A refinement of the benzoic acid structure at room temperature

C7H602, monoclinic, P21/e, a = 5.510(5), b = 5 . 1 5 7 ( 6 ) , c = 2 1 . 9 7 3 ( 8 ) ,&, f l = 9 7 . 4 1 ( 8 ) ° , Z = 4 , Dc= 1.31 Mg m -a. The cyclic hydrogenbonded dimers, arranged around the symmetry centres, are nearly planar, the torsion angle around the C C O 2 H bond being 1.5°. The benzene C atoms are coplanar within + 0 . 0 0 4 / k and the C C bond lengths average-1.388 (6) /k. The internal benzene angles deviate very slightly from the ideal value of 120 ° , ranging from 119.7 (4) to 120.5 (4) ° (mean 120.1°). The two C O bond lengths are nearly equal, their values being 1.263(3) and 1.275(4) /k. Correspondingly, the C C O angles are 118.8 (3) and 118.0 (4) °. The O . . . O bridge distance is 2.633 (3)/k. The C O bond lengths and the difference Fourier map indicate that there is a mixture of the two possible orientations of the dimer at the same crystal site. Introduetlon. After many attempts suitable single crystals were obtained from an alcohol-water mixture by slow evaporation. Cell parameters were determined by refinement of 21 reflexions on an automated single-crystal Siemens AED diffractometer. Intensity data were collected using a crystal of approximate dimensions 0.1 × 0.1 x 0.3 ram, mounted along b. Three-dimensional data were collected at 293 K using Mo Ka radiation by the 0-20 scan technique, within the limiting sphere of 0 = 27 °. A total of 469 independent reflexions having I > 2o(1) were collected and used in the calculations; Lorentz and polarization factors were applied but no correction was made for absorption. The structure was refined anisotropically starting from the parameters of the non-hydrogen atoms from the two-dimensional determination (Sim, Robertson & Goodwin, 1955). The benzene H atoms were placed in chemically reasonable positions ( C H , 1.00/k) and all occurred in regions of positive electron charge density on the difference Fourier map. On this map, an electron density peak of 0.2 e/~-3 appeared just in the middle of the O(1 ) -O(2 ' ) line (no other peak * To whom correspondence should be addressed. >0.1 e/~-3 was detected). This peak was assumed (see below) to be due to the overlap of two H atoms, with occupancy factors of one half ( O H , 0.90 ,&_, and / C O H , 119°). The final block-diagonal-matrix anisotropic refinement, including the contribution of the H atoms held constant (B = 6A2), gave a final R of 0 .038. t The final weighting scheme was W = I/(A + I Fol + BIFol2), where A = 3.5 and B = 0.012 were chosen so as to maintain w(IFol IFcl) 2 essentially constant over all ranges of IFol and sin 0/2. Atomic scattering factors were those of Moore (1963) and calculations were carried out with programs written by Albano, Domenicano & Vaciago (1966). The final atomic coordinates are given in Table 1 and the numbering scheme is shown in Fig. 1 together with bond lengths and angles. t Lists of structure factors and anisotropic thermal parameters have been deposited with the British Library Lending Division as Supplementary Publication No. SUP 35192 (4 pp.). Copies may be obtained through The Executive Secretary, International Union of Crystallography, 5 Abbey Square, Chester CH 1 2HU, England. Table 1. Fractional atomic coordinates (x 104, for H x l03) with estimated standard deviations in parentheses