Crystal structures of p-chlorobromobenzene and p-chloroiodobenzene

1-Cl-4-Br-benzeue, ClBrC6H4, M-r=191.4 P2(1)/c, a=4.0238(6), b=5.8034(10), c=15.083(2), beta=112.23(1)degrees V = 326.03 Angstrom(3), Z = 2, D-x = 1.9 50 g cm(-3), lambda(Mo K alpha) = 0.71073 Angstrom, mu = 6.594 mm(-1), F(00 0) = 184, T = 220 K, final R = 0.046 for 980 observed reflections. 1-Cl-4-I-benzene, ClIC6H4, M-r = 238.4, P2(1)/c, a = 4.1762(4), b = 5.8814( 9), c = 15.790(2), beta = 113.42(1)degrees V= 355.88 Angstrom(3), Z = 2, D- x = 2.225 g cm(-3), lambda(Mo K alpha) = 0.71073 Angstrom, mu = 4.765 mm(-1 ), F(000) = 220, T = 230 K, final R = 0.041 for 1078 observed reflections. Both in PCBB and PCIB the centrosymmetric array is brought about by the sta tistical superposition of molecules occupying regular lattice sites but irr egularly pointing in opposite directions. Both systems share an isomorphic crystalline structure in which molecules are almost stacked along the a-axi s. (C) 1999 Elsevier Science B.V. All rights reserved.