d11Kd33). These values for PANP range from K0.07 to 0.46. This indicates that electron density is not localized in any particular bond but is spread over the entire carbon tetrahedron.

13

C chemical shift tensor dii, the anisotropy dCSA, the asymmetry parameter h, span U and skew k for the PANP

Carbon

d11

d22

d33

diso

dCSA

U

h

k

C2 C6 C8 C5 C4,13,11 C10, 12 C9 C3

263.3665 240.5264 238.9012 220.7431 230.5925 231.2937 201.1983 191.2337

167.0544 184.5887 166.3499 140.9580 161.3019 150.0377 133.1679 128.7025

53.9923 37.1166 45.310 72.9929 10.6225 8.6746 1.1088 1.7452

161.462 154.037 150.175 144.876 134.145 129.917 111.792 107.220

107.4687 116.9204 104.865 71.8831 123.5225 121.2424 110.6832 105.4748

209.3742 203.3604 193.5912 147.7502 219.97 222.6191 200.0895 189.4885

K0.4293 K0.6131 K0.5066 K0.3079 K0.6634 K0.6112 K0.6564 K0.6639

0.0801 0.4607 0.2606 K0.0796 0.3104 0.2111 0.3205 0.3401

Chemical shifts are given in ppm relative to TMS. See text for definitions of diso, dCSA, U, h and k.

Table 7 Calculated GIAO/B3LYP and experimental chemical shift (ppm) of the PANP and calculated isotropic shielding constants of the reference nuclei (TMS) Geom.

X-ray geometry

B3LYP geometry

Exp. data

GIAO/B3LYP/ TMS

6-311G 190.65

6-311CCG 190.50

6-311G** 184.54

6-311G 190.65

6-311CCG 190.50

6-311G** 184.54

C2 C3 C4 C5 C6 C8 C9 C10 C11 C12 C13

168.26 105.52 130.35 150.69 165.31 158.8 111.99 125.73 130.39 124.81 132.17

171.0 106.92 131.15 151.13 166.85 160.99 113.67 126.09 130.91 125.38 132.98

169.83 104.54 131.46 149.22 166.13 158.56 112.1 125.42 129.41 124.34 131.29

165.04 133.1 142.15 152.16 165.25 161.06 116.89 135.03 142.38 136.38 147.17

167.59 133.26 143.25 152.98 166.44 163.36 119.13 135.65 142.81 136.94 148.11

167.06 131.49 144 150.27 166.69 160.49 117.28 134.68 141.45 135.75 146.01

161.53 107.13 133.68 144.90 153.89 150.16 111.92 129.86 132.23 129.86 131.98

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have been obtained with 6-311G** for carbons C15, C2, C6, with 6-311G for C2, C6, and with 6-311CCG for C15, C2 and C6. The calculated isotropic shielding constants si, were transformed to chemical shifts relative to TMS for 13C by diZsTMSKsi. They were taken from calculations at the same computational level (the absolute isotropic shielding constants of the reference nuclei—TMS are summarized in Table 7). The following general statements can be drawn from the NMR studies:

Fig. 6. Plot of experimental 13C CPMAS chemical shifts vs. chemical shifts in the CDCl3 solution.

Calculated and experimental 13C CPMAS chemical shifts of the aromatic carbons are shown in Table 7. Calculations of chemical shifts have been performed (with GIAO/B3LYP; 6-311G, 6-311CCG and 6-311G** bases) for two input data: † crystallographic data based on X-ray diffraction, and † geometrical parameters from GIAO/DFT-B3LYP optimization with 6-31G(d,p) basis set. The obtained results are very different, (see Table 6 and Fig. 6). For the input crystallographic data, the chemical shifts are very similar to experimental data (13C CPMAS). A correlation of experimental chemical shifts vs. absolute shieldings (calculated on 6-311G** basis) is satisfactory (R2Z0.9668, Fig. 7). For carbons C14, C17, C18, C1, the values are downfield shifted by about 4–12 ppm with respect to C15, C16, C2 and C6. Generally, the best results

Fig. 7. Plot of experimental CPMAS chemical shifts vs. absolute shieldings for carbon nuclei of the PANP (equation: drelZK0.7606sabsC171.68, number of points used 11, correlation coefficient R2Z0.9668).

