Redetermination of 1,4-dimethoxybenzene

The structure of the centrosymmetric title compound, C8H10O2, originally determined by Goodwin et al. [Acta Cryst.(1950), 3, 279–284], has been redetermined to modern standards of precision to aid in its use as a model compound for 13C chemical-shift tensor measurements in single-crystal NMR studies. In the crystal structure, a C—H⋯O interaction helps to establish the packing.

The structure of the centrosymmetric title compound, C 8 H 10 O 2 , originally determined by Goodwin et al. [Acta Cryst. (1950), 3, 279-284], has been redetermined to modern standards of precision to aid in its use as a model compound for 13 C chemical-shift tensor measurements in single-crystal NMR studies. In the crystal structure, a C-HÁ Á ÁO interaction helps to establish the packing.
Data collection: COLLECT (Hooft, 1998); cell refinement: DENZO-SMN (Otwinowski & Minor, 1997); data reduction: DENZO-SMN; program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: WinGX (Farrugia, 1999) and ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: publCIF (Westrip, 2009 (Carter et al., 1988). In 1950 Goodwin et al. obtained the first X-ray diffraction structure for 1,4-dimethoxybenzene. This structure (R-factor = 0.12) is shown in Fig. 1 and reported an unusual H-C-C angle of 75.7°, which prompted the acquisition of a second structure (Carter et al., 1988). More typical H-C-C angles were observed with this new refinement and this structure (R-factor = 0.067) was used to assign tensor orientations in the single-crystal NMR analysis. Inadvertently, the second structure was never submitted to the Cambridge Crystallographic database. Here, the acquisition of a third structure is reported to correct this oversight. The new structure (R-factor = 0.038) is shown in Fig. 2. The unit-cell and space group of the previous studies are confirmed.
Acquisition of this third, more accurate, structure is beneficial to NMR studies because the 13 C chemical shift tensor data of 1,4-dimethoxybenzene continue to serve as a standard to evaluate new chemical-shift tensor measurement methods as well as to assess electronic structure methods for computing magnetic properties of molecules.

Refinement
The H atoms were located in difference maps and their positions and U iso values were freely refined. Fig. 1. The structure of (I) according to Goodwin et al. (1950).

Special details
Experimental. The program DENZO-SMN (Otwinowski & Minor, 1997) uses a scaling algorithm (Fox & Holmes, 1966) which effectively corrects for absorption effects. High redundancy data were used in the scaling program hence the 'multi-scan' code word was Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.