Determination of 13C Chemical Shift Anisotropy Tensors and

Article pubs.acs.org/JPCA

Determination of 13C Chemical Shift Anisotropy Tensors and Molecular Order of 4-Hexyloxybenzoic Acid Nitin P. Lobo,† M. Prakash,‡ T. Narasimhaswamy,‡ and K. V. Ramanathan*,§ †

Department of Physics, Indian Institute of Science, Bangalore-560012, India Chemical Lab & Polymer Lab, CSIR-Central Leather Research Institute, Adyar, Chennai-600 020, India § NMR Research Center, Indian Institute of Science, Bangalore-560012, India ‡

ABSTRACT: 4-Alkoxy benzoic acids belong to an important class of thermotropic liquid crystals that are structurally simple and often used as starting materials for many novel mesogens. 4-Hexyloxybenzoic acid (HBA) is a homologue of the same series and exhibits an enantiotropic nematic phase. As this molecule could serve as an ideal model compound, high resolution 13C NMR studies of HBA in solution, solid, and liquid crystalline phases have been undertaken. In the solid state, two-dimensional separation of undistorted powder patterns by effortless recoupling (2D SUPER) experiments have been carried out to estimate the magnitude of the components of the chemical shift anisotropy (CSA) tensor of all the aromatic carbons. These values have been used subsequently for calculating the orientational order parameters in the liquid crystalline phase. The CSA values computed by density functional theory (DFT) calculations showed good agreement with the 2D SUPER values. Additionally, 13C−1H dipolar couplings in the nematic phase have been determined by separated local field (SLF) spectroscopy at various temperatures and were used for computing the order parameters, which compared well with those calculated by using the chemical shifts. It is anticipated that the CSA values determined for HBA would be useful for the assignment of carbon chemical shifts and for the study of order and dynamics of structurally similar novel mesogens in their nematic phases.

■

addition to chemicals shifts, 13C−1H dipolar couplings can also be obtained by using techniques like the separated local field (SLF) NMR spectroscopy, leading to a wealth of information about the mesogens such as alignment of the molecule, orientational order, and the dynamics of the core and the sidechain units.7 Typically, a two-ring-based calamitic mesogen consists of about 20 carbons.8 Of them, 12 or more carbons belong to the aromatic core unit, and the remaining belong to the terminal chains. Owing to symmetries present in the molecule such as the C2 symmetry of the phenyl rings, the number of carbon resonance lines arising from the mesogen is less than the total number of carbons.9 In spite of the rather small number of carbons in the spectrum, their assignment in the aligned phase is not straightforward. The position of the resonance lines depends on factors such as (i) the sign of the diamagnetic susceptibility anisotropy of the mesogenic molecule that will determine the orientation of the molecule in the magnetic field, (ii) the local and overall dynamics of the molecule as reflected

INTRODUCTION Thermotropic liquid crystals are a distinct class of advanced materials in which the orientational order and flow properties are closely related.1 These materials are also sensitive to external stimuli such as electric and magnetic fields, which has led to many practical applications including their use in display devices.2 The remarkable properties exhibited by them are the result of the intermolecular interactions arising out of the presence of structurally different segments and the shape anisometry.3 Calamitics are by far the most systematically investigated mesogens, although many new molecular shapes are being added to the pool of thermotropic liquid crystals.4 An understanding of the organization of these materials at the molecular level is vital for designing new mesogens with novel morphologies. Toward this end, high-resolution solid-state 13C NMR has emerged as an important tool for characterization of a wide variety of calamitic mesogens.5 13C NMR has the advantage that the measurements are made with natural abundance samples and does not require any special sample preparation. 13C chemical shifts span a large range and well resolved spectral features observed for different segments of the mesogens allow monitoring the dynamics at individual sites over a range of temperatures and through different phases.6 In © 2012 American Chemical Society

Received: March 9, 2012 Revised: June 21, 2012 Published: June 21, 2012 7508

dx.doi.org/10.1021/jp302291u | J. Phys. Chem. A 2012, 116, 7508−7515

The Journal of Physical Chemistry A

Article

Figure 1. (a) Molecular structure of HBA. (b) SUPER pulse sequence for determining the CSA parameter of HBA. (c) SAMPI-4 pulse sequence used for obtaining the 2D-SLF spectrum in the mesophase.