1. Carbon C14 of the pyridine ring and carbon C1 of the phenyl ring exhibit strong lowering of the electron density. They are in the ipso position to the –NaN– azo group that has strong double character. 2. Carbon C15 of the pyridine ring is strongly screened in comparison with the unsubstituted pyridine. Their chemical shift differs in 13 ppm. This effect results from the combining influence of two chromophors: growing electron density in the meta position of the nitro group and ortho position of the azo group. Carbon C16 of the pyridine ring also is sensitive on the ortho influence of the nitro group exhibiting weak screen effect. The difference between the chemical shift of this carbon in the compound studied and free pyridine is 2 ppm. 3. Atom C17 of the pyridine ring bonded to the nitro group exhibits strong chemical shift in comparison with the free pyridine. It follows from the strong acceptor properties of the nitro group that is in the studied compound coplanar with the pyridine ring. Two oxygen atoms of the nitro group have similar negative charges that are compensated through the positive charges on the nitrogen and C14 atoms. The nitrogen of the nitro group and C17 carbon of the pyridine ring are bounded via the double CaN bond. 4. C2 and C6 atoms of the phenyl ring, both placed in the ortho position to the azo group, differ in their magnetic properties C2 differ in 17 ppm in comparison to the free pyridine but the C6 absorbs in significantly lower magnetic field. 5. The NMR studies allow to propose the following electron structure of the PANP molecule in the solid state (Fig. 8).

Fig. 8. The electron structure of the PANP molecule.

J. Michalski et al. / Journal of Molecular Structure 744–747 (2005) 377–392

391

These data suggest that the molecule takes the planar trans-configuration of the azo bond and both rings. 6. The comparison of the NMR spectra of the PANP in the solid state and CDCl3 solution proves that C2 and C6 atoms in the dissolved sample are magnetically equivalent giving the single line at 12.83 ppm that splits in the solid sample into 111.92 (C2) and 131.98 ppm (C6) [65]. The C3 and C5 carbons are equivalent both in the solid and liquid state. The equivalence of the C2 and C6 as well as C3 and C5 atoms for the dissolved molecule could be the results of the fast rotation of the phenyl ring around the C–N bond.

5. The possibility of a hydrogen bond formation in the PANP structure has been suggested by the X-ray studies since the C–H/O intermolecular contacts appear in the unit cell of this compound. The IR and Raman spectra do not confirm the existence of such hydrogen bond in the solid PANP. 6. The NMR measurements confirm the results obtained from the vibrational and X-ray studies. They propose the electronic structure of the compound studied based on the electronic nonequivalence of the carbon atoms of the pyridine and phenyl rings that are nearest to the azobridge.

5. Conclusions

Acknowledgements

The structural X-ray, vibrational and NMR studies as well as DFT calculations of the PANP allow to draw the following conclusions on the structure and properties of the compound studied:

This work has been supported by the Polish Committee for Scientific Research in the frame of Grant No. 4 T09B 113 24.

1. The azo-bond in the PANP appears in the transconfiguration with nearly coplanar arrangement of the azo-bridge as well as pyridine and phenyl ring. 2. The n(NaN) vibrations of the PANP participate in two bands in the 1400–1500 cmK1 region (at ca. 1485 and 1469 cmK1) and their contribution takes the values 11 and 44%, respectively. It means that the greatest n(NaN) character appears for the 1469 cmK1 mode observed as strong Raman line and weak intensity IR band. The clear splitting of the n(NaN) vibration into two components has also been observed for other azo-compounds [66–68]. This effect can be explained as a results of the dynamical coupling between the n(NaN) vibrations and vibrations of the two massive rings linked through the azo-bridge. 3. The other characteristic IR and Raman bands of the fPNNfB azo-bridge are those described in Table 4 and Fig. 4. They are observed in the 1250–1300 cmK1 range and correspond to the stretching vibrations of two C–N bonds of this system as n(CNNC). The contribution of the C–N stretch to these modes is of the order 10–30%. These modes are coupled with the n(f) and d(f) modes of the pyridine and phenyl rings. 4. The next characteristic bands of the azo-bond are as follows: † In-plane bending d(CNNC) in the 800–950 cmK1 region, † Out-of-plane bending g(CNN) in the 500–580 cmK1 region, † Torsions t(CNNC) in the 180–350 cmK1 region, It should be noted that the contribution of the –NaN– bond vibrations to the bands in the FIR region is very large due to participation of the azo-bridge in the mutual vibrations of the massive fragments of the molecule.

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