T - American Chemical Society

J. Phys. Chem. B 1999, 103, 9269-9278

9269

Crystal Structures of Symmetrical Tetra-n-Alkyl Ammonium and Phosphonium Halides. Dissection of Competing Interactions Leading to “Biradial” and “Tetraradial” Shapes David J. Abdallah,† Robert E. Bachman,† Jerry Perlstein,‡ and Richard G. Weiss*,† Department of Chemistry, Georgetown UniVersity, Washington, District of Columbia 20057-1227, and Center for Photoinduced Charge Transfer, Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627 ReceiVed: March 25, 1999; In Final Form: June 10, 1999

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.

Introduction The diversity of molecular organizations in lyotropic mixtures and neat samples of ammonium and phosphonium salts with four n-alkyl chains can be attributed to several opposing factors. Dispersion forces between chains and electrostatic interactions between oppositely charged centers provide attractive interactions. Repulsive forces include hydrophobic-ionic or electrostatic interactions between ionic centers of the same charge. For instance, aqueous solutions of ammonium salts with one or two long alkyl chains are known to aggregate into micelles1 and bilayers.2,3 Ammonium and phosphonium salts with three octadecyl chains and a fourth shorter alkyl chain form enantiotropic and thermotropic liquid-crystalline phases,4 and those with four long alkyl chains form lyotropic gels with organic liquids.5 Crystals of ammonium salts with one long alkyl chain and “simple” counterions are packed in interdigitated layers in which the alkyl groups adopt all-transoid (extended) conformations.6 The cationic headgroups are localized in a lyophobic “plane”. When necessary, two interdigitated chains can tilt to make a projection onto the layer plane comparable to the cross-sectional area of one headgroup. By comparison, the conformations of ammonium bromide salts with two methyl groups and two long n-alkyl chains are not fully extended and not interdigitated.7 One long chain has two consecutive gauche bends near to the cationic center. This allows the long chains to maximize their dispersive interactions by pairing. In this way, tilting of the paired chains with respect to the ionic plane can accommodate a space comparable to a single headgroup without interdigitation. Little is known about the molecular packing of charged surfactants with a single atom headgroup and three or four long * To whom correspondence should be addressed (fax: (202) 687-6209; E-mail: [email protected]). † Georgetown University. ‡ University of Rochester.

alkyl chains. For steric reasons, the packing arrangements such as those in molecules with one or two long chains are not possible. Thus, the only crystal structures we have been able to find for ammonium and phosphonium cations having three or four long alkyl chains (i.e., g7 carbon atoms) and relatively simple anions are tri-decylmethylammonium bis(4,5-di-mercapto-1,3-di-thiol-2-thionato(2-)-S4,S5)aurate(1-),8 tetra-decylammonium tetra-phenylborate,9 and benzyl-tri-octadecylammonium bromide.4b Of these, the first two do not have their alkyl chains in a parallel arrangement (i.e., at least two chains of one molecule lying next to each other in partially or completely extended conformations). The benzylammonium salt does have alkyl chains in a parallel arrangement, however, by allowing one long all-transoid chain to interdigitate along a nearly perpendicular axis to the pair of parallel chains, each of which has consecutive gauche bends between C2-C3 and C3C4; the overall “zigzag” lamellar packing arrangement is neither a classical monolayer nor bilayer structure. Here, we report that chains of tetra-n-alkylammonium and phosphonium salts (Table 1) can be parallel in pairs (as in salts with two long alkyl chains). However, the overall packing arrangements always resemble a monolayer in which the charged centers lie in the middle of the layer. Structural features, such as intramolecular chain pairing and molecular packing, are analyzed quantitatively by force-field energy computation and molecular modeling. Specific attention is placed on how the conformations of the chains and their packing are influenced by chain length, the nature of the cationic center, and the size of the anion. Experimental Section Materials. Solvents were reagent grade and used as received. Purities of materials are according to the supplier. Potassium ethyl xanthate, mp 210 °C (dec), was synthesized according to literature procedures.10 tri-Octadecylphosphine and tri-decylphosphine were gifts from Cytec, Inc.11 Over time, they

10.1021/jp9910338 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/18/1999

9270 J. Phys. Chem. B, Vol. 103, No. 43, 1999

Abdallah et al.

TABLE 1: Chemical Formulas of tetra-Alkylammonium and Phosphonium Salts

a

(H(CH2)n)4Y+X-

n

Y

X

4NI 10PBr 12NCl‚H2Oa 12NBr 12NI 16NI 18NI 18PI

4 10 12 12 12 16 18 18

N P N N N N N P

I Br Cl Br I I I I

Monohydrate by X-ray crystallography; see text.

developed oxides which could be separated easily from the desired phosphonium salts via selective crystallization. tetraHexadecylammonium bromide (16NBr; 98%; Aldrich) and hydriodic acid (49% aqueous; Baker) were used as received. tetra-Octadecylammonium iodide (18NI) was available from previous work.4 Single crystals were grown by diffusion of acetone into a CHCl3 solution. 1-Bromodecane (98%) and 1-iodooctadecane (95%) from Aldrich and tetra-dodecylammonium chloride (12NCl‚H2O; 97%; mp 80.7-82.7 °C) and tetradodecylammonium bromide (12NBr; 99%; mp 88.1-91.8 °C) from Fluka were used as received. Single crystals were grown by vapor diffusion of ether into an ethyl acetate solution (12NCl‚ H2O) and hexane into a benzene solution (12NBr). Single crystals of tetra-dodecylammonium iodide (12NI; Lancaster; 98%; mp 114.5-116.6 °C; lit. mp 116 °C12) were grown by diffusion of methanol into a chloroform solution. tetra-Hexadecylammonium Iodide (16NI). To a solution of 331 mg (0.33 mmol) 16NBr in 20 mL of CHCl3 was added 343 mg (2.14 mmol) potassium ethyl xanthate.10b After being stirred for 24 h, the yellow solution was filtered. The intermediate, tetra-hexadecylammonium ethyl xanthate, was isolated by evaporating the filtrate to residue and recrystallizing it from acetone/hexane: 223 mg (64%) of a yellow semicrystalline solid;1H NMR (CDCl3), δ 4.49 (q, J ) 7.2 Hz, 2H), 3.38 (m, 8H), 1.69 (m, 8H), 1.37 (t, J ) 7.21 Hz, 3H), 1.25 (m, 104H), 0.88 (t, J ) 6.3 Hz, 12H). In the dark, 1 mL (5.9 mmol) concentrated hydriodic acid was added to a solution of 223 mg (0.215 mmol) tetra-hexadecylammonium ethyl xanthate in 10 mL CHCl3. After being stirred for 1 h, the solution was extracted with HPLC grade water (5 × 20 mL) and the organic layer was reduced to a solid residue on a rotary evaporator. Recrystallization from tert-butyl methyl ether afforded 182 mg (53%) of a white solid: mp 111.8-113.5 °C; 1H NMR (CDCl3), δ 3.35 (m, 8H), 1.68 (m, 8H), 1.25 (m, 104H), 0.88 (t, J 6.6 Hz, 12H). Single crystals were obtained by diffusion of benzene into a CHCl3 solution. tetra-Decylphosphonium Bromide (10PBr). Inside a sealed glovebag purged with nitrogen, 1 mL (6.58 mmol) 1-bromodecane, 10 mL CHCl3, and 500 mg (0.94 mmol) tri-decylphosphine were transferred to a round-bottom flask. The solution was stirred for 3 days under nitrogen and heated to 35 °C for the last 2 h. 31P NMR spectra of aliquots of the reaction mixture in CDCl3 were used to monitor the progress of the reaction. The tri-decylphosphine peak at -31.8 ppm slowly disappeared and a new peak at 33.1 ppm appeared. After the solvent was removed, the remaining white solid was repeatedly recrystallized from diethyl ether to afford 536 mg (85%) of a white crystalline platelike solid: mp 53.6-57.0 °C; 1H NMR (CDCl3), δ 2.45 (m, 2H), 1.51 (s, 4H), 1.27 (s, 12H), 0.88 (t, 3H) ppm; 31P NMR (CDCl3), δ 33.11 ppm. Single crystals were obtained from ether. tetra-Octadecylphosphonium Iodide (18PI). Inside a sealed glovebag purged with nitrogen, 1 g (2.63 mmol) 1-iodoocta-

