A low-temperature redetermination of the monoclinic β-form of cyclo­octaselenium

Monoclinic β-cyclo­octaselenium, Se₈, crystallizes as discrete eight-membered ring mol­ecules. The low-temperature (150 K) crystal structure is composed of cyclic crown-shaped mol­ecules, similar to those of α- and γ-Se₈. The average Se—Se bond length is 2.308 (8) Å, the average bond angle is 105.7 (8)° and the average torsion angle is 101 (2)°. In contrast with α-Se₈, the structure of β-Se₈ does not show elongation of the ring at low temperatures.

Key indicators

• Single-crystal X-ray study
• T = 150 K
• Mean [sigma](Se-Se) = 0.001 Å
• R factor = 0.040
• wR factor = 0.095
• Data-to-parameter ratio = 24.9

checkCIF/PLATON results

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PLAT230_ALERT_2_C Hirshfeld Test Diff for    Se1    -   Se2     ..       5.14 su

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            FO or CO or EO for organic.
            From the CIF: _publ_requested_category   EO
            From the CIF: _chemical_formula_sum :Se8

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Computing details

Data collection: KappaCCD Server Software (Nonius, 1997); cell refinement: DENZO–SMN (Otwinowski & Minor, 1997); data reduction: DENZO–SMN; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg & Berndt, 1999); software used to prepare material for publication: WinGX (Farrugia, 1999).

beta-monoclinic cyclooctaselenium

Crystal data

Se8	F(000) = 1088
Mr = 631.68	Dx = 4.482 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 9298 reflections
a = 9.2004 (18) Å	θ = 2.2–26.0°
b = 8.0000 (16) Å	µ = 31.14 mm^−1
c = 12.735 (3) Å	T = 150 K
β = 92.95 (3)°	Prism, red
V = 936.1 (3) Å^3	0.15 × 0.08 × 0.05 mm
Z = 4

Data collection

Nonius KappaCCD diffractometer	1821 independent reflections
Radiation source: fine-focus sealed tube	1463 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.078
φ scans, and ω scans with κ offsets	θmax = 26.0°, θmin = 2.2°
Absorption correction: ψ scan (SHELXTL; Bruker, 2001)	h = −10→11
Tmin = 0.062, Tmax = 0.211	k = −9→9
9298 measured reflections	l = −15→15

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.040	Secondary atom site location: difference Fourier map
wR(F^2) = 0.095	w = 1/[σ^2(Fo^2) + (0.0413P)^2 + 5.4999P] where P = (Fo^2 + 2Fc^2)/3
S = 1.09	(Δ/σ)max = 0.001
1821 reflections	Δρmax = 1.13 e Å^−3
73 parameters	Δρmin = −1.23 e Å^−3

Special details

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 (Ų)

	x	y	z	Uiso*/Ueq
Se1	0.06355 (10)	0.68641 (11)	0.91290 (7)	0.0288 (2)
Se2	0.25586 (11)	0.77285 (11)	1.02241 (8)	0.0304 (3)
Se3	0.25827 (11)	0.60084 (11)	1.16858 (7)	0.0279 (2)
Se4	0.44886 (10)	0.41857 (11)	1.14620 (7)	0.0256 (2)
Se5	0.34045 (11)	0.17089 (11)	1.09083 (7)	0.0270 (2)
Se6	0.35943 (10)	0.16250 (11)	0.90960 (7)	0.0240 (2)
Se7	0.13558 (10)	0.24704 (12)	0.84010 (7)	0.0297 (3)
Se8	0.16495 (11)	0.52018 (13)	0.78818 (7)	0.0329 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Se1	0.0278 (5)	0.0309 (5)	0.0278 (5)	0.0072 (4)	0.0030 (4)	0.0012 (4)
Se2	0.0354 (6)	0.0211 (5)	0.0350 (6)	−0.0020 (4)	0.0058 (4)	0.0020 (4)
Se3	0.0337 (6)	0.0245 (5)	0.0261 (5)	0.0009 (4)	0.0075 (4)	−0.0018 (4)
Se4	0.0241 (5)	0.0279 (5)	0.0246 (5)	−0.0011 (4)	0.0005 (4)	−0.0008 (4)
Se5	0.0358 (6)	0.0223 (5)	0.0236 (5)	−0.0006 (4)	0.0068 (4)	0.0026 (3)
Se6	0.0281 (5)	0.0208 (5)	0.0235 (5)	0.0008 (4)	0.0057 (4)	0.0002 (3)
Se7	0.0246 (5)	0.0364 (6)	0.0283 (5)	−0.0029 (4)	0.0022 (4)	−0.0082 (4)
Se8	0.0342 (6)	0.0398 (6)	0.0251 (5)	0.0104 (4)	0.0040 (4)	0.0043 (4)

Geometric parameters (Å, º)

Se1—Se2	2.3021 (15)	Se4—Se5	2.3121 (13)
Se1—Se8	2.3052 (14)	Se5—Se6	2.3244 (13)
Se2—Se3	2.3141 (14)	Se6—Se7	2.3010 (15)
Se3—Se4	2.3095 (14)	Se7—Se8	2.3029 (15)
Se2—Se1—Se8	105.37 (5)	Se4—Se5—Se6	105.92 (5)
Se1—Se2—Se3	106.50 (5)	Se7—Se6—Se5	105.19 (5)
Se4—Se3—Se2	104.60 (5)	Se6—Se7—Se8	105.87 (5)
Se3—Se4—Se5	105.09 (5)	Se7—Se8—Se1	106.97 (5)
Se8—Se1—Se2—Se3	−99.24 (6)	Se4—Se5—Se6—Se7	−99.75 (6)
Se1—Se2—Se3—Se4	104.27 (6)	Se5—Se6—Se7—Se8	102.57 (5)
Se2—Se3—Se4—Se5	−103.15 (6)	Se6—Se7—Se8—Se1	−103.24 (6)
Se3—Se4—Se5—Se6	100.28 (6)	Se2—Se1—Se8—Se7	98.21 (6)