Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536807034873/cf2117sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S1600536807034873/cf2117Isup2.hkl |
CCDC reference: 657816
Key indicators
- Single-crystal X-ray study
- T = 180 K
- Mean (C-C) = 0.002 Å
- R factor = 0.026
- wR factor = 0.051
- Data-to-parameter ratio = 28.0
checkCIF/PLATON results
No syntax errors found
Alert level B PLAT029_ALERT_3_B _diffrn_measured_fraction_theta_full Low ....... 0.96
Alert level C PLAT230_ALERT_2_C Hirshfeld Test Diff for C12 - C13 .. 5.43 su PLAT230_ALERT_2_C Hirshfeld Test Diff for C15 - C16 .. 5.98 su
Alert level G REFLT03_ALERT_4_G Please check that the estimate of the number of Friedel pairs is correct. If it is not, please give the correct count in the _publ_section_exptl_refinement section of the submitted CIF. From the CIF: _diffrn_reflns_theta_max 32.10 From the CIF: _reflns_number_total 6122 Count of symmetry unique reflns 3530 Completeness (_total/calc) 173.43% TEST3: Check Friedels for noncentro structure Estimate of Friedel pairs measured 2592 Fraction of Friedel pairs measured 0.734 Are heavy atom types Z>Si present yes PLAT033_ALERT_2_G Flack Parameter Value Deviates 2 * su from zero. 0.28 PLAT860_ALERT_3_G Note: Number of Least-Squares Restraints ....... 1
0 ALERT level A = In general: serious problem 1 ALERT level B = Potentially serious problem 2 ALERT level C = Check and explain 3 ALERT level G = General alerts; check 0 ALERT type 1 CIF construction/syntax error, inconsistent or missing data 3 ALERT type 2 Indicator that the structure model may be wrong or deficient 2 ALERT type 3 Indicator that the structure quality may be low 1 ALERT type 4 Improvement, methodology, query or suggestion 0 ALERT type 5 Informative message, check
The title compound was synthesized as described in the literature for the analogous compound 2,3-bis(2-cyanoethylthio)-6,7-dimethyltetrathiafulvalene (Binet et al., 1996). The red crystals (mp. 424 K) of the studied compound were isolated by slow evaporation of a solution in acetonitrile.
H atoms were located in a difference map then positioned geometrically and refined using a riding model with C—H distances set to 0.97 Å. A common Uiso(H) was refined and converged to a value of 0.037 (2) Å2.
The search for molecular systems liable to afford interesting electrical properties – such as metallic or even superconducting behaviour – follows several strategies. One of them is to use unsymmetrically substituted tetrathiafulvalenes as building blocks (Fabre, 2000; Yamada & Sugimoto, 2004; Batail, 2004). When these target molecules are to be functionalized with hydroxyl or amine groups (e.g. in order to obtain H-bond networks) one of the possible synthesis strategies implies the use of a precursor bearing cyanoethylthio groups attached to the tetrathiafulvalene (TTF) core (Binet et al., 1996). The title compound was synthesized in this context and its crystal structure was determined to ascertain that the expected precursor was really obtained. The molecular structure is shown in Fig. 1. The main features of the structure are as follows. The two cyanoethylthio groups protrude on both sides of the TTF core, almost perpendicular to the external S3/S4/C5/C6/S7/S8 plane (Fig. 2). The TTF core is not planar and shows a boat conformation: the two C3S2 rings are folded around the S···S hinges. The central C1/C2/S1/S2/S3/S4 group is planar; the external S1/S2/C3/C4 and S3/S4/C5/C6 planes make dihedral angles of 12.19 (6)° and 22.70 (4)° respectively with the central plane (Fig. 2). The crystal investigated was an inversion twin, with contributions of 0.72:0.28 (4) for the twin domains. In this crystal structure there are no unusual intermolecular interactions, and no packing effect can be invoked to explain the folding of the TTF core (Fig. 3).
