Study of cetyltrialkylammonium bromide and tribromide salts in the solid phase R. Caminiti,*a M. Carbone,b G. Mancinic and C. Saduna aDept. of Chemistry, Istituto Nazionale di Fisica della Materia, University of Rome ‘L a Sapienza’, P.le A. Moro, 5 00185 Rome, Italy bDept. of Chemical Sciences and T echnologies, University ‘T or Vergata’, V ia della Ricerca Scientifica 1, 00133 Rome, Italy cCentro CNR di Studio sui Meccanismi di Reazione, C/O Dept.of Chemistry, University of Rome ‘L a Sapienza’, P.le A. Moro, 5 00185 Rome, Italy Some cetyltrialkylammonium tribromide salts have been studied in the solid phase using Raman spectroscopy and energy dispersive X-ray diraction. The first technique detected the presence of asymmetric tribromides while the latter technique revealed the degree of asymmetry of the tribromide units (dierence in the bond lengths between external and central Br atoms), by application of a subtraction method.In this method the spectra of the corresponding monobromide salts were recorded and dierences between tribromide and monobromide curves isolated the tribromide contributions. The existence of asymmetrical and symmetrical tribromide ions has been established and the degree of asymmetry was then correlated to the steric hindrance and electronegativity of the ammonium substituents. The values of the BrMBr bond distances have been deduced; the most asymmetric tribromide has BrMBr distances at 2.38 and 2.66 A° , while the symmetric tribromide has both BrMBr bond lengths equal to 2.52 A° .A linear geometry is confirmed for the tribromide ions.The study of polyhalides is important from an applied and a Cetyltrymethylammonium bromide (CTAB) is one of the most studied cationic surfactants, used in many fields such as fundamental point of view. Polyhalides are typically used in oxidation reactions of organic molecules as well as for doping micellar catalysis, medicine and detergency.This surfactant forms micellar aggregates in aqueous solution which are polymers in order to change their conductivity.1 The number of atoms in a polyhalogen unit can be quite responsible for its useful physico-chemical properties. The corresponding tribromide salt as well as the others we investi- large and depends on the nature of the halogen atom as well as on the hosting structure.Units containing up to seven2 and gated are surfactants used in bromination reactions.11–15 The studied quaternary ammonium salts all contain the eight3 atoms have been found for polyiodides and up to ten for polybromides.4 cetyl (C16H33) fragment but dier in the head group so that we investigated the eect of head group variation on the In all cases a better knowledge of the polybromides structure helps in the understanding of their properties and possible polyhalogen asymmetry. Study of the polyhalogen asymmetry is usually carried out applications.As far as tribromides are concerned, structural studies showed that the tribromide ions usually have linear by a combination of Raman spectroscopy and X-ray diraction. The stretching vibration frequencies are correlated to the geometry which can be either symmetric, with the same bond lengths between the external and the central Br atoms number of atoms in the polyhalogen unit as well as to their structure.For a tribromide the Raman spectra can be used to [Br(1)MBr(2)=Br(2)MBr(3)], such as in [(CH3)3NH+]2 Br-Br3-5 or asymmetric with dierent lengths between the deduce whether the ion is symmetric or asymmetric.The X-ray diraction technique can give information about external and the central Br atoms [Br(1)MBr(2) Br(2)MBr(3)], such as in CsBr3,6 with a dierence of 0.33 A° ). the structure, thus, in the present case, an exact evaluation of the degree of asymmetry can be achieved. The degree of asymmetry is even higher in PBr7 where the Br3- units show bond lengths of 2.39 and 2.91 A° .7 The deg- In the present study we employed both techniques.In particular the degree of asymmetry of the tribromide was ree of asymmetry (the dierence between the BrMBr bond lengths) is not constant in dierent tribromides, but depends determined by large angle X-ray scattering (LAXS). LAXS is a powerful technique for determining structural parameters of on the counter ion and on the host structure. Several reports, based on Raman spectroscopic evidence, non-crystalline