as SUPER (Separation of Undistorted Powder patterns by Effortless Recoupling), a two-dimensional (2D) solid-state NMR method.16 The SUPER experiment is known to yield fairly accurate values of the magnitude of the CSA tensor in the principal axis frame, provided the carbon signals in the spectrum do not overlap. In this work, we apply the SUPER technique for determining the CSA values of aromatic core carbons of 4-hexyloxybenzoic acid (HBA) (Figure 1a), a nematogen belonging to the well-known family of 4-alkoxy benzoic acids and an important starting material for many calamitic mesogens.17 The values obtained from the experiment are compared with those derived from the DFT. The obtained CSA values have been used for estimating the orientational order parameters of the liquid crystal at different temperatures, which are compared with those obtained using 13C−1H dipolar couplings. We expect that the results will also be useful for investigating dynamics and order in other liquid crystalline systems containing molecular fragments similar to HBA.18

in the order parameters of the mesogens, and (iii) the magnitude and orientation of the anisotropic chemical shift (CSA) tensor of individual carbons.10−12 In order to assign the spectral lines and to gain insight into molecular orientation and order, it is therefore desirable to have precise information about the CSA tensors of carbons in different segments of the mesogens. Due to limited availability of such information in the literature,13 the assignment is very often carried out by comparing the chemical shift values of structurally similar mesogens. An alternate approach that is available for the determination of CSA is the use of quantum chemical methods such as the density functional theory (DFT). Significant success has been achieved by this method, which has been applied to several liquid crystalline systems.14 However, the use of the DFT approach in some cases results in considerable deviations; for example, in the case of carbons of the ester and the azomethine linking units in some of the systems of our interest, differences of as much as 3−6 ppm between the calculated and experimental isotropic chemical shifts were observed. The use of the corresponding CSA values in such cases has to be made with caution. In this scenario, obtaining CSA values experimentally for carbons of structurally simple mesogens that are often used as the building blocks for construction of new mesogens15 could be a useful step leading to the assignment of spectra of larger molecules containing similar structural units. Here we measure the CSA of carbons of a model compound by using the experimental technique known

■

EXPERIMENTAL SECTION HBA was purchased from Aldrich, USA, and was further purified by recrystallization from heptane. Solid-state NMR experiments were carried out on the sample using a Bruker Avance-III 500 WB NMR spectrometer. The proton and carbon resonance frequencies were 500.17 and 125.79 MHz, respectively. The 2D SUPER experiment (Figure 1b) to obtain the 13C CSA was performed in the solid state of HBA at room 7509



Article

dwell time of 32 μs. A shifted sine bell window function was applied to the time domain data, and the spectra were processed in the phase sensitive mode.

temperature and at a spinning speed (ωr) of 5 kHz using a 4 mm triple resonance magic angle spinning (MAS) probe. The spectrum gives a correlation between anisotropic CSA powder patterns and isotropic chemical shifts in the F1 and F2 dimensions, respectively. The SUPER pulse sequence has the advantage that a powder pattern spanning several tens of kilohertz can be scaled down to a narrow spectral range corresponding to a spinning frequency of the order of 5 kHz. After the polarization transfer to carbon during CP, the 13C CSA interactions are recoupled by a pair of two 13C 2π pulses during the t1 period. The anisotropic CSA patterns obtained in the F1 dimension are scaled. The scaling factor depends on the pulse initiation and termination times as a ratio of the rotor period τr.16 These have been selected so as to obtain a scaling factor of 0.155. Correspondingly, the 13C 2π radio frequency (rf) power is given by ω1,C = 12.12 ωr, which was satisfied by choosing the rf strength to be 61 kHz . A continuous wave (cw) proton decoupling field of 150 kHz was applied on the 1H channel during the recoupling of 13C magnetization during the 2π pulses to avoid the signal dephasing by the heteronuclear couplings. Total suppression of spinning sidebands (TOSS)19 and γ-integral20 was used before the acquisition to obtain the sideband-free isotropic spectra. During acquisition, TPPM-1521 decoupling sequence with an rf strength of 100 kHz was used. The number of t1 increments was 32, with 256 scans per t1 increment and a repetition time of 4s. The data were acquired using the States-TPPI quadrature detection method and processed accordingly. The spectra in the liquid crystalline phase of the sample were obtained in the range between 100 and 153 °C using a double resonance probe equipped with a 5 mm horizontal solenoid coil for static samples. The sample was aligned by first heating to the isotropic phase and then cooling to the mesophase for NMR measurements. The 13C spectra in the nematic phase were obtained by using a Hartmann−Hahn cross-polarization pulse sequence with a contact time of 2 ms following the 90° proton pulse of width of 4 μs. SPINAL-6422 decoupling was employed during carbon signal acquisition using a proton decoupling power of 30 kHz. To avoid sample heating, the recycle delay between each FID was kept as 8 s. Each spectrum was obtained with 256 scans. For measuring the 13C−1H dipolar couplings of the molecule in the mesophase, the SAMPI-4 pulse sequence23 (Figure 1c) was applied on the oriented sample under static conditions. Details of the application of the pulse sequence to liquid crystalline systems have been described in earlier publications.18,24The method yields a 2D spectrum with carbon chemical shifts along the F2 dimension and the proton−carbon dipolar oscillation frequncies along the F1 dimension. For polarization inversion, a contact time τ of 2 ms was used. The spectra were recorded by using 62.5 kHz of rf for both the proton and carbon channels during the t1 period. During the t2 period, a broadband heteronuclear decoupling pulse scheme SPINAL-64 with 30 kHz decoupling strength was applied on protons. τ1 and τ2 were adjusted to be equal to 7π/4ω1 and 6π/ 4ω1,18,23 where ω1 is the rf field strength, and were respectively 14 and 12 μs. The scale factor of the sequence was estimated experimentally by using the proton coupled 13C spectrum of chloroform oriented in the liquid crystal N-(4-ethoxybenzylidine)-4-n-butylaniline (EBBA) as a reference and was observed to be close to 1. Typically, 16 transients were used for each t1 period with a recycle delay of 15 s between scans to avoid sample heating, and 128 t1 increments were employed with a