For general background on molecular metals based on tetrathiafulvalene (TTF) derivatives, see: Fabre (2000); Yamada & Sugimoto (2004); Batail (2004). For the synthesis of the title compound, see: Binet et al. (1996). Analogous precursors are used to obtain functionalized TTF derivatives (Legros et al., 2000; Benbellat et al., 2006) and oligo-TTF (Carcel et al., 2006).
Data collection: CrysAlis CCD (Oxford Diffraction, 2006); cell refinement: CrysAlis RED (Oxford Diffraction, 2006); data reduction: CrysAlis RED; program(s) used to solve structure: SIR92 (Altomare et al., 1993); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPIII (Burnett & Johnson, 1996), CAMERON (Watkin et al., 1993) and ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999).
C16H16N2S6 | Dx = 1.519 Mg m−3 |
Mr = 428.67 | Melting point: 424 K |
Orthorhombic, Pca21 | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: P 2c -2ac | Cell parameters from 6238 reflections |
a = 30.4738 (14) Å | θ = 3.0–32.1° |
b = 8.8963 (4) Å | µ = 0.73 mm−1 |
c = 6.9150 (3) Å | T = 180 K |
V = 1874.68 (15) Å3 | Block, orange |
Z = 4 | 0.40 × 0.25 × 0.15 mm |
F(000) = 888 |
Oxford Diffraction Xcalibur diffractometer with CCD detector | 6122 independent reflections |
Radiation source: fine-focus sealed tube | 5344 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.026 |
φ–ω scans | θmax = 32.1°, θmin = 3.0° |
Absorption correction: numerical [using a multifaceted crystal model based on expressions derived by Clark & Reid (1995)] | h = −37→45 |
Tmin = 0.83, Tmax = 0.90 | k = −13→12 |
19080 measured reflections | l = −9→10 |
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.026 | H-atom parameters constrained |
wR(F2) = 0.051 | w = 1/[σ2(Fo2) + (0.0248P)2] where P = (Fo2 + 2Fc2)/3 |
S = 0.96 | (Δ/σ)max = 0.003 |
6122 reflections | Δρmax = 0.34 e Å−3 |
219 parameters | Δρmin = −0.25 e Å−3 |
1 restraint | Absolute structure: Flack (1983), with 2592 Friedel pairs |
Primary atom site location: structure-invariant direct methods | Absolute structure parameter: 0.28 (4) |
C16H16N2S6 | V = 1874.68 (15) Å3 |
Mr = 428.67 | Z = 4 |
Orthorhombic, Pca21 | Mo Kα radiation |
a = 30.4738 (14) Å | µ = 0.73 mm−1 |
b = 8.8963 (4) Å | T = 180 K |
c = 6.9150 (3) Å | 0.40 × 0.25 × 0.15 mm |
Oxford Diffraction Xcalibur diffractometer with CCD detector | 6122 independent reflections |
Absorption correction: numerical [using a multifaceted crystal model based on expressions derived by Clark & Reid (1995)] | 5344 reflections with I > 2σ(I) |
Tmin = 0.83, Tmax = 0.90 | Rint = 0.026 |
19080 measured reflections |