systems, since it can provide information about the short-range order.11–26,30 In particular energy dispersive support the existence of tribromide and pentabromide anions.4,8–10 X-ray diraction (EDXD)18–20,22,25,26,29–31 has been found to be a suitable tool in the investigation of such systems due to Studies on cetyltrimethylammonim tribromide (CTAB3) are reported in ref. 4. The authors indicate the presence, in this its high speed and reliability compared to traditional angular scanning X-ray diraction (ADXD). molecule, of two Raman bands at 153 and 205 cm-1, which they attribute to BrMBr stretching vibrations, concluding that For this purpose, X-ray diraction patterns of the corresponding monobromide samples were collected, and dierences the Br(1)MBr(2) and Br(2)MBr(3) distances are dierent.No structural studies on powdered CTA bromide or tribromide evaluated between the curves relative to the tribromide salts and the corresponding monobromide ones. In this way the have been reported. We investigate here the degree of asymmetry of the tribrom- contribution of the tribromide unit was isolated and analysed.ide ion in CTAB3 (sample G1) and in three other quaternary ammonium salts; cetylquinuclidinium tribromide (CQB3, sample G3), cetyltripropylammonium tribromide (CTPAB3, Experimental sample G5) and cetyldimethyl(2-hydroxyethyl)ammonium Materials tribromide (CDEAB3, sample G7). The CTAB (sample G2), which is commercially available (Fluka) was purified by crystallization from absolute alcohol * Email: r.caminiti@caspur.it J.Mater. Chem., 1997, 7(8), 1331–1337 1331and diethyl ether. The cetylquinuclidinium bromide (sample fall outside of the used energy range. The stability of the voltage power supply for X-rays is better than 0.1%. G4) was prepared as reported in ref. 27. The cetyltripropylammonium bromide and the cetyldimethyl(2-hydroxylethyl)am- The X-ray tube operates at 50 kV and 40 mA for recording the spectra of all samples while conditions of 45 keV and monium bromide (samples G6 and G8) were prepared by standard quaternization procedures. 35 mA were also used for sample G5. X-Ray detection was accomplished using an EG&G liquid- The corresponding yellow–orange tribromides were prepared through reaction of the monobromides with Br2, as nitrogen cooled ultrapure Ge SSD (ORTEC, model 92X), connected to a PC 286 via ADCAM hardware and Maestro reported in ref. 31.Elemental analyses for H, C, N and Br were performed by II software, which performs the necessary analogue-to-digital conversions and amplifications.In order to obtain the relation the Microanalytical Service of the Area della Ricerca di Roma of the CNR, giving results in agreement with the formulae of between the photon energy vs. the channel number in the multichannel analyser (MCA), the absorption edge of Ba, In, the materials (Table 1). Pd and Eu were used. The linear relationship between the photon energy and the channel number was good.Techniques Raman spectra. FT Raman spectra (resolution ±4 cm-1) of Measurements the tribromide salts were recorded on a FRA 106 FT-Raman accessory, mounted on a Bruker IFS 66 FT-IR vacuum The transmission geometry has been employed, since it allows an easier correction for the sample absorption.29,30 For instrument, operating with an exciting frequency of 1064 nm (Nd5YAG laser) and with a germanium diode detector cooled measurement of the incident beam spectrum, which is necessary in the energy dispersive method, we used the tube at 50 or at liquid N2 temperature.Power levels of the laser source varied between 20 and 100 mW. The solid samples were packed 45 kV and 2 mA, while the dimension of the slits was 100 mm×160 mm. into a suitable cell and then fitted into the compartment designed to use 180° scattering geometry.No sample decompo- Fig. 1 shows the primary beam spectrum as well as transmission spectra of samples G7 and G8 measured under the sition was observed during the experiments. same conditions. Each of the three measurements was recorded over 15000 s. In order to exclude the fluorescence lines from X-Ray diraction patterns.The dispersive X-ray diraction experiments were carried out by employing a non-commercial the spectrum as well as regions where the intensity is strongly absorbed by the sample, the range used in the data analysis X-ray energy scanning