■

COMPUTATIONAL DETAILS The gas phase molecular geometries of HBA were optimized by using DFT-based Becke’s three parameter hybrid exchange functional and Lee−Yang−Parr correlation functional 25 (B3LYP) method employing the 6-31G(d) basis set. The NMR chemical shifts were calculated at the B3LYP/6311+G(2d,p) level of theory using the gauge invariant atomic orbitals (GIAO) method to circumvent the gauge problem using B3LYP/6-31G(d) geometry.26 Cheeseman recommended the 6-311+G(2d,p) basis set for NMR chemical shifts after scrutiny of various basis sets, and hence the same was selected for the present study.27 The scale factor determined for B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d) methodology from the previous study was used for the calculation of 13C NMR chemical shifts.28 Tetramethylsilane (TMS) chemical shifts calculated at the same level of theory were used as the reference. All calculations were carried out using the Gaussian 03W suite of programs.29

■

RESULTS AND DISCUSSIONS The molecular structure of HBA along with carbon numbers is shown in Figure 1a. The molecule exists as a dimer owing to

Figure 2. (a) Proton-decoupled 13C NMR spectrum of HBA dissolved in CDCl3 at 25 °C. (b) 13C CPTOSS spectrum with 5 kHz spinning speed at 25 °C in the solid phase. (c) 13C CP spectrum of the static oriented sample of HBA at 120 °C in the nematic phase.

the intermolecular hydrogen bonding through the carboxylic acid group.30 The mesogen exhibits a nematic phase,31 and the transition temperatures are 105.4 °C (TCr−N) and 153.2 °C (TN−I), respectively, as determined from hot stage optical polarizing microscopy. Solution NMR. Figure 2a shows the proton-decoupled 13C NMR spectrum of HBA dissolved in CDCl3. The spectrum shows five lines in the region 113−173 ppm and six lines in the 7510



Article

Table 1. 13C Chemical Shifts (ppm) of HBA in Solution, Solid, and Nematic Phases 128 °C

138 °C

148 °C

152 °C

C. no.

solution C.S. (ppm) δsol

solid CPTOSS (ppm)

C.S. (ppm) δlc

120 °C AIS (ppm) (δlc- δsol)

C.S. (ppm)

C.S. (ppm)

C.S. (ppm)

C.S. (ppm)

