Tetra­thia­fulvalene revisited

Batsanov, A.S.

doi:10.1107/S0108270106022554

organic compounds

STRUCTURAL
CHEMISTRY

ISSN: 2053-2296

Volume 62| Part 8| August 2006| Pages o501-o504

doi:10.1107/S0108270106022554

Tetrathiafulvalene revisited

Andrei S. Batsanov ^a ^*

^aDepartment of Chemistry, University of Durham, South Road, Durham DH1 3LE, England
^*Correspondence e-mail: a.s.batsanov@durham.ac.uk

(Received 19 April 2006; accepted 13 June 2006; online 22 July 2006)

Monoclinic (α) tetrathiafulvalene [systematic name: 2-(1,3-dithiol-2-ylidene)-1,3-dithiole], C₆H₄S₄, undergoes a reversible second-order phase transition at ca 190 K through a displacive modulation with doubling of the a parameter. The low-temperature phase (γ) contains two crystallographically non-equivalent (but centrosymmetric) molecules of very similar geometry.

Comment

The electron donor 2,2′-bis(1,3-dithiole), or tetrathiafulvalene (TTF), synthesized by Wudl et al. (1970 ), is of paramount importance as a component of charge-transfer (CT) salts and complexes, organic metals, and superconductors. Knowing its precise structural parameters is obviously useful, especially since Reith et al. (1988 ) and Clemente & Marzotto (1996 ) have demonstrated simple linear relations between the degree of CT and bond lengths in the TTF molecule.

TTF appears in two polymorphic modifications, viz. the orange monoclinic α form (or TTF1) and the yellow triclinic β form (or TTF2). The crystal structure of α-TTF was determined at room temperature (Cooper et al., 1971 , 1974 ) and never revisited, evidently because of the high quality of the original study. The β phase, which is more stable at elevated temperatures (Bozio et al., 1979 ; Venuti et al., 2001 ), was discovered by LaPlaca et al. (1975 ), who reported its lattice parameters (Weidenborner et al., 1977 ). The full structure determination was carried out by Ellern et al. (1994 ), also at room temperature. The low-temperature crystallography of TTF remained unexplored, although β-TTF was known to convert occasionally into α-TTF after slow cooling to 80 K (Venuti et al., 2001). Temkin et al. (1977 ) studied the Raman spectra of α-TTF at 2, 80 and 300 K, and observed ten low-frequency (intermolecular) modes at low temperatures, instead of the six expected for two molecules per unit cell. A more detailed Raman spectroscopic study of both α- and β-TTF from 300 to 80 K was carried out by Venuti et al. (2001), who reported no unusual effects.

We undertook a variable-temperature study of α-TTF to obtain the standard bond lengths free of the spurious shortening caused by thermal libration (for which no corrections had been made in the earlier studies). On cooling the sample below 190 K, additional reflections emerged, which corresponded to the doubling of the lattice parameter a. The sample remained a single crystal and the transition was fully reversible; five cycles of cooling/warming across it did not cause any appreciable deterioration of the crystal quality. No further changes of the diffraction pattern were detected down to 95 K. Full sets of data were collected at 290, 150 and 98 K and used for structure determination. At 290 K, our results agree with those of Cooper et al. (1971, 1974). The molecule (Fig. 1a) lies at a crystallographic inversion centre and participates in an infinite stack, running parallel to the y axis, with rigorously uniform interplanar separations of 3.60 Å (calculated between the central C₂S₄ groups).

For the low-temperature phase, all `new' reflections (with odd h) were systematically weaker than the `old' (with even h). At 150 K, the intensities differ by an average factor of 13, but at 98 K only by a factor of 8. Nevertheless, the `new' reflections are not negligible. Thus, 62% of reflections with h odd have I greater than 3σ(I) at 150 K, and 72% at 98 K (cf. 85 and 88% for h even). At 150 K, the mean I/σ(I) ratio is 24.1 for h even and 9.2 for h odd; at 98 K, this ratio is, respectively, 26.4 and 13.5.

Thus, at around 190 K, α-TTF undergoes a second-order phase transition into a new low-temperature modification, γ-TTF. From systematic absences, the latter has P2₁/n symmetry rather than P2₁/c. For example, the 98 K data set contained 374 reflections of h0l type with both h and l odd, giving the average I/σ(I) ratio of 22.1. Of these, 342 reflections (91%) had I greater than 3σ(I). On the other hand, 386 reflections of h0l type with h odd and l even had the average I/σ(I) equal to 0.7; only 59 reflections (15%) had I exceeding 3σ(I), and those only very slightly. The mean absolute intensities of the former and the latter class related as 91:1.

The structure of the low-temperature phase was solved successfully in the space group P2₁/n, with two crystallographically non-equivalent molecules (A and B; Fig. 1b) lying at inversion centres at (0, $[1\over2]$ , $[1\over2]$ ) and ( $[1\over2]$ , $[1\over2]$ , $[1\over2]$ ). Thus, the asymmetric unit comprises two half-molecules, which would be related by an a/2 translation (equivalent to an a translation in α-TTF) but for a slight misalignment (Fig. 1c). At 150 K, atoms C3 of A and B deviate by 0.053 (1) Å from an ideal a/2 translation, atoms S1 and S2 by 0.180 (1) and 0.173 (1) Å, and the peripheral atoms C1 and C2 by 0.248 (2) and 0.245 (2) Å. At 98 K, the deviations slightly increase, viz. 0.064 (2) Å for C3, 0.218 (1) Å for S1, 0.210 (1) Å for S2, 0.300 (2) Å for C1 and 0.299 (2) Å for C2. The mean deviation for all non-H atoms increases from 0.18 Å at 150 K to 0.22 Å at 98 K. The angle between the central C₂S₄ planes of molecules A and B increases from 2.3 (1)° at 150 K to 2.8 (1)° at 98 K, and the angle between the long axes of these molecules from 4.5 (1) to 5.4 (1)°. Owing to the smallness of these deviations, γ-TTF can be regarded as displacively modulated α-TTF. In fact, using only the (low-temperature) data with h even, the structure of γ-TTF can be solved and refined as a 1:1 disordered structure having a lattice similar to that of the α phase and P2₁/c symmetry. The refinement converges at nearly the same R factor (on half the number of reflections, of course).

An alternative model of γ-TTF was tested, assuming one independent molecule occupying a general position [with the molecular centroid at ca ( $[1\over4]$ , 0, 0)]. The refinement was unstable and produced absurd atomic displacement parameters and an R value greater than 0.12. Refinements in P2₁/c symmetry were equally unsuccessful.

In both α and γ phases, the TTF molecule shows a small but significant chair-like distortion, folding along the S1⋯S2 vector by 2.0 (1)° in the α phase and by 2.3 (1) and 1.8 (1)° in molecules A and B of the γ phase. The same conformation was found in β-TTF, which (unlike the α and γ phases) does not have a stacking motif, whereas in the gas phase, TTF adopts a boat conformation, with a 13.5° folding of both rings (Hargittai et al., 1994 ). Both ab initio calculations (Batsanov et al., 1995 ) and a survey of the Cambridge Structural Database (Wang et al., 1997 ) indicate that the TTF molecule is indeed rather flexible conformationally.