diractometer,25,26,30 with a solid-state detector, constructed at the Department of Chemistry, was restricted between channel 330 and 680, which corresponds to an energy range of 22.123–45.735 keV.In order to cover a University of Rome ‘La Sapienza’, Powder Diraction Laboratory.29 In the design of the machine the goniometer is suciently wide region of s-space (s=1.014Esin h) the diracted X-ray photons from samples were collected at dierent scat- substituted by two rotating arms, which can be moved independently by a motor in the range of 2h -5° to 120°, by a tering angles.Measurement angles and used energy ranges are listed in Table 2. program from our group written in BASIC. The X-ray optical path is defined by four Huber 20 mm variable slits mounted The scattering intensity was obtained over the range s= 0.2–16.2 A° -1. The sample (a pellet of thickness 2 mm) was on the arms.The distance between the X-ray source and the sample, equal to that between the sample and the detector is 20 cm. The geometry of our machine is such that both reflection and transmission intensities can be measured. Advantages of the energy dispersive method over the conventional angle dispersive method have been described18–20,22,29–31 and the applicability of the method has already been widely described in the literature.18–20,29,31–33 A more detailed description of the apparatus has been given in ref. 29 and 30.The X-ray source is a Seifert tube (3 kW) with a tungsten target, which provides X-radiation in the energy range 0–60 keV. The W L lines in the energy range 8–11 keV and the fluorescent X-rays from Br (11.91, 13.29 keV) Table 1 Elemental analysisa Fig. 1 Energy spectra of the primary beam (a), the transmittance of the G8 (b) and the G7 (c) samples vs. channel number in the MCA sample C(%) N(%) H(%) Br(%) G1 (CTAB3) 42.70 2.8 8.23 42.97 Table 2 Scattering parameters associated with the minimum (43.53) (2.67) (8.07) (45.72) (22.123 eV) and maximum values (45.735 eV) of the energy for each G2 (CTAB) 61.57 3.99 12.02 24.07 measurement angle (62.62) (3.84) (11.62) (21.92) G3 (CQB3) 47.81 2.50 7.55 42.14 h/degrees smin smax (47.93) (2.43) (8.04) (41.59) G4 (CQB) 65.61 3.21 10.07 21.11 21 8.04 16.62 (66.32) (3.36) (11.13) (19.18) 15.5 5.99 12.39 G5 (CTPAB3) 50.31 2.12 8.55 38.69 10.5 4.08 8.45 (49.35) (2.3) (8.95) (39.40) 8.0 3.12 6.45 G6 (CTPAB) 66.01 3.21 11.97 18.81 5.0 1.96 4.04 (66.93) (3.12) (12.13) (17.81) 3.5 1.37 2.83 G7 (CDEAB3) 43.01 2.48 7.88 43.45 3.0 1.17 2.43 (43.34) (2.53) (8.00) (43.25) 2.0 0.78 1.62 G8 (CDEAB) 61.02 3.49 11.09 20.39 1.5 0.59 1.21 (60.9) (3.55) (11.24) (20.26) 1.0 0.39 0.80 0.5 0.19 0.41 aCalculated values in parentheses. 1332 J. Mater. Chem., 1997, 7(8), 1331–1337placed at the centre of the goniometer. The measuring time at with each angle was set so as to obtain a minimum of 50000 counts per channel.I(E,h)=ICoh(E,h)+ E¾I0(E¾)P(E¾, h)AInc(E, E¾, h)IInc(E¾, h) EI0(E)P(E, h)ACoh(E, h) Fig. 2 shows, as an example, the diracted intensity vs. the channel number at h=21, for sample G7. The intensity is (2) plotted in logarithmic scale in order to show also the Br fluorescence lines. Restricting the channel ranges means that where h is the scattering angle, E is the photon energy revealed the Br fluorescence lines can be excluded from the data by the detector and E¾ is the initial energy of a photon analysis range.inelastically scattered at the observed energy E; K is the scale Fig. 3 shows diracted intensities for sample G7 at dierent factor between the intensity reaching the detector and the collection angles.The transmittance of the sample at h=0° intensity scattered by a stoichiometric unit of sample; I0(E) is has been made under the same experimental conditions as that the energy spectrum of the primary beam measured at h=0°; for the primary beam. The two measurements are necessary to P(E,h) is the polarization factor by a scattering of primary determine the sample linear absorption coecient, which varies radiation with polarization P(E), ICoh(E,h) is the total scattered to a significant extent with the X-ray energy.From the relation elastic intensity and to which three dierent terms contribute [eqn. (3)], where Sicifi2(s) is the self-scattering intensity; ci is It(E)/I0(E)=exp[-m(E)t] the concentration of the dierent species, i1(s) and i2(s) are the we obtain the experimental values of exp[-m(E)t] used in intensities of interfering waves scattered by atom pairs belong- eqn.