1 2

163.7 114.2

233.5 141.9

69.8 27.7

231.3 141.0

227.9 139.8

223.1 138.0

219.5 136.8

3 4 5 α β γ δ ω χ

132.3 121.4 172.2 68.3 29.1 25.6 31.5 22.6 14.0

163.4 109.7 119.5 132.4 122.8 171.7 68.3 31.4 29.9 33.9 24.7 15.4

166.7 193.1 213.3 61.7 22.0 20.3 25.4 19.8 10.5

34.4 71.7 41.1 −6.6 −7.1 −5.3 −6.1 −2.8 −3.5

165.6 190.8 211.9 61.9 22.2 20.4 25.6 19.8 10.6

163.9 187.5 209.7 62.2 22.7 20.8 26.1 19.9 10.9

161.6 182.6 206.7 62.7 23.3 21.2 26.7 20 11.2

160.0 179.0 204.5 63.1 23.7 21.6 27.1 20.2 11.4

spectrum in the solution phase, the solid state spectrum (Figure 2b) reveals six lines in the low frequency region. The chemical shifts are slightly different in solution and in the solid state, which may be attributed to packing effects. More strikingly, the spectrum in the solid state shows six peaks in the 108−172 ppm region, in contrast to only five lines in the solution spectrum. A close examination reveals that the C2 carbon which appeared at 114.2 ppm in solution splits in to two peaks, 2′ and 2″, in the solid state spectrum with chemical shifts of 109.7 and 119.5 ppm. This splitting is due to a locked orientation of the hexyloxy group with respect to the benzene ring, as noticed for structurally similar mesogens in solid state 13 C NMR.33 Similar observation have been made earlier for the ortho carbons in 1,4-dimethoxybenzene in the solid state with splitting of the lines attributed to shielding by the methoxy group for one of the carbons and deshielding of the other carbon.34 Preliminary confirmation of the above assignments were also obtained from a proton double-quantum-carbon single-quantum correlation experiment, the details of which will be published elsewhere. In the high temperature phases, namely, in the liquid crystalline phase and isotropic phase, these two lines collapse into a single line, with the chemical shift in solution being very close to the center of the two lines. CSA Measurement by SUPER. Figure 3 shows a typical 2D SUPER spectrum of HBA. Since the aromatic core of the mesogenic molecule is the focus of many of the studies of the liquid crystalline phase, the discussion here is restricted to these carbons. The F1 cross-section gives CSA powder patterns of different carbons in the core unit and is depicted in Figure 4 (blue dotted lines). In order to determine the principal components of the CSA tensor, the simulation of the experimental CSA powder patterns were carried out using the WSOLIDS simulation package,35 and the results are shown as red solid lines (Figure 4) for direct comparison. The CSA parameters for carbons 1−5 of HBA obtained from SUPER are listed in Table 2. For the methylene carbon at position 2, the CSA values corresponding to both the peaks at 109.7 and 119.5 ppm were obtained, and these are listed as 2′ and 2″ in Table 2. It is observed that in both the solution and the nematic phases, these carbons together give rise to a single peak. In solution the corresponding peak appears near the center of the two peaks in the solid phase, indicating motional averaging of the chemical shifts of 2′ and 2″. In the liquid crystalline phase also, as described below, a single peak corresponding to 2′ and 2″ appears, but with a larger shift compared to that in solution. Here, for the purpose of estimating order parameters in the

Figure 3. Typical 2D SUPER spectrum (sheared) in the solid phase of HBA at room temperature and at a spinning rate of 5 kHz using the pulse sequence in Figure 1b.

region 13−69 ppm. The existence of carboxylic acids as dimers in solution is very well-known.32 In the present case, only one set of carbon peaks are observed, indicating that the molecule exists as a symmetric dimer. The lines that appear in the 113− 173 ppm region are assigned to the carbons of the phenyl ring and to the carboxyl carbon, and those in the 13−69 ppm region are assigned to carbons of the hexyloxy chain. Among the lines observed in the spectrum, the protonated carbons of the phenyl ring show two highly intense peaks at 114.2 and 132.3 ppm, while the quaternary carbons are noticed at 121.4 and 163.7 ppm. The line at 172.2 ppm is assigned to the carboxyl carbon. Among the terminal hexyloxy chain carbons, the OCH2 carbon is seen at 68.3 ppm as a distinct peak, while the methyl carbon is noticed at 14.0 ppm. The other four methylene carbons showed four lines in the region 22−32 ppm. The above assignment of the signals was confirmed by comparing the experimental spectrum with the iterated spectrum generated by the ACD Chemsketch (version 3.0) software and also by standard 2D NMR measurements. Further confirmation for the aromatic part of the spectrum was also available from the DFT calculations presented elsewhere. The chemical shift values along with the carbon numbers are listed in Table 1. 13 C CP/MAS Spectrum of HBA. The 13C CP/TOSS spectrum of the HBA in the solid phase is obtained by spinning the sample at 5 kHz at room temperature. As in the case of the 7511



Article

Figure 4. Plots of F1 slices from the 2D SUPER spectrum for carbon sites 1, 2′, 2″, 3, 4, and 5. Dashed blue lines are experimental, and red solid lines denote theoretical simulations of CSA powder patterns using the WSOLIDS simulation package.

Table 2. CSA Tensors for Carbon Sites of HBA Determined by SUPER Compared with Calculated CSA Tensor Values Obtained Using Quantum Chemical Calculationa CSA(ppm) C. No 1 2

3 4 5

SUPER QCC (2′) SUPER (2′) QCC (2″)SUPER (2″) QCC SUPER (Average of 2′ and 2″) SUPER QCC SUPER QCC SUPER QCC

δ11

δ22

δ33

δiso

248.6 240.6 187.6 190.3 200.1 198.7 193.9

172.9 174.2 129.8 119.0 137.8 123.5 133.8

68.7 69.9 11.7 8.8 20.5 23.0 16.1

163.4 161.6 109.7 106.0 119.5 115.1 114.6

228.4 236.6 211.8 201.9 232.9 241.6

157.2 150.9 135.5 134.9 178.5 161.8

11.8 7.6 21.1 24.4 103.7 106.3

132.4 131.7 122.8 120.4 171.7 169.9

θb 4.8 113.7 61.5

60.9

Figure 5. Correlation of experimental SUPER and calculated DFT chemical-shift parameters (δ11, δ22 and δ33) for HBA. The best-fit straight line corresponds to a slope of 0.99 and an intercept of zero.