decane, 10 mL CHCl3, and 1 g (1.26 mmol) trioctadecylphosphine were transferred to a round-bottom flask. The mixture was stirred in the dark under N2. Progress of the reaction was monitored as above: a new peak appeared in the 31P NMR spectra at 33.0 ppm. After 7 days at room temperature, the reaction mixture was cooled to 0 °C. A white solid, collected using a vacuum filtration apparatus packed in ice, was recrystallized from tert-butyl methyl ether and a small amount of CHCl3 to afford 945 mg (64%) of a white solid: mp 97.6-99.3 °C; 1H NMR (CDCl ), δ 2.42 (s, 2H), 1.61 (s, 4H), 1.27 (s, 28H), 3 0.89 (t, 3H) ppm; 31P NMR (CDCl3), δ 33.02 ppm; single crystals were obtained from diffusion of benzene into a CHCl3 solution. Instrumentation. Melting points were measured on a Leitz 585 SM-LUX-POL microscope equipped with crossed polars, a Leitz 350 heating stage, and an Omega HH21 microprocessor thermometer connected to a J-K-T thermocouple. NMR spectra were recorded on a Varian 300 MHz spectrometer interfaced to a Sparc UNIX computer using Mercury software. Chemical shifts were referenced to an internal tetramethylsilane (TMS) (1H) standard or an external 85% H3PO4 standard (31P). Packing Analysis and Modeling. Packing-energy analysis for each crystal structure and molecular modeling were done on a Silicon Graphics Indigo Workstation with an R8000 processor. Code for the crystal packing-energy analysis was written in Fortran 77 and integrated into CHEM-X/CHEMLIB13 for viewing the structures. Molecular simulations of conformers of varying length were done using MACROMODEL14 and the MM2 force-field. X-ray Analyses. Single-crystal X-ray diffraction data were collected on a Siemens SMART CCD diffractometer using Mo KR radiation (λ ) 0.71073). Structures were determined by direct methods and refined against F2 using SHELXLT/PC v5.0 software suite.15 Crystals suitable for X-ray analysis were found only after repeated attempts using a variety of empirical methods. Single crystals obtained by the diffusion method were analyzed by placing a 5 mL glass vial of 1-2 mg of the salts dissolved in a good solvent into a 50 mL screw-cap jar containing a nonsolvent at room temperature. Additional nonsolvent was added to the jar if after 1 day no crystals were formed. In all cases, the colorless crystals had one dimension less than 0.02 mm, making them appear platelike or ribbonlike and easily deformed. When mounted onto glass fibers using epoxy, many of the crystals appeared to bend to the surface of the epoxy, rendering them useless for analysis. Inspections before and after each analysis at low temperature gave no evidence of shattering that would be associated with a firstorder phase transition. All crystals appeared to be the same, except 12NI for which no solution could be found for data collected at low temperature (173 K). The crystal became “foggy” when cooled, possibly indicating a that solid-solid transition may have occurred. Therefore, data were collected at room temperature. Important crystallographic parameters are collected in Table 2. Results A search of the Cambridge Structural Database for compounds with four long n-alkyl chains attached to a charged or uncharged tetrahedral center gave few “hits”. Although tetraheptylphosphonium iodide is listed, no coordinates or structural information are available.16 Some of the bond distances and angles reported for tetra-decylammonium tetra-phenylborate9 are chemically unreasonable; however, the overall conformation, which is of primary importance here, is discussed. The structure

Tetra-n-Alkyl Ammonium and Phosphonium Halides

J. Phys. Chem. B, Vol. 103, No. 43, 1999 9271

Figure 1. Molecular conformations from the X-ray crystal structures of (a) 10PBr, (b) 12NCl‚H2O, (c) 12NBr, and (d) 18PI. Conformations of the cations are either tetraradial (a) or biradial (b, c, and d). Biradial conformations have two sets of paired chains. 12NI (not shown) is tetraradial and resembles the structure of 10PBr. 16NI and 18PI (not shown) are biradial and resemble the structure of 18NI.

TABLE 2: Crystallographic Data for Ammonium and Phosphonium Salts formula molecular mass crystal system space group a/Å b/Å c/Å R/° β/° γ/° Dc/g cm-3 Z F(000) µ/mm-1 crystal size/mm data collection T/K θmin, θmax/° total data observed data refinement no. of refined parameters final R final wR2 final R (all data) goodness of fit

10PBr

18PI

12NCl‚H2O

12NBr

12NI

16NI

18NI

C40H84PBr 675.98 monoclinic C2/c (no. 15) 13.6708(1) 13.8227(1) 45.9482(3) 90 90.053(1) 90 1.034 8 2992 1.01 0.52 × 0.40 × 0.02

C72H148PI 1171.85 triclinic P1 (no. 2) 9.7367(3) 9.7614(3) 40.1848(13) 91.050(1) 91.141(1) 91.106(1) 1.019 2 1296 0.47 0.48 × 0.35 × 0.02

C48H102NClO 744.76 monoclinic P21/n (no. 14) 5.4370(1) 50.7958(7) 17.8681(3) 90 92.420(1) 90 1.003 4 1688 0.11 0.48 × 0.19 × 0.02

C48H100NBr 771.23 monoclinic C2/c (no. 15) 13.2418(1) 13.0768(1) 55.5295(2) 90 93.219(1) 90 1.067 8 3440 0.89 0.45 × 0.45 × 0.02

C48H100NI 818.23 triclinic P1 (no. 2) 14.1973(0) 14.2949(2) 52.8104(8) 85.850(1) 89.684(0) 89.952(1) 1.017 8 3584 0.63 0.50 × 0.40 × 0.02