(1) and (2) for absorption corrections.30,31 ing to the same and dierent domains respectively; s is the The total intensity I¾ scattered by a sample and observed by scattering parameter and is defined by eqn. (4), the energy dispersive detector in approximation of single ICoh(E,h)=.n cifi2(s)+i1(s)+i2(s) (3) scattering and transmission geometry25,30,31 can be expressed as: s=4p sin h/l=1.014 E sin h (4) I¾(E,h)=KI0(E)P(E,h)ACoh(E,h)I(E,h) (1) where E is expressed in keV and s in A° -1. A more detailed description of the terms reported in eqn. (1) and (2) has been given in ref. 25 and 29–31. Data treatment After correction of the collected experimental data, for escape peak suppression, the intensity data were handled, as described by Nishikawa and Iijima,31 by means of our DIF1 program written in Fortran IV.Normalization to a stoichiometric unit of volume containing one Br atom was performed. Radial distribution functions D(r), were calculated from the static structure functions i(s) [eqn.(5)], according to eqn. (6). i(s)=Icoh(E,h)-.icifi2(s) (5) D(r)=4pr2r0+2rp-1Psmax 0 s.i(s).M(s) sin(rs) ds (6) Fig. 2 EDXD profile of the sample G7 obtained at h=21 In this equation r0=[Sinifi(0)]2V -1, where V is the stoichiometric unit of volume chosen, ni=number of atoms i per unit volume, and fi the scattering factor per atom i. The sharpening factor is given by eqn.(7). M(s)={fBr2(0)/fBr2(s)} exp(-0.005 s2) (7) In order to determine the BrMBr bond lengths in the tribromomide ions, the peaks referring to the BrMBr interactions were isolated by a subtraction method.34–36 By subtracting the radial distribution function of the monobromide ammonium salts from the corresponding tribromide salts, we have obtained a dierence radial distribution function that contains information selectively about the Br first neighbours in the range 0–3 A° .The resulting dierence curves are indicated as D(r)Br3 -D(r)Br. The subtraction operation is valid only if the organic structure is unaltered by the presence of two extra bromine atoms, so that its contribution to the radial distribution function remains the same both in the mono- and in the tribromide salts.The absence, in the dierence curve, of extra oscillations with respect to the original curves or of negative peaks indicates the correctness of this hypothesis. Theoretical peaks were calculated, by a corresponding Fourier transformation of the theoretical intensities for pairs of interactions between atoms p and q (Debye functions) Fig. 3 EDXD profiles of the sample G7 obtained at dierent collection angles [eqn.(8)], using the same sharpening factor and the same smax J. Mater. Chem., 1997, 7(8), 1331–1337 1333value as for the experimental data and assuming the root mean in (C6H5)4As+Br3-,39 and the resulting constant force is lowered significantly from that in Br2. square deviation to be spq. The Raman spectrum of Br2 in benzene,8 yields a funda- ipq(s)=.fpfq sin(rpqs) (rpqs)-1 exp(-0.5 s2pqs2) (8) mental stretching frequency at 306 cm-1, while, for symmetrical Br3- units (with equal BrMBr distances) such as in (n- Since our goal was the determination of BrMBr distances within the tribromide ion we considered the Debye functions C4H9)N+Br3-, the actual BrMBr stretching frequency is 179 cm-1, and is the average of symmetrically and antisym- only for BrMBr first neighbour interactions.metrically coupled normal modes.39 The vibration frequencies dier for asymmetrical Br3- units Data analysis and the two stretching modes are observed separately. In CsBr3,6 which shows two dierent BrMBr bond distances of Raman spectra 2.44 and 2.77 A° , the vibration frequencies were observed at The Raman spectra of the studied samples are shown in Fig. 4 140 and 208 cm-1. In a more asymmetrical tribromide such and observed vibration frequencies of the Raman-active bands as PBr4+Br3-,7 where the BrMBr bond lengths are in the are collected in Table 3, where also a comparison to literature range 2.39–2.91 A° , the stretching frequencies are observed40 at data for symmetric tribromides is made.Large shifts in both 249–135 cm-1. the wavenumber and intensity of the bands are observed and Apparently, the splitting between the vibration