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 c hains containing 10-18 carbon atoms. All of the salts crystallize as stacke d monolayers with an "ionic plane" consisting of an array of anions and pos itively charged N or P atoms in the middle of each layer. The four alkyl ch ains of each molecule adopt either a biradial or a tetraradial shape. In th e biradial salts, the chains are paired and each pair is projected on oppos ite sides of the ionic plane. In the tetraradial case, the chains are unpai red 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 an alyses, coupled with entropy calculations based on conformational statistic s, predict correctly that the tetraradial --> biradial shape transformation will occur when the alkyl chains reach ca. 12 carbon atoms in length. Deta iled analyses of the several factors responsible for the crystal packing in these salts and comparisons with salts containing either four shorter chai ns or one or two long chains are presented.