R[F2 > 2σ(F2)] = 0.026 | H-atom parameters constrained |
wR(F2) = 0.051 | Δρmax = 0.34 e Å−3 |
S = 0.96 | Δρmin = −0.25 e Å−3 |
6122 reflections | Absolute structure: Flack (1983), with 2592 Friedel pairs |
219 parameters | Absolute structure parameter: 0.28 (4) |
1 restraint |
Experimental. Cooling Device: Oxford Instruments Cryojet. Excalibur (Oxford Diffraction) four-circle Kappa geometry diffractometer equipped with an area CCD detector. Crystal-detector distance (mm): 70.0 |
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. |
x | y | z | Uiso*/Ueq | ||
S1 | 0.480152 (12) | 0.18707 (4) | 0.71619 (6) | 0.02209 (8) | |
S2 | 0.504941 (12) | 0.20238 (5) | 1.12758 (6) | 0.02285 (9) | |
S3 | 0.392867 (12) | −0.00712 (4) | 0.83085 (6) | 0.02141 (8) | |
S4 | 0.416522 (12) | 0.01365 (4) | 1.24278 (6) | 0.02122 (8) | |
S7 | 0.296701 (12) | −0.05864 (4) | 0.90997 (6) | 0.02033 (8) | |
S8 | 0.325115 (12) | −0.04576 (4) | 1.38378 (6) | 0.01986 (8) | |
C1 | 0.46686 (5) | 0.13945 (17) | 0.9560 (2) | 0.0192 (3) | |
C2 | 0.43072 (5) | 0.06014 (17) | 1.0032 (2) | 0.0187 (3) | |
C3 | 0.52257 (4) | 0.31129 (16) | 0.7803 (2) | 0.0199 (3) | |
C4 | 0.53333 (5) | 0.31976 (17) | 0.9663 (2) | 0.0202 (3) | |
C5 | 0.35007 (4) | −0.02832 (16) | 0.9997 (2) | 0.0166 (3) | |
C6 | 0.36087 (4) | −0.01888 (16) | 1.1878 (2) | 0.0169 (3) | |
C7 | 0.54676 (5) | 0.38981 (19) | 0.6199 (3) | 0.0270 (3) | |
H71 | 0.5285 | 0.4690 | 0.5670 | 0.0369 (14)* | |
H72 | 0.5530 | 0.3187 | 0.5172 | 0.0369 (14)* | |
C8 | 0.58959 (6) | 0.4571 (2) | 0.6940 (3) | 0.0354 (4) | |
H81 | 0.6115 | 0.3785 | 0.7037 | 0.0369 (14)* | |
H82 | 0.6001 | 0.5313 | 0.6023 | 0.0369 (14)* | |
C9 | 0.58370 (6) | 0.53047 (19) | 0.8900 (3) | 0.0345 (4) | |
H91 | 0.5616 | 0.6086 | 0.8805 | 0.0369 (14)* | |
H92 | 0.6111 | 0.5772 | 0.9288 | 0.0369 (14)* | |
C10 | 0.56982 (5) | 0.41627 (19) | 1.0434 (3) | 0.0289 (4) | |
H101 | 0.5947 | 0.3537 | 1.0780 | 0.0369 (14)* | |
H102 | 0.5600 | 0.4685 | 1.1587 | 0.0369 (14)* | |
C11 | 0.29071 (5) | −0.25865 (18) | 0.9490 (2) | 0.0237 (3) | |
H111 | 0.2962 | −0.2813 | 1.0841 | 0.0369 (14)* | |
H112 | 0.2608 | −0.2882 | 0.9196 | 0.0369 (14)* | |
C12 | 0.32253 (5) | −0.34943 (19) | 0.8223 (3) | 0.0280 (4) | |
H121 | 0.3174 | −0.3249 | 0.6875 | 0.0369 (14)* | |
H122 | 0.3524 | −0.3209 | 0.8536 | 0.0369 (14)* | |
C13 | 0.31747 (5) | −0.5116 (2) | 0.8493 (2) | 0.0278 (4) | |
C14 | 0.31976 (5) | 0.14875 (18) | 1.4600 (2) | 0.0235 (3) | |
H141 | 0.3046 | 0.1527 | 1.5833 | 0.0369 (14)* | |
H142 | 0.3487 | 0.1920 | 1.4774 | 0.0369 (14)* | |
C15 | 0.29443 (5) | 0.24071 (19) | 1.3115 (3) | 0.0292 (4) | |
H151 | 0.3064 | 0.2212 | 1.1839 | 0.0369 (14)* | |
