ELECTRON-DENSITY DISTRIBUTION IN CRYSTALS OF P-NITROBENZENE DERIVATIVES

The electron-density distributions in five nitrobenzene derivatives have been examined by the multipole expansion method based on X-ray diffraction data measured with Mo Kalpha radiation (lambda = 0.70926 angstrom) at 120 K. p-Dinitrobenzene, C6H4N2O4, (I), M(r) = 168.1, monoclinic, P2(1)/n, a = 10.941 (2), b = 5.3813 (5), c = 5.6684 (4) angstrom, beta = 92.116 (8)-degrees, V 333.51 (7) angstrom3, Z = 2, D(x) = 1.67 Mg m-3, mu = 0.135 mm-1, R = 0.030 for 2045 unique reflections. 4-Nitrobenzoic acid, C7H5NO4, (11), M(r) = 167.1, monoclinic, A2/a, a = 12.857 (1), b = 5.0272 (2), c = 20.997 (2) angstrom, beta = 97.072 (8)-degrees, V = 1346.8 (2) angstrom3, Z = 8, D(x) = 1.65 Mg M-3, mu = 0.130 mm-1, R = 0.030 for 3930 reflections. 4-Nitrobenzamide, C7H6N2O3, (III), M(r) = 166.1, monoclinic, P2(1)/c, a = 7.393 (2), b = 6.8005 (9), c = 13.814 (2) angstrom, beta = 90.88 (1)-degrees, V 694.4 (2) angstrom3, Z = 4, D, = 1.59 Mg m-3, mu = 0.119 mm-1, R = 0.033 for 3811 reflections. 4-Nitrobenzaldehyde oxime, C7H6N2O3, (IV), M(r) = 166.1, monoclinic, P2(1)/c, a = 6.2336(6), b = 4.8377 (5), c = 24.352 (2) angstrom, beta = 94.87 (8)-degrees, V = 731.7 (1) angstrom3, Z = 4, D(x) = 1.51 Mg M-3, mu = 0.113 mm-1, R = 0.043 for 2493 reflections. 4-Nitroaniline, C6H6N2O2, (V), M(r) = 138.1, monoclinic, P2(1)/n, a = 12.122(2), b = 6.0276(9), c = 8.487 (1) angstrom, beta = 92.72 (1)-degrees, V = 619.4 (2) angstrom3, Z = 4, D(x) = 1.48 Mg m-3, mu = 0.107 mm-1, R = 0.040 for 2573 reflections. Single-crystal neutron diffraction studies were also made for (I) at 120 K, and for (II) at both 120 and 302 K. For the nitro group, the N-O bonding electrons and the lone pairs of the 0 atoms are clearly observed. The pi-donating nature of the amino group in (V) can be seen by the polarization of the electron density of the benzene ring. The carboxyl H atom in (II) and the oxime H atom in (IV) have pronounced positive effective charges, which are reflected in the relatively large thermal parameters in the X-ray conventional refinement using neutral-atom scattering factors. Neutron diffraction study of (II) indicated that the disorder of the COOH group due to the double proton transfer is 11 (3)% at 302 K and is not observed at 120 K within experimental error.