173 0.89, 28.29 47083 10538

173 1.01, 28.26 42272 17848

173 1.21, 28.35 53668 12012

173 0.73, 28.51 38168 11566

298 1.16, 28.42 104682 48634

298 1.16, 28.76 38343 16043

298 2.06, 23.25 19442 10538

380

667

468

201

801

595

671

0.0594 0.1030 0.1108 1.095

0.0700 0.1403 0.2501 0.934

0.1075 0.2255 0.249 1.162

0.2036 0.4215 0.2848 1.240

0.2515 0.4783 0.5585 1.573

0.0826 0.2051 0.2392 1.007

0.0845 0.1689 0.1137 1.228

of tetra-butylammonium iodide (4NI) is known17 and was used as a model for the conformation about the central atom in our computational analyses (vide infra). Our X-ray data for 12NBr and 12NI are of relatively poor quality, resulting in some uncertainty in bond distances and atomic coordinates. HoweVer, this uncertainty does not extend to the oVerall molecular conformations that are of primary interest to this work. Other structures were refined anisotropically. In all cases, the intensity of diffracted X-rays was low because of the small size of the crystals and some deformation when the crystals were mounted onto glass fibers. Coordinates, bond distances, and angles are included as Supporting Information. Molecular Conformations. X-ray crystallographic results for our tetra-n-alkylammonium and phosphonium salts are sum-

marized in Table 2. Structures can be classified as “tetraradial” or “biradial” according to how the four alkyl chains extend (Figure 1). Both types have a tetrahedral arrangement for the four R-carbon atoms adjacent to the cationic center. Chains in molecules of the tetraradial type (10PBr and 12NI) are alltransoid (Figure 1a). The overall conformations of these cations are not tetrahedral because twists about the N-C bonds at the cationic center extend chains at angles that do not correspond to 109.5°. 12NCl‚H2O, 12NBr, 16NI, 18NI, and 18PI are biradial because chains are paired in two sets (Figure 1b). Conformations of the chains are all-transoid only beyond the fourth carbon atom from the cationic center, and intervening bonds adopt one or two gauche bends. The biradial conformations have a total of four gauche bends (excluding dihedral angles about the quaternary center), two within each pair of


Figure 2. Representation of the central portion of the X-ray crystal structure of the cationic portion of an 18PI molecule. The tetrahedron is defined by the four R-carbon atoms surrounding the phosphorus atom in the center. Dark C-C bonds are gauche and the others are transoid.

chains. The location of the gauche bends appears to be dependent on the counterion. Here “gauche” is used rather loosely because some of the dihedral angles extend up to 75°. Deviations of this sort are common in sterically hindered flexible chains. The structures of 16NI and the two compounds with the longest chains, 18NI and 18PI, are essentially the same. The only apparent difference is the covalent bond distances between N-C (1.525 Å) and P-C (1.805 Å). The cations are in biradial conformations. Two of the four-alkyl chains contain two consecutive gauche bends at C2-C3 and C3-C4 and two chains are all-transoid. The remainder of each bent chain is all-transoid and oriented parallel to one of the all-transoid chains. Figure 2 shows the gauche arrangement around the cation centers of the molecules. The two sets of chains extend in opposite directions from the center, making the overall shape of the cation rodlike rather than tetraradial. 12NCl‚H2O and 12NBr are also biradial, but their structures differ somewhat from those of 16NI, 18NI, and 18PI. The overall shape of the cations is like a bent rod (see Figure 1c). Pairing in 12NCl‚H2O and 12NBr results from one gauche bend in each of the four chains. One occurs at C2-C3 in one chain and the other is at C3-C4 in the second chain of each pair (Figure 3). However, the chains of 12NCl‚H2O and 12NBr extend differently from the nitrogen atom. Although 12NCl‚H2O was purchased as an anhydrous salt and was recrystallized from anhydrous solvents, X-ray analyses showed that one water molecule per cation was present. Water acts as a bridge between ionic centers because both the oxygennitrogen (4.36 Å) and oxygen-chloride distances (3.19 Å) are shorter than 4.63 Å, the nitrogen-chloride distance. Molecular Packing. The charged centers of the cation and anion assemble into ordered ionic planes that are sandwiched between the hydrophobic chain regions. The shortest distances between a charged center of the cation and anion (dC-A) of the salts examined here are slightly longer than those reported for salts with shorter chains (Table 3). The cation-anion distance is always larger than twice the ionic radius of the anion (the larger of the atomic cationic and anionic radii). van der Waals contact between the charged centers is not possible because the cation is sterically encumbered by the R-methylene groups; the

Abdallah et al. anion does come into contact with some hydrogens on the R and β-carbons. The chains of both the “tetraradial” and “biradial” cations appear to assemble into lattices that make “pockets” of different sizes to accommodate the anions. The volume in which the anions fit into the cation lattice, the “pocket size”, is approximated as a sphere of volume VP. The radius of the sphere (rp) is the distance from the center of the anion to the center of the nearest hydrogen atoms minus the van der Waals radius (rvdW) of hydrogen (1.17 Å).18 A comparison between the pocket sizes and van der Waals volumes of the anions (VA), calculated from ionic radii (rion),19 is made in Table 4. Note that the calculated sizes of the pockets are (unrealistically) smaller than the volumes of the anions. This is because our method for calculation of rp assumes no penetration of the anion beyond the first point of contact with the nearest hydrogen atom. Clearly, this is not the case and, in fact, there may be attractive hydrogenbonding interactions between the nearby hydrogen atoms and the anions.20 Regardless, the calculations provide a qualitative comparison of pocket sizes. The VP value for 12NCl‚H2O is not included in Table 4 because the volume occupied by the “anion” must include the presence of a molecule of water. As a result of extensive hydrogen bonding of the water molecule with chloride and electrostatic interactions with the positively charged nitrogen atom, a spherical model for the pocket in this case is inappropriate. Salts with both tetraradial and biradial shapes exhibit layered packing (Figure 4). The spacings (d), defined as the perpendicular distance between single layers, are collected in Table 5. Salts with tetraradial shapes extend the alkyl chains in crisscross patterns at an angle φ with respect to the normal of the ionic plane defined by the anionic and cationic centers. All chains of one molecule terminate at the same distance from the ionic plane within a layer (i.e., there is a “methyl surface”). The biradial salts assemble in layered patterns as well. However, pairs of chains are parallel and adjacent. For these structures, φ is the angle of a pair of chains with respect to the normal to theionic plane. These chains also constitute a methyl surface. The values of φ for both tetraradial and biradial structures are included in Table 5. 12NCl‚H2O and 12NBr pack with their longest dimension (the distance from the ends of the paired chains) orthogonal to the lamellar surfaces. However, each pair of chains makes a nonorthogonal angle with respect to a lamellar surface due to the overall bowed conformation of the cations. Analysis of Molecular Shapes by Molecular Mechanics. To dissect the energetics associated with the biradial f tetraradial conformation change, we performed a series of molecular mechanics calculations based on tetra-n-alkylammonium iodide salts for even-numbered chain lengths of 4-18 carbon atoms. tetra-n-Alkylammonium iodide salts of intermediate chain lengths were assumed to resemble either the tetraradial shape of 12NI or the biradial shape of 18NI. To compute the free energy change for the transformation (∆Gn, where n is the number of carbon atoms per chain) at 300 K, we made three simplifying assumptions: (a) the equilibrium tetraradial conformation of chains of any length is all-transoid; (b) the equilibrium biradial conformation for chains of any length is that of 18NI; and (c) the entropy change can be computed from the conformational entropy making all conformations for a given chain length equal in energy (a microcanonical ensemble). Results are shown in Table 6. Energy Contributions to the Chain Conformation. The approximate enthalpy difference between tetraradial and biradial



Figure 3. Representation of portions of cationic parts near charged centers by X-ray crystallography: (a) 12NCl‚H2O and (b) 12NBr. The tetrahedra are defined by the four R-carbon atoms surrounding the nitrogen centers. Dark C-C bonds are in gauche conformations and the others are transoid. The lower portions of the two structures are oriented to be nearly congruent. This comparative projection allows differences between the two cationic shapes to be viewed clearly in the upper portions of the figures.

