14N and 81Br Quadrupolar Nuclei as Sensitive NMR Probes of n

Aug 12, 2009 - 81Br NMR spectra were recorded at ν0(81Br) ≡ 202.60 MHz (B0 = 17.6 T) ... The chemical shift reference used here (δiso = 0 ppm) is ...
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N and 81Br Quadrupolar Nuclei as Sensitive NMR Probes of n-Alkyltrimethylammonium Bromide Crystal Structures. An Experimental and Theoretical Study Bruno Alonso,*,† Dominique Massiot,‡ Pierre Florian,‡ Henrich H. Paradies,§ Philippe Gaveau,† and Tzonka Mineva† Institut Charles Gerhardt de Montpellier, Mate´riaux AVance´s pour la Catalyse et la Sante´, ICGM-MACS, UMR 5253 CNRS-ENSCM-UM2-UM1, 8 rue de l’Ecole Normale, 34296 Montpellier cedex 5, France, CEMHTI, CNRS UPR3079 UniVersite´ d’Orle´ans, 1D aV. de la Recherche Scientifique, 45071 Orle´ans cedex 2, France, and The UniVersity of Salford, Joule Physics Laboratory, School of Computing, Science and Engineering, Materials Research Institute, Manchester, M 5 4WT, United Kingdom ReceiVed: March 27, 2009; ReVised Manuscript ReceiVed: July 6, 2009

This is the first time a comprehensive study has been carried out on n-alkyltrimethylammonium bromide salts using 14N and 81Br solid state NMR, X-ray diffraction, and theoretical calculations. The investigation represents a necessary step toward further 14N and 81Br NMR characterization of the environment of cationic and anionic groups in materials, accounting for the amphiphilic properties of cationic surfactants. The NMR spectra of five CxH2x+1(CH3)3N+Br- polycrystalline samples with different n-alkyl chain lengths (x ) 1, 12, 14, 16, 18) were recorded and modeled. The 14N and 81Br quadrupolar coupling interaction parameters (CQ, ηQ) were also estimated from spectrum modeling and from computer simulation. The obtained results were discussed in depth making use of the experimental and reoptimized crystal structures. In the study, both 14N and 81Br nuclei were found to be sensitive probes for small structural variations. The parameters which influence the NMR properties the most are mobility, deviation of C-N-C bond angles from Td angles, and variations in r(N-Br) distances. I. Introduction Because of their amphiphilic properties, quaternary ammonium salts are widely used in various applications, e.g., detergents, textile softeners, household materials, liquid crystals, and processed materials.1,2 In particular, the single chained n-alkyltrimethylammonium bromide salts have been successfully employed in the synthesis of new inorganic mesoporous materials using cationic surfactants as templates.3,4 Since this seminal discovery, continuous efforts have been devoted toward the combination of soft chemistry and self-assembly processes, and by stimulating considerable interest in their structure, properties, formation mechanisms, and possible applications.5-9 The significance of the interactions between the positively charged headgroup, the counteranions, and other chemical and/ or physical entities, e.g., size and shape polydispersity of the apparent aggregates, nature of the solvent, and effective colloidal charges, have also been highlighted.10-20 NMR of 14N, 79/81Br, or 35/37Cl nuclei would be a very informative technique for the characterization of both physical and chemical properties of these cationic amphiphiles in various dispersions. Nevertheless, very little is known with respect to the relation to the 14N NMR analysis of the formation of the materials21-23 and even less for the final materials.24 Similarly, nothing has been reported to our knowledge on the characterization of the counteranions Br- or Cl- using either 79/81Br or 35/37Cl NMR, probably because of the large quadrupolar coupling constants, CQ. We believe that additional information can be gained by studying these and other materials through solid state NMR methods involving these * Corresponding author. E-mail: [email protected]. Phone: +33 (0)4 67 16 34 68. Fax: +33 (0)4 67 16 34 70. † Institut Charles Gerhardt de Montpellier. ‡ Universite´ d’Orle´ans. § The University of Salford.

