The structure of 4-nitroaniline has been refined three-dimensionally with visually-estimated Cu Ka data. Positional and thermal parameters for the carbon, nitrogen, and oxygen atoms, and positional parameters for the hydrogen atoms, were obtained by least squares. The standard errors in bond lengths not involving hydrogen are 0.006-0.007 A; the corresponding errors in bond angles are about 0.4 °. Intramolecular bond distances were corrected for the effects of thermal motion. These distances are discussed, and compared with those in related molecules. The molecular geometry supports the view that there is a small but significant contribution of a quinonoid resonance form to the structure of the molecule, although the amino group appears to interact with the aromatic ring to a greater extent than does the nitro group. The nitro group shows a pronounced torsional oscillation, the r.m.s, amplitude being about 14°; this sort of motion seems to be characteristic of nitro groups in aromatic compounds.

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A unique low-temperature, full-circle goniometer for single-crystal X-ray diffraction has been constructed with the novel adaptation of a closed-cycle refrigerator in such a manner that the cold finger remains stationary during the q~ and Y. rotations. This has been made possible through the development of a flexible heattransfer link between the top of the ~p shaft and the cold station. The new design concept incorporates several important features: (1) the vacuum shroud and radiation shields also remain stationary so as to reduce the X-ray windows to narrow slots; (2) the sample crystal is enclosed in a black, nearly isothermal cavity (Hohlraum); and (3) the q~ shaft is mechanically decoupled from the thermal contraction of the cold finger and other parts. The new system has considerable advantages over presently existing low-temperature adaptations for X-ray diffraction: (1) the goniometer operates between 300 and 18 K without expenditure of liquid cryogen and with minimal attendance; (2) the system is capable of operating for long periods of time with minimal risk of interruption of the cooling cycle; (3) absorption of X-rays by the windows is minimal and independent of the Bragg angle; and (4) data collection at any temperature down to its present lowest limit of 18 K is virtually as easy as with any conventional roomtemperature diffractometer. At the time of writing the instrument has been in use for over 10 000 h and has measured over 100000 X-ray reflections from single crystals.

Crystals of (CHs)aNO are monoclinic, space group C2/m, 4 molecules in a unit cell with lattice constants a = 10.054, b = 8.793, c = 5.006 A and fl = 91.04 °. The required molecular symmetry is m. Fourier and least-squares refinement on 514 visually estimated intensities were used to obtain optimum values for 23 positional and 23 thermal parameters. The value of R is 7.4%. The bond distances found are (values including the libration correction are given in parentheses): N-O = 1.388_+0.005 (1.404), average C-N=1.477 _+0.006 (1.495), and average C-H=I.01 _+0.07 /~. The bond angles at the nitrogen atom are O-N-C = 110.0 _+ 0.6 ° and average C-N-C = 109.0 +0-6 °. The C-N and N-O bond distances are compared with those in other compounds.

Crystal data are given for synthetic mixed bariumstrontium sulfates, and a powder pattern for (Ba 050 Sr 050 )SO 4 is presented. Taken with previously published data for the end-member sulfates, we verify the nonlinear behavior of the unit-cell edges with composition, and present tables of interpolated data from which d-spacings for intermediate compositions can be calculated. The use of these data for analysis of complex natural samples by use of the 121 peak is described.