We obtain dipolar couplings via a novel application of evolutionary algorithms solving for multip... more We obtain dipolar couplings via a novel application of evolutionary algorithms solving for multiple spin-systems simultaneously and automatically from the NMR spectra of several solutes in several nematic and smectic liquid crystal solvents. The order parameters obtained from the dipolar couplings are used to test a novel Hamiltonian that includes two Maier–Saupe nematic terms plus Kobayashi–McMillan smectic A terms. It
The NMR of orientationally ordered solutes in liquid-crystalline solvents gives accurate values o... more The NMR of orientationally ordered solutes in liquid-crystalline solvents gives accurate values of order parameters, and these in turn can be interpreted in terms of the anisotropic intermolecular potential that causes the orientational ordering . A wealth of experimental evidence leads to the conclusion that a single mechanism is incapable of describing solute orientational order in different liquid crystals. We shall demonstrate that experimental results can be rationalized in terms of two and only two independent secondrank Maier-Saupe like mechanisms whose relative importance varies with liquid-crystal solvent . There is strong indication that the dominant mechanism involves short-range interactions that depend on solute size and shape. The second mechanism is less certain, and various possibilities will be discussed with the favourite involving the molecular quadrupole.
Quantum irreversible decoherence behaviour in open quantum systems with few degrees of freedom: A... more Quantum irreversible decoherence behaviour in open quantum systems with few degrees of freedom: Application to 1H NMR reversion experiments in nematic liquid crystals
Recent discoveries of the role of alkane flexibility in determining liquid-crystal behaviour are ... more Recent discoveries of the role of alkane flexibility in determining liquid-crystal behaviour are surveyed. With the impetus for understanding the alkane conformational problem established, recent model dependent (1)H NMR work on the topic will be reviewed where progress is made but the need to circumvent models eventually becomes evident. A closer look at the rigid basic units of alkanes will provide the way forward where it is shown that the orientational ordering and anisotropic potentials of these molecules dissolved in liquid crystals scale with each other. Once this relationship is established, a series of works using anisotropic and isotropic (1)H NMR spectroscopy to study alkane conformational statistics will be covered, wherein the influence of the gas, isotropic condensed and anisotropic condensed phases will be described.
The proton NMR spectra of n-pentane orientationally ordered in two nematic liquid-crystal solvent... more The proton NMR spectra of n-pentane orientationally ordered in two nematic liquid-crystal solvents are studied over a wide temperature range and analysed using covariance matrix adaptation evolutionary strategy. Since alkanes possess small electrostatic moments, their anisotropic intermolecular interactions are dominated by short-range size-and-shape effects. As we assumed for n-butane, the anisotropic energy parameters of each n-pentane conformer are taken to be proportional to those of ethane and propane, independent of temperature. The observed temperature dependence of the n-pentane dipolar couplings allows a model-free separation between conformer degrees of order and conformer probabilities, which cannot be achieved at a single temperature. In this way for n-pentane 13 anisotropic energy parameters (two for trans trans, tt, five for trans gauche, tg, and three for each of gauche+ gauche+, pp, and gauche+ gauche-, pm), the isotropic trans-gauche energy difference Etg and its te...
Orientational order parameters determined from (1)H NMR spectroscopy of solutes in liquid crystal... more Orientational order parameters determined from (1)H NMR spectroscopy of solutes in liquid crystals that form both nematic and smectic A phases are used to determine the solute smectic A order parameters and the smectic-nematic coupling term. For the analysis, it is necessary to know the nematic part of the potential in the smectic A phase: various ways of extrapolating parameters from the nematic phase to the smectic phase are explored.