frequencies appear to correlate with the degree of asymmetry in the two can be correlated to the degree of asymmetry and can be BrMBr bond lengths. considered a qualitative method to deduce whether the tribro- The vibrational spectra of tribromides can be viewed in mide ion is symmetrical or asymmetrical.terms of interaction of Br2 with Br- with the latter acting as Within this simple picture, samples G1 and G7, with stretch- a Lewis base and Br2 as a Lewis acid. Upon complexation of ing vibration frequencies of 153, 205 and 140, 223 cm-1, Br2 with Br-, BrMBr antibonding molecular orbitals are respectively, contain asymmetrical Br3- units, with most prob- populated and the BrMBr bonds weakened; hence the BrMBr ably, a higher degree of asymmetry in sample G7 (larger distance is increased from 2.3 A° in Br2 37 to 2.53 A° , for instance, splitting between the vibration frequencies).Sample G3 might contain a symmetrical Br3- unit since a coupled vibration frequency is observed at 163 cm-1.This value is somewhat dierent from the value of 179 cm-1 reported for (n-C4H9)N+Br3-,39 but this may simply depend on the BrMBr bond distance, which we have calculated for the sample G3, but is not reported for (n-C4H9)N+Br3-. Sample G5 shows somewhat unusual behaviour in that the two vibration frequencies of 168 and 226 cm-1 do not follow the usual trend, of a lower stretching frequency for the n1 vibration and higher frequency for n3 compared to the average frequency for symmetric units. Furthermore at increasing asymmetry the intensity of the peak corresponding to the n3 vibration increases as does as the intensity ratio n3/n1 (see ref. 4 Fig. 1). For sample G7, instead, the n1 vibration is more intense. To be sure that both the peaks refer to the sample and not, for instance, to a symmetric Br3- (with a stretching vibration at 168 cm-1), with an impurity yielding a peak at higher vibration frequency a newly prepared sample was analysed, which yielded a similar Raman spectrum and diraction pattern.In this case we consider the Raman spectrum is not sucient to discriminate whether the tribromide unit is asymmetrical. The splitting of the frequencies for sample G7 hints rather at resonance eects, which can occur when two vibrations have closely spaced frequencies X-Ray diraction patterns Observed structure functions, in the form s.i (s).M(s) are shown in Fig. 5 for samples G1 and G2. Below 3 A° -1 the curves for the monobromide and tribromide salts show similar structures, Fig. 4 Raman spectra of the tribromide salts.Sample G3 shows one peak only and hints at symmetric tribromide. The samples G1, G5 and G7 show two peaks, which suggest the presence of asymmetric tribromides, though the splitting and the intensity ratios are not the same Table 3 Stretching frequencies of the tribromide Raman-active vibrations sample cation n1/cm-1 n3/cm-1 ref. G1 CTA 153 205 CTA 153 205 4 G3 CQ 163 G5 CTPA 168 226 Fig. 5 Observed structure function s.i (s).M(s) of samples G2 and G1. G7 CDEA 140 223 The structure function of the tribromide sample shows much wider Cs+ 138 213 8 oscillations compared to the corresponding monobromide one. 1334 J. Mater. Chem., 1997, 7(8), 1331–1337whereas much wider oscillations are seen for the tribomide in the contribution of a higher scattering species, namely a BrMBr interaction.The literature data related to BrMBr distances in the range 3–15 A° -1, hinting at the presence of extra interactions not present in the monobromides, most probably due tribomide ions, both for symmetric and asymmetric units are in agreement with such a value, therefore we consider that the to the first neighbour BrMBr interactions. No significant dierence in the reduced intensities is displayed by dierent comparison of this peak to theoretical peak shapes yields the BrMBr bond distances.samples. The radial distribution functions in the Di form [Di(r)= Another typical feature in the tribromides RDF is a peak at ca. 5.1 A° , which we ascribe to Br(1),Br(3) interactions, since D(r)-4pr2r0] are shown in Fig. 6 for the monobromides in and Fig. 7 for the tribromides in the range 0–15 A° . However it is rather intense and has no correspondence in the monobromide curves. The position of this peak confirms that the anion we only analysed the region 0–3 A° where we can distinguish the first-neighbour