A model-free temperature-dependent conformational study of n-pentane in nematic liquid crystals

Abstract

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 E tg and its temperature coefficient E ′ tg are obtained. The value obtained for the extra energy associated with the proximity of the two methyl groups in the gauche + gauche − conformers (the pentane effect) is sensitive to minute details of other assumptions and is thus fixed in the calculations. Conformer populations are affected by the environment. In particular, anisotropic interactions increase the trans probability in the ordered phase. C 2015 AIP Publishing LLC. [http://dx

NMR of orientationally ordered short-chain hydrocarbons

“…Here β is the anisotropic part of some solute electronic property and G ZZ ðLCÞ is the axially symmetric liquidcrystal field of a uniaxial medium. The conformer populations are affected by the environment; in particular, as seen in Figure 2 BOTTOM, nematic-phase anisotropic interactions increase the trans probability, consistent with the nematic ordering preferring elongated solutes [7].…”

“…Note that the isotropic contribution is identical for both liquid crystals. Reproduced with permission of APS from [7].…”

“…Since in the NMR spectra of solutes dissolved in nematic phases the direct dipolar couplings that exist between every pair of magnetic nuclei are generally no longer averaged to zero, the spectral complexity increases at an alarming rate as a function of the number of magnetic spins, as illustrated in Figure 1 [1][2][3][4][5][6][7][8].…”

Recent NMR Studies of Thermotropic Liquid Crystals

“…Hence there are 13 independent anisotropic energy parameters at each temperature, while their temperature dependences are taken to be identical to that of the ethane Gβ by scaling. Sixteen sets of dipolar couplings measured for n-pentane in 1132 at temperatures between 258.5 and 333.5 K and six sets of dipolar couplings measured for n-pentane in 5CB at temperatures between 273.5 and 298.5 K were used to investiagate its conformational probability using the above model-free approach [357]. The conformers tt and tg of n-pentane in 1132 have their S zz 's decreasing with increasing temperature as expected, and the order of tt is always higher than that of tg at the same T. It was quite impressive that an RMS difference of about 7 Hz was obtained between experimental and fitted dipolar couplings for the combined 22 spectra, a number that up to now has been considered excellent for fitting results to a single spectrum when using some model for the ordering potential.…”

Evolutionary algorithms and nuclear magnetic resonance of oriented molecules

“…Since dipolar couplings occur between every pair of magnetic nuclei, the complexity of the resulting NMR spectra increases spectacularly with the number of nuclei, in our case 1 H. As an example, we show in Figure the NMR spectra of partially oriented members of a series of alkanes, starting with the relatively simple CH 4 (methane), C 2 H 6 (ethane), C 3 H 8 (propane), and the more complex C 4 H 10 ( n ‐butane), C 5 H 12 ( n ‐pentane), and C 6 H 14 ( n ‐hexane). Even a superficial inspection of the NMR spectra of the longer alkanes shows their extreme complexity …”