An Unusual Methylene Aziridine Refined in P21/c and the Nonstandard Setting P21/n (G. C. Feast, J. Haestier, L. W. Page, J. Robertson, A. L. Thompson & D. J. Watkin)

Acta Cryst. (2009), C65, o635-o638.    [ doi:10.1107/S0108270109046952 ]

The unusual methylene aziridine 6-tert-butyl-3-oxa-2-thia-1-azabicyclo[5.1.0]oct-6-ene 2,2-dioxide, C₉H₁₅NO₃S, was found to crystallize with two molecules in the asymmetric unit. The structure was solved in both the approximately orthogonal and the oblique settings of space group No. 14, viz. P2₁/n and P2₁/c, respectively. A comparison of these results clearly displayed an increase in the correlation between coordinates in the ac plane for the oblique cell. The increase in the corresponding covariances makes a significant contribution to the standard uncertainties of derived parameters, e.g. bond lengths. Since there is yet no CIF definition for the full variance-covariance matrix, there are clear advantages to reporting the structure in the nonstandard space-group setting.

Publisher’s copy: IUCr