Although γ-TTF contains two symmetrically non-equivalent types of stacks, the differences between their geometries are negligible. The mean interplanar separation is 3.53 Å at 150 K and 3.51 Å at 98 K. In α-TTF, atom S1 forms with its symmetry equivalent an inter-stack contact, S1⋯S1ⁱⁱ, of 3.400 (1) Å, considerably shorter than twice the van der Waals radius of S (1.81 Å; Rowland & Taylor, 1996 ), while S2 is `wedged' between two molecules of the adjacent stack, at equal distances [3.575 (1) Å] from S2ⁱⁱⁱ and S2^iv (symmetry codes as in Figs. 1 and 2). In γ-TTF, this pattern remains essentially the same. The S1⋯S1ⁱⁱ contact becomes S1A⋯S1Bⁱⁱ of 3.353 (1) (150 K) or 3.341 (1) Å (98 K). The S2⋯S2ⁱⁱⁱ and S2⋯S2^iv contacts become S2A⋯S2Bⁱⁱⁱ and S2A⋯S2B^iv, respectively, which are no longer symmetrically equivalent but nevertheless are equidistant within experimental error, averaging 3.514 (1) Å at 150 K and 3.498 (1) Å at 98 K.

The observed and libration-corrected [by the TLS model of Schomaker & Trueblood (1968 ) in PLATON (Spek, 2003 )] bond distances are listed in Table 1, and in Table 2 the average values are compared with the earlier results. It is worth noting that the `outer' S—C1,2 bonds are significantly shorter than the `inner' S—C3 bonds, and the difference is even more pronounced after the libration correction and/or at low temperature.

The May 2006 version of the Cambridge Structural Database (Allen & Taylor, 2004 ) lists over 150 structures containing unsubstituted TTF groups. Most of these materials are salts or CT complexes, in which the TTF group bears a net charge varying between 0 and +1 (Salmeron-Valverde et al., 2003 ; Salmeron-Valverde & Bernes, 2005 , and references therein). Accumulation of positive charge substantially alters the molecular geometry, the S—C bonds contracting and the C=C bonds lengthening in comparison with neutral TTF (Reith et al., 1988; Clemente & Marzotto, 1996). However, low-temperature studies of some cocrystals, where CT is ruled out by spectroscopic and conductivity evidence, have revealed a TTF molecular geometry very similar to the present results (see Table 1).

Venuti et al. (2001) have made interesting predictions of the structure and energy of solid TTF by quasi-harmonic lattice dynamics, using three different molecular-mechanical models, viz. a 6-exp type atom–atom potential (W) parametrized by Williams & Cox (1984 ), the same with added Coulombic term (W+C) and another 6-exp potential specially devised by Della Valle et al. (1999 ) for bis(ethylenedithio)tetrathiafulvalene systems, also with a Coulombic term (ET+C). Both the W+C and the ET+C models predict that, on cooling from 300 to 0 K, the a and b cell parameters will contract but the c parameter will expand, whereas the W method predicts a uniform contraction of all three parameters (see Table 3). Since the space-group symmetry was taken as datum, the phase transition obviously could not be forseen. Apart from this, our results show better agreement with W than with Coulombic corrected models. It is also noteworthy that W accurately predicted the β angle, which in the other two models was off the mark by 2–5°.

The same authors determined phonon wavenumbers of TTF from Raman data at 300 and 295 K, and at eight other temperatures ranging from 200 to 80 K. It is noteworthy that between 80 and 190 K each wavenumber shows a gentle linear dependence on temperature, but the values for 200 K are uniformly shifted down from the corresponding regression lines, which may be a manifestation of the phase transition. Unfortunately, no measurements were taken between 200 and 295 K, which makes comparison with the present results difficult.

For the preliminary publication on the phase traisition in TTF, see Batsanov et al. (1998 ).

Figure 1
The molecular structure of TTF, showing (a) the α phase at 290 K, (b) the γ phase at 98 K, and (c) the overlap of molecules A and B (shifted by a/2) in the γ phase. Atomic displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) −x, −y + 1, −z + 1; (ii) −x + 1, −y + 1, −z + 1.]

Figure 2
The crystal packing in γ-TTF. Dashed lines indicate the unit cell of α-TTF. [Symmetry codes: (ii) −x + 1, −y + 1, −z + 1; (iii) −x + $[{1\over 2}]$ , − $[{1\over 2}]$ + y, $[{3\over 2}]$ − z; (iv) −x + $[{1\over 2}]$ , y + $[{1\over 2}]$ , −z + $[{3\over 2}]$ ].

Experimental

Commercial TTF was recrystallized from heptane at room temperature.

TTF at 290 K

Crystal data

C₆H₄S₄
M_r = 204.33
Monoclinic, P 2₁ /c
a = 7.352 (2) Å
b = 4.0181 (11) Å
c = 13.901 (4) Å
β = 101.426 (10)°
V = 402.5 (2) Å³
Z = 2
D_x = 1.686 Mg m⁻³
Mo Kα radiation
μ = 1.09 mm⁻¹
T = 290 (2) K
Block, orange
0.20 × 0.16 × 0.14 mm

Data collection

Siemens SMART 1000 CCD area-detector diffractometer
ω scans
Absorption correction: integration (XPREP in SHELXTL; Bruker, 2001 ); R_int = 0.040 before correction T_min = 0.786, T_max = 0.875
3571 measured reflections
926 independent reflections
813 reflections with I > 2σ(I)
R_int = 0.034
θ_max = 27.5°

Refinement

Refinement on F²
R[F² > 2σ(F²)] = 0.028
wR(F²) = 0.065
S = 1.12
926 reflections
55 parameters
All H-atom parameters refined
w = 1/[σ²(F_o²) + (0.0224P)² + 0.1703P] where P = (F_o² + 2F_c²)/3
(Δ/σ)_max = 0.001
Δρ_max = 0.32 e Å⁻³
Δρ_min = −0.16 e Å⁻³
Extinction correction: SHELXL97
Extinction coefficient: 0.132 (7)

TTF at 150 K

Crystal data

C₆H₄S₄
M_r = 204.33
Monoclinic, P 2₁ /n
a = 14.641 (3) Å
b = 3.933 (1) Å
c = 13.832 (3) Å
β = 100.98 (1)°
V = 781.9 (3) Å³
Z = 4
D_x = 1.736 Mg m⁻³
Mo Kα radiation
μ = 1.13 mm⁻¹
T = 150 (2) K
Block, orange
0.20 × 0.16 × 0.14 mm

Data collection

Siemens SMART 1000 CCD area-detector diffractometer
ω scans
Absorption correction: integration (XPREP in SHELXTL; Bruker, 2001); R_int = 0.040 before correction T_min = 0.779, T_max = 0.875
8728 measured reflections
2072 independent reflections
1757 reflections with I > 2σ(I)
R_int = 0.033
θ_max = 29.0°

Refinement

Refinement on F²
R[F² > 2σ(F²)] = 0.025
wR(F²) = 0.066
S = 1.16
2072 reflections
108 parameters
All H-atom parameters refined
w = 1/[σ²(F_o²) + (0.024P)² + 0.3828P] where P = (F_o² + 2F_c²)/3
(Δ/σ)_max = 0.001
Δρ_max = 0.43 e Å⁻³
Δρ_min = −0.20 e Å⁻³
Extinction correction: SHELXL97
Extinction coefficient: 0.0300 (15)

TTF at 90 K

Crystal data

C₆H₄S₄
M_r = 204.33
Monoclinic, P 2₁ /n
a = 14.625 (3) Å
b = 3.909 (1) Å
c = 13.812 (3) Å
β = 100.90 (1)°
V = 775.4 (5) Å³
Z = 4
D_x = 1.750 Mg m⁻³
Mo Kα radiation
μ = 1.14 mm⁻¹
T = 98 (2) K
Block, orange
0.20 × 0.16 × 0.14 mm