TABLE 3: Shortest Distances from Halides (A) to Charged Atoms of Cation (C) with Selected Values from the Literature for Other Salts salt

conformation

dC-A/(Å)

lit. dC-A/(Å)

10PBr 12NCl‚H2O 12NBr 12NI 16NI 18NI 18PI

tetraradial biradial biradial tetraradial biradial biradial biradial

4.86 4.63 4.74 4.95 4.81 4.76 4.90

4.15a 4.42b 4.43c 4.76d 4.76d 4.76d 4.63e

a tri-Methyl-2-phenylethylphosphonium bromide: Riddell, F. G.; Rogerson, M.; Tumbull, W. B.; Fulop, F. J. Chem. Soc. Perkin Trans. 1997, 2, 95. b tetra-Ethylammonium chloride monohydrate: Loehlin, J. H.; Kvick, A. Acta Cryst. 1978, B34, 3488. c 2-Carboxylethyl-trimethylammonium bromide: Yip, W.-H.; Ru-Ji, W.; Mak, T. C. W. Acta. Cryst. 1990, C46, 717. d tetra-Ethylammonium iodide: Vincent, B. R.; Knop, O.; Linden, A.; Cameron, T. S.; Robertson, K. N. Can. J. Chem. 1988, 66, 3060. e di-Ethyl-1-tetramethylenephosphonium iodide: Gomelya, N. D.; Feshchenko, N. G.; Chemega, A. N.; Antipin, M. Y.; Struchkov, Y. T.; Boldeshu, I. E. Zh. Obshch. Khim 1985, 55, 1733.

TABLE 4: Van der Waals Volumes of Anions and Calculated “Pocket” Sizes pocketa a

salt

rp (Å)


1.76 1.58 1.74 1.93 1.84 1.86 1.87

VP

anion (Å3)

22.8 22.1 30.1 26.1 27.0 27.4

rion (Å)

VB (Å3)

VP/VA

1.96 1.81 1.96 2.20 2.20 2.20 2.20

31.5 24.8 (37.3)b 31.5 44.6 44.6 44.6 44.6

0.72 0.70 0.68 0.59 0.60 0.61

a See text for details. b The sum of the van der Waals volumes of chloride and water. The latter is based on rH2O ) 1.44 Å: Dorsey, N. E. Properties of Ordinary Water-Substance; Reinhold: New York, 1940, p 42.

conformations was computed in several steps. First, atomic coordinates for a single cation-anion 18NI pair were read into the MACROMODEL program and the energy was minimized using the MM2 force-field until the energy gradient was less than 0.01 kcal/mol-Å. The final result is the equilibrium energy

for the 18NI in a biradial conformation. Then the energies of biradial conformations of the shorter chain lengths were obtained by truncating successively each of the four chains by two methylene units (starting from the chain termini) of 18NI. The energy of each homologue was minimized as for 18NI. Second, to compute the energies of the corresponding tetraradial conformations, all of the gauche bends in the 18NI crystal structure were rotated until the chains were all-transoid. This conformation for 18NI and the shorter homologues were then energy minimized as described above for the biradial conformations. Finally, the enthalpy changes for the tetraradial f biradial transformations were obtained by difference (Table 6). Biradial shapes are favored when the carbon chain lengths are greater than 8. As an example of the energy “components”, the differences in each energy component are shown in Table 7 for 6NI and 18NI. For 18NI, the bulk of the difference comes from the van der Waals attractive interaction between the chains which is offset by some torsional and bending energy. Clearly, as the chains become shorter, the vdW term decreases in absolute magnitude; with fewer atoms/chain, there is less attraction between chains. Clearly, the preference for the biradial shapes of 18NI is driven by the van der Waals component. As the chains are shortened, the van der Waals component decreases while some of the other components favoring tetraradial shapes increase. Entropic Contributions to Chain Bending. Assumption (c) above was used to obtain the entropy contributions. Each chain of a cation contains n C-C and N-C bonds (excluding the end methyl linkage). With four independent chains per cation, the total number of bonds is 4n. If we assume that only gauche or transoid conformations are possible (no distinction was made between gauche + or gauche -), the total number of possible conformations for a tetraradial cation (Ωtetraradial) is 24n. This is an overestimate because many conformations that place chains so that they overlap in physically unrealistic ways can be eliminated. In the biradial state, chain pairing requires that bond conformations change together; the maximum number of bondpairs is 2n. With 2 conformations/bond-pair, the number of viable and unrealistic biradial conformations, Ωbiradial, is 22n.


Abdallah et al.

Figure 4. Molecular packing of (a) tetraradial 10PBr and (b) biradial 18PI from X-ray analyses. Carbon and hydrogen atoms have been removed from the top layers to show the planes defined by the ions. d is the perpendicular distance between ionic planes.

TABLE 5: Layer Thicknesses (d), Tilt Angles (O), and Calculated Extended Chain Lengths (dex) of Salts salt

da/(Å)

φb/(°)

dcex(Å)


23.3 25.4 27.8 26.4 35.1 39.5 40.2

36 45 36 38 36 35 35

15.4 17.7 17.7 17.7 22.7 25.2 25.5

a Perpendicular distance between two ionic planes. b The angle made by a single chain (tetradial) or pair of chains (biradial) with respect to the normal to the ionic layer. c Calculated from the center of the cation (N or P) to the terminal methyl hydrogen plus the van der Waals radius of hydrogen.

The entropy difference for a tetraradial f biradial transformation in this approximation is shown in eq 1.

∆S ) Sbiradial - Stetraradial ) R ln(Ωbiradial/Ωtetraradial) ) -2nR ln 2 (1) Thus, the entropy contribution for the tetraradial/biradial transformation is assumed to depend principally on chain length. The entropy contribution to ∆Gn shifts the “crossover” between tetraradial and biradial shapes from chain lengths of 8 carbon atoms to those of 12 carbon atoms (Table 6 at T ) 300 K). Analysis of Molecular Packing. The packing features of the structures determined by X-ray diffraction were quantitatively analyzed by MM2 force-field calculations. 4NI and the six salts for which a single crystal could be grown are considered here. The analysis computes the crystal lattice energy, which is the negative of the energy necessary to separate the molecules of a crystal to infinity. The “packing unit” was taken to be the crystallographic asymmetric unit. It consists of one ion-pair, except for 12NI (where the asymmetric unit contains four ion-pairs) and 4NI (where the asymmetric unit contains two ion-pairs). The packing units are assembled into the lattice (given by the space group) such that they are completely surrounded by other molecules. The interaction of the packing unit with all of the other molecules can then be computed as the lattice energy/

TABLE 6: Calculated Tetraradial f Biradial Energetic Changes (kcal/mol) for tetra-Alkylammonium Iodide Homologues

a

salt

∆H

T∆S a

∆Gn

4NI 6NI 8NI 10NI 12NI 14NI 16NI 18NI

1.82 2.28 -1.31 -5.34 -9.1 -12.95 -16.8 -20.67

-2.47 -4.11 -5.75 -7.4 -9.04 -10.69 -12.33 -13.99

4.29 6.39 4.44 2.06 -0.06 -2.26 -4.47 -6.68

∆S was calculated using eq 1. T ) 300 K.

packing unit. If the packing unit consists of more than one ionpair, the negative of the energy necessary to separate the ionpairs is added to this energy to obtain the lattice energy/ion-pair. The interaction potential used to find the lattice energy for an ion-pair with no hydrogen bonding is given by eq 2.

