Refinement of obverse/reverse twins

Herbst-Irmer, R.; Sheldrick, G.M.

doi:10.1107/S0108768102001039

Refinement of obverse/reverse twins

Two examples of structures of obverse/reverse twins are presented. Example (1) is the structure of 2,2,4,4,6,6-hexa-tert-butylcyclotrisiloxane. It crystallizes in R $\overline 3$ c. Example (2) is the structure of [{Li(Me₃Si)₃CAlF₃(thf)}₃LiF(thf)], tris[lithiuim tetrahydrofuran (trimethylsilyl)methyltrifluoroaluminate]lithium fluoride tetrahydrofuran. It crystallizes in R3. Additional to the obverse/reverse twinning this structure shows merohedral twinning. It will be shown how these two structures can be refined with the SHELXL program.

Keywords: obverse/reverse twins; merohedral twinning; SHELXL.

Read article Similar articles

Supporting information

	Crystallographic Information File (CIF) https://doi.org/10.1107/S0108768102001039/jz0014sup1.cif Contains datablocks 1, 2
	Structure factor file (CIF format) https://doi.org/10.1107/S0108768102001039/jz00141sup2.hkl Raw intensity data: hkl Fsquared obs u(Fsquared obs) before any averaging or structuring for treatment as a twin for structure (1)
	Structure factor file (CIF format) https://doi.org/10.1107/S0108768102001039/jz00142sup3.hkl Raw intensity data: hkl Fsquared obs u(Fsquared obs) before any averaging or structuring for treatment as a twin for structure (2) data set 1
	Structure factor file (CIF format) https://doi.org/10.1107/S0108768102001039/jz00142sup4.hkl Raw intensity data: hkl Fsquared obs u(Fsquared obs) before any averaging or structuring for treatment as a twin for structure (2) data set 2

Computing details top

Data collection: Siemens SMART (V. 4.202, Siemens 1996) for (1). Cell refinement: Siemens SAINT (V. 4.050, Siemens 1996) for (1). Data reduction: Siemens SAINT (V. 4.050, Siemens 1996) for (1). For both compounds, program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997). Molecular graphics: Bruker AXS SHELXTL (V. 5.10) for (1). Software used to prepare material for publication: SHELXL97 (Sheldrick, 1997) for (1).

(1) top

Crystal data top

C₂₄H₅₄O₃Si₃	F(000) = 1584
M_r = 474.94	D_x = 1.111 Mg m⁻³
Trigonal, R3c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -R 3 2"c	Cell parameters from 849 reflections
a = 10.0793 (7) Å	θ = 2.5–25°
c = 48.409 (4) Å	µ = 0.19 mm⁻¹
V = 4259.1 (6) Å³	T = 133 K
Z = 6

Data collection top

Stoe-Siemens-Huber four circle diffractometer	849 independent reflections
Radiation source: fine-focus sealed tube	751 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.037
Detector resolution: 8.192 pixels/mm pixels mm^-1	θ_max = 25.2°, θ_min = 2.5°
phi scan	h = −10→9
Absorption correction: multi-scan	k = −11→11
	l = 0→57
47973 measured reflections

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.035	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.090	H atoms treated by a mixture of independent and constrained refinement
S = 1.11	w = 1/[σ²(F_o²) + (0.0527P)² + 2.7315P] where P = (F_o² + 2F_c²)/3
849 reflections	(Δ/σ)_max < 0.001
51 parameters	Δρ_max = 0.39 e Å⁻³
0 restraints	Δρ_min = −0.33 e Å⁻³

Special details top

Experimental. Intensities were measured with a Siemens CCD area detector

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Si	0.84589 (6)	0.3333	0.0833	0.0157 (2)
O	0.6667	0.17946 (18)	0.0833	0.0259 (5)
C3	1.0225 (2)	0.2288 (2)	0.05245 (4)	0.0285 (5)
H3A	1.0699	0.2298	0.0347	0.043*
H3B	0.9403	0.1244	0.0565	0.043*
H3C	1.0999	0.2637	0.0671	0.043*
C2	1.0871 (2)	0.5007 (2)	0.04538 (4)	0.0275 (5)
H2A	1.1412	0.5020	0.0285	0.041*
H2B	1.1588	0.5352	0.0610	0.041*
H2C	1.0452	0.5693	0.0432	0.041*
C	0.9555 (2)	0.3364 (2)	0.05089 (3)	0.0201 (4)
C1	0.8500 (2)	0.2840 (2)	0.02550 (4)	0.0275 (5)
H1A	0.8036	0.3488	0.0234	0.041*
H1B	0.7691	0.1771	0.0280	0.041*
H1C	0.9098	0.2925	0.0090	0.041*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Si	0.0160 (3)	0.0144 (4)	0.0161 (3)	0.00718 (18)	0.00040 (13)	0.0008 (3)
O	0.0222 (10)	0.0277 (8)	0.0261 (9)	0.0111 (5)	0.0001 (7)	0.0001 (4)
C3	0.0323 (11)	0.0282 (11)	0.0305 (10)	0.0195 (10)	0.0067 (8)	0.0026 (8)
C2	0.0257 (11)	0.0258 (11)	0.0289 (9)	0.0114 (9)	0.0084 (8)	0.0080 (8)
C	0.0203 (10)	0.0184 (9)	0.0220 (8)	0.0100 (8)	0.0039 (7)	0.0015 (7)
C1	0.0295 (11)	0.0328 (11)	0.0205 (9)	0.0157 (9)	0.0028 (8)	−0.0013 (8)

Geometric parameters (Å, º) top

Si—O	1.6932 (8)	O—Siⁱⁱⁱ	1.6932 (8)
Si—Oⁱ	1.6932 (8)	C3—C	1.539 (2)
Si—Cⁱⁱ	1.9115 (16)	C2—C	1.542 (2)
Si—C	1.9116 (16)	C—C1	1.536 (2)

O—Si—Oⁱ	104.98 (12)	C1—C—C3	105.60 (14)
O—Si—Cⁱⁱ	109.63 (6)	C1—C—C2	107.87 (14)
Oⁱ—Si—Cⁱⁱ	110.99 (6)	C3—C—C2	108.69 (15)
O—Si—C	110.99 (6)	C1—C—Si	110.88 (12)
Oⁱ—Si—C	109.63 (6)	C3—C—Si	113.70 (12)
Cⁱⁱ—Si—C	110.49 (11)	C2—C—Si	109.86 (11)
Si—O—Siⁱⁱⁱ	135.02 (12)

Symmetry codes: (i) −y+1, x−y, z; (ii) x−y+1/3, −y+2/3, −z+1/6; (iii) −x+y+1, −x+1, z.

(2) top