The structure of firefly luciferin has been confirmed by X-ray diffraction analysis. The compound, ClIH803N2S2, forms orthorhombic crystals, space group P2x2121, with cell dimensions a=9.410, b= 22.956, and e= 5-370 A: there are four molecules in the unit cell. The structure was refined by leastsquares calculations to an R index of 0.054 for 841 non-zero reflections collected on a diffractometer with Co Ke radiation.

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The crystal structure of bis [diethyl bis-(1-pyrazolyl)has been determined to a reliability index R= 5"40 %. The space group is P21/c with cell parameters a=7.566 (7), b=9.749 (8), c= 15-809 (15)/~, and ,8=91.67 (5) °. N.m.r. data had previously been interpreted as indicating four equivalent hydrogens, two pairs of methylene hydrogens in flanking positions with respect to nickel. The X-ray structure places one hydrogen of each methylene pair in the apical position either above or below the square planar Ni ligands.

1229smoothly from visual models to packing refinement to 'before and after' views, all within a few minutes. This would not only lead to quicker model evaluation, but would also help avoid the occasional problem of false minima that can arise when PCK5 is used blindly. The crystal and molecular structure of ~-hydroxy-l,l-dimethylpyrrolidinium bromide 0~-phenyl-2-thiopheneglycolate (e heteronium bromide) Cash 22NOaSBr, was determined by single-crystal X-ray diffraction techniques. The crystals were orthorhombic, Din= 1"5 g cm -a, space group Pna2~, Z=4, a= 18-596 (1), b= 8.049 (9), and c= 12.297 (8)/~. The structure was solved using 1671 observed independent reflections to yield a final R value of ?-044.

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