Elattice )

1 nb 1 (E + Eel) + Epack 2n n

(2)

Enb is the van der Waals contribution (always attractive), Eel is the electrostatic contribution (can be attractive or repulsive), Epack is the packing unit contribution to the total lattice energy/ ion-pair (Elattice), n is the number of ion-pairs in the packing unit, and the 1/2 is inserted to avoid double counting. Enb and Eel represent the interaction energy of the packing unit with the other molecules and Epack is the interaction of the molecules within the packing unit. The terms in eq 2 are computed from an atom-atom potential given by the MM2 force-field expression of Allinger21 as used in MACROMODEL (eqs 3 and 4):

Enb )

[

Aij 2.90 × 105 exp ∑ i,j Eel )

(

qiqj

∑ i,j (r)r

) ( )]

-12.50rij Bij

- 2.25

(r) ) 0rij ij

Bij rij

6

(3)

(4)



TABLE 7: Contributions to the Tetraradial f Biradial Intramolecular Transition Energy (kcal/mol)14 6NI 18NI

stretcha

bendb

torsionc

imp. tors.d

x-termse

vdWf

electro.g

totalh

0.14 0

1.47 1.81

2.14 2.48

0 0

0.15 0.18

-1.6 -25.11

-0.01 -0.03

2.29 -20.67

a Bond stretching energy. b Bond bending energy. c Bond rotation energy. d Improper torsion energy. e Bond-stretch-bend interaction energy. f van der Waals 1-4 interaction energy. g Coulomb 1-4 interaction energy. h Total energy.

TABLE 8: KAP Analyses of tetra-n-Alkyl Ammonium and Phosphonium Salts salt 4NI

10PBr

12NCl‚H2O

12NBr

12NI

16NI

18NI

18PI

stage

structure typea

0 1 2 3 3* 0 1 2 3 3* 0 1 2 3 3* 0 1 2 3 3* 0 1 2 3 3* 0 1 2 3 3* 0 1 2 3 3* 0 1 2 3 3*

two ion pairs transl. chain (a-b diagonal) transl. layer (a-b plane) C2 crystal crystal minus the anionsb single ion pair 2-fold chain (b-axis) 2-fold layer (a-b plane) C2/c crystal crystal minus the anionsb single ion pair transl. chain(a-axis) glide layer (a-c plane) P21/n crystal crystal minus the anionsb single ion pair 2-fold chain (a-axis) 2-fold layer (a-b plane) C2/c crystal crystal minus the anionsb four ion pairs transl. chain (b-axis) transl. layer (a-b plane) P1 crystal crystal minus the anionsb single ion pair inversion chain (a-axis) inversion layer (a-b plane) P1 crystal crystal minus the anionsb single ion pair inversion chain (b-axis) inversion layer (a-b plane) P1 crystal crystal minus the anionsb single ion pair inversion chain (b-axis) inversion layer (a-b plane) P1 crystal crystal minus the anionsb

van der Waals energyc (kcal/mol)

Coulomb energyd (kcal/mol)

total energye (kcal/mol)

energy anisotropyf

-4.8 -12.2 -22.7 -35.3 -27.1

-0.85 -13.1 -16.5 -16.2

-5.65 -25.3 -39.2 -51.5

0.49 0.76

-31.5 -84.5 -95.3 -87.5

0.1 -0.2 -0.1

-31.4 -84.7 -95.4

0.33 0.89

-59.2 -91 -101.8 -98.2

13.3 -9.7 -9.8

-45.9 -100.7 -111.6

0.41 0.9

-37.7 -96.4 -105.2 -100.7 -31.9 -56.8 -92.2 -99.2 -88.8

-7.1 -19.4 -20.6

-44.8 -115.8 -125.8

0.36 0.92

-6.1 -11.5 -16.2 -16.3

-38 -68.3 -108.4 -115.5

0.59 0.94

-52.6 -118.9 -127 -117.1

-13 -16.2 -16.2

-65.6 -135.1 -143.2

0.46 0.94

-76.5 -129 -134.1 -131.1

-13.1 -13.5 -13.5

-89.6 -142.5 -147.6

0.61 0.97

-76.9 -130 -140.1 -128.2

0.6 1.2 1.3

-76.3 -128.8 -138.8

0.55 0.93

transl ) translational; a,b and c correspond to the crystallographic coordinates. b Energy after removing anions from a lattic and neutralizing the charge of the cation; by definition, Ecoulomb ) 0. c Enb/2n from eq 2. d Eel/2n from eq 2. e Elattice from eq 2. f Ratio of total energy for each stage to total energy of Stage 3. a

where i is an atom in the packing unit and j is an atom in the other molecules of the lattice, rij is the distance between atoms i and j, Aij and Bij are parameters given by the force-field, qi and qj are assigned empirical charges, and (r) is a distancedependent dielectric constant with 0 ) 1.0. The empirical charges are those of Gasteiger22 when available; otherwise, those of Skorczyk23 are used. Although the electrostatic term can be large, it is generally not the deciding contribution to the packing geometry (as opposed to the absolute energy). The packing energy was analyzed in terms of Kitaigorodskii’s Aufbau Principle (KAP) which was described previously,24 and the results are summarized in Table 8. The purpose of KAP is to find quantitatively the lowest energy substructures which make up the complete crystal structure. Close packing is applied to the low energy shape of the packing unit and it is treated as a rigid structure. The substructures are called Stages and are numbered 0-3. Each substructure is a local minimum of the

local interaction potential. Stage 0 is the packing unit itself, made up of the asymmetric unit of the crystal structure. When there is only one ion-pair, Epack makes no contribution to eq 2. For 4NI and 12NI, a finite energy contribution to the crystal lattice energy is added by computing the interaction between the ion-pairs within the packing unit. For these salts, 12NI, with four ion-pairs in Stage 0, is the most complex structure. Stage 1 is the lowest energy for a one-dimensional strand of molecules. In this Stage, the ion-pairs are packed with a single repeat vector. It is easily found by examining the interaction of the packing unit with its lowest energy nearest neighbor ionpairs. The lowest energy interactions usually have one of four distinct symmetry types (translation, glide, 21 screw, or inversion).25 All of the salts examined here display one of these four symmetry types. Stage 2 is the lowest energy two-dimensional layer structure displayed by all the salts. In this Stage, the ion-pairs are packed

