2-Pyridone, C5H5NO, crystal structure refinements at 295 Κ and 120 Κ, experimental and theoretical deformation density studies

2-Pyridone, C5H5NO, has been studied by single crystal diffraction at 295 Κ and 120 Κ using X-ray and neutron diffraction methods. The crystal structure of this compound was first studied by Penfold [Acta Crystallogr. 6, 591 — 600 (1953)] but described in an unusual setting. We confirmed this structure at both temperatures and refined the structure in the usual setting. Deformation density studies based on the 120 Κ data were performed using the multipole and charge cloud models. They are compared with the theoretical (4 —31G + BF) dynamic deformation density. Introduction The crystal structure of 2-pyridone was first described by Penfold (1953), in an unusual setting (Ohms et al., 1982a). Detailed structural knowledge on the tautomeric 3-hydroxypyridine (Ohms et al., 1983) as well as on the analogous 2-thiopyridone (Ohms et al., 1982b) is available. Below we present the structural results of new X-ray and neutron diffraction studies on 2-pyridone at two temperatures. X — X and Μ — A (of the multipole as well as of the charge cloud variety) deformation densities are derived and compared to the theoretical (4 —31G -IBF) dynamic counterpart. The question of the 2-pyridone/2-hydroxypyridine tautomerism will be addressed (Scanlan et al., 1983). * Part 22 of comparison of observed and calculated electron densities. For part 21, see: Breitenstein, Μ. , Mawhorter, R. J., Meyer, H. and Schweig, Α. , submitted for publication 1 8 6 U. O h m s et al. : Structure refinement of 2-pyridone Table 1. Crystal data and experimental details of 2-pyridone. P2l2l2l. 7. = 4 Temperature [K] 295 295 120 120 Source X-rays Neutrons X-rays Neutrons o[A] 13.657(4) 13.645(7) 13.564(3) 13.564(5) b 5.912(2) 5.900(3) 5.792(2) 5.795(3) c 5.701(2) 5.692(3) 5.598(2) 5.604(3) Wave length [A] 0.71069 0.9203 0.71069 0.9203 sin A 0.81 0.70 0.90 0.823 Scan method Θ/2Θ ω 0/20 ω Measured reflections 6548 1810 4668 2269 Unique reflections 1030 789 1628 1262 Observed reflections 647 625 1174 981 χ used for /obs > χ σ(Ι) 2.0 2.5 2.0 3.0 Internal R 0.025 0.056 0.038 0.041 Number of parameters for free a tom model refinement 64 110 84 110 R(Fobs) 0.034 0.041 0.041 0.041 RJw = 1/o(/obs)] 0.036 0.039 0.062 0.026 Experimental A crystal of the dimensions 0.2 χ 0.2 χ 0.3 mm, grown from the melt by the Bridgman method, was used for the X-ray study at 295 K. Other single crystals grown from methanolic solutions were used for the other diffraction studies. Their dimensions were 3.1 χ 3.8 χ 3.8 mm for the neutron investigations and 0.2 χ 0.15 χ 0.2 mm for the low temperature X-ray study. Details concerning the diffraction measurements are listed in Table 1. No phase transition occurs in the temperature range from 100 Κ up to 295 K. This was confirmed by a X-ray powder diffraction study using the GuinierSimon technique. The lattice constants were obtained by least-squares refinements of the setting angles of reflexions at higher Bragg angles. For the X-ray study at 120 Κ the crystal was cooled by a nitrogen gas stream (cooling device: NCD1, Leyboldt-Hereaus). The sample temperature was calibrated by the phase transition of potassium dihydrogen phosphate at 122 Κ and monitored during the measurement by means of a Cu/CuNi thermo-couple. The neutron diffraction studies were performed at the research reactor FR2 at the Kernforschungszentrum Karlsruhe. We used a Helium closed-cycle refrigerator for the 120 Κ study (cooling device: Oxford instruments), the temperature was controlled by direct heating. Standard reflexions were recorded during all four data acquisitions, a decrease of intensity was not observed in any case. U. Ohms et al.: Structure refinement of 2-pyridone 187 Refinement of data Least-squares refinements have been carried out in case of X-ray data using the X-ray 72 programs (Stewart, 1972). During the refinement of the 295 Κ data the hydrogen positional and isotropic thermal parameters were fixed. In case of the 120 Κ data they were refined with a modified hydrogen scattering curve (Stewart et al., 1965). For the nitrogen, oxygen and the carbon atoms the scattering curves were taken from the International Tables for X-Ray Crystallography (1974). Comparisons of the molecular plane after calculations of difference Fourier syntheses show higher bond peaks at 120 Κ due to the smaller thermal smearing at the lower temperature (cf. Figs. 1 a and 1 b). For the refinements of the neutron data sets we used programs of the X-ray 76 system (Stewart et al., 1976). The weights were calculated to l/a(F0) (Rees, 1977). This weighting scheme was used for all refinements. The scattering lengths were bc = 6 . 648410" 1 5 m, b^ = 9.36 10" 1 5 m, bQ = 5.8 10" 1 5 m and bH = — 3.7409 10" 1 5 m (Koester, 1977). The resulting parameters of all four refinements are given in Table 2. Discussion of molecular and crystal structure The bond lengths and angles of the molecule in the asymmetric unit at 120 Κ are shown in Figure 2. At both temperatures the tautomeric pyridone form is proved. The bond lengths and angles are in the same range as in 5-chloro-2pyridone (Kvick et al., 1972) and in 2-pyridone contained in the molecular complex 2-pyridone with 6-chloro-2-hydroxypyridine (Almlöf et al., 1971). The theoretical calculated molecule (Scanlan et al., 1983) contains longer carbon single bonds and shorter carbon double bonds than the experimental determined bond lengths in the crystalline phase. The hydroxy form exists in the liquid and in the gaseous phase, but the pyridone form is the more stable (Bodor et al., 1970). In the crystal structure of 2-pyridone the molecules are connected by Ν Η . Ο hydrogen bonds to form chains. The data of the hydrogen bond evaluated from the neutron diffraction are at 120Κ: Ν . Ο ' 2.762(2)Ä, H ( N ) . Ο' 1.764(4)Ä, angle N H ( N ) 0 ' 175.3(2)°. O' at the position 1 / 2 + χ , — y, 1/2 + ζ. In contrast other structures with a 2-pyridone molecule and the structure of the sulfur analogeous 2-thiopyridone contain no chains but dimers, which are formed by hydrogen bonds. The molecular packing is presented in Figure 3. Comparison of X-ray and neutron parameters There are no differences between the positional parameter sets of the X-ray and neutron diffraction studies within their estimated standard deviations. 188 Ü. Ohms et al.: Structure refinement of 2-pyridone Fig. l a . Plane of molecule in the difference Fourier synthesis at 295 K. Contour line interval = 0.1 e/A, positive contours = full lines, negative contours = dashed lines, zero contour = dotted line Fig. 1 b. Plane of molecule in the difference Fourier synthesis at 120 K, contour lines as in Fig. 1 a U. Ohms et al.: Structure refinement of 2-pyridone 189 ·< c 1—• υ => I "δ . 5 (Λ C ν Ζ ~ .> Λ OG > t) ü S ί £ S ·2 C α u . 2 ε δ <= Έ. 2 « 3 ' S eo Λ Ό C Ό Ο Ü £ Μ II " (Α Ό UJ CO Ο ο