Crystal structures of symmetrical tetra-n-alkyl ammonium and phosphonium halides.: Dissection of competing interactions leading to "biradial" and "tetraradial" shapes

We analyzed experimentally and by calculations the crystal packing of eight ammonium and phosphonium halide salts, each with four equivalent n-alkyl chains containing 10-18 carbon atoms. All of the salts crystallize as stacked monolayers with an "ionic plane" consisting of an array of anions and positively charged N or P atoms in the middle of each layer. The four alkyl chains of each molecule adopt either a biradial or a tetraradial shape. In the biradial salts, the chains are paired and each pair is projected on opposite sides of the ionic plane. In the tetraradial case, the chains are unpaired and take the share of a crude tetrahedron. There are,gauche bends along at least some of the chains of each salt. MM2 force-field computational analyses, coupled with entropy calculations based on conformational statistics, predict correctly that the tetraradial --> biradial shape transformation will occur when the alkyl chains reach ca. 12 carbon atoms in length. Detailed analyses of the several factors responsible for the crystal packing in these salts and comparisons with salts containing either four shorter chains or one or two long chains are presented.