Crystal Structures of Symmetrical Tetra-n-Alkyl Ammonium and Phosphonium Halides. Dissection of Competing Interactions Leading to “Biradial” and “Tetraradial” Shapes

Abstract

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 shape 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 f 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.

Alkyltributylphosphonium chloride ionic liquids: synthesis, physicochemical properties and crystal structure

“…Elemental carbon and hydrogen contents (wt%) were analysed using a Perkin-Elmer Series II CHNS/O 2400 CHN Elemental Analyser, and provided analysis within ±0.3 wt% error. Halide content was determined by dioxygen flask, 45 combustion followed by halide titration with aquous mercury(II) nitrate, using a dicarbazide indicator. Water content.…”

“…The 31 P{ 1 H} NMR spectra of the alkyltributylphosphonium chlorides (see Table 3 As the tertiary tributylphosphine signal is at -31.06 ppm (most of tertiary alkylphosphines exhibit signals in the δ -30 to -60 ppm range), monitoring of the reactions was very straightforward. The purity of the starting phosphine was determined by 31 P NMR 45 spectroscopy as ca. 95%.…”

The quest for the simplest possible organogelators and some properties of their organogels

“…From single crystal x-ray diffraction studies on several of these salts, the separation between cationic and anionic centers minus the van der Waals radius of nitrogen or phosphorus, as appropriate, is always smaller for the nNA salts than the corresponding nPA salts 38 . However, we are reluctant to draw a strong conclusion from this observation because the morph of an LMOG in its gels may differ from that in solids obtained by bulk recrystallizations 40 .…”

“…Salts with the longest chains provide gels with the lowest CGCs and the highest T gel values, regardless of cooling protocol. If the packing of gelator molecules in gel strands is microscopically separated into lipophilic and lipophobic regions, as in their bulk solid states 38 , longer alkyl chain lengths will increase the aggregate stabilities as a result of larger London dispersion forces. Table 1) nNBr/hexadecane gels at 0.01 and 0.08 mol L -1 gelator concentrations.…”

Crystalline structure and thermotropic behavior of alkyltrimethylphosphonium amphiphiles

“…To the best of our knowledge, the few studies carried out to date on phosphonium-based surfactants concern salts bearing two, three or four long alkyl chains, [15][16][17][18] whereas no study has been addressed to examine those containing only one long alkyl chain except that of Kanazawa et al which was devoted to evaluate the antimicrobial properties of the chloride salts of some of these compounds. 19 In this paper we wish to report on a series of alkyltrimethylphosphonium bromide surfactants, abbreviated as nATMPBr (Scheme 1) with the alkyl chain being linear and containing an even number of carbon atoms (n) ranging from 12 to 22.…”