Data collection

Siemens SMART 1000 CCD area-detector diffractometer
ω scans
Absorption correction: integration (XPREP in SHELXTL; Bruker, 2001); R_int = 0.036 before correction T_min = 0.778, T_max = 0.874
8696 measured reflections
2064 independent reflections
1842 reflections with I > 2σ(I)
R_int = 0.029
θ_max = 29.0°

Refinement

Refinement on F²
R[F² > 2σ(F²)] = 0.022
wR(F²) = 0.060
S = 1.12
2064 reflections
108 parameters
All H-atom parameters refined
w = 1/[σ²(F_o²) + (0.0245P)² + 0.4237P] where P = (F_o² + 2F_c²)/3
(Δ/σ)_max = 0.001
Δρ_max = 0.52 e Å⁻³
Δρ_min = −0.23 e Å⁻³
Extinction correction: SHELXL97
Extinction coefficient: 0.0307 (14)

Table 1
Observed and libration-corrected^a bond distances (Å)

T (K)	S1—C1	S2—C2	S1—C3	S2—C3	C1=C2	C3=C3ⁱ
290	1.736 (2)	1.735 (2)	1.7556 (17)	1.7569 (17)	1.319 (3)	1.341 (3)
Corrected	1.741	1.741	1.763	1.764	1.325	1.345
150, A^b	1.7462 (17)	1.7444 (17)	1.7607 (14)	1.7613 (14)	1.329 (2)	1.345 (3)
Corrected	1.749	1.747	1.765	1.765	1.332	1.347
150, B	1.7450 (17)	1.7453 (17)	1.7577 (14)	1.7608 (14)	1.328 (2)	1.349 (3)
Corrected	1.748	1.749	1.761	1.765	1.331	1.351
98, A	1.7482 (14)	1.7467 (14)	1.7613 (12)	1.7612 (13)	1.3337 (19)	1.348 (2)
Corrected	1.750	1.748	1.764	1.764	1.336	1.349
98, B	1.7447 (14)	1.7477 (14)	1.7609 (13)	1.7625 (13)	1.3349 (19)	1.346 (2)
Corrected	1.747	1.749	1.763	1.765	1.337	1.347
Notes: (a) using the TLS model (Schomaker & Trueblood, 1968); (b) A and B signify the two independent molecules.

Table 2
Mean bond distances (Å) in TTF

Phase	T (K)	S—C1,2	S—C3	C1=C2	C3=C3ⁱ
α-TTF^a	295	1.731 (2)	1.757 (2)	1.314 (3)	1.349 (3)
α-TTF^b	290	1.736 (2)	1.756 (2)	1.319 (3)	1.341 (3)
Corrected	290	1.741	1.764	1.325	1.345
γ-TTF^b	150	1.745 (2)	1.760 (2)	1.329 (2)	1.347 (3)
Corrected	150	1.748	1.764	1.332	1.349
γ-TTF^b	98	1.747 (1)	1.761 (1)	1.334 (2)	1.347 (2)
Corrected	98	1.749	1.764	1.337	1.348
β-TTF^c	298	1.731 (6)	1.755 (3)	1.309 (4)	1.337 (4)
Gas^d	433	1.739 (4)	1.758 (4)	1.338 (4)	1.354 (5)
TTF·Q^e	150	1.740 (7)	1.763 (4)	1.328 (2)	1.336 (4)
TTF·OFN^f	120	1.749 (1)	1.765 (2)	1.328 (2)	1.352 (2)
Notes: (a) Cooper et al. (1971, 1974); (b) this work, libration corrections using the TLS model; (c) Ellern et al. (1994); (d) electron diffraction study by Hargittai et al. (1994); (e) Batsanov et al. (1994 ), Q is 1-oxo-2,6-dimethyl-4-dicyanomethylenecyclohexa-2,5-diene; (f) Batsanov et al. (2001 ), OFN is octafluoronaphthalene.

Table 3
Experimental and calculated unit-cell parameters (Å, °) of α- and γ-TTF

Source	T (K)	a	b	c	β
X-ray^a	295	7.364	4.023	13.922	101.42
X-ray^b	290	7.352 (2)	4.018 (1)	13.901 (4)	101.43 (1)
X-ray^b	150	14.641 (3)	3.933 (1)	13.832 (3)	100.98 (1)
X-ray^b	98	14.625 (3)	3.909 (1)	13.812 (3)	100.90 (1)
W^c	300	7.582	3.859	14.205	101.64
W^c	0	7.426	3.764	14.184	101.29
W+C^c,d	300	7.340	4.429	13.764	105.40
W+C^c,d	0	7.252	3.987	13.931	103.65
ET+C^c,d	300	7.375	4.204	13.894	106.66
ET+C^c,d	0	7.244	4.067	14.040	106.35
Notes: (a) Cooper et al. (1971, 1974), no s.u. values were published; (b) this work; (c) calculations by Venuti et al. (2001); (d) molecule treated as flexible.

All H atoms were refined isotropically [Csp²—H = 0.88 (3) and 0.94 (2) Å at 290 K, and 0.91 (2)–0.95 (2) Å at low temperatures]. The highest peaks of residual electron density (which increase from 0.20–0.32 e Å⁻³ at 290 K to 0.32–0.43 e Å⁻³ at 150 K to 0.32–0.52 e Å⁻³ at 98 K) lie near the mid-points of the C—S and C=C bonds, while the deepest `holes' are found in the area of pπ orbitals of the C atoms. The relatively large (ca 2.2) ratio of the maximum/minimum electron density may be due to the anisotropy of extinction, for which only an isotropic correction was applied.

For all determinations, data collection: SMART (Siemens, 1995 ); cell refinement: SMART; data reduction: SAINT (Siemens, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 2001).

Supporting information

Crystal structure: contains datablocks global, I_290, I_150, I_90. DOI: 10.1107/S0108270106022554/ga3012sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S0108270106022554/ga3012I_290sup2.hkl

Structure factors: contains datablock I_150. DOI: 10.1107/S0108270106022554/ga3012I_150sup3.hkl

Structure factors: contains datablock I_90. DOI: 10.1107/S0108270106022554/ga3012I_90sup4.hkl

Comment top

The electron donor 2,2'-bis(1,3-dithiole), or tetrathiafulvalene (TTF), synthesized by Wudl et al. (1970), is of paramount importance as a component of charge-transfer (CT) salts and complexes, organic metals, and superconductors. Knowing its precise structural parameters is obviously useful, especially since Reith et al. (1988) and Clemente & Marzotto (1996) have demonstrated simple linear relations between the degree of CT and bond lengths in the TTF molecule.

TTF appears in two polymorphic modifications, viz. the orange monoclinic α form (or TTF1) and the yellow triclinic β form (or TTF2). The crystal structure of α-TTF was determined at room temperature (Cooper et al., 1971, 1974) and never revisited, evidently because of the high quality of the original study. The β phase, which is more stable at elevated temperatures (Bozio et al., 1979; Venuti et al., 2001), was discovered by LaPlaca et al. (1975), who reported its lattice parameters (Weidenborner et al., 1977). The full structure determination was carried out by Ellern et al. (1994), also at room temperature. The low-temperature crystallography of TTF remained unexplored, although β-TTF was known to convert occasionally into α-TTF after slow cooling to 80 K (Venuti et al., 2001). Temkin et al. (1977) studied the Raman spectra of α-TTF at 2, 80 and 300 K, and observed ten low-frequency (intermolecular) modes at low temperatures, instead of the six expected for two molecules per unit cell. A more detailed Raman spectroscopic study of both α- and β-TTF from 300 to 80 K was carried out by Venuti et al. (2001), who reported no unusual effects.