The NMR spectra of the three solutes ortho-, meta-, and para-dichlorobenzene in the nematic and s... more The NMR spectra of the three solutes ortho-, meta-, and para-dichlorobenzene in the nematic and smectic A phases of the liquid crystals 8CB and 8OCB are analyzed to yield two orientational order parameters for each solute. Extrapolation of the asymmetry in the energy parameters that describe the orientational ordering in the nematic phase are used to provide estimates of the strength of the nematic potential in the smectic A phase. The experimentally determined asymmetry of the orientational order parameters in the smectic A phase is then used in conjunction with Kobayashi-McMillan theory applied to solutes to give information about the smectic A layering and the nematic/smectic A coupling. In both smectic A solvents, the solute smectic coupling constant, τ, is negative (with the origin fixed at the center of the smectic layer) for all solutes. The signs and relative values of τ indicate that the ortho and para solutes favor the interlayer region while the meta solute is more evenly distributed throughout the layers.
The determination of accurate structures of relatively small molecules dissolved in liquid-crysta... more The determination of accurate structures of relatively small molecules dissolved in liquid-crystal solvents is no trivial matter. Extensive vibrational corrections to the observed dipolar couplings are required. Vibrational force fields are often available, but the usual harmonic corrections are strictly limited to small-amplitude internal motions. Moreover, the need to also apply anharmonic corrections and to include the elusive vibration-reorientation interaction is problematic and can only be fulfilled for a very limited set of small molecules. In this paper we discuss the implications for the accuracy of the structure of larger molecules for which this information is not available. We discuss the examples of azulene and biphenylene, and set realistic limits on their proton structures, derived from 1 H dipolar couplings extracted from NMR spectra obtained in different liquid-crystal solvents and analyzed with sophisticated evolutionary algorithms.
The proton NMR spectra of ethane, propane, and n-butane codissolved and orientationally ordered i... more The proton NMR spectra of ethane, propane, and n-butane codissolved and orientationally ordered in two liquid-crystal solvents that exhibit both nematic (N) and smectic A (SmA) phases (one of which also has a reentrant nematic (RN) phase) are analyzed using CMA-ES (covariance-matrix adaptation evolution strategy). To deal with problems arising from the broad liquid-crystal background signal, a smoothed experimental spectrum is fitted to a smoothed calculated spectrum. The ethane and propane dipolar couplings and anisotropic energy parameters scale with each other, and the n-butane couplings are assumed to behave likewise. This restriction on the relative values of the n-butane energy parameters facilitates a fit to the temperature dependence of the n-butane dipolar couplings, in which the six n-butane energy parameters (three for trans and three for gauche), the isotropic trans-gauche energy difference E(tg), and its temperature coefficient E(tg)', and the methyl CCH angle decrease are obtained. Unlike earlier studies, the fit does not employ a model for the anisotropic intermolecular potential. The nematic potential in the SmA phase of the liquid-crystal mixture 6OCB/8OCB is estimated by interpolated values obtained from the ethane spectra for the N and RN regions. Analysis of results for the SmA phase involves adding a nematic-smectic coupling prefactor and a smectic prefactor. Spectra obtained in the liquid-crystal 8OCB are calculated with no adjustable parameters, scaling the potentials using the ethane values, and the agreement between experiment and calculation is outstanding. Conformer populations are affected by the environment, and the isotropic solute-solvent interactions result in increased gauche populations, whereas anisotropic interactions increase the trans probability. The trans probability in the SmA phase is slightly lower than expected from the nematic results.
From the dipolar couplings obtained by NMR spectroscopy we have calculated the order parameters o... more From the dipolar couplings obtained by NMR spectroscopy we have calculated the order parameters of a wide variety of solutes in the nematic and smectic A phases of the liquid crystals 8CB and 8OCB. These measurements are then rationalized with the previously tested two Maier-Saupe Kobayashi-McMillan interaction potential from which smectic order parameters are calculated.