1.9 33.5

The uncertainties in the experimental values are ±5 ppm for protonated carbons and ±3 ppm for quaternary carbons. bθ is the angle between the 1-axis of the CSA tensor principal axes frame and the para-axis of the aromatic ring derived from QCC.

a

°C. Spectra were also recorded at 128 °C, 138 °C, 148 and 152 °C. The carbons from the phenyl ring and the carboxyl carbon that were seen in the solution in the range of 113−173 ppm Table 3. Local Order Parameters of the Phenyl Ring Obtained from CSA

oriented phase, a simple average of the CSA tensor values for the 2′ and 2″ carbons has been utilized. These average values are also given in Table 2. The CSA tensor components obtained using DFT calculations are also given in Table 2. A plot of the experimental and calculated values (Figure 5) shows that these values match reasonably well. 13 C NMR of HBA in the Nematic Phase. The sample of HBA was heated to its isotropic phase in the magnetic field, and the spectra were recorded while cooling the sample. Figure 2c shows the 13C NMR spectrum obtained by cross-polarization from protons of the static sample in its nematic phase at 120

calculated AIS (ppm)

a

7512

T (°C)

S′zz

S′xx − S′yya

1

2

3

4

120 128 138 148 152

0.73 0.71 0.69 0.63 0.57

0.082 0.077 0.072 0.066 0.053

65.0 63.2 61.3 55.9 50.4

29.4 28.5 27.5 25.1 22.4

35.4 34.3 33.1 30.3 26.9

68.1 66.1 64.1 58.6 52.7

The sign of S'xx - S'yy is negative. dx.doi.org/10.1021/jp302291u | J. Phys. Chem. A 2012, 116, 7508−7515


Article

Table 4. Dipolar Oscillation Frequencies (in kHz) of Carbons in the Aromatic Core Unit of HBA at Different Temperatures in the Mesophase Dipolar oscillation frequencies (in kHz) C. No.

120 °C

128 °C

138 °C

148 °C

1 2 3 4

1.74 3.13 3.03 1.70

1.68 3.03 2.92 1.65

1.64 2.86 2.75 1.59

1.50 2.61 2.52 1.44

discussed latter is missing, confirming the assignment. From the measurements at different temperatures in the nematic phase, alignment-induced chemical shifts (AIS) have been calculated as δlc − δsoln, where δlc and δsoln are the chemical shifts in the mesophase and in solution, respectively. Table 1 provides typical AIS values for all the carbons in the neamtic phase at 120 °C. It is observed in Table 1 that the ring quaternary carbons, namely, C1 and C4, exhibit higher AIS in comparison to methine carbons (C2 and C3) as observed in other similar systems.18 The 13C chemical shifts can be used to determine the order parameters in the mesophase. In the present case, the CSA information available from the SUPER experiment for the phenyl ring of the mesogen has been used to estimate the order parameters of the phenyl ring. Two order parameters, viz., S′zz and (S′xx − S′yy) are required, corresponding to the C2 symmetry of the ring. The AIS in the liquid crystalline phase is related to the components of the chemical shift tensor δij and the local order parameter according to36

Figure 6. The variation of the chemical shifts of carbons in the aromatic core as a function of temperature of the HBA mesogen in its mesophase. Experimental values are represented as points. The calculated from eq 1values are presented as continuous curves and correspond from top to bottom to carbons numbered as 1, 4, 3, and 2.

have now moved to the high-frequency region of 140−234 ppm. In contrast, the terminal chain carbons noticed in the 13− 69 ppm range in the solution shifted slightly to the lowfrequency region of 9−64 ppm. The appearance of well resolved intense peaks and the observed changes in the chemical shift values on going to the mesophase suggest that the molecules are aligned parallel to the field. The phenyl ring methine carbons showed two peaks like in the solution spectrum for the ortho carbons. The signals corresponding to the two quaternary carbons of the phenyl ring and the carboxyl carbon are noticed in the region of 180−240 ppm. The carboxyl carbon has been assigned to the peak appearing at around 213 ppm. This is based on its low intensity resulting from the absence of nearby protons and consequent inefficiency of the cross-polarization process. For the same reason, the corresponding cross-peak in the 2D-SLF plot

δ lc − δsoln = (2/3)S′zz [P2(cos β)(δ11 − δ22) + (1/2)(δ22 − δ33)] − (1/3)(S′xx − S′ yy ) [δ33 − (cos 2 β)δ22 − (sin 2 β)δ11]