We undertook a variable-temperature study of α-TTF to obtain the standard bond lengths free of the spurious shortening caused by thermal libration (for which no corrections had been made in the earlier studies). On cooling the sample below 190 K, additional reflections emerged, which corresponded to the doubling of the lattice parameter a. The sample remained a single-crystal and the transition was fully reversible; five cycles of cooling/warming across it did not cause any appreciable deterioration of the crystal quality. No further changes of the diffraction pattern were detected down to 95 K. Full sets of data were collected at 290, 150 and 98 K and used for structure determination. At 290 K, our results agree with those of Cooper et al. (1971, 1974). The molecule (Fig. 1a) lies at a crystallographic inversion centre and participates in an infinite stack, running parallel to the y axis, with rigorously uniform interplanar separations of 3.60 Å (calculated between the central C₂S₄ groups).

For the low-temperature phase, all `new' reflections (with odd h) were systematically weaker than the `old' (with even h). At 150 K, the intensities differ by an average factor of 13, but at 98 K only by a factor of 8. Nevertheless, the `new' reflections are not negligible. Thus, 62% of reflections with h odd have I > 3σ(I) at 150 K, and 72% at 98 K (cf. 85% and 88% for h even). At 150 K, the mean I/σ(I) ratio is 24.1 for h even and 9.2 for h odd; at 98 K, respectively, 26.4 and 13.5.

Thus, at around 190 K, α-TTF undergoes a second-order phase transition into a new low-temperature modification, γ-TTF. From systematic absences, the latter has P2₁/n symmetry rather than P2₁/c. For example, the 98 K data set contained 374 reflection of h0l type with both h and l odd, giving the average I/σ(I) ratio of 22.1. Of these, 342 reflections (91%) had I > 3σ(I). On the other hand, 386 reflections h0l with h odd and l even, had the average I/σ(I) = 0.7; only 59 reflections (15%) had I exceeding 3σ(I), and those very slightly. The mean absolute intensities of the former and the latter class related as 91:1.

The structure of the low-temperature phase was solved successfully in space group P2₁/n, with two crystallographically non-equivalent molecules (A and B; Fig. 1b) lying at inversion centres 0 1/2 1/2 and 1/2 1/2 1/2. Thus the asymmetric unit comprises two half-molecules, which would be related by an a/2 translation (equivalent to an a translation in α-TTF) but for a slight misalignment (Fig. 1c). At 150 K, atoms C3 of A and B deviate by 0.053 (1) Å from an ideal a/2 translation, atoms S1 and S2 by 0.180 (1) and 0.173 (1) Å, and the peripheral atoms C1 and C2 by 0.248 (2) and 0.245 (2) Å. At 98 K, the deviations slightly increase, viz. 0.064 (2) Å for C3, 0.218 (1) Å for S1, 0.210 (1) Å for S2, 0.300 (2) Å for C1 and 0.299 (2) Å for C2. The mean deviation for all non-H atoms increases from 0.18 Å at 150 K to 0.22 Å at 98 K. The angle between the central C₂S₄ planes of molecules A and B increases from 2.3 (1)° at 150 K to 2.8 (1)° at 98 K, and the angle between the long axes of these molecules from 4.5 (1) to 5.4 (1)°, respectively. Owing to the smallness of these deviations, γ-TTF can be regarded as displacively modulated α-TTF. In fact, using only the (low-temperature) data with h even, the structure of γ-TTF can be solved and refined as a 1:1 disordered structure having a lattice similar to that of the α-phase and P2₁/c symmetry. The refinement converges at nearly the same R factor (on half the number of reflections, of course).

An alternative model of γ-TTF was tested, assuming one independent molecule occupying a general position (with the molecular centroid at ca 1/4, 0, 0). The refinement was unstable and produced absurd atomic displacement parameters and R > 0.12. Refinements in P2₁/c symmetry were equally unsuccessful.

In both α and γ phases, the TTF molecule shows a small but significant chair-like distortion, folding along the S1···S2 vector by 2.0 (1)° in the α-phase and by 2.3 (1) and 1.8 (1)° in molecules A and B of the γ-phase. The same conformation was found in β-TTF which (unlike the α and γ phases) does not have a stacking motif, whereas in the gas phase, TTF adopts a boat conformation with the 13.5° folding of both rings (Hargittai et al., 1994). Both ab initio calculations (Batsanov et al., 1995) and a survey of the Cambridge Structural Database (Wang et al., 1997) indicate that the TTF molecule is indeed rather flexible conformationally.

Although γ-TTF contains two symmetrically non-equivalent types of stacks, the differences of their geometry are negligible. The mean interplanar separation is 3.53 Å at 150 K and 3.51 Å at 98 K. In α-TTF, atom S1 forms with its symmetrical equivalent an inter-stack contact, S1···S1ⁱⁱ, of 3.400 (1) Å, considerably shorter than twice the van der Waals radius of S (1.81 Å; Rowland & Taylor, 1996), while S2 is `wedged' between two molecules of the adjacent stack, at equal distances [3.575 (1) Å] from S2ⁱⁱⁱ and S2^iv. In γ-TTF, this pattern remains essentially the same. The S1···S1ⁱⁱ contact becomes S1A···S1Bⁱⁱ of 3.353 (1) Å (150 K) or 3.341 (1) Å (98 K). The S2···S2ⁱⁱⁱ and S2···S2^iv contacts become S2A···S2Bⁱⁱⁱ and S2A···S2B^iv, respectively, which are no longer symmetically equivalent but nevertheless are equidistant within experimental error, averaging 3.514 (1) at 150 K and 3.498 (1) Å at 98 K (see Fig. 2).

The observed and libration-corrected [by the the TLS model of Schomaker & Trueblood (1968)] bond distances are listed in Table 1, and in Table 2 the average values are compared with the earlier results. It is worth noting that the `outer' S—C1,2 bonds are significantly shorter than the `inner' S—C3 bonds, and the difference is even more pronounced after the libration correction and/or at low temperature.

The current (May 2006) version of the Cambridge Structural Database (Allen & Taylor, 2004) lists over 150 structures containing unsubstituted TTF groups. Most of these materials are salts or CT complexes, in which the TTF group bears a net charge varying between 0 and +1 (Salmeron-Valverde et al., 2003; Salmeron-Valverde & Bernes, 2005, and references therein). Accumulation of positive charge substantially alters the molecular geometry, the S—C bonds contracting and C═C bonds lengthening in comparison with the neutral TTF (Reith et al., 1988; Clemente & Marzotto, 1996). However, low-temperature studies of some cocrystals, where CT is ruled out by spectroscopic and conductivity evidence, have revealed a TTF molecular geometry very similar to the present results (see Table 1).