H152 | 0.2640 | 0.2087 | 1.3116 | 0.0369 (14)* | |
C16 | 0.29636 (5) | 0.40254 (19) | 1.3501 (2) | 0.0258 (3) | |
N1 | 0.31410 (5) | −0.63897 (17) | 0.8655 (2) | 0.0380 (4) | |
N2 | 0.29893 (5) | 0.52864 (16) | 1.3764 (2) | 0.0336 (3) |
U11 | U22 | U33 | U12 | U13 | U23 | |
S1 | 0.02102 (17) | 0.02499 (19) | 0.02027 (18) | −0.00603 (15) | −0.00037 (16) | 0.00098 (16) |
S2 | 0.02127 (18) | 0.0250 (2) | 0.02230 (18) | −0.00701 (15) | −0.00391 (16) | 0.00209 (16) |
S3 | 0.01632 (16) | 0.0290 (2) | 0.01892 (16) | −0.00528 (15) | 0.00099 (14) | −0.00146 (16) |
S4 | 0.01668 (16) | 0.0275 (2) | 0.01951 (17) | −0.00304 (14) | −0.00214 (15) | 0.00305 (17) |
S7 | 0.01445 (16) | 0.02107 (18) | 0.02546 (18) | −0.00232 (14) | −0.00238 (15) | 0.00118 (16) |
S8 | 0.02178 (17) | 0.01618 (16) | 0.02161 (18) | −0.00153 (13) | 0.00558 (15) | 0.00128 (15) |
C1 | 0.0177 (7) | 0.0195 (7) | 0.0205 (7) | 0.0006 (6) | −0.0012 (6) | −0.0006 (6) |
C2 | 0.0149 (7) | 0.0214 (7) | 0.0196 (7) | 0.0001 (6) | −0.0011 (6) | 0.0002 (6) |
C3 | 0.0159 (7) | 0.0173 (7) | 0.0264 (8) | −0.0001 (5) | 0.0023 (6) | 0.0004 (6) |
C4 | 0.0145 (7) | 0.0177 (7) | 0.0285 (8) | −0.0012 (5) | −0.0003 (6) | 0.0003 (6) |
C5 | 0.0126 (7) | 0.0164 (7) | 0.0209 (7) | −0.0015 (5) | 0.0017 (6) | 0.0003 (6) |
C6 | 0.0150 (6) | 0.0157 (7) | 0.0200 (7) | 0.0003 (5) | 0.0028 (5) | −0.0001 (6) |
C7 | 0.0267 (8) | 0.0246 (9) | 0.0298 (8) | −0.0023 (6) | 0.0070 (7) | 0.0045 (7) |
C8 | 0.0269 (9) | 0.0337 (10) | 0.0455 (12) | −0.0107 (7) | 0.0107 (8) | 0.0008 (8) |
C9 | 0.0285 (9) | 0.0288 (9) | 0.0460 (11) | −0.0112 (7) | 0.0063 (9) | −0.0028 (8) |
C10 | 0.0228 (8) | 0.0262 (9) | 0.0378 (10) | −0.0061 (7) | −0.0060 (7) | −0.0009 (8) |
C11 | 0.0218 (8) | 0.0209 (8) | 0.0283 (9) | −0.0052 (6) | 0.0017 (6) | −0.0011 (6) |
C12 | 0.0336 (9) | 0.0242 (9) | 0.0262 (8) | −0.0020 (7) | 0.0038 (7) | −0.0020 (7) |
C13 | 0.0315 (9) | 0.0299 (9) | 0.0221 (9) | 0.0002 (7) | −0.0011 (7) | −0.0039 (7) |
C14 | 0.0308 (8) | 0.0188 (8) | 0.0209 (7) | 0.0022 (6) | 0.0035 (7) | −0.0003 (6) |
C15 | 0.0348 (9) | 0.0202 (8) | 0.0326 (9) | 0.0032 (7) | −0.0057 (8) | −0.0008 (7) |
C16 | 0.0262 (8) | 0.0271 (9) | 0.0242 (8) | 0.0071 (6) | −0.0007 (7) | 0.0004 (7) |
N1 | 0.0528 (10) | 0.0283 (8) | 0.0329 (9) | 0.0008 (7) | −0.0007 (8) | −0.0026 (7) |
N2 | 0.0392 (8) | 0.0253 (8) | 0.0364 (9) | 0.0075 (6) | −0.0032 (7) | 0.0001 (7) |
S1—C3 | 1.7573 (15) | C8—H81 | 0.970 |
S1—C1 | 1.7590 (15) | C8—H82 | 0.970 |
S2—C1 | 1.7514 (15) | C9—C10 | 1.529 (3) |
S2—C4 | 1.7558 (16) | C9—H91 | 0.970 |
S3—C5 | 1.7605 (14) | C9—H92 | 0.970 |
S3—C2 | 1.7631 (15) | C10—H101 | 0.970 |