(1)

For the phenyl ring, we have chosen the z-axis to be the paraaxis of the ring, the x-axis in the plane of the ring, and the y-axis perpendicular to the plane. The angle β is 60° for protonated

Figure 7. 13C−1H 2D-SLF spectrum of HBA mesogen in the nematic phase at 120 °C (left). Cross sections corresponding to the phenyl ring carbons are also shown (right). 7513



Article

obtained from the spectrum for the aromatic carbons are listed in Table 4 for different temperatures. These frequencies correspond directly to the dipolar coupling only for an isolated C−H pair.38 For more than two protons coupled to a carbon, the dipolar coupling information needs to be extracted from the oscillation frequency.11 Thus, in the case of the ortho- and the meta- carbons, the experimentally measured dipolar frequency f D actually has two components, namely, a coupling of the carbon to the ipso proton DC−Hi and also to the proton at the ortho position DC−Ho. The measured frequency is then obtained as [(DC−Hi)2 + (DC−Ho)2]1/2. The quaternary carbons have dipolar couplings to two equivalent protons in the ortho position. Since the meta protons are far away, their coupling may be neglected. In this case, the oscillation frequency may be obtained as √2·(DC−Ho). The experimentally determined four oscillation frequencies for the phenyl ring have been used to obtain the two order parameters using the relation9b,11 3 ⎡1 DC−H = −(hγCγH/4π 2rCH )⎢ (3 cos2 θz − 1)S′zz ⎣2

+

⎤ 1 (cos2 θx − cos2 θy)(S′xx − S′ yy )⎥ ⎦ 2

(2)

where γC and γH are the gyromagnetic ratios of carbon and hydrogen nuclei, rC−H is the distance between the nuclei C and H, and θx , θy, and θz are the angles the C−H vector makes with the coordinate axes. From the SLF spectra at four different temperatures, the order parameters have been obtained. A comparison of the order parameters obtained by the use of CSA values and dipolar couplings shown in Figure 8a,b indicates that these values match very well.

■

CONCLUSIONS Solid state 13C NMR studies of HBA were carried out in the solid as well as in the nematic phases. The 13C CP/TOSS spectrum in the solid state showed a splitting of the C2 carbon resonance, which indicated a preferential orientation of the hexyloxy chain with respect to the benzene ring. This splitting vanished in the nematic and isotropic phases. The variable temperature 13C NMR studies in the nematic phase provided AIS, and using the CSA determined from 2D SUPER experiment, the orientational order parameters were calculated. The CSA values were also computed by DFT, which were compared with the experimental data. The agreement between theory and experiment was found to be good. Additionally, the 13 C−1H dipolar couplings were obtained and were used for confirming the assignment of carbon chemical shifts in the nematic phase as well as for calculating the molecular order. The order parameters obtained this way compared well with those computed from chemical shifts. In HBA, the intermolecular hydrogen bonding interactions lead to the formation of a dimer, and the present studies support the presence of a symmetric dimer. The CSA values determined in the present work for HBA are expected to be useful for chemical shift assignment of structurally similar novel mesogens.

Figure 8. Comparison of order parameters (a) S′zz and (b) |S′xx −S′yy| from dipolar coupling and chemical shifts as a function of temperature for HBA in the mesophase.

carbons, and β is 0° for unprotonated (or quaternary) carbons of the phenyl ring. The estimated order parameters are listed in Table 3. Figure 6 shows the plot of calculated chemical shifts along with experimental values as a function of temperature, and the data fit reasonably well. The order parameter values determined for HBA at different temperatures are consistent with values reported for other calamitic mesogens.37 2D-SLF. As a further means of confirming the order parameters obtained from the use of experimentally determined CSA, the 2D SLF NMR experiment on the aligned mesogen in the nematic phase has been carried out using the SAMPI-4 pulse sequence.23 The experiment provides a correlation between 13C chemical shifts and the corresponding 13C−1H dipolar couplings depicted respectively along the F2 and F1 dimensions. The SLF spectra of HBA have been obtained at four different temperatures, viz., 120, 128, 138, and 148 °C. A typical spectrum obtained at 120 °C is shown in Figure 7. The spectrum shows well-resolved dipolar cross-peaks along the F1 axis for all the carbons except the carboxyl carbon, which has no protons in the immediate neighborhood. For the phenyl ring, two 13C−1H dipolar contours with a large separation along the dipolar dimension arising from methine carbons is observed, whereas the contours corresponding to the quaternary carbons show smaller separation. The dipolar oscillation frequencies