abundant quadrupolar nuclei: 14N (I ) 1, 99.6% abund.) on one side which is representative of the polar head groups and 79/81 Br (I ) 3/2, 50.7/49.3% abund.) or 35Cl (I ) 3/2, 75.8% abund.) on the other side, which are representatives of the counteranions. Therefore, our first experimental approach was to study the widely used polycrystalline n-alkyltrimethylammonium bromide salts CxH2x+1(CH3)3N+Br- by means of 14N and 81Br solid state NMR which may exploit new ways for possible studies of more complex materials in the near future. The characterization of solid samples by 14N MAS NMR has been attracting more attention over the last 10 years,25-27 facing the problem of large quadrupolar coupling constants CQ encountered in the case of amino acid functionality. In parallel, different indirect methods (spin-1/2 detection) have been developed to estimate 14N NMR interactions,28-31 and very recently to record 14N spectra.32-34 Overtone 14N NMR spectroscopy,35-37 aiming at acquiring directly double quantum spectra, has been investigated by several groups over the last years but has not succeeded yet in reaching the required efficiency for their use in real material science applications. For tetraalkylammonium salts, only a few recent studies have been devoted to their detailed solid state 14N MAS NMR characterization38,39 which complete earlier data obtained under static conditions.40,41 In the case of n-alkyltrimethylammonium bromide salts CxH2x+1(CH3)3N+Br- with long chains (x > 4), 14N static NMR studies have been published earlier for the hexadecyltrimethylammonium bromide (HTAB, x ) 16) and decyltrimethylammonium bromide (x ) 10).41 In all of these studies, the values of the quadrupolar coupling constant CQ were always small and significantly below 1 MHz, allowing a direct observation of 14N using standard solid state NMR spectrometers. In the case of solid state NMR characterization involving 79/81 Br nuclei, there have been only a few reports published and

10.1021/jp9027904 CCC: $40.75  2009 American Chemical Society Published on Web 08/12/2009

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recently reviewed,42 but virtually no information exists on n-alkyltrimethylammonium bromide salts. Nowadays, the access to high magnetic fields opens new opportunities for the characterization of materials by solid state NMR of half integer quadrupolar nuclei.43 It thus becomes possible to characterize and explore the Br environment by solid state NMR. The purpose of this contribution is to analyze by means of 14 N and 81Br MAS NMR n-alkyltrimethylammonium bromide salts CxH2x+1(CH3)3N+Br- with different alkyl chain lengths (x ) 1, 12, 14, 16, 18) and to evaluate the effect of their structure on the NMR parameters using crystallographic data and theoretical calculations. Particular attention has been devoted to nuclear quadrupolar coupling interaction parameters which are highly informative and accessible from experiments and calculations. II. Experimental Methods II.1. Chemical Compounds. Tetramethylammonium bromide (TMAB, pur. g 99%), dodecyltrimethylammonium bromide (DTAB, pur. g 98%), tetradecyltrimethylammonium bromide (TTAB, pur. g 99%), hexadecyltrimethylammonium bromide (HTAB, pur. g 99%), and octadecyltrimethylammonium bromide (OTAB, pur. g 97%) were used as purchased from Sigma-Aldrich. Before and after use, a single crystal phase is observed by XRD for these powdered compounds. II.2. Solid State NMR. II.2.1. Spectrum Acquisition. 14N NMR spectra were recorded at a Larmor frequency ν0(14N) ≡ 28.91 MHz (B0 ) 9.4 T) on a widebore Varian Infinity 400 spectrometer using 7.5 mm zirconia rotors spun at νMAS ranging from 1 to 6 kHz. A single pulse sequence was employed with 18° pulse angles (RF field strength ∼25 kHz), 0.5 s of recycling delays, and 1H decoupling during acquisition (RF field strength ∼50 kHz). For some compounds, 14N NMR experiments were also performed at ν0(14N) ≡ 54.19 MHz (B0 ) 17.6 T) on a widebore Bruker Avance 750 spectrometer using 4 mm rotors. 14 N chemical shifts are here referenced using δiso ) 0 ppm for the signal of solid NH4Cl spun at νMAS ) 5 kHz, which is at -342.4 ppm relative to nitromethane.26 14N spectra can be very sensitive to slightly off-MAS conditions.26,44 The magic angle was set here so as to anneal the splitting of the TMAB center band, as done with other compounds in previous works.26 It has been verified that the spinning sideband pattern (SSB) does not change when varying the irradiation frequency (50 kHz offset). Complementary 15N{1H} cross-polarization (CP) MAS spectra were recorded at a Larmor frequency ν0(15N) ≡ 40.55 MHz (B0 ) 9.4 T) on a widebore Varian Infinity spectrometer using 7.5 mm zirconia rotors spun at νMAS ranging from 0.5 to 1 kHz. The conditions used are similar to those published earlier.38 The CP contact time was fixed to 3 ms, and RF field strengths were about 50 kHz. 81 Br NMR spectra were recorded at ν0(81Br) ≡ 202.60 MHz (B0 ) 17.6 T) on a widebore Bruker Avance 750 spectrometer using 2.5 and 4 mm zirconia rotors spun at νMAS ranging from 0 to 30 kHz. 81Br spectra were recorded using Hahn echoes: 45° pulse-τ-90° pulse-τ-Acq., where τ is synchronized with MAS and in the range (33-143) × 10-6 s, and using a RF field strength of 50 kHz. For the highest νMAS, it has been verified that the single pulse spectra yielded the same results. 1 H decoupling during acquisition (RF field strength ∼50 kHz) was only used for static conditions. We have also verified that no 1H decoupling during acquisition was necessary during the acquisition of 81Br signals for the rotating samples (νMAS g 7 kHz). Here, νMAS are almost 3 times the values of the more intense