BrMBr interactions. The distribution func- Br3- has a linear geometry, as suggested in a previous EXAFS study.1 tions of the monobromide samples show two peaks at ca. 1.5 and 2.5 A° arising from C,C, and NMC bond distances (ca. Fig. 8(a) shows the radial distribution function of samples G1 and G2 in the range 0–5.5 A° , compared to the theoretical 1.5 A° ), and the non-directly bound C,C and NMC distances, with sp3 hybridization (ca. 2.5 A° ) calculated for the organic peak shapes calculated for the monobromide.In this calculation we have considered interactions of the type CMX, with substituents of the ammonium cation. In the radial distribution functions of the tribromides, the X=C, N, O between all the first and second neighbours. Particularly for the C,C interaction we considered the CTAB area of the peak at ca. 2.5 A° is much larger, compared to the corresponding monobromide curves, therefore cannot be structure determined by Campanelli and Scaramuzza41 in which the CMX distances used in the calculations are reported. simply ascribed to the C,C interaction, but it must contain The experimental curve is well reproduced by the calculated peak shapes in the 0–2.7 A° range, indicating that the considered interactions are sucient to simulate the RDF of the monobromide ammonium salts.Therefore the subtraction of the monobromide RDF from the corresponding tribromide RDF simply yields the peaks related to BrMBr interactions. If any change in the cation geometry occurs when coordinated to a tribromide, instead of a monobromide ion or if some interactions are present in the monobromide ion, which disappear for the tribromide, this would result, either in a residual peak and/or in a negative peak.The peak at ca. 1.5 A° can be used to determine if the subtraction operation is valid. As shown in Fig. 8(b) for sample G1 the dierence radial distribution function is fairly flat in the range 0–2 A° and the dierence radial distribution functions for the other samples show the same characteristics.We can, therefore, consider that the subtraction of the monobromide Fig. 6 Radial distribution functions of the form D(r)-4pr2r0 of the monobromide samples Fig. 8 (a) Dotted line, D(r) of the sample G2 in the range 0–5.5 A° ; continuous line, D(r) of the sample G1; dashed line, theoretical peak shape calculated by introducing in the Debye formula the first- and second-neighbour interactions of the ammonium cation, which is common to both samples.(b) Dierence curve between the radial distribution functions of sample G1 and G2 [D(r)G1-D(r)G2]. The dierence curve in the range 0–2 A° is rather flat, therefore all Fig. 7 Radial distribution functions of the form D(r)-4pr2r0 of the contributions of the ammonium cation have been removed by subtraction. tribromides samples J.Mater. Chem., 1997, 7(8), 1331–1337 1335curve isolates only the BrMBr contribution to the tribromide RDF and that the calculation of theoretical peak shapes, which reproduces the peak at ca. 2.5 A° in the dierence curve should give exact BrMBr distances. This operation was performed for all the samples and the corresponding BrMBr distances are reported in Table 4, together with mean square root used in the calculation.In Fig. 9 the experimental and theoretical dierence curves have been reported for all the sample pairs, and show good agreement. The presence of a peak at ca. 5.1 A° in the tribromide RDF allows us to infer the Br3- geometry.1 This peak, which we consider correlated to a Br,Br interaction, corresponds to the sum of the BrMBr distances in both Fig. 10 Comparison between observed (solid lines) and calculated (dotted lines) structure functions s.i(s).M(s) for G1 and G3 symmetric and asymmetric units. The only possible geometry that would yield peaks at a distance twice the fundamental one, is linear.1 The theoretical s.i(s).M(s) curves have been calculated by introducing only interactions related to the Br3- unit, in the Debye formula [eqn.