S4—C2 | 1.7617 (16) | C10—H102 | 0.970 |
S4—C6 | 1.7620 (14) | C11—C12 | 1.536 (2) |
S7—C5 | 1.7616 (15) | C11—H111 | 0.970 |
S7—C11 | 1.8089 (16) | C11—H112 | 0.970 |
S8—C6 | 1.7552 (15) | C12—C13 | 1.463 (2) |
S8—C14 | 1.8162 (16) | C12—H121 | 0.970 |
C1—C2 | 1.3479 (19) | C12—H122 | 0.970 |
C3—C4 | 1.330 (2) | C13—N1 | 1.143 (2) |
C3—C7 | 1.504 (2) | C14—C15 | 1.523 (2) |
C4—C10 | 1.503 (2) | C14—H141 | 0.970 |
C5—C6 | 1.344 (2) | C14—H142 | 0.970 |
C7—C8 | 1.524 (2) | C15—C16 | 1.465 (2) |
C7—H71 | 0.970 | C15—H151 | 0.970 |
C7—H72 | 0.970 | C15—H152 | 0.970 |
C8—C9 | 1.515 (3) | C16—N2 | 1.139 (2) |
C3—S1—C1 | 94.77 (7) | C8—C9—H91 | 109.3 |
C1—S2—C4 | 94.95 (7) | C10—C9—H91 | 109.3 |
C5—S3—C2 | 94.18 (7) | C8—C9—H92 | 109.3 |
C2—S4—C6 | 94.13 (7) | C10—C9—H92 | 109.3 |
C5—S7—C11 | 101.03 (7) | H91—C9—H92 | 108.0 |
C6—S8—C14 | 98.62 (7) | C4—C10—C9 | 109.78 (15) |
C2—C1—S2 | 123.02 (12) | C4—C10—H101 | 109.7 |
C2—C1—S1 | 122.83 (11) | C9—C10—H101 | 109.7 |
S2—C1—S1 | 114.15 (8) | C4—C10—H102 | 109.7 |
C1—C2—S4 | 123.44 (11) | C9—C10—H102 | 109.7 |
C1—C2—S3 | 123.28 (12) | H101—C10—H102 | 108.2 |
S4—C2—S3 | 113.28 (8) | C12—C11—S7 | 111.60 (11) |
C4—C3—C7 | 124.49 (14) | C12—C11—H111 | 109.3 |
C4—C3—S1 | 117.44 (11) | S7—C11—H111 | 109.3 |
C7—C3—S1 | 117.83 (11) | C12—C11—H112 | 109.3 |
C3—C4—C10 | 123.92 (15) | S7—C11—H112 | 109.3 |
C3—C4—S2 | 117.35 (11) | H111—C11—H112 | 108.0 |
C10—C4—S2 | 118.63 (12) | C13—C12—C11 | 112.28 (14) |
C6—C5—S3 | 116.98 (11) | C13—C12—H121 | 109.1 |
C6—C5—S7 | 125.19 (11) | C11—C12—H121 | 109.1 |
S3—C5—S7 | 117.83 (9) | C13—C12—H122 | 109.1 |
C5—C6—S8 | 125.92 (11) | C11—C12—H122 | 109.1 |
C5—C6—S4 | 117.04 (10) | H121—C12—H122 | 107.9 |
S8—C6—S4 | 116.95 (9) | N1—C13—C12 | 178.0 (2) |
C3—C7—C8 | 110.74 (14) | C15—C14—S8 | 111.22 (11) |
C3—C7—H71 | 109.5 | C15—C14—H141 | 109.4 |
C8—C7—H71 | 109.5 | S8—C14—H141 | 109.4 |
C3—C7—H72 | 109.5 | C15—C14—H142 | 109.4 |
C8—C7—H72 | 109.5 | S8—C14—H142 | 109.4 |
H71—C7—H72 | 108.1 | H141—C14—H142 | 108.0 |
C9—C8—C7 | 111.63 (14) | C16—C15—C14 | 112.62 (14) |
C9—C8—H81 | 109.3 | C16—C15—H151 | 109.1 |
C7—C8—H81 | 109.3 | C14—C15—H151 | 109.1 |
C9—C8—H82 | 109.3 | C16—C15—H152 | 109.1 |
C7—C8—H82 | 109.3 | C14—C15—H152 | 109.1 |
H81—C8—H82 | 108.0 | H151—C15—H152 | 107.8 |
C8—C9—C10 | 111.53 (15) | N2—C16—C15 | 177.89 (18) |
Experimental details
Crystal data | |
Chemical formula | C16H16N2S6 |
Mr | 428.67 |
Crystal system, space group | Orthorhombic, Pca21 |
Temperature (K) | 180 |
a, b, c (Å) | 30.4738 (14), 8.8963 (4), 6.9150 (3) |
V (Å3) | 1874.68 (15) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.73 |