The NMR spectra of n-pentane as solute in the liquid crystal 5CB are measured at several temperat... more The NMR spectra of n-pentane as solute in the liquid crystal 5CB are measured at several temperatures in the nematic phase. Atomistic molecular dynamics simulations of this system are carried out to predict the dipolar couplings of the orientationally ordered pentane, and the spectra predicted from these simulations are compared with the NMR experimental ones. The simulation predictions provide an excellent starting point for analysis of the experimental NMR spectra using the covariance matrix adaptation evolutionary strategy. This shows both the power of atomistic simulations for aiding spectral analysis and the success of atomistic molecular dynamics in modeling these anisotropic systems.
NMR spectra of ethane, propane, and n-butane as solutes in the nematic liquid crystals 4-n-pentyl... more NMR spectra of ethane, propane, and n-butane as solutes in the nematic liquid crystals 4-n-pentyl-4 -cyanobiphenyl (5CB) and Merck ZLI 1132 (1132 are investigated over a wide temperature range. The ratios of dipolar couplings of ethane to propane are constant over the entire temperature range. Assuming that this constancy applies to the butane conformers facilitates the separation of probability from order parameter. This separation allows the investigation of conformational distribution without the need of invoking any model for the anisotropic intermolecular potential. The results give an order matrix that is consistent with that predicted from model potentials that describe the orientational potential in terms of short-range size and shape effects. The isotropic intermolecular potential contribution to the trans-gauche energy difference E tg is found to be temperature dependent with the values and variation in agreement with that found when the same results are analyzed using the chord model for anisotropic interactions [A. C. J. Weber and E. E. Burnell, Chem. Phys. Lett. 506, 196 (2011)]. The fit obtained for 9 spectra in 5CB (63 dipolar couplings) has an RMS difference between experimental and calculated dipolar couplings of 2.7 Hz, while that for the 16 spectra in 1132 (112 couplings) is 6.2 Hz; this excellent fit with nine adjustable parameters suggests that the assumption of equal temperature dependencies of the order parameters for ethane, propane, and each conformer of butane is correct. Also the fit parameters (E tg and the methyl angle increase) obtained for 1132 and 5CB agree. The results indicate that the chord model, which was designed to treat hydrocarbon chains, is indeed the model of choice for these chains. The temperature variation of E tg provides a challenge for theoreticians. Finally, even better fits to the experimental dipolar couplings are obtained when the energy in the Boltzmann factor is used for scaling ethane to butane results. However, in this case the values obtained for E tg differ between 1132 and 5CB.
The complexity of 1 H NMR spectra of solutes in partially ordered solvents such as liquid crystal... more The complexity of 1 H NMR spectra of solutes in partially ordered solvents such as liquid crystals increases rapidly with the number of spins. Spectra of simple solutes with sufficient symmetry and containing not too many spins (typically 66) are readily analysed. The analysis of larger spin systems is more difficult, and often impossible. In this Letter, we present the application of a general automated genetic algorithm to solving highly complex spin systems with minimal operator intervention. The robustness of the method is demonstrated for the nine-spin system p-bromo-biphenyl, a solute interconverting between two symmetry-related conformations.
a b s t r a c t Experimental dipolar couplings are obtained from the 1 H NMR spectra of n-butane ... more a b s t r a c t Experimental dipolar couplings are obtained from the 1 H NMR spectra of n-butane dissolved in a liquid crystal over an 80°temperature range with Covariance Matrix Adaptation Evolutionary Strategies (CMA-ES). These observables are rationalized using a size-and-shape potential [E.E. Burnell, C.A. de Lange, Chem. Rev. 98 (1998) 2359], CI (2k), and with a modified chord model [D.J. Photinos, E.T. Samulski, H. Toriumi, J. Phys. Chem. 94 (1990) 4694], CCd, for the orientational ordering potential. The trans-gauche energy difference, E tg , is temperature dependent when using the CCd model and temperature independent when using the CI (2k) model for the orientational ordering potential.