■

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest. 7514



■

Article

(16) Liu, S.-F.; Mao, J.-D.; Schmidt-Rohr, K. J. Magn. Reson. 2002, 155, 15−28. (17) (a) Neubert, M. E.; Carllno, L. T.; Fishel, D. L.; D’sidocky, R. M. Mol. Cryst. Liq. Cryst. 1980, 59, 253−272. (b) Coates, D.; Gray, G. W. Mol. Cryst. Liq. Cryst. 1976, 37, 249−262. (18) Kalaivani, S.; Narasimhaswamy, T.; Das, B. B.; Lobo, N. P.; Ramanathan, K. V. J. Phys. Chem. B 2011, 115, 11554−11565. (19) Dixon, W. T. J. Chem. Phys. 1982, 77, 1800−1809. (20) deAzevedo, E. R.; Hu, W.-G.; Bonagamba, T. J.; Schmidt-Rohr, K. J. Chem. Phys. 2000, 112, 8988−9001. (21) Bennett, A. E.; Rienstra, C. M.; Auger, M.; Lakshmi, K. V.; Griffin, R. G. J. Chem. Phys. 1995, 103, 6951−6958. (22) Fung, B. M.; Khitrin, A. K.; Ermolaev, K. J. Magn. Reson. 2000, 142, 97−101. (23) Nevzorov, A. A.; Opella, S. J. J. Magn. Reson. 2007, 185, 59−70. (24) Das, B. B.; Ajitkumar, T. G.; Ramanathan, K. V. Solid State Nucl. Magn. Reson. 2008, 33, 57−63. (25) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B. 1988, 37, 785−789. (26) Ditchfield, R. J. Chem. Phys. 1972, 56, 5688−5691. (27) Cheeseman, J. R.; Trucks, G. W.; Keith, T. A.; Frisch, M. J. J. Chem. Phys. 1996, 104, 5497−5509. (28) Aliev, A. E.; Murias, D. C.; Zhou, S. J. Mol. Struct. (THEOCHEM) 2009, 893, 1−5. (29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C. et al. Gaussian 03, revision E.01; Gaussian, Inc.: Wallingford, CT, 2004. (30) (a) Nosikova, L. A.; Kudryashova, Z. A.; Iskhakova, L. D.; Tsivadze, A. Y. Russ. J. Phys. Chem. A 2007, 81, 1263−1266. (b) Rowell, J. C.; Philips, W. D.; Melby, L. R.; Panar, M. J. Chem. Phys. 1965, 43, 3442−3454. (31) (a) Gray, G. W.; Jones, B. J. Chem. Soc. 1953, 4179−4180. (b) Bennett, G. M.; Jones, B. J. Chem. Soc. 1939, 420−425. (c) Jensen, J.; Grundy, S. C.; Bretz, S. L.; Hartley, C. S. J. Chem. Educ. 2011, 88, 1133−1136. (32) (a) Schrier, E.; Pottle, M.; Scheraga, H. A. J. Am. Chem. Soc. 1964, 86, 3444−3449. (b) Yamamoto, K.; Nishi, N. J. Am. Chem. Soc. 1990, 112, 549−558. (33) Kato, T.; Uryu, T. Mol. Cryst. Liq. Cryst. 1991, 195, 1−14. (34) Maricq, M. M.; Waugh, J. S. J. Chem. Phys. 1979, 70, 3300− 3316. (35) Eichele, K. WSolids1, ver. 1.20.18, Universität Tübingen: Tübingen, Germany, 2012. (36) Dong, R. Y. J. Phys. Chem. B 2009, 113, 1933−1939. (37) (a) Narasimhaswamy, T.; Monette, M.; Lee, D. K.; Ramamoorthy, A. J. Phys. Chem. B 2005, 109, 19696−19703. (b) Pratima, R.; Ramanathan, K. V. J. Magn. Reson. A 1996, 118, 7−10. (38) Nagaraja, C. S.; Ramanathan, K. V. Liq. Cryst. 1999, 26, 17−21.

ACKNOWLEDGMENTS T.N. would like to thank Dr. A. B. Mandal, Director, CLRI, for his support and encouragement. The authors thank Dr. V. Subramanian and Mr. E. Varathan for their help with quantum chemical calculations. The partial financial support from NWP23 is duly acknowledged. The use of the Bruker AV-III-500 Solid State NMR spectrometer funded by the Department of Science and Technology, New Delhi, at the NMR Research Centre, Indian Institute of Science, Bangalore, is gratefully acknowledged.