Figure 1. 14N spectra (B0 ) 9.4 T; ν0 ≡ 28.91 MHz) of nalkyltrimethylammonium bromide salts CxH2x+1(CH3)3N+Br-: (a) TMAB (x ) 1), (b) DTAB (x ) 12), (c) TTAB (x ) 14), (d) HTAB (x ) 16), (e) OTAB (x ) 18). The MAS frequency νMAS was fixed to (a) 1 kHz or (b-e) 5 kHz. 81

Br-1H dipolar couplings ( |V11| > |V22|. Following Haeberlen notations and IUPAC recommendations,46 the chemical shift anisotropy (CSA) interaction tensor is defined by the isotropic chemical shift δiso, the anisotropy ∆CSA, and the asymmetry ηCSA parameters. These parameters are defined here by

δiso ) (δ11 + δ22 + δ33)/3; ∆CSA ) (δ33 - δiso); ηCSA ) (δ22 - δ11)/(δ33 - δiso) where |δ33 - δiso| > |δ11 - δiso| > |δ22 - δiso|. The Euler angles (φ, χ, ψ) describe the orientation of the CSA tensor with respect to the EFG tensor. In the case of powders, the angle intervals can be restricted47,48 and we used same common intervals as in SIMPSON software.49 II.2.3. Spectrum Modeling. Spectra were modeled using the DmFit software50 freely accessible from the Internet.51 For both 14 N and 81Br spectra, the model function denoted “Int2Quad” in the DmFit software was employed. This model assumes ideal excitation and detection and takes into account the quadrupolar and CSA interactions, with their Euler angles and the effect of the MAS spinning rate. The related NMR parameters can be fixed or estimated from an iterative fitting procedure, following previous analysis and fitting procedures.52 More details about

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Figure 2. Modeling of 14N spectra: (a) TMAB (x ) 1), (b) DTAB (x ) 12), (c) TTAB (x ) 14), (d) HTAB (x ) 16), (e) OTAB (x ) 18). For each sample, the experimental spectrum is shown at the top, the model in the middle, and the difference between both of them at the bottom. The spectra and acquisition conditions are the same as those in Figure 1.

TABLE 1: 14N NMR Parameters of n-Alkyltrimethylammonium Bromide Salts CxH2x+1(CH3)3N+Br- from Previous Works and This Worka crystal salt

δisob (ppm)

CQ (kHz)

TMAB (x ) 1) id id id dTAB (x ) 10) DTAB (x ) 12) TTAB (x ) 14) HTAB (x ) 16) id OTAB (x ) 18)

9.2

26.2 26.5 26.6 ( 0.2 25.2 116.0 ( 1.0 107.2 97.4 97.8 98.0 ( 1.0 96.9

a

13.0 13.0 13.0 13.0

ηQ

ν(14N) (MHz)