9276 J. Phys. Chem. B, Vol. 103, No. 43, 1999 into a layer with two repeat vectors. The lowest energy layers are computed via the interaction energy of the Stage 1 chains. There are usually only seven types of layers;26 our salts display four of the seven types. Stage 3 is the complete three-dimensional crystal structure whose symmetry type is given by its space group. Also included in Table 8 is the Enb energy (Stage 3*), representing packing of the cations after the anions are removed from a lattice and the charge on the cations is neutralized. A visual examination of these salt structures especially in Stage 2, indicates that the anion is inserted into holes in a cation lattice (vide ante). Therefore, the energy of the cation lattice alone was computed. From this energy an approximate energy per CH2 group is estimated by dividing the cation lattice potential by the number of carbon atoms (Table 9). The energy anisotropy (Table 8) is defined as the ratio of the energy of Stage 1 or Stage 2 to Stage 3. It is a measure of the degree to which a particular Stage deviates from the packing of spherical molecules. In a cubic packing arrangement, there would be 12 nearest neighbors for each sphere in Stage 3, 6 in Stage 2, and 2 in Stage 1. Thus, perfectly isotropic energy ratios (i.e., for a perfect cubic assembly) would be 16% for Stage 1 and 50% for Stage 2. The deviations of molecular shapes from a sphere have an intrinsic influence on the magnitude of the anisotropies. Discussion The conformation of an alkyl chain in a crystalline phase is usually all-transoid,27 like that favored in solutions.28 However, crystallized chains of appreciable length, such as those in polyethylene, tend to bend at regularly spaced intervals.29 A large cyclic paraffin, cyclotetratriacontane, which crystallizes with its sides paired (like polyethylene) has bends at each end, consisting of consecutive gauche, gauche, transoid, gauche, gauche (GGTGG) bonds.30 In fact, replacement of the quaternary heteroatom of 16NI, 18NI, and 18PI with a methylene unit would produce a GGTGG sequence of bonds. Okuyama et al.7a,c were the first to recognize the similarities between the chain bending within an amphiphile and polyethylene. However, the same replacement for crystalline 12NCl‚H2O and 12NBr results in a GTGGTG arrangement of bonds.31 Conformations of Alkyl Chains in Amphiphiles. The tendency of long methylene chains to assume all-transoid conformations in the solid state may be compromised if higher energy gauche conformations are offset by favorable intramolecular interactions elsewhere in the molecule. In our biradial salts, energetic increases due to gauche bends are compensated by attractive intramolecular van der Waals interactions between chains in a pair. Each bend consists of two gauche bends at positions near the cationic center. Each gauche bend along a chain is calculated to increase the relative energy by ca. 0.7 kcal mol-1.9 Thus, the relative van der Waals interactions of paired chains must be worth at least 1.4 kcal mol-1 in biradially shaped salts. Bending of chains may also bring about more favorable intermolecular interactions. In the reported structure of tetradecylammonium tetra-phenylborate,9 which is not biradial, three of its four chains adopt a gauche bend to capitalize on dispersion forces between chains of different cations. A bent alkane-chain conformation is also exhibited by di-octadecylammonium bromide.32 Headgroups align in an ionic plane and the two chains extend into the lipophilic layer at an angle 65° to the normal of the ionic plane in a parallel arrangement. Although these bends do not produce intramolecular chain pairing like

Abdallah et al. TABLE 9: Cation Shapes in Crystal Lattices and the Calculated Interaction Energy per Mole of Methylene Units (EvdW/CH2) salt

cation type

space group

EvdW/CH2 (kcal/mol)

4NI 10PBr 12NCl‚H2O 12NBr 12NI 16NI 18NI 18PI

tetraradial tetraradial tetraradial biradial biradial biradial biradial biradial

C2 C2/c P1 P21/n C2/c P1 P1 P1

1.7 2.2 1.9 2 2.1 1.8 1.8 1.8

di-methyl-di-octadecylammonium bromide, 12NBr, 12NCl‚ H2O, 16NI, 18NI, and 18PI, the parallel arrangement of chains produces a similar lipophilic region. Although we were unable to obtain suitable crystals of salts with all of the chain lengths in the 7-18 carbon atom range, the ones for which X-ray structures are available indicate that the tetraradial f biradial change occurs at a rather specific length. Because X-ray analyses demonstrate that 12NI is tetraradial and 16NI is biradial, it would be desirable to examine the structures of ammonium iodides of intermediate chain lengths. Our calculations based on biradial shapes, using 18NI as a model, predict that the critical chain length is 12 carbon atoms long. As shown in Table 6, the calculated gain in free energy from the T∆S term (due to chain pairing) almost equals the decreases in ∆H (from van der Waals interactions). Also, opposing the conformational change from tetraradial to biradial shapes in addition to entropic considerations is the increased enthalpy from gauche bends along the chains. These analyses assume that the biradial shapes differ only in the length of the alkyl chains, but not the angles between pairs of chains as projected from a cationic center. When the alkyl chains are long, the nature of the anion appears to define the conformation of the biradial cation (vide infra). Entropy always favors the tetraradial shape. Because its contribution to ∆Gn decreases with temperature, some tetraradial salts at room temperature may undergo morphological phase transitions to biradial molecular shapes at lower temperatures. This is especially true of salts whose chains are near the critical length. Our inability to collect a suitable X-ray data set for 12NI at 173 K, where ∆G12 is calculated to be -3.89 kcal/mol, may be related to such an effect. At 300 K, where an X-ray data set was obtained, ∆G12 is predicted to be -0.06 kcal/mol. The before and after appearances of single crystals for which diffraction data were collected at subambient temperatures were the same. 12NBr and 12NI are known by differential scanning calorimetry (DSC) measurements to display solid-solid transitions below room temperature.33 Cation Size Effects. The structures of 18NI and 18PI demonstrate that the size of the cation center has a minor effect on the overall shape of the cation. The pairing of their alkyl chains is similar to that for cations of ammonium bromide salts with two long alkyl chains, di-methyl-di-tetradecylammonium and di-methyl-di-octadecylammonium bromide.7a,c However, the pairing motif is found twice in the four long-chain cations and results in the cation having two lypophilic regions separated by an ionic plane; our salts aggregate into a monolayer rather than a bilayer type structure. Anion Size Effects. The biradial structures, 12NCl‚H2O, 12NBr, 16NI, and 18NI reveal an appreciable influence of the anion size and shape on the overall conformation of the cation. The origin of the influence lies in the arrangement of the four gauche bends distributed among the four chains of each cation.