From the dipolar couplings of orientationally ordered n-butane obtained by NMR spectroscopy we ha... more From the dipolar couplings of orientationally ordered n-butane obtained by NMR spectroscopy we have calculated conformer probabilities using the modified Chord (Cd) and Size-and-Shape (CI) models to estimate the conformational dependence of the order matrix. All calculation methods make use of GAUSSIAN 03 structures for the gauche and trans conformers. Calculations were performed for both the Rotational Isomeric State (RIS) approximation, as well as a continuous gas-phase potential for the dihedral angle rotation. Conformational probability distribution functions for butane as a solute in the ordered liquid-crystal solvent are obtained.
We obtain dipolar couplings via a novel application of evolutionary algorithms solving for multip... more We obtain dipolar couplings via a novel application of evolutionary algorithms solving for multiple spinsystems simultaneously and automatically from the NMR spectra of several solutes in several nematic and smectic liquid crystal solvents. The order parameters obtained from the dipolar couplings are used to test a novel Hamiltonian that includes two Maier-Saupe nematic terms plus Kobayashi-McMillan smectic A terms. It is shown that this Hamiltonian can rationalize the NMR experiments with physically reasonable smectic order parameters and Hamiltonian prefactors.
1 H NMR and optical microscopy have been used to study four solutes dissolved in samples of a bin... more 1 H NMR and optical microscopy have been used to study four solutes dissolved in samples of a binary mixture of 4-n-hexyloxy-4′-cyanobiphenyl (6OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB) that are close to the region of the phase diagram where the smectic-A (SmA) and re-entrant nematic (RN) phases exist. Optical microscopy clearly indicates that one of the four studied samples shows the phase sequence of isotropic-nematic-SmA-RN. The derived solute order parameters were interpreted by means of two Maier-Saupe mechanisms in conjunction with the Kobayashi-McMillan theory in the case of the SmA phase. The novel feature of this study is that the nematic potential is extrapolated from the nematic to the SmA phase based on concentrations of the 6OCB-8OCB mixtures. The derived smectic order parameters for each of the studied solutes clearly show a maximum absolute magnitude near the centre of the SmA temperature range. The different partitioning of these solutes in the binary mixture is also discussed.
We obtain dipolar couplings via a novel application of evolutionary algorithms solving for multip... more We obtain dipolar couplings via a novel application of evolutionary algorithms solving for multiple spin-systems simultaneously and automatically from the NMR spectra of several solutes in several nematic and smectic liquid crystal solvents. The order parameters obtained from the dipolar couplings are used to test a novel Hamiltonian that includes two Maier–Saupe nematic terms plus Kobayashi–McMillan smectic A terms. It
The NMR of orientationally ordered solutes in liquid-crystalline solvents gives accurate values o... more The NMR of orientationally ordered solutes in liquid-crystalline solvents gives accurate values of order parameters, and these in turn can be interpreted in terms of the anisotropic intermolecular potential that causes the orientational ordering . A wealth of experimental evidence leads to the conclusion that a single mechanism is incapable of describing solute orientational order in different liquid crystals. We shall demonstrate that experimental results can be rationalized in terms of two and only two independent secondrank Maier-Saupe like mechanisms whose relative importance varies with liquid-crystal solvent . There is strong indication that the dominant mechanism involves short-range interactions that depend on solute size and shape. The second mechanism is less certain, and various possibilities will be discussed with the favourite involving the molecular quadrupole.
Quantum irreversible decoherence behaviour in open quantum systems with few degrees of freedom: A... more Quantum irreversible decoherence behaviour in open quantum systems with few degrees of freedom: Application to 1H NMR reversion experiments in nematic liquid crystals
Recent discoveries of the role of alkane flexibility in determining liquid-crystal behaviour are ... more Recent discoveries of the role of alkane flexibility in determining liquid-crystal behaviour are surveyed. With the impetus for understanding the alkane conformational problem established, recent model dependent (1)H NMR work on the topic will be reviewed where progress is made but the need to circumvent models eventually becomes evident. A closer look at the rigid basic units of alkanes will provide the way forward where it is shown that the orientational ordering and anisotropic potentials of these molecules dissolved in liquid crystals scale with each other. Once this relationship is established, a series of works using anisotropic and isotropic (1)H NMR spectroscopy to study alkane conformational statistics will be covered, wherein the influence of the gas, isotropic condensed and anisotropic condensed phases will be described.