■

REFERENCES

(1) (a) Kato, T.; Mizoshita, N.; Kishimoto, K. Angew. Chem., Int. Ed. 2006, 45, 38−68. (b) Tschierske, C. Chem. Soc. Rev. 2007, 36, 1930− 1970. (c) Goodby, J. W.; Saez, I. M.; Cowling, S. J.; Gortz, V.; Draper, M.; Hall, A. W.; Sia, S.; Cosquer, G.; Lee, S.-E.; Raynes, P. Angew. Chem., Int. Ed. 2008, 47, 2754−2787. (2) Pauluth, D; Tarumi, K. J. Mater. Chem. 2004, 14, 1219−1227. (3) (a) Gray, G. W. Molecular Structure and Properties of Liquid Crystals; Academic Press: New York, 1962. (b) Goodby, J. W.; Saez, I. M.; Cowling, S. J.; Gasowska, J. S.; MacDonald, R. A.; Sia, S.; Watson, P.; Toyne, K. J.; Hird, M.; Lewis, R. A.; Lee, S.-E.; Vaschenko, V. Liq. Cryst. 2009, 36, 567−605. (4) (a) Demus, D. Mol. Cryst. Liq. Cryst. 2001, 364, 25−91. (b) Achalkumar, A. S.; Hiremath, U. S.; Shankar Rao, D. S.; Yelamaggad, C. V. Liq.Cryst. 2011, 38, 1563−1589. (c) Lehmann, M. In Top. Curr. Chem.; Tschierske, C., Ed.; Springer-Verlag: Heidelberg, 2012; Vol. 318, pp 193−224. (5) (a) Dong, R. Y. Nuclear Magnetic Resonance Spectroscopy of Liquid Crystals; World Scientific: Singapore, 2010. (b) Ramamoorthy, A., Ed. Thermotropic Liquid Crystals: Recent Advances; Springer: Dordrecht, Netherlands, 2007. (c) Fung, B. M. Prog. Nucl. Magn. Reson. Spectrosc. 2002, 41, 171−186. (d) Ramanathan, K. V.; Sinha, N. Monatsh. Chem. 2002, 133, 1535−1548. (6) (a) Pines, A.; Chang, J. J. Phys. Rev. A 1974, 10, 946−949. (b) Dvinskikh, S. V.; Yamamoto, K.; Scanu, D.; Deschenaux, R.; Ramamoorthy, A. J. Phys. Chem. B 2008, 112, 12347−12353. (c) Dvinskikh, S. V.; Luz, Z.; Zimmermann, H.; Maliniak, A.; Sandstrom, D. J. Phys. Chem. B 2003, 107, 1969−1976. (7) (a) Tan, C.; Fung, B. M. J. Phys. Chem. B 2003, 107, 5787−5790. (b) Sinha, N; Ramanathan, K. V. Chem. Phys. Lett. 2000, 332, 125− 130. (c) Sinha, N; Ramanathan, K. V.; Berdague, P.; Judeinstein, P.; Bayle, J. P. Liq. Cryst. 2002, 29, 449−457. (8) (a) Collings, P. J. Liquid Crystals: Nature’s Delicate Phase of Matter; Princeton University Press: New Jeresy, 2002. (b) Pestov, S. M., Ed. Physical Properties of Liquid Crystals; Landolt-Bornstein Numerical Data and Functional Relationships in Science and Technology- New Series; Springer: Berlin, 2003; Vol. VIII/5A. (9) (a) Pines, A.; Chang, J. J. J. Am. Chem. Soc. 1974, 96, 5590−5591. (b) Fung, B. M.; Afzal, J.; Foss, T. L.; Chau, M.-H. J. Chem. Phys. 1986, 85, 4808−4814. (10) Burnell, E. E.; de Lange, C. A., Eds. NMR of Ordered Liquids; Kluwer Academic Publsihers: The Netherlands, 2003. (11) Xu, J.; Fodor-Csorba, K.; Dong, R. Y. J. Phys. Chem. A 2005, 109, 1998−2005. (12) Nakai, T.; Fujimora, H.; Kuwari, D.; Miyajima, S. J. Phys. Chem. B 1999, 103, 417−425. (13) Veeman, W. S. Prog. Nucl. Magn. Reson. Spectrosc. 1984, 16, 193−235. (14) (a) Geppi, M.; Marini, A.; Veracini, C. A.; Urban, S.; Czub, J.; Kuczyński, W.; Dabrowski, R. J. Phys. Chem. B 2008, 112, 9663−9676. (b) Dong, R. Y.; Geppi, M.; Marini, A.; Hamplova, V.; Kaspar, M.; Veracini, C. A.; Zhang, J. J. Phys. Chem. B 2007, 111, 9787−9794. (15) Demus, D.; Goodby, J. W.; Gray, G. W.; Spiess, H.-W.; Vill, V., Eds. Handbook of Liquid Crystals; Wiley-VCH: Weinheim, Germany, 1998. 7515


Determination of 13C Chemical Shift Anisotropy Tensors and

Recommend Documents