Tetra-n-Alkyl Ammonium and Phosphonium Halides The specific locations of the bends are given in the Molecular Conformations part of the Results section. Different locations of bends in the chains of the biradial structures allow the pocket size to fit diverse anions. For example, the iodide salts, 16NI, 18NI, and 18PI, have the same arrangement of gauche bends which differs from the arrangement of the bromide salt, 12NBr. The arrangement of gauche bends of the chloride salt, 12NCl‚ H2O, is still different from the bromide and iodide salts. However, a perfect fit may not be possible and some anions may be more tightly packed in their pockets than others. When the anions cannot be accommodated well within a biradial pocket, the shape of the cations may revert to tetraradial. This may be why two of the tetra-dodecylammonium cations are biradial and 12NI is tetraradial. Qualitative comparison between pocket sizes of the tetraradial 12NI and the biradial 16NI, 18NI, and 18PI may provide insights into the differences among the 12NX salts (Table 4). The pocket size of the 12NI is larger. If all salts were the same shape, at least in the vicinity of the ionic planes, their pockets would have the same volumes. That they are of different shapes may be a consequence of the inability of the smaller van der Waals interaction energy in the packing of the shorter ammonium iodide to compensate for the added strain of placing the iodide into a smaller pocket, such as that of the longer ammonium iodide. Note that the VP/VA ratios of the three tetraradial salts in Table 4 are consistently larger than the ratios of the three biradial salts. Given the small number of examples, we are reluctant to make a strong generalization that tetraradial pockets are relatively larger than biradial ones; the data are very suggestive but additional examples are needed to examine this trend more carefully. The biradial shape of the tetra-dodecylammonium cation of 12NCl‚H2O indicates that the total anionic volume of a chloride ion and a water molecule, ca. 37.3 Å3, is distributed so that the perturbation at the headgroup does not require a transformation to a tetraradial shape. Whether a 12N cation with a truly spherical anion of this volume would remain biradial is unknown. If our arguments concerning why some of the 12NX are biradial and others are tetraradial are correct, the limiting VA must be between 31.5 Å3 (12NBr) and 44.6 Å3 (12NI) at this chain length. Packing Considerations. Parallel chain packing arrangements such as those found in ammonium salts with one or two long alkyl chains are not possible for our tetra-n-alkylammonium and phosphonium salts if their headgroups are arranged in bilayers (like those of salts with one or two long alkyl chains). There is inadequate space to accommodate four mutually parallel chains unless headgroup interactions are disrupted drastically. In addition, the long chains cannot be packed snugly near the single atom headgroups because they are projected from it tetrahedrally. Biradial packing arrangements for the tetraalkylated salts allow parallel chain packing, but only in a pairwise sense. The charged centers of the tetra-alkylated salts are localized in a plane, and the pairs of chains are projected above and below the plane at angles φ to its normal. The magnitude of φ is related to the cross-sectional area the chains project onto the plane. Tilting allows the alkyl chains to balance the area occupied by the headgroups. It may also contribute to packing stabilization in solid, layered phases. However, the vast majority of n-alkanes pack with their long axes orthogonal to layer planes.27a,34 The approximate area of a chain pair projected onto the plane is 2Ao/cos(φ), where Ao is the cross-sectional area of untilted chain.35 We have chosen not to use the calculated values for

J. Phys. Chem. B, Vol. 103, No. 43, 1999 9277 the projected areas in our quantitative analyses because they must follow from the natures of the tetraradial or biradial packing modes. None of the chains of a tetra-alkylated salt is coparallel with the others in tetraradial structures. Chains maximize their contacts intermolecularly by adopting an interweaving pattern. Our stepwise analysis of the crystal packing starts with defining the packing unit from X-ray crystallographic studies. In all cases, except for 4NI and 12NI, this unit consists of a single cation-anion pair. KAP analyses24 for the seven salts whose X-ray structures are reported provides the total energy and energy components associated with each substructure in the overall crystal structure (Table 8). No pattern of energy differences was found between tetraradial and biradial structures. Both types exhibit a pronounced energy anisotropy in the substructures (as expected from the anisotropic shapes of the ion-pairs). van der Waals interactions dominate the energetics of onedimensional strands of molecules in Stage 1 substructures (except for the shortest chained salt, 4NI, where Coulombic interactions make the largest contribution); the van der Waals energy of the longest alkyl chain salts is more than five times the Coulombic contribution. The energy anisotropy calculated for this Stage deviates far from the value of 16% for a cubic type structure (which many simpler salts display36). The two-dimensional arrays of molecules in Stage 2 substructures also show a pronounced deviation from the ideal energy anisotropy, reflecting that most of the crystal energy is in the layer packing. The anion in all cases is buried in the two-dimensional Stage 2 cation layer structure, and no additional interaction of the anion with the remainder of the crystal lattice can occur. The large electrostatic component of the layer lattice energy is not structure determining. Although the magnitude of this energy depends on the charge distribution used in the computations, the same substructures are obtained regardless of the choice of partial charges for the electrostatic term. The small total energy change between a Stage 2 monolayer and a Stage 3 crystal reflects the small increase in van der Waals interactions as “methyl surfaces” of layers come together. Such small differences are expected for a lamellar type packing arrangement. The van der Waals energy per methylene unit (Evdw/CH2), was determined by dividing the Stage 3* energy (see Table 8) by the number of carbon atoms in a structure. Evdw/CH2 remains ca. 1.9 kcal/mol regardless of cation size, type of anion, space group packing, and whether the cation is tetraradial or biradial (Table 9). Thus, the average interaction of one CH2 with all others is independent of the packing details and does not depend on whether the cation is tetraradial or biradial. This conclusion also supports our hypothesis that the conformation of the cation is due primarily to intramolecular factors, and that chain bending within the cation is dependent mostly on the length of the alkyl chains. Conclusions We have combined experimental and computational data to dissect the factors responsible for the molecular shapes and packing in crystals of ammonium and phosphonium halide salts with four equivalent n-alkyl chains. All of the salts pack in multilayered, noninterdigitated assemblies that place the ionic centers in a common plane at the center of a layer. Individual molecules within a layer adopt either biradial (chains paired intramolecularly) or tetraradial (chains unassociated intramolecularly) shapes.

9278 J. Phys. Chem. B, Vol. 103, No. 43, 1999 Perhaps fortuitously, the calculations are able to predict the approximate chain length at which the cations change from biradial to tetraradial shapes when entropic factors are included statistically. Near the critical chain lengths where the biradialtetraradial change occurs, the difference between the van der Waals volume allotted to an anion and the size of the “pocket” provided for it may determine which shape is adopted. If this approach is general, it will have significant predictive value, allowing interpolations and extrapolations from known structures to unknown ones. Moreover, the results suggest that salts of this sort may have interesting materials applications because they exhibit an alternation of charged planes separated by low dielectric layers at regular intervals (as in a microcapacitor). The distance between charged planes can be controlled by n-alkyl chain lengths and the distribution of charges within a plane can be modified by altering the anion and cation headgroups. Thus, crystals of these salts can be tailored to individual applications and can be oriented for the anisotropic transport of charges. Acknowledgment. R.G.W. gratefully acknowledges the National Science Foundation for its support of this research. J.P. thanks the National Science Foundation Center for Photoinduced Charge Transfer (CHE-9120001) for partial support of this research. Purchase of the X-ray diffractometer was made possible by grants from the National Science Foundation (CHE9115394) and Georgetown University. The authors are deeply indebted to Dr. Allan Robertson of Cytec for useful discussions and a generous gift of tri-alkylphosphines. Supporting Information Available: Complete crystallographic information for the seven ammonium and phosphonium salts, including tables of positional and equivalent displacment parameters, bond metricals, and anisotropic displacement parameters as well as an X-ray crystallographic file, in CIF format. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lee, Y. S.; Sujadi, D.; Rathman, J. F. Langmuir 1996, 12, 6202. (2) Kunitake, T.; Okahata, Y. J. Am. Chem. Soc. 1977, 99, 3860. (3) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans 2 1976, 272, 1525. (4) (a) Lu, L.; Sharma, N.; Nagana Gowda, G. A.; Khetrapal, C. L.; Weiss, R. G. Liquid Crystals 1997, 22, 23. (b) Lu, L. Ph.D. Thesis, Georgetown University, Washington, DC, 1997. (c) Abdallah, D. J.; Robertson, A.; Hsu, H.-F.; Weiss, R. G. J. Am. Chem. Soc., submitted. (5) (a) Lu, L.; Weiss, R. G. Chem. Commun. 1996, 2030. (b) Lu, L.; Weiss, R. G. Langmuir 1995, 11, 3630. (c) Abdallah, D. J.; Weiss, R. G. Chem. Mater., in press. (6) Crystal structures. (a) n-Decylammonium chloride: Pinto, A. V. A.; Vencato, I.; Gallardo, H. A.; Mascarenhas, Y. P. Mol. Cryst. Liq. Cryst. 1987, 149, 29. (b) n-Dodecylammonium chloride: Silver, J.; Marsh, P. J.; Frampton, C. S. Acta Crystallogr. 1995, C51, 2432. (c) n-Undecylammonium chloride monohydrate: Silver, J.; Martin, S.; Marsh, P. J.; Frampton, C. S. Acta Crystallogr. 1996, C52, 1261. (d) n-Dodecylammonium bromide: Lunden, B. M. Acta Crystallogr. 1974, B30, 1756. (e) Hexadecyltri-methylammonium bromide: Campanelli, A. R.; Scaramuzza, L. Acta Crystallogr. 1986, C42, 1380. (f) Dodecyl-tri-methylammonium bromide:

Abdallah et al. Kamitori, S.; Sumimoto, Y.; Vongbupnimit, K.; Noguchi, K.; Okuyama, K. Mol. Cryst. Liq. Cryst. 1997, 300, 31. (7) A search of the Cambridge Structural Database for tetra-alkyl salts with a group 5A cation, a halide anion, and two n-alkyl groups of eight or more carbon atoms, revealed few structures. (a) di-Methyl-di-tetradecylammonium bromide monohydrate: Okuyama, K.; Iijima, N.; Hirabayashi, K.; Kunitake, T.; Kusunoki, M. Bull. Chem. Soc. Jpn. 1988, 61, 2337. (b) di-Methyl-di-octadecylammonium bromide: Kajiyama, T.; Kumano, A.; Takayanagi, M.; Kunitake, T. Chem. Lett. 1984, 915. (c) di-Methyl-dioctadecylammonium bromide monohydrate: Okuyama, K.; Soboi, Y.; Iijima, N.; Hirabayashi, K.; Kunitake, T.; Kajiyama, T. Bull. Chem. Soc. Jpn. 1988, 61, 1485. (8) Honda, K.; Goto, M.; Kurahashi, M.; Miura, Y.; Nakamura, T.; Matsuyoshi, M.; Kawabata, Y. Anal. Sci. 1990, 6, 927. (9) Abbot, A. P.; Claxton, T. A.; Fawcett, J.; Harper, J. C. J. Chem. Soc., Faraday Trans. 1996, 92, 1747. (10) (a) Rao, S. R. Xanthates and Related Compounds; Marcel Dekker: New York, 1971. (b) A method based on that described in: Supulveda, L.; Cabrera, W.; Gamboa, C.; Meyer, M. J. Colloid Interface Sci. 1987, 117, 460. (11) Cytec Canada, Inc., Niagara Falls, Ontario, Canada. (12) Eriksen, S. P.; Tuck, L. D.; Oneto, J. F. J. Org. Chem. 1960, 25, 849. (13) CHEMX/CHEMLIB is a molecular modeling program distributed by the Oxford Molecular Group at http://www.oxmol.com/chemdesign/ products.html. (14) MACROMODEL molecular modeling program, version 5. Mohamadi, F.; Richards, N. G. J.; Guida, W. C.; Liskamp, R.; Lipton, M.; Caufield, C.; Chang, G.; Hendrickson, T.; Still, W. C. J. Comput. Chem. 1990, 11, 440. (15) Siemens Analytical X-ray Instruments, Madison, WI. (16) Chalii, V. P.; Krason, Y. P.; Makovetskii, Y. P. Zh. Strukt. Khim. 1975, 16, 917. (17) Wang, Q.; Habenschuss, A.; Xenopoulus, A.; Wunderlich, B. Mol. Cryst. Liq. Cryst. 1995, 264, 115. (18) Bondi, A. J. Phys. Chem. 1964, 68, 441. (19) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. (20) (a) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W. H. Freeman: San Francisco, 1960. (b) Joesten, M. D.; Schaad, L. J. Hydrogen Bonding; Marcel Dekker: New York, 1974. (21) Allinger, N. L. J. Am. Chem. Soc. 1977, 99, 8127. (22) Gasteiger, J.; Marsili, M. Tetrahedron 1980, 36, 3219. (23) Skorczyk, R. Acta Crystallogr. 1976, A3, 447. (24) (a) Perlstein, J. J. Am. Chem. Soc. 1992, 114, 1955. (b) Perlstein, J. J. Am. Chem. Soc. 1994, 116, 455. (c) Perlstein, J. J. Am. Chem. Soc. 1994, 116, 11420. (d) Perlstein, J.; Steppe, K.; Vaday, S.; Ndip, E. M. N. J. Am. Chem. Soc. 1996, 118, 8433. (25) Scaringe, R. P.; Perez, S. J. J. Phys. Chem. 1987, 91, 2394. (26) Scaringe, R. P. Electron Crystallography of Organic Molecules; Scaringe, R. P., Ed.; Kluwer: Dordrecht, 1990; p 85. (27) (a) Nyburg, S. C.; Gerson, A. R. Acta Crystallogr. 1992, B48, 103 and references therein. (b) Valiyaveettil, S.; Enkelmann, V.; Mullen, K. J. Chem. Soc., Chem. Commun. 1994, 2097. (c) Koh, L. L.; Xu, Y.; Gan, L. M.; Chew, C. H.; Lee, K. C. Acta Crystallogr. 1993, C49, 1032. (28) Zhang, Y.; Venable, R. M.; Pastor, R. W. J. Phys. Chem. 1996, 100, 2652, and references therein. (29) (a) Clark, E. S. Polymeric Materials; Relationship Between Structure and Mechanical BehaVior; American Society for Metals: Metal Parks, OH, 1975; p 12. (b) Wunderlich, B. Macromolecular Physics; Academic Press: New York, 1973; p 248. (30) Kay, H. F.; Newman, B. A. Acta Crystallogr. 1968, B24, 615. (31) This arrangement may also be responsible for chain “looping” near the fold-surface of a polyethylene microcrystallite. (32) Nyburg, S. C. Acta Crystallogr. 1996, C52, 192. (33) Xenopoulos, A.; Cheng, J.; Yasuniwa, M.; Wunderlich, B. Mol. Cryst. Liq. Cryst. 1992, 214, 63. (34) Small, D. M. Handbook of Lipid Research 4, The Physical Chemistry of Lipids Plenum: New York, 1986; p183. (35) Weidemann, G.; Brezesinski, G.; Vollhardt, D.; Mohwald, H. Langmuir 1998, 14, 6485.

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