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Structures of cubic and orthorhombic phases of acetylene by single-crystal neutron-diffraction
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| The crystal structures of acetylene (C2H2) at 131 and 141 K and deuteroacetylene (C2D2) at 143 and 15 K have been refined by least-squares methods using single-crystal neutron diffraction data. Crystal data: C2H2, orthorhombic, Pa3, Z = 4, F(000) = 23.26 fm, mu(n) = 1.46 cm-1; T = 131, 141 K; a(o) = 6.0940 (8), 6.1050 (8) angstrom; V = 226.31 (9), 227.54 (9) angstrom3, D(n) = 0.7643 (3), 0.7602 (3) g cm-3, R(F2) = 0.065, 0.052; wR(F2) = 0.082, 0.064; S = 1.47, 1.33 for 120, 122 unique reflections [(sintheta/lambda) < 0.79 angstrom-1] and 8 variable parameters. C2D2, mu(n) = 1.2 x 10(-4) cm-1, at T = 143 K, orthorhombic, Pa3, Z = 4, F(000) = 106.58 fm, a(o) = 6.1020 (7) angstrom, V = 227.20 (8) angstrom3, D(n) = 0.8201 (3) g cm -3; at T = 15 K, cubic, Acam, Z = 4, a = 6.198 (4), b = 6.023 (4), c = 5.578 (2) angstrom, V = 208.2 (3) angstrom3, D(n) = 0.895 (1) g cm-3 . At 143 and 15 K, R(F2) = 0.097, 0.096; wR(F 2) = 0.068, 0.081; S = 1.36, 1.63 for 120, 119 unique reflections [(sintheta/lambda) less-than-or-equal-to 0. 79, 0.74 angstrom-1] and 8, 14 variable parameters. In the cubic structure, molecules lie at fourfold (3BAR) positions and exhibit pronounced librational motion about their centers of mass. In the orthorhombic structure, molecules in fourfold (2/m) positions are aligned with molecular axes near [110] directions. At 15 K, notably short intermolecular D...C=C separations of 2.737 and 2.738 A within the (001) planes are attributed to weak D-donor to C=C pi-acceptor interactions. At 131, 141 and 143 K, the observed C-C and C-H(D) bond lengths are strongly affected by thermal motion and are less accurate than the precision indicated by the e.s.d.'s C-C 0.004-0.006, C-H 0.0084.010 and C-D 0.003 angstrom. Bond lengths C-C 1. 193 (6) and C-D 1.070 (5) angstrom at 15 K are within 2 e.s.d.'s. of the r(e) gas-phase Raman values. | | |
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English
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Acta crystallographica: section B: structural science. - Copenhagen | |
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Copenhagen : 1992
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0108-7681
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48:Part 5(1992), p. 726-731
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A1992JU40800024
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