The proton NMR spectra of n-pentane orientationally ordered in two nematic liquid-crystal solvent... more The proton NMR spectra of n-pentane orientationally ordered in two nematic liquid-crystal solvents are studied over a wide temperature range and analysed using covariance matrix adaptation evolutionary strategy. Since alkanes possess small electrostatic moments, their anisotropic intermolecular interactions are dominated by short-range size-and-shape effects. As we assumed for n-butane, the anisotropic energy parameters of each n-pentane conformer are taken to be proportional to those of ethane and propane, independent of temperature. The observed temperature dependence of the n-pentane dipolar couplings allows a model-free separation between conformer degrees of order and conformer probabilities, which cannot be achieved at a single temperature. In this way for n-pentane 13 anisotropic energy parameters (two for trans trans, tt, five for trans gauche, tg, and three for each of gauche+ gauche+, pp, and gauche+ gauche-, pm), the isotropic trans-gauche energy difference Etg and its te...
Orientational order parameters determined from (1)H NMR spectroscopy of solutes in liquid crystal... more Orientational order parameters determined from (1)H NMR spectroscopy of solutes in liquid crystals that form both nematic and smectic A phases are used to determine the solute smectic A order parameters and the smectic-nematic coupling term. For the analysis, it is necessary to know the nematic part of the potential in the smectic A phase: various ways of extrapolating parameters from the nematic phase to the smectic phase are explored.
The NMR spectra of the three solutes ortho-, meta-, and para-dichlorobenzene in the nematic and s... more The NMR spectra of the three solutes ortho-, meta-, and para-dichlorobenzene in the nematic and smectic A phases of the liquid crystals 8CB and 8OCB are analyzed to yield two orientational order parameters for each solute. Extrapolation of the asymmetry in the energy parameters that describe the orientational ordering in the nematic phase are used to provide estimates of the strength of the nematic potential in the smectic A phase. The experimentally determined asymmetry of the orientational order parameters in the smectic A phase is then used in conjunction with Kobayashi-McMillan theory applied to solutes to give information about the smectic A layering and the nematic/smectic A coupling. In both smectic A solvents, the solute smectic coupling constant, τ, is negative (with the origin fixed at the center of the smectic layer) for all solutes. The signs and relative values of τ indicate that the ortho and para solutes favor the interlayer region while the meta solute is more evenly distributed throughout the layers.
The determination of accurate structures of relatively small molecules dissolved in liquid-crysta... more The determination of accurate structures of relatively small molecules dissolved in liquid-crystal solvents is no trivial matter. Extensive vibrational corrections to the observed dipolar couplings are required. Vibrational force fields are often available, but the usual harmonic corrections are strictly limited to small-amplitude internal motions. Moreover, the need to also apply anharmonic corrections and to include the elusive vibration-reorientation interaction is problematic and can only be fulfilled for a very limited set of small molecules. In this paper we discuss the implications for the accuracy of the structure of larger molecules for which this information is not available. We discuss the examples of azulene and biphenylene, and set realistic limits on their proton structures, derived from 1 H dipolar couplings extracted from NMR spectra obtained in different liquid-crystal solvents and analyzed with sophisticated evolutionary algorithms.
