MESOPHASES OF ALKYLAMMONIUM SALTS .6. THE CRYSTAL-STRUCTURES OF TETRA-N-BUTYLAMMONIUM BROMIDE AND IODIDE

The structures of tetra-n-butylammonium bromide,N(C4H9)(4)Br (abbrevia ted: 4Br) and tetra-n-butylammonium iodide, N(C4H9)(4)I (abbreviated:4 1) have been determined. For 4Br: Mw = 322.4 Da, c-centered monoclinic space group C2/c, a=14.409(5)Angstrom, b=13.901(2)Angstrom, c=20.345( 9)Angstrom, beta=110.18(3)Angstrom, V=3729.4 Angstrom(3), Z=8, Dc=1.14 8g/cm(3) F(000)=1392, R=0.098 for 1311 reflections. For 4I: Mw=369.4pa , c-centered monoclinic space group C2, a=14.482(4)Angstrom, b=14.318( 4)Angstrom, c=19.975(7)Angstrom, beta=110.86(2), V=3870 Angstrom(3), Z =8, Dc=1.230 g/cm(3), F(000)=1536, R=0.142 for 1899 reflections. The s tructure analysis shows that the n-butyl chains are distorted to diffe rent degrees from their all-trans conformations and are connected thro ugh the quaternary nitrogen with one or two gauche- bonds. The plastic crystalline mesophase for 41 seems to be possible because of a lower packing fraction that produces larger deviations of the bond-rotation angles from the intramolecular energy-minimum, not found for 4Br. The condis crystal mesophase for 4Br, in turn, may be due to a more interm eshing cation structure, possible because of the higher packing fracti on with the Br- anion. Higher homologues of the tetra-n-alkylammonium salts form only condis-crystal mesophases, lower homologues, only plas tic-crystal mesophases. The low-temperature crystal structures of all salts of this homologous series contain similar cation conformations. The n-butyl and longer alkyl chains form layer-like crystals with meth yl surfaces. The somewhat poor fit of computed and experimental struct ures does not arise from experimental error, but is due to static pack ing disorder.