νMAS (kHz)

T/K

ref

0.00 ( 0.02 0.0 0.85 ( 0.02 0.90 0.90 0.84 0.90 ( 0.02 0.84

28.9 26 19.5 21.7 19.5 28.9 28.9 28.9 19.5 28.9

1 0 0 0 0 5 5 5 0 5

∼293 296 290 293 293 ∼293 ∼293 ∼293 293 ∼293

this work ref 40 ref 41 ref 86 ref 41 this work this work this work ref 41 this work

0.04

Unless otherwise reported, the errors bars are of (0.2 ppm for δiso, (1 kHz for CQ, and (0.01 for ηQ. b Relative to NH4Cl span at 4 kHz.

the use of the fitting procedures are presented in the Results section. The DmFit program has been modified so as to estimate mathematical fitting errors from covariance matrix and/or Monte Carlo analysis.53-55 They correspond to a 95% confidence limit. II.2.4. Theoretical Calculations. Optimized geometrical parameters and EFG of TMAX (X ≡ Cl, Br, I), NH4Cl, and HTAB bulk structures were computed in the frame of the periodic density functional (DF) approach with the Crystal 06 computer package.56 The calculations were carried out at T ) 0 K. Therefore, only the low temperature phase (symmetry P4/nmm space group) which is common for the three TMAX salts was considered, though different unit cell parameters or even crystal phases at various temperatures and degrees of hydration have been proposed.57-59 However, for the present study, the most detailed TMAB crystal structure data set was taken from ref 59, and for TMAC and TMAI, the published values of Wyckoff for the a and c lattice vectors were used.57 The initial structural parameters of the HTAB P21 unit cell are those of polymorph III,45 which are very close to our XRD results. The atomic positions in the TMAX, HTAB, and reference NH4Cl unit cells were fully optimized employing the B3LYP60,61 approximation for the exchange-correlation functional. The basis set of double-ζ quality (6-31d1)62 was used to describe N, C, and H atoms; the Cl, Br, and I atoms were described with the HAYWLC63 effective core potential basis set in the geometry

optimization. EFG tensors in the principle axes of the compounds were computed for the optimized structures. EFG values at the Br nucleus in TMAB and HTAB crystals were obtained from single point energy calculations for the optimized geometry using all electron double-ζ basis, DZVP.64 EFG tensors were also computed for [N(CH3)4X8]7- and [N(CH3)4]+ clusters cut from the optimized bulk structures for the TMAX salts and centered at the N atom (see Figure 8b). These cluster calculations were carried out again with the B3LYP functional using g03 code.65 Initial test calculations for the TMAB cluster were performed with the 6-311g(d,p),66-68 IGLOIII,69 and Aug-cc-pVTZ70,71 basis sets. As the NMR property variation was less than 8%, the 6-311g(d,p) basis set was employed for the results presented in this work. Nuclear quadruple moments (eQ) of 26.2 and 2.04 Q/fm2, respectively,72 have been used for the 81Br and 14N CQ calculations. II.3. X-ray Methods. II.3.1. Powder Diffraction. XRD diffractograms at small angle were recorded in a conventional θ-θ Bragg-Brentano configuration (Ni-filtered Cu KR1,2 ) 1.5418 Å) on a Bruker-AXS D8 Advance Powder X-ray diffractometer fitted with a linear Vantec-1 detector. The maximum lengths lmax of the fully extended CxH2x+1(CH3)3N+ cations have been estimated using two different approximations: empirical values normally used in surfactant systems73 and crude force field geometric optimization.74 lmax values were used to

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Figure 3. Variations of (a) δiso, (b) CQ, and (c) ηQ parameters as a function of x for (() 14N and (0) 81Br modeled spectra of n-alkyltrimethylammonium bromide salts CxH2x+1(CH3)3N+Br-. For 14N CQ and ηQ parameters, (∆) additional points are taken from ref 41.