Venuti et al. (2001) have made interesting predictions of the structure and energy of solid TTF by quasi-harmonic lattice dynamics, using three different molecular-mechanical models, viz. a 6-exp type atom–atom potential (W) parametrized by Williams & Cox (1984), the same with added coulombic term (W+C) and another 6-exp potential specially devised by Della Valle et al. (1999) for bis(ethylenedithio)tetrathiafulvalene systems, also with a coulombic term (ET+C). Both the W+C and ET+C models predicted that, on cooling from 300 to 0 K, the a and b cell parameters would contract but the c parameter would expand, whereas the W method predicted a uniform contraction of all three parameters (see Table 3). Since the space-group symmetry was taken as datum, the phase transition obviously could not be forseen. Apart from this, our results show better agreement with W than with coulombic corrected models. It is also noteworthy that W accurately predicted the β angle, which in the other two models was off the mark by 2–5°.

The same authors determined phonon wavenumbers of TTF from Raman data at 300 and 295 K, and at eight other temperatures ranging from 200 to 80 K. It is noteworthy that, between 80 and 190 K, each wavenumber shows a gentle linear dependence on temperature, but the values for 200 K are uniformly shifted down from the corresponding regression lines, which may be a manifestation of the phase transition. Unfortunately, no measurements were taken between 200 and 295 K, which makes comparison with the present results difficult.

For the preliminary publication [concerning what?] see Batsanov et al. (1998).

Experimental top

Commercial TTF was recrystallized from heptane at room temperature.

Computing details top

For all compounds, data collection: SMART (Siemens, 1995); cell refinement: SMART; data reduction: SAINT (Siemens, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 2001); software used to prepare material for publication: SHELXTL.

Figures top

	Fig. 1. The molecular structure of TTF: (a) γ phase at 98 K, (b) α phase at 290 K, and (c) overlap of molecules A and B (shifted by a/2) in the γ phase. Atomic displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) −x, 1 − y, 1 − z, (ii) 1 − x, 1 − y, 1 − z.]
	Fig. 2. The crystal packing in γ-TTF. Dashed lines indicate the unit cell of α-TTF. [Symmetry codes: (iii) 1/2 − x, y − 1/2, 3/2 − z, (iv) 1/2 − x, y + 1/2, 3/2 − z. ii as in Fig. 1?]

(I_290) 2-(1,3-dithiol-2-ylidene)-1,3-dithiole top

Crystal data top

C₆H₄S₄	F(000) = 208
M_r = 204.33	D_x = 1.686 Mg m⁻³
Monoclinic, P2₁/c	Melting point: 392 K
Hall symbol: -P 2ybc	Mo Kα radiation, λ = 0.71073 Å
a = 7.352 (2) Å	Cell parameters from 404 reflections
b = 4.0181 (11) Å	θ = 9–22°
c = 13.901 (4) Å	µ = 1.09 mm⁻¹
β = 101.426 (10)°	T = 290 K
V = 402.5 (2) Å³	Block, orange
Z = 2	0.20 × 0.16 × 0.14 mm

Data collection top

Siemens SMART 1000 CCD area-detector diffractometer	926 independent reflections
Radiation source: fine-focus sealed tube	813 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.034
Detector resolution: 8 pixels mm^-1	θ_max = 27.5°, θ_min = 2.8°
ω scans	h = −9→8
Absorption correction: integration (XPREP in SHELXTL; Bruker, 2001), R_int = 0.040 before correction	k = −5→4
T_min = 0.786, T_max = 0.875	l = −18→18
3571 measured reflections

Refinement top

Refinement on F²	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F² > 2σ(F²)] = 0.028	All H-atom parameters refined
wR(F²) = 0.065	w = 1/[σ²(F_o²) + (0.0224P)² + 0.1703P] where P = (F_o² + 2F_c²)/3
S = 1.12	(Δ/σ)_max = 0.001
926 reflections	Δρ_max = 0.32 e Å⁻³
55 parameters	Δρ_min = −0.16 e Å⁻³
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.132 (7)

Crystal data top

C₆H₄S₄	V = 402.5 (2) Å³
M_r = 204.33	Z = 2
Monoclinic, P2₁/c	Mo Kα radiation
a = 7.352 (2) Å	µ = 1.09 mm⁻¹
b = 4.0181 (11) Å	T = 290 K
c = 13.901 (4) Å	0.20 × 0.16 × 0.14 mm
β = 101.426 (10)°

Data collection top

Siemens SMART 1000 CCD area-detector diffractometer	926 independent reflections
Absorption correction: integration (XPREP in SHELXTL; Bruker, 2001), R_int = 0.040 before correction	813 reflections with I > 2σ(I)
T_min = 0.786, T_max = 0.875	R_int = 0.034
3571 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.028	0 restraints
wR(F²) = 0.065	All H-atom parameters refined
S = 1.12	Δρ_max = 0.32 e Å⁻³
926 reflections	Δρ_min = −0.16 e Å⁻³
55 parameters

Special details top

Experimental. The data collection nominally covered full sphere of reciprocal space, by a combination of 5 sets of ω scans; each set at different ϕ and/or 2θ angles and each scan (10 sec exposure) covering 0.3° in ω. Crystal to detector distance 4.52 cm. The absense of crystal decay was monitored by repeating 50 initial frames at the end of data collection and comparing 36 strongest duplicate reflections.

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.

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

	x	y	z	U_iso*/U_eq
S1	0.28816 (6)	0.65547 (14)	0.50738 (4)	0.04534 (19)
S2	0.03956 (7)	0.69867 (13)	0.64978 (3)	0.04436 (19)
C1	0.3765 (3)	0.8232 (6)	0.62223 (18)	0.0532 (5)
C2	0.2655 (3)	0.8415 (6)	0.68590 (16)	0.0540 (6)
C3	0.0677 (2)	0.5732 (4)	0.53254 (12)	0.0325 (4)
H1	0.492 (4)	0.894 (7)	0.631 (2)	0.075 (8)*
H2	0.300 (4)	0.925 (7)	0.750 (2)	0.073 (8)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
S1	0.0315 (3)	0.0565 (3)	0.0500 (3)	−0.0063 (2)	0.01273 (19)	0.0000 (2)
S2	0.0502 (3)	0.0531 (3)	0.0309 (2)	−0.0031 (2)	0.01062 (19)	−0.0053 (2)
C1	0.0386 (11)	0.0514 (13)	0.0626 (13)	−0.0087 (10)	−0.0066 (9)	−0.0002 (10)
C2	0.0590 (13)	0.0526 (13)	0.0433 (11)	−0.0034 (10)	−0.0071 (9)	−0.0062 (9)
C3	0.0316 (8)	0.0354 (8)	0.0317 (8)	0.0006 (7)	0.0095 (6)	0.0025 (7)

Geometric parameters (Å, º) top

S1—C1	1.736 (2)	C1—C2	1.319 (3)
S1—C3	1.7556 (17)	C1—H1	0.88 (3)
S2—C2	1.735 (2)	C2—H2	0.94 (3)
S2—C3	1.7569 (17)	C3—C3ⁱ	1.341 (3)

C1—S1—C3	94.69 (10)	C1—C2—H2	124.7 (16)
C2—S2—C3	94.66 (10)	S2—C2—H2	117.2 (16)
C2—C1—S1	118.06 (17)	C3ⁱ—C3—S1	122.66 (17)
C2—C1—H1	126.6 (18)	C3ⁱ—C3—S2	122.90 (17)
S1—C1—H1	115.3 (18)	S1—C3—S2	114.45 (9)
C1—C2—S2	118.11 (17)

Symmetry code: (i) −x, −y+1, −z+1.