The proton NMR spectra of ethane, propane, and n-butane codissolved and orientationally ordered i... more The proton NMR spectra of ethane, propane, and n-butane codissolved and orientationally ordered in two liquid-crystal solvents that exhibit both nematic (N) and smectic A (SmA) phases (one of which also has a reentrant nematic (RN) phase) are analyzed using CMA-ES (covariance-matrix adaptation evolution strategy). To deal with problems arising from the broad liquid-crystal background signal, a smoothed experimental spectrum is fitted to a smoothed calculated spectrum. The ethane and propane dipolar couplings and anisotropic energy parameters scale with each other, and the n-butane couplings are assumed to behave likewise. This restriction on the relative values of the n-butane energy parameters facilitates a fit to the temperature dependence of the n-butane dipolar couplings, in which the six n-butane energy parameters (three for trans and three for gauche), the isotropic trans-gauche energy difference E(tg), and its temperature coefficient E(tg)', and the methyl CCH angle decrease are obtained. Unlike earlier studies, the fit does not employ a model for the anisotropic intermolecular potential. The nematic potential in the SmA phase of the liquid-crystal mixture 6OCB/8OCB is estimated by interpolated values obtained from the ethane spectra for the N and RN regions. Analysis of results for the SmA phase involves adding a nematic-smectic coupling prefactor and a smectic prefactor. Spectra obtained in the liquid-crystal 8OCB are calculated with no adjustable parameters, scaling the potentials using the ethane values, and the agreement between experiment and calculation is outstanding. Conformer populations are affected by the environment, and the isotropic solute-solvent interactions result in increased gauche populations, whereas anisotropic interactions increase the trans probability. The trans probability in the SmA phase is slightly lower than expected from the nematic results.
From the dipolar couplings obtained by NMR spectroscopy we have calculated the order parameters o... more From the dipolar couplings obtained by NMR spectroscopy we have calculated the order parameters of a wide variety of solutes in the nematic and smectic A phases of the liquid crystals 8CB and 8OCB. These measurements are then rationalized with the previously tested two Maier-Saupe Kobayashi-McMillan interaction potential from which smectic order parameters are calculated.
The NMR spectra of n-pentane as solute in the liquid crystal 5CB are measured at several temperat... more The NMR spectra of n-pentane as solute in the liquid crystal 5CB are measured at several temperatures in the nematic phase. Atomistic molecular dynamics simulations of this system are carried out to predict the dipolar couplings of the orientationally ordered pentane, and the spectra predicted from these simulations are compared with the NMR experimental ones. The simulation predictions provide an excellent starting point for analysis of the experimental NMR spectra using the covariance matrix adaptation evolutionary strategy. This shows both the power of atomistic simulations for aiding spectral analysis and the success of atomistic molecular dynamics in modeling these anisotropic systems.
NMR spectra of ethane, propane, and n-butane as solutes in the nematic liquid crystals 4-n-pentyl... more NMR spectra of ethane, propane, and n-butane as solutes in the nematic liquid crystals 4-n-pentyl-4 -cyanobiphenyl (5CB) and Merck ZLI 1132 (1132 are investigated over a wide temperature range. The ratios of dipolar couplings of ethane to propane are constant over the entire temperature range. Assuming that this constancy applies to the butane conformers facilitates the separation of probability from order parameter. This separation allows the investigation of conformational distribution without the need of invoking any model for the anisotropic intermolecular potential. The results give an order matrix that is consistent with that predicted from model potentials that describe the orientational potential in terms of short-range size and shape effects. The isotropic intermolecular potential contribution to the trans-gauche energy difference E tg is found to be temperature dependent with the values and variation in agreement with that found when the same results are analyzed using the chord model for anisotropic interactions [A. C. J. Weber and E. E. Burnell, Chem. Phys. Lett. 506, 196 (2011)]. The fit obtained for 9 spectra in 5CB (63 dipolar couplings) has an RMS difference between experimental and calculated dipolar couplings of 2.7 Hz, while that for the 16 spectra in 1132 (112 couplings) is 6.2 Hz; this excellent fit with nine adjustable parameters suggests that the assumption of equal temperature dependencies of the order parameters for ethane, propane, and each conformer of butane is correct. Also the fit parameters (E tg and the methyl angle increase) obtained for 1132 and 5CB agree. The results indicate that the chord model, which was designed to treat hydrocarbon chains, is indeed the model of choice for these chains. The temperature variation of E tg provides a challenge for theoreticians. Finally, even better fits to the experimental dipolar couplings are obtained when the energy in the Boltzmann factor is used for scaling ethane to butane results. However, in this case the values obtained for E tg differ between 1132 and 5CB.