estimate the maximum unit cell parameter cmax corresponding to ideal bilayers with perpendicular alkyl chains. II.3.2. Crystal Structure Determination. The crystal structure of HTAB (polymorph III) used for calculations was determined from colorless plate-shaped crystal grown at 20-25 °C from purely organic solvents comprised of ether/ethylacetate (90/ 10%v/v), having approximate dimensions of 0.40 × 0.30 × 0.01 mm3, and mounted on a glass fiber. Measurements were made on a Rigaku AFC5R diffractometer with graphic monochromatic Cu KR radiation and a 12 KW rotating anode generator. More information (about experimental conditions, atomic coordinates, U values, intramolecular distances, and torsion or conformational angles) is available as Supporting Information (S1). The structures of polymorphs I, II, and III of HTAB are described elsewhere.45

III. Results III.1. Experimental NMR Spectra and Modeling. III.1.1. 14N. We have studied the following n-alkyltrimethylammonium bromide salts CxH2x+1(CH3)3N+Br- by solid state 14 N MAS NMR: tetramethylammonium bromide (TMAB, x ) 1), dodecyltrimethylammonium bromide (DTAB, x ) 12), tetradecyltrimethylammonium bromide (TTAB, x ) 14), hexadecyltrimethylammonium bromide (HTAB, x ) 16), and octadecyltrimethylammonium bromide (OTAB, x ) 18). Their 14 N MAS spectra recorded at B0 ) 9.4 T (ν0 ≡ 28.91 MHz) are presented in Figure 1. The extent of the observed SSB mostly depends on the intensity of the quadrupolar coupling (QUAD) interaction. A static quadrupolar coupling, characterized by its intensity CQ and its asymmetry parameter ηQ, gives rise at first

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Figure 4. 81Br spectra (B0 ) 17.6 T; ν0 ≡ 202.60 MHz, νMAS ) 14 kHz) of n-alkyltrimethylammonium bromide salts CxH2x+1(CH3)3N+Br-: (a) TMAB (x ) 1), (b) DTAB (x ) 12), (c) TTAB (x ) 14), (d) HTAB (x ) 16), (e) OTAB (x ) 18).

Figure 6. Modeling of 81Br spectra of HTAB (x ) 16): (a) νMAS ) 30 kHz, (b) νMAS ) 14 kHz, (c) νMAS ) 7 kHz, (d) νMAS ) 0 kHz. The experimental spectrum is shown at the top, the model in the middle, and the difference between both of them at the bottom.

Figure 5. Modeling of 81Br spectra of TMAB (x ) 1): (a) νMAS ) 14 kHz, (b) νMAS ) 7 kHz, (c) νMAS ) 0 kHz. The experimental spectrum is shown at the top, the model in the middle, and the difference between both of them at the bottom.

order to a symmetric spinning sideband manifold whose sideband intensities allow measuring the interaction.25-27,75 We have modeled these 14N spectra by considering only the quadrupolar interaction. The results are presented in Figure 2 and summarized in Table 1. The differences between the experimental spectra and the models are small (Figure 2) but always nonsymmetrical. The chemical shift anisotropy (CSA) interaction, if significant, introduces an asymmetry of the spinning sideband intensities, as reported earlier for other nitrogen containing crystals.25,27 From 15N CPMAS measurements ran in parallel, estimated ∆CSA values are small here (close

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TABLE 2:

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Br NMR Parameters of n-Alkyltrimethylammonium Bromide Salts CxH2x+1(CH3)3N+Br- a

81

TMAB (x ) 1) average standard deviation DTAB (x ) 12)

average standard deviation TTAB (x ) 14)

average standard deviation HTAB (x ) 16)

average standard deviation OTAB (x ) 18)

average standard deviation

MAS (kHz)

δisob (ppm)

CQ (MHz)

ηQ

∆CSA (ppm)

ηCSA

φ (deg)

χ (deg)

ψ (deg)

14 7 0

46.1 45.6 45.4 45.7 0.3 59 57 59 59 58.6 0.9 57 55.3 57.2 57.5 56.7 0.8 56 55 56 57 55.9 0.8 55 53.4 54.2 54.3 54.2 0.5

6.03 6.03 6.03 6.03 0.002 7.27 7.32 7.53 7.44 7.39 0.10 7.57 7.57 7.94 7.89 7.74 0.17 8.01 8.03 8.07 8.00 8.03 0.03 7.99 8.04 8.14 8.13 8.08 0.07

0.026 0.025 0.013 0.02