(I_150) 2-(1,3-dithiol-2-ylidene)-1,3-dithiole top

Crystal data top

C₆H₄S₄	F(000) = 416
M_r = 204.33	D_x = 1.736 Mg m⁻³
Monoclinic, P2₁/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 375 reflections
a = 14.641 (3) Å	θ = 10.3–28.5°
b = 3.933 (1) Å	µ = 1.13 mm⁻¹
c = 13.832 (3) Å	T = 150 K
β = 100.98 (1)°	Block, orange
V = 781.9 (3) Å³	0.20 × 0.16 × 0.14 mm
Z = 4

Data collection top

Siemens SMART 1000 CCD area-detector diffractometer	2072 independent reflections
Radiation source: fine-focus sealed tube	1757 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.033
Detector resolution: 8 pixels mm^-1	θ_max = 29.0°, θ_min = 1.9°
ω scans	h = −19→19
Absorption correction: integration (XPREP in SHELXTL; Bruker, 2001), R_int = 0.040 before correction	k = −5→5
T_min = 0.779, T_max = 0.875	l = −18→18
8728 measured reflections

Refinement top

Refinement on F²	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F² > 2σ(F²)] = 0.025	All H-atom parameters refined
wR(F²) = 0.066	w = 1/[σ²(F_o²) + (0.024P)² + 0.3828P] where P = (F_o² + 2F_c²)/3
S = 1.16	(Δ/σ)_max = 0.001
2072 reflections	Δρ_max = 0.43 e Å⁻³
108 parameters	Δρ_min = −0.20 e Å⁻³
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0300 (15)

Crystal data top

C₆H₄S₄	V = 781.9 (3) Å³
M_r = 204.33	Z = 4
Monoclinic, P2₁/n	Mo Kα radiation
a = 14.641 (3) Å	µ = 1.13 mm⁻¹
b = 3.933 (1) Å	T = 150 K
c = 13.832 (3) Å	0.20 × 0.16 × 0.14 mm
β = 100.98 (1)°

Data collection top

Siemens SMART 1000 CCD area-detector diffractometer	2072 independent reflections
Absorption correction: integration (XPREP in SHELXTL; Bruker, 2001), R_int = 0.040 before correction	1757 reflections with I > 2σ(I)
T_min = 0.779, T_max = 0.875	R_int = 0.033
8728 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.025	0 restraints
wR(F²) = 0.066	All H-atom parameters refined
S = 1.16	Δρ_max = 0.43 e Å⁻³
2072 reflections	Δρ_min = −0.20 e Å⁻³
108 parameters

Special details top

Experimental. The data collection nominally covered full sphere of reciprocal space, by a combination of 5 sets of ω scans; each set at different ϕ and/or 2θ angles and each scan (15 sec exposure) covering 0.3° in ω. Crystal to detector distance 4.52 cm. The absense of crystal decay was monitored by repeating 50 initial frames at the end of data collection and comparing 101 strongest duplicate reflections.

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

	x	y	z	U_iso*/U_eq
S1A	0.14418 (2)	0.65757 (11)	0.50070 (3)	0.01971 (11)
S2A	0.02459 (3)	0.70504 (11)	0.65059 (2)	0.01926 (11)
C1A	0.19272 (11)	0.8288 (4)	0.61548 (12)	0.0232 (3)
H1A	0.2525 (15)	0.903 (6)	0.6201 (15)	0.035 (6)*
C2A	0.13898 (11)	0.8499 (4)	0.68256 (11)	0.0237 (3)
H2A	0.1581 (15)	0.940 (6)	0.7463 (16)	0.044 (6)*
C3A	0.03491 (9)	0.5755 (4)	0.53126 (9)	0.0148 (3)
S1B	0.64591 (2)	0.65584 (11)	0.51396 (3)	0.01981 (11)
S2B	0.51267 (3)	0.70455 (11)	0.64999 (2)	0.01908 (11)
C1B	0.68574 (11)	0.8291 (4)	0.63049 (12)	0.0238 (3)
H1B	0.7469 (16)	0.911 (6)	0.6475 (15)	0.043 (6)*
C2B	0.62601 (11)	0.8497 (4)	0.69147 (11)	0.0241 (3)
H2B	0.6389 (15)	0.939 (6)	0.7552 (16)	0.042 (6)*
C3B	0.53295 (9)	0.5749 (4)	0.53407 (10)	0.0151 (3)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
S1A	0.01441 (18)	0.0254 (2)	0.02012 (19)	−0.00306 (14)	0.00525 (13)	−0.00033 (15)
S2A	0.02089 (19)	0.0235 (2)	0.01372 (18)	−0.00194 (14)	0.00420 (13)	−0.00262 (13)
C1A	0.0169 (7)	0.0245 (8)	0.0256 (7)	−0.0046 (6)	−0.0023 (6)	0.0003 (6)
C2A	0.0238 (8)	0.0243 (8)	0.0197 (7)	−0.0022 (6)	−0.0042 (6)	−0.0025 (6)
C3A	0.0139 (6)	0.0176 (7)	0.0133 (6)	0.0000 (5)	0.0040 (5)	0.0011 (5)
S1B	0.01413 (18)	0.0245 (2)	0.02155 (19)	−0.00299 (13)	0.00532 (13)	−0.00046 (15)
S2B	0.02094 (19)	0.0232 (2)	0.01364 (18)	−0.00138 (14)	0.00473 (13)	−0.00235 (13)
C1B	0.0179 (7)	0.0229 (8)	0.0272 (8)	−0.0033 (6)	−0.0038 (6)	−0.0014 (7)
C2B	0.0248 (8)	0.0244 (8)	0.0199 (7)	−0.0012 (6)	−0.0035 (6)	−0.0045 (6)
C3B	0.0144 (6)	0.0174 (7)	0.0144 (6)	−0.0004 (5)	0.0048 (5)	0.0005 (5)

Geometric parameters (Å, º) top

S1A—C1A	1.7462 (17)	S1B—C1B	1.7450 (17)
S1A—C3A	1.7607 (14)	S1B—C3B	1.7577 (14)
S2A—C2A	1.7444 (17)	S2B—C2B	1.7453 (17)
S2A—C3A	1.7613 (14)	S2B—C3B	1.7608 (14)
C1A—C2A	1.329 (2)	C1B—C2B	1.328 (2)
C1A—H1A	0.91 (2)	C1B—H1B	0.94 (2)
C2A—H2A	0.94 (2)	C2B—H2B	0.93 (2)
C3A—C3Aⁱ	1.345 (3)	C3B—C3Bⁱⁱ	1.349 (3)

C1A—S1A—C3A	94.79 (7)	C1B—S1B—C3B	94.79 (7)
C2A—S2A—C3A	94.73 (7)	C2B—S2B—C3B	94.56 (7)
C2A—C1A—S1A	117.81 (12)	C2B—C1B—S1B	117.77 (12)
C2A—C1A—H1A	128.2 (13)	C2B—C1B—H1B	122.2 (13)
S1A—C1A—H1A	113.9 (13)	S1B—C1B—H1B	120.0 (13)
C1A—C2A—S2A	118.05 (12)	C1B—C2B—S2B	118.10 (12)
C1A—C2A—H2A	124.5 (14)	C1B—C2B—H2B	125.7 (14)
S2A—C2A—H2A	117.4 (14)	S2B—C2B—H2B	116.2 (14)
C3Aⁱ—C3A—S1A	122.73 (14)	C3Bⁱⁱ—C3B—S1B	122.53 (14)
C3Aⁱ—C3A—S2A	122.68 (14)	C3Bⁱⁱ—C3B—S2B	122.73 (14)
S1A—C3A—S2A	114.58 (8)	S1B—C3B—S2B	114.74 (8)

Symmetry codes: (i) −x, −y+1, −z+1; (ii) −x+1, −y+1, −z+1.