(8)], using the parameters For instance the dierence between the Br(1)MBr(2) and Br(2)MBr(3) distances is 0.00 A° in sample G3 and 0.28 A° in reported in Table 4. From this we can see how, despite the similarities between the samples (they are all ammonium salts sample G7. Furthermore the latter value is comparable to the situation where Cs+ is the counter ion.with a long aliphatic chain) symmetric (G3), slightly asymmetric (G5), and highly asymmetric (G1, G7) structures are all The degree of asymmetry in the tribromide ions is correlated to their geometry with respect to the cation as well as to its observed. Finally, theoretical curves are compared to experimental electronegativity. The asymmetry can be viewed as a preferential interaction of one of the external Br atoms with the cation, ones in Fig. 10 for samples G1 and G3. It is of interest how the use of these interactions is already which can induce a polarization of the BrMBr bonds. The eect of the electronegativity is quite evident for instance in sucient to reproduce the oscillations of the structure functions in the entire range, that is, by contrast quite flat for the CsBr3,6 where an electropositive atom such as Cs+, causes a high degree of asymmetry.corresponding monobromides, clearly indicating that the Br3- ion gives the main contribution to the scattered intensity. In quaternary ammonium salts with hydrocarbon chains, electronegativity is not particularly important for the BrMBr bond polarization and the asymmetry is simply connected to Results and Discussion the orientation of the linear Br3- unit relative to the nitrogen atom.A completely symmetrical surrounding of the nitrogen In the previous sections we showed that the studied samples display dierent degrees of asymmetry of the tribromide ion. atom leads to a symmetric tribromide such as in (n- C4H9)N+Br3-.39 Table 4 BrMBr bond lengths in Br3- and the corresponding root The investigated samples all contain a long aliphatic chain mean square deviation.s1, s2 and s3 refer to the Br(1)MBr(2), (the cetyl fragment), which introduces an element of asymmetry Br(2)MBr(3) and Br(1)MBr(3) bond lengths respectively around the nitrogen atom, since the other substituents around the nitrogen are small. In the cetyltrimethylammonium salt sample Br(1)MBr(2) Br(2)MBr(3) Br(1)MBr(3) s1=s2 s3 this degree of asymmetry is high and, correspondingly, the /A° /A° /A° tribromide asymmetry is high.In the cetyltri-n-propyl G1 2.45 2.66 5.11 0.08 0.15 ammonium salt the length of the substituents is already G3 2.52 2.52 5.04 0.11 0.15 sucient to prevent a preferential interaction of one Br end G5 2.48 2.55 5.03 0.10 0.14 with the nitrogen and this tribromide is almost symmetric.G7 2.38 2.66 5.04 0.08 0.15 The behaviour of the cetylquinuclidinium ammonium salt is interesting, in that despite its apparent geometrical asymmetry a symmetric tribromide is observed. Most probably the aliphatic chains are forced into a ring geometry, leaving space for the nitrogen atom to adopt a similar interaction with both external Br atoms.The high degree of asymmetry for the cetyldimethyl(2- hydroxyethyl)ammonium is not surprising, since the geometrical asymmetry around the nitrogen atom is accompanied by the presence of an electronegative element, oxygen, which can polarize the BrMBr bonds. It is interesting to note the good correspondence between the Raman spectra and the X-ray diraction patterns, with a single stretching vibration frequency for symmetric tribromides and split vibration frequencies for highly asymmetric tribromide units.The slight asymmetry in sample G5 might have induced, as previously suggested, a Fermi resonance. In summary we studied the dependence of the degree of asymmetry of tribromides on the substituents of ammonium salts, by Raman spectroscopy and large angle scattering.The results show that the electronegativity of the ammonium substituent can polarize one Br external atom, yielding BrMBr bonds of dierent length. However even with non-polar substituents it is possible to obtain asymmetric tribromides, depending on the steric hin- Fig. 9 Dierence radial distribution curves: solid lines, experimental curves; dotted lines, theoretical curves drance and on the possibility for the tribromide units to 1336 J.Mater. Chem., 1997, 7(8), 1331–1337G. Paschina, G. Piccaluga and G. Pinna, J. Mater. Sci. L ett., 1988, interact with the nitrogen atom either asymmetrically or 7, 407. symmetrically. In all cases the tribromide geometry was linear. 18 T. Egami, J.Mater.Sci., 1978, 13, 2587. 19 T. Egami, in Glassy metals I, ed. H. J. Gunterodt and H. Beck, Springer Verlag, Berlin, 1981, p. 25. 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