An unusual methylene aziridine refined inP2₁/cand the nonstandard settingP2₁/n

Abstract

The unusual methylene aziridine 6-tert-butyl-3-oxa-2-thia-1-azabicyclo[5.1.0]oct-6-ene 2,2-dioxide, C(9)H(15)NO(3)S, 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. P2(1)/n and P2(1)/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.

Linear transformations of variance/covariance matrices

“…Our re-refinement was performed with SHELXL (Sheldrick, 2008). The refined parameters obtained from SHELXL and from Feast et al (2009) were identical within standard uncertainties. The result of the P2 1 =n refinement is given in the left-hand column of Table 4 and of the P2 1 =c refinement in the right-hand column.…”

“…It has been reported that refinement in oblique cells has increased correlation between selected parameters, potentially making the refinement unstable (Dunitz, 1995). The molecule methylene aziridine (Feast et al, 2009) can be described in space group P2 1 =n with cell parameters a = 13.8593 (3), b = 10.5242 (2), c = 14.8044 (4) Å , = 92.0014 (7) or space group P2 1 =c with cell parameters a = 13.8594 (2), b = 10.5243 (2), c = 19.9230 (3) Å , = 132.0439 (7) . In the oblique cell setting, Feast et al (2009) highlight an increase of the correlation between refined parameters.…”

“…The molecule methylene aziridine (Feast et al, 2009) can be described in space group P2 1 =n with cell parameters a = 13.8593 (3), b = 10.5242 (2), c = 14.8044 (4) Å , = 92.0014 (7) or space group P2 1 =c with cell parameters a = 13.8594 (2), b = 10.5243 (2), c = 19.9230 (3) Å , = 132.0439 (7) . In the oblique cell setting, Feast et al (2009) highlight an increase of the correlation between refined parameters. For our study we rerefined the structure based on the deposited reflection data.…”

“…The original refinement (Feast et al, 2009) had been performed with the CRYSTALS package (Betteridge et al, 2003). Our re-refinement was performed with SHELXL (Sheldrick, 2008).…”

“…These are related by linear transformations. Feast et al (2009) argue that a setting with an oblique angle increases the correlation between parameters and difficulties can arise during the refinement. Taking the variance/covariance matrix into account in the linear transformation, the results of the refinement in the transformed setting can be compared with the transformed results.…”