(I_90) 2,2'-bis(1,3-dithiole), low-temparature (γ) phase top

Crystal data top

C₆H₄S₄	F(000) = 416
M_r = 204.33	D_x = 1.750 Mg m⁻³
Monoclinic, P2₁/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 426 reflections
a = 14.625 (3) Å	θ = 10.4–28.5°
b = 3.909 (1) Å	µ = 1.14 mm⁻¹
c = 13.812 (3) Å	T = 98 K
β = 100.90 (1)°	Block, orange
V = 775.4 (5) Å³	0.20 × 0.16 × 0.14 mm
Z = 4

Data collection top

Siemens SMART 1000 CCD area-detector diffractometer	2064 independent reflections
Radiation source: fine-focus sealed tube	1842 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.029
Detector resolution: 8 pixels mm^-1	θ_max = 29.0°, θ_min = 1.9°
ω scans	h = −19→19
Absorption correction: integration (XPREP in SHELXTL; Bruker, 2001), R_int = 0.036 before correction	k = −5→5
T_min = 0.778, T_max = 0.874	l = −18→18
8696 measured reflections

Refinement top

Refinement on F²	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F² > 2σ(F²)] = 0.022	All H-atom parameters refined
wR(F²) = 0.060	w = 1/[σ²(F_o²) + (0.0245P)² + 0.4237P] where P = (F_o² + 2F_c²)/3
S = 1.12	(Δ/σ)_max = 0.001
2064 reflections	Δρ_max = 0.52 e Å⁻³
108 parameters	Δρ_min = −0.23 e Å⁻³
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0307 (14)

Crystal data top

C₆H₄S₄	V = 775.4 (5) Å³
M_r = 204.33	Z = 4
Monoclinic, P2₁/n	Mo Kα radiation
a = 14.625 (3) Å	µ = 1.14 mm⁻¹
b = 3.909 (1) Å	T = 98 K
c = 13.812 (3) Å	0.20 × 0.16 × 0.14 mm
β = 100.90 (1)°

Data collection top

Siemens SMART 1000 CCD area-detector diffractometer	2064 independent reflections
Absorption correction: integration (XPREP in SHELXTL; Bruker, 2001), R_int = 0.036 before correction	1842 reflections with I > 2σ(I)
T_min = 0.778, T_max = 0.874	R_int = 0.029
8696 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.022	0 restraints
wR(F²) = 0.060	All H-atom parameters refined
S = 1.12	Δρ_max = 0.52 e Å⁻³
2064 reflections	Δρ_min = −0.23 e Å⁻³
108 parameters

Special details top

Experimental. The data collection nominally covered full sphere of reciprocal space, by a combination of 5 sets of ω scans; each set at different ϕ and/or 2θ angles and each scan (17 sec exposure) covering 0.3° in ω. Crystal to detector distance 4.52 cm. The absense of crystal decay was monitored by repeating 50 initial frames at the end of data collection and comparing 136 strongest duplicate reflections.

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

	x	y	z	U_iso*/U_eq
S1A	0.14421 (2)	0.65820 (9)	0.49940 (2)	0.01315 (10)
S2A	0.02559 (2)	0.70689 (9)	0.65080 (2)	0.01276 (10)
C1A	0.19376 (9)	0.8309 (4)	0.61409 (10)	0.0154 (3)
H1A	0.2544 (13)	0.903 (5)	0.6194 (13)	0.025 (5)*
C2A	0.14031 (10)	0.8524 (4)	0.68212 (10)	0.0158 (3)
H2A	0.1614 (13)	0.943 (5)	0.7456 (14)	0.027 (5)*
C3A	0.03522 (8)	0.5756 (3)	0.53117 (9)	0.0104 (2)
S1B	0.64640 (2)	0.65572 (9)	0.51548 (2)	0.01315 (10)
S2B	0.51112 (2)	0.70619 (9)	0.65007 (2)	0.01271 (10)
C1B	0.68524 (9)	0.8306 (4)	0.63223 (10)	0.0163 (3)
H1B	0.7483 (13)	0.906 (5)	0.6485 (13)	0.029 (5)*
C2B	0.62445 (9)	0.8526 (4)	0.69292 (10)	0.0159 (3)
H2B	0.6391 (13)	0.945 (5)	0.7575 (14)	0.029 (5)*
C3B	0.53263 (8)	0.5749 (3)	0.53437 (9)	0.0105 (2)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
S1A	0.00975 (16)	0.01711 (17)	0.01306 (16)	−0.00216 (11)	0.00333 (11)	−0.00036 (12)
S2A	0.01369 (16)	0.01562 (17)	0.00918 (15)	−0.00165 (11)	0.00267 (11)	−0.00180 (11)
C1A	0.0118 (6)	0.0157 (6)	0.0171 (6)	−0.0026 (5)	−0.0017 (5)	0.0011 (5)
C2A	0.0162 (6)	0.0154 (6)	0.0137 (6)	−0.0015 (5)	−0.0026 (5)	−0.0016 (5)
C3A	0.0103 (5)	0.0121 (6)	0.0094 (5)	0.0004 (4)	0.0036 (4)	0.0004 (4)
S1B	0.00965 (16)	0.01617 (17)	0.01408 (16)	−0.00195 (11)	0.00336 (11)	−0.00062 (12)
S2B	0.01381 (16)	0.01531 (17)	0.00940 (15)	−0.00088 (11)	0.00320 (11)	−0.00153 (11)
C1B	0.0132 (6)	0.0163 (6)	0.0172 (6)	−0.0023 (5)	−0.0027 (5)	−0.0003 (5)
C2B	0.0164 (6)	0.0166 (6)	0.0130 (6)	−0.0008 (5)	−0.0019 (5)	−0.0025 (5)
C3B	0.0107 (5)	0.0112 (6)	0.0100 (5)	−0.0001 (4)	0.0033 (4)	0.0004 (4)

Geometric parameters (Å, º) top

S1A—C1A	1.7482 (14)	S1B—C1B	1.7447 (14)
S1A—C3A	1.7613 (12)	S1B—C3B	1.7609 (13)
S2A—C2A	1.7467 (14)	S2B—C2B	1.7477 (14)
S2A—C3A	1.7612 (13)	S2B—C3B	1.7625 (13)
C1A—C2A	1.3337 (19)	C1B—C2B	1.3349 (19)
C1A—H1A	0.921 (19)	C1B—H1B	0.953 (19)
C2A—H2A	0.942 (19)	C2B—H2B	0.95 (2)
C3A—C3Aⁱ	1.348 (2)	C3B—C3Bⁱⁱ	1.346 (2)

C1A—S1A—C3A	94.86 (6)	C1B—S1B—C3B	94.87 (6)
C2A—S2A—C3A	94.80 (6)	C2B—S2B—C3B	94.70 (6)
C2A—C1A—S1A	117.70 (11)	C2B—C1B—S1B	117.81 (10)
C2A—C1A—H1A	127.6 (11)	C2B—C1B—H1B	124.2 (11)
S1A—C1A—H1A	114.7 (11)	S1B—C1B—H1B	118.0 (11)
C1A—C2A—S2A	117.94 (10)	C1B—C2B—S2B	117.89 (10)
C1A—C2A—H2A	123.2 (11)	C1B—C2B—H2B	123.8 (12)
S2A—C2A—H2A	118.9 (11)	S2B—C2B—H2B	118.3 (11)
C3Aⁱ—C3A—S2A	122.69 (12)	C3Bⁱⁱ—C3B—S1B	122.50 (12)
C3Aⁱ—C3A—S1A	122.67 (12)	C3Bⁱⁱ—C3B—S2B	122.81 (12)
S2A—C3A—S1A	114.64 (7)	S1B—C3B—S2B	114.69 (7)

Symmetry codes: (i) −x, −y+1, −z+1; (ii) −x+1, −y+1, −z+1.