The complexity of 1 H NMR spectra of solutes in partially ordered solvents such as liquid crystal... more The complexity of 1 H NMR spectra of solutes in partially ordered solvents such as liquid crystals increases rapidly with the number of spins. Spectra of simple solutes with sufficient symmetry and containing not too many spins (typically 66) are readily analysed. The analysis of larger spin systems is more difficult, and often impossible. In this Letter, we present the application of a general automated genetic algorithm to solving highly complex spin systems with minimal operator intervention. The robustness of the method is demonstrated for the nine-spin system p-bromo-biphenyl, a solute interconverting between two symmetry-related conformations.
a b s t r a c t Experimental dipolar couplings are obtained from the 1 H NMR spectra of n-butane ... more a b s t r a c t Experimental dipolar couplings are obtained from the 1 H NMR spectra of n-butane dissolved in a liquid crystal over an 80°temperature range with Covariance Matrix Adaptation Evolutionary Strategies (CMA-ES). These observables are rationalized using a size-and-shape potential [E.E. Burnell, C.A. de Lange, Chem. Rev. 98 (1998) 2359], CI (2k), and with a modified chord model [D.J. Photinos, E.T. Samulski, H. Toriumi, J. Phys. Chem. 94 (1990) 4694], CCd, for the orientational ordering potential. The trans-gauche energy difference, E tg , is temperature dependent when using the CCd model and temperature independent when using the CI (2k) model for the orientational ordering potential.
From the dipolar couplings of orientationally ordered n-butane obtained by NMR spectroscopy we ha... more From the dipolar couplings of orientationally ordered n-butane obtained by NMR spectroscopy we have calculated conformer probabilities using the modified Chord (Cd) and Size-and-Shape (CI) models to estimate the conformational dependence of the order matrix. All calculation methods make use of GAUSSIAN 03 structures for the gauche and trans conformers. Calculations were performed for both the Rotational Isomeric State (RIS) approximation, as well as a continuous gas-phase potential for the dihedral angle rotation. Conformational probability distribution functions for butane as a solute in the ordered liquid-crystal solvent are obtained.
We obtain dipolar couplings via a novel application of evolutionary algorithms solving for multip... more We obtain dipolar couplings via a novel application of evolutionary algorithms solving for multiple spinsystems simultaneously and automatically from the NMR spectra of several solutes in several nematic and smectic liquid crystal solvents. The order parameters obtained from the dipolar couplings are used to test a novel Hamiltonian that includes two Maier-Saupe nematic terms plus Kobayashi-McMillan smectic A terms. It is shown that this Hamiltonian can rationalize the NMR experiments with physically reasonable smectic order parameters and Hamiltonian prefactors.
1 H NMR and optical microscopy have been used to study four solutes dissolved in samples of a bin... more 1 H NMR and optical microscopy have been used to study four solutes dissolved in samples of a binary mixture of 4-n-hexyloxy-4′-cyanobiphenyl (6OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB) that are close to the region of the phase diagram where the smectic-A (SmA) and re-entrant nematic (RN) phases exist. Optical microscopy clearly indicates that one of the four studied samples shows the phase sequence of isotropic-nematic-SmA-RN. The derived solute order parameters were interpreted by means of two Maier-Saupe mechanisms in conjunction with the Kobayashi-McMillan theory in the case of the SmA phase. The novel feature of this study is that the nematic potential is extrapolated from the nematic to the SmA phase based on concentrations of the 6OCB-8OCB mixtures. The derived smectic order parameters for each of the studied solutes clearly show a maximum absolute magnitude near the centre of the SmA temperature range. The different partitioning of these solutes in the binary mixture is also discussed.
Uploads
Papers by Adrian Weber