Conformational equilibrium and orientational ordering: 1H-nuclear magnetic resonance of butane in a nematic liquid crystal

Abstract

Articles you may be interested inMild hydration of didecyldimethylammonium chloride modified DNA by 1H-nuclear magnetic resonance and by sorption isotherm J. Appl. Phys. 113, 044702 (2013); 10.1063/1.4789011 Ultrasound-order director fluctuations interaction in nematic liquid crystals: A nuclear magnetic resonance relaxometry study J. Chem. Phys. 118, 9037 (2003); 10.1063/1.1566735Order fluctuations of the director in nematic thermotropic liquid crystals studied by nuclear magnetic resonance dipolar relaxationIn this study we use multiple-quantum 1 H-NMR spectroscopy to study butane, the simplest flexible alkane, dissolved in a nematic solvent. An analysis of the highly accurate 1 H dipolar coupling constants gives important information about conformational and orientational behavior, including the trans-gauche energy difference, E tg , and the conformer probabilities and order parameters. An essential component of the analysis involves the use of mean-field models to describe the orientational ordering of solutes in a nematic solvent. Several models were found to adequately describe the molecular ordering, including the chord model of Photinos et al. ͓D. J. Photinos, E. T. Samulski, and H. Toriumi, J. Phys. Chem. 94, 4688 ͑1990͔͒ and recent versions of a model proposed by Burnell and co-workers ͓D. S. Zimmerman and E. E. Burnell, Mol. Phys. 78, 687 ͑1993͔͒. It was found that E tg lies in the range 2.1-3.0 kJ/mol, which is significantly below most experimental estimates of the gas-phase value. An attempt to describe more realistically the conformational states by including torsional fluctuations about the rotational isomeric states did not significantly improve the quality of the fits or alter the results. Finally, the anisotropic component of the solute-solvent interaction was found to perturb only marginally the conformational probabilities from the isotropic values.

Evolutionary algorithms to solve complicated NMR spectra

“…The value E tg Ϸ 550 cal/ mol agrees with current wisdom for this energy and is consistent with values obtained for butane 5 and longer chain alkanes 9,37 from NMR experiments analyzed using the chord model. The extra energy associated with the second gauche rotation in the g + g − conformer, which is expected to exhibit strong steric hindrance of the methyl groups, is Ϸ400 cal/mol.…”

“…21 For the structure of each conformer we used the same bond angles and bond lengths that were used for a previous analysis of butane. 5 Finally, we require estimates of the pentane conformer order matrices in the two liquid-crystal solvents magic mixture and 1132. Solute orientational order in the magic mixture is dominated by a single orientational mechanism that depends on solute size and shape alone.…”

“…Multiple-quantum NMR spectroscopy 3,4 has been used for the analysis of quite complicated spin systems, but the nature of the technique leads to spectral parameters that have large associated errors; however, accurate spectral parameters can be obtained using the inaccurate Multiple-quantum parameters as initial guesses for the analysis of the more precise single-quantum highresolution spectrum. 5,6 In an alternative approach, two methods that deploy deuteron isotopic substitution have been used. In the first, order parameters are determined for each C-D bond from the deuteron NMR spectrum of the orientationally ordered species.…”

Efficient analysis of highly complex nuclear magnetic resonance spectra of flexible solutes in ordered liquids by using molecular dynamics

“…The NMR spectrum of n-butane could only be solved when time-consuming multiple quantum methods that required much expertise were employed. 15 Other methods, based at least in part on isotopic substitution, have also been used for studying longer alkanes 16 but, although valued for the wealth of conformational information they provide, 17,18 they have not found widespread use. An analysis using conventional methods of the NMR spectrum of n-pentane in a nematic phase (having of order 20 000 transitions) has not been reported.…”

A Multiple-Quantum1H NMR Study of Conformational Biasing of Biphenyl in a Nematic Liquid Crystal

“…The logical approach, then, is to use the informa-the internal potential. We obtain the dipolar couplings from tion obtained from the MQ spectra as a starting point to an analysis of the complex one-quantum spectrum after first solve the complicated one-quantum spectrum (6)(7)(8), where solving an eight-quantum spectrum to obtain relatively acculinewidths can be as low as 1 Hz.…”