Experimental details

	(I_290)	(I_150)	(I_90)
Crystal data
Chemical formula	C₆H₄S₄	C₆H₄S₄	C₆H₄S₄
M_r	204.33	204.33	204.33
Crystal system, space group	Monoclinic, P2₁/c	Monoclinic, P2₁/n	Monoclinic, P2₁/n
Temperature (K)	290	150	98
a, b, c (Å)	7.352 (2), 4.0181 (11), 13.901 (4)	14.641 (3), 3.933 (1), 13.832 (3)	14.625 (3), 3.909 (1), 13.812 (3)
β (°)	101.426 (10)	100.98 (1)	100.90 (1)
V (Å³)	402.5 (2)	781.9 (3)	775.4 (5)
Z	2	4	4
Radiation type	Mo Kα	Mo Kα	Mo Kα
µ (mm⁻¹)	1.09	1.13	1.14
Crystal size (mm)	0.20 × 0.16 × 0.14	0.20 × 0.16 × 0.14	0.20 × 0.16 × 0.14

Data collection
Diffractometer	Siemens SMART 1000 CCD area-detector diffractometer	Siemens SMART 1000 CCD area-detector diffractometer	Siemens SMART 1000 CCD area-detector diffractometer
Absorption correction	Integration (XPREP in SHELXTL; Bruker, 2001), R_int = 0.040 before correction	Integration (XPREP in SHELXTL; Bruker, 2001), R_int = 0.040 before correction	Integration (XPREP in SHELXTL; Bruker, 2001), R_int = 0.036 before correction
T_min, T_max	0.786, 0.875	0.779, 0.875	0.778, 0.874
No. of measured, independent and observed [I > 2σ(I)] reflections	3571, 926, 813	8728, 2072, 1757	8696, 2064, 1842
R_int	0.034	0.033	0.029
(sin θ/λ)_max (Å⁻¹)	0.650	0.682	0.682

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.028, 0.065, 1.12	0.025, 0.066, 1.16	0.022, 0.060, 1.12
No. of reflections	926	2072	2064
No. of parameters	55	108	108
H-atom treatment	All H-atom parameters refined	All H-atom parameters refined	All H-atom parameters refined
Δρ_max, Δρ_min (e Å⁻³)	0.32, −0.16	0.43, −0.20	0.52, −0.23

Computer programs: SMART (Siemens, 1995), SMART, SAINT (Siemens, 1995), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), SHELXTL (Bruker, 2001), SHELXTL.

Observed and libration-corrected^a bond distances (Å) top

T (K)	S1—C1	S2—C2	S1—C3	S2—C3	C1═C2	C3═C3ⁱ
290	1.736 (2)	1.735 (2)	1.7556 (17)	1.7569 (17)	1.319 (3)	1.341 (3)
Corrected	1.741	1.741	1.763	1.764	1.325	1.345
150, A^b	1.7462 (17)	1.7444 (17)	1.7607 (14)	1.7613 (14)	1.329 (2)	1.345 (3)
Corrected	1.749	1.747	1.765	1.765	1.332	1.347
150, B	1.7450 (17)	1.7453 (17)	1.7577 (14)	1.7608 (14)	1.328 (2)	1.349 (3)
Corrected	1.748	1.749	1.761	1.765	1.331	1.351
98, A	1.7482 (14)	1.7467 (14)	1.7613 (12)	1.7612 (13)	1.3337 (19)	1.348 (2)
Corrected	1.750	1.748	1.764	1.764	1.336	1.349
98, B	1.7447 (14)	1.7477 (14)	1.7609 (13)	1.7625 (13)	1.3349 (19)	1.346 (2)
Corrected	1.747	1.749	1.763	1.765	1.337	1.347

Notes: (a) using the TLS model (Schomaker & Trueblood, 1968); (b) A and B signify two independent molecules.

Mean bond distances (Å) in TTF top

Phase	T (K)	S—C1,2	S—C3	C1═C2	C3═C3ⁱ
α-TTF^a	295	1.731 (2)	1.757 (2)	1.314 (3)	1.349 (3)
α-TTF^b	290	1.736 (2)	1.756 (2)	1.319 (3)	1.341 (3)
Corrected	290	1.741	1.764	1.325	1.345
γ-TTF^b	150	1.745 (2)	1.760 (2)	1.329 (2)	1.347 (3)
Corrected	150	1.748	1.764	1.332	1.349
γ-TTF^b	98	1.747 (1)	1.761 (1)	1.334 (2)	1.347 (2)
Corrected	98	1.749	1.764	1.337	1.348
β-TTF^c	298	1.731 (6)	1.755 (3)	1.309 (4)	1.337 (4)
Gas^d	433	1.739 (4)	1.758 (4)	1.338 (4)	1.354 (5)
TTF·Q^e	150	1.740 (7)	1.763 (4)	1.328 (2)	1.336 (4)
TTF·OFN^f	120	1.749 (1)	1.765 (2)	1.328 (2)	1.352 (2)

Notes: (a) Cooper et al. (1971, 1974); (b) this work, libration corrections using the TLS model; (c) Ellern et al. (1994); (d) electron diffraction study by Hargittai et al. (1994); (e) Batsanov et al. (1994), Q is 1-oxo-2,6-dimethyl-4-dicyano- methylenecyclohexa-2,5-diene; (f) Batsanov et al. (2001), OFN is octafluoronaphthalene.

Experimental and calculated unit-cell parameters (Å, °)of α- and γ-TTF top

source	T (K)	a	b	c	β
X-ray^a	295	7.364	4.023	13.922	101.42
X-ray^b	290	7.352 (2)	4.018 (1)	13.901 (4)	101.43 (1)
X-ray^b	150	14.641 (3)	3.933 (1)	13.832 (3)	100.98 (1)
X-ray^b	98	14.625 (3)	3.909 (1)	13.812 (3)	100.90 (1)
W^c	300	7.582	3.859	14.205	101.64
W^c	0	7.426	3.764	14.184	101.29
W+C^c,d	300	7.340	4.429	13.764	105.40
W+C^c,d	0	7.252	3.987	13.931	103.65
ET+C^c,d	300	7.375	4.204	13.894	106.66
ET+C^c,d	0	7.244	4.067	14.040	106.35

Notes: (a) Cooper et al. (1971, 1974), no s.u. were published; (b) this work; (c) calculations by Venuti et al. (2001); (d) molecule treated as flexible.

Acknowledgements

The author thanks Professor M. R. Bryce for a sample of crystalline TTF and Professor J. A. K. Howard for valuable advice.

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ISSN: 2053-2296

Volume 62| Part 8| August 2006| Pages o501-o504

doi:10.1107/S0108270106022554