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The compounds tetrakis(trimethylsilyl)methane C[Si(CH3)3]4 (TC) and tetrakis(trimethylsilyl)silane Si[Si(CH3)3]4 (TSi) have crystal structures with the molecules in a cubic closed-packed (c.c.p.) stacking. At room temperature both structures have space group Fm{\bar 3}m (Z = 4) with a = 13.5218 (1) Å, V = 2472.3 (1) Å3 for TSi, and a = 12.8902 (2) Å, V = 2141.8 (1) Å3 for TC. X-ray scattering data can be described by a molecule with approximately sixfold orientational disorder, ruling out a structure with free rotating molecules. Upon cooling, TSi exhibits a first-order phase transition at Tc = 225 K, as is characterized by a jump of the lattice parameter of Δa = 0.182 Å and by an exothermal maximum in differential scanning calorimetry (DSC) with ΔH = 11.7 kJ mol−1 and ΔS = 50.0 J mol−1 K−1. The structure of the low-temperature phase is refined against X-ray powder data measured at 200 K. It has space group P213 (Z = 4), a = 13.17158 (6) Å and V = 2285.15 (2) Å3. The molecules are found to be ordered as a result of steric interactions between neighboring molecules, as is shown by analyzing distances between atoms and by calculations of the lattice energy in dependence on the orientations of the molecules. TC has a phase transition at Tc1 = 268 K, with Δa1 = 0.065 Å, ΔH1 = 3.63 kJ mol−1 and ΔS1 = 13.0 J mol−1 K−1. A second first-order phase transition occurs at Tc2 = 225 K, characterized by Δa2 = 0.073 Å, ΔH2 = 6.9 kJ mol−1 and ΔS2 = 30.0 J mol−1 K−1. The phase transition at higher temperature has not been reported previously. New NMR experiments show a small anomaly in the temperature dependence of the peak positions in NMR to occur at Tc2. Rietveld refinements were performed for the low-temperature phase measured at T = 150 K [space group P213, lattice parameter a = 12.609 (3) Å], and for the intermediate phase measured at T = 260 K [space group Pa{\bar 3}, lattice parameter a = 12.7876 (1) Å]. The low-temperature phase of TC is formed isostructural to the low-temperature phase of TSi. In the intermediate phase the molecules exhibit a twofold orientational disorder.

Supporting information

cif

Crystallographic Information File (CIF)
Contains datablocks global, TSi295, TSi200, TC295, TC260, TC150

hkl

Structure factor file (CIF format)
Contains datablock TSi295

hkl

Structure factor file (CIF format)
Contains datablock TC295

hkl

Structure factor file (CIF format)
Contains datablock TSi200

hkl

Structure factor file (CIF format)
Contains datablock TC150

hkl

Structure factor file (CIF format)
Contains datablock TC260

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Text file
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CCDC references: 138568; 138569; 138570; 138571; 138572

Computing details top

(TSi295) top
Crystal data top
Si(Si(CH3)3)4V = 2472.3 (1) Å3
Mr = 320.87Z = 4
Cubic, Fm3mDx = 0.862 Mg m3
Hall symbol: -F 4 2 3Synchrotron radiation, λ = 1.14946 (2) Å
a = 13.5218 (1) ÅT = 295 K
Data collection top
h = l =
k =
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
Si0.00.00.00.203 (13)
Si(1)0.028 (2)0.086 (1)0.154 (1)0.203 (13)
C(1)0.072 (3)0.130 (3)0.228 (4)0.203 (13)1.25 (5)
C(2)0.113 (3)0.168 (2)0.168 (2)0.203 (13)1.25 (5)
C(3)0.038 (1)0.038 (1)0.256 (2)0.203 (13)1.25 (5)
(TSi200) top
Crystal data top
Si(Si(CH3)3)4V = 2285.15 (2) Å3
Mr = 320.87Z = 4
Cubic, P213Dx = 0.932 Mg m3
Hall symbol: P 2ac 2ab 3Synchrotron radiation, λ = 1.14946 (2) Å
a = 13.17158 (6) ÅT = 200 K
Data collection top
h = l =
k =
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Si(0)0.00480.00480.00480.0279 (5)
Si(1)0.1072 (2)0.1072 (2)0.1072 (2)0.0279 (5)
Si(2)0.0661 (2)0.1602 (2)0.0078 (2)0.0279 (5)
C(11)0.2452 (3)0.0899 (5)0.0877 (4)0.119 (2)
C(21)0.1134 (5)0.1936 (5)0.1436 (4)0.119 (2)
C(22)0.0361 (5)0.2510 (4)0.0396 (6)0.119 (2)
C(23)0.1905 (5)0.1711 (6)0.0710 (4)0.119 (2)
(TC295) top
Crystal data top
C(Si(CH3)3)4V = 2141.8 (1) Å3
Mr = 304.79Z = 4
Cubic, Fm3mDx = 0.945 Mg m3
Hall symbol: -F 4 2 3Synchrotron radiation, λ = 0.82488 (2) Å
a = 12.8902 (1) ÅT = 295 K
Data collection top
h = l =
k =
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
C(0)0000.101 (13)
Si(1)0.021 (3)0.073 (2)0.130 (2)0.15 (3)
C(1)0.067 (4)0.126 (4)0.225 (5)0.101 (13)1.35 (8)
C(2)0.114 (3)0.157 (2)0.157 (2)0.101 (13)1.35 (8)
C(3)0.039 (2)0.039 (2)0.240 (3)0.101 (13)1.35 (8)
(TC260) top
Crystal data top
C(Si(CH3)3)4V = 2091.0 (1) Å3
Mr = 304.79Z = 4
Cubic, Pa3Dx = 0.968 Mg m3
Hall symbol: -P 2ac 2ab 3Synchrotron radiation, λ = 0.82488 (2) Å
a = 12.7876 (1) ÅT = 260 K
Data collection top
h = l =
k =
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
C(0)0.00.00.00.265 (19)
Si(1)0.0847 (3)0.0847 (3)0.0847 (3)0.317 (8)0.5
Si(2)0.0352 (10)0.1415 (6)0.0073 (17)0.317 (8)0.5
C(11)0.2172 (7)0.0166 (13)0.1032 (14)0.174 (3)0.75
C(21)0.113 (2)0.1878 (15)0.1119 (18)174 (3)0.75
C(22)0.0370 (13)0.2667 (7)0.0511 (11)174 (3)0.75
C(23)0.1467 (17)0.1474 (14)0.107 (2)174 (3)0.75
(TC150) top
Crystal data top
C(Si(CH3)3)4V = 2005 (1) Å3
Mr = 304.79Z = 4
Cubic, P213Dx = 1.01 Mg m3
Hall symbol: P 2ac 2ab 3Synchrotron radiation, λ = 0.82488 (2) Å
a = 12.609 (3) ÅT = 150 K
Data collection top
h = l =
k =
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
C(0)0.00.00.00.252 (19)
Si(1)0.0881 (2)0.0881 (2)0.0881 (2)0.0252 (9)
Si(2)0.0548 (2)0.1425 (2)0.0005 (3)0.0252 (9)
C(11)0.2380 (3)0.0500 (7)0.0666 (7)0.0252 (9)
C(21)0.0966 (5)0.1785 (3)0.1495 (4)0.0252 (9)
C(22)0.0417 (5)0.2425 (2)0.0548 (5)0.0252 (9)
C(23)0.1905 (4)0.1418 (4)0.0846 (5)0.0252 (9)
 
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