Crystal size (mm) | 0.40 × 0.25 × 0.15 |
Data collection | |
Diffractometer | Oxford Diffraction Xcalibur diffractometer with CCD detector |
Absorption correction | Numerical [using a multifaceted crystal model based on expressions derived by Clark & Reid (1995)] |
Tmin, Tmax | 0.83, 0.90 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 19080, 6122, 5344 |
Rint | 0.026 |
(sin θ/λ)max (Å−1) | 0.748 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.026, 0.051, 0.96 |
No. of reflections | 6122 |
No. of parameters | 219 |
No. of restraints | 1 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.34, −0.25 |
Absolute structure | Flack (1983), with 2592 Friedel pairs |
Absolute structure parameter | 0.28 (4) |
Computer programs: CrysAlis CCD (Oxford Diffraction, 2006), CrysAlis RED (Oxford Diffraction, 2006), CrysAlis RED, SIR92 (Altomare et al., 1993), SHELXL97 (Sheldrick, 1997), ORTEPIII (Burnett & Johnson, 1996), CAMERON (Watkin et al., 1993) and ORTEP-3 (Farrugia, 1997), WinGX (Farrugia, 1999).
The search for molecular systems liable to afford interesting electrical properties – such as metallic or even superconducting behaviour – follows several strategies. One of them is to use unsymmetrically substituted tetrathiafulvalenes as building blocks (Fabre, 2000; Yamada & Sugimoto, 2004; Batail, 2004). When these target molecules are to be functionalized with hydroxyl or amine groups (e.g. in order to obtain H-bond networks) one of the possible synthesis strategies implies the use of a precursor bearing cyanoethylthio groups attached to the tetrathiafulvalene (TTF) core (Binet et al., 1996). The title compound was synthesized in this context and its crystal structure was determined to ascertain that the expected precursor was really obtained. The molecular structure is shown in Fig. 1. The main features of the structure are as follows. The two cyanoethylthio groups protrude on both sides of the TTF core, almost perpendicular to the external S3/S4/C5/C6/S7/S8 plane (Fig. 2). The TTF core is not planar and shows a boat conformation: the two C3S2 rings are folded around the S···S hinges. The central C1/C2/S1/S2/S3/S4 group is planar; the external S1/S2/C3/C4 and S3/S4/C5/C6 planes make dihedral angles of 12.19 (6)° and 22.70 (4)° respectively with the central plane (Fig. 2). The crystal investigated was an inversion twin, with contributions of 0.72:0.28 (4) for the twin domains. In this crystal structure there are no unusual intermolecular interactions, and no packing effect can be invoked to explain the folding of the TTF core (Fig. 3).