Exploiting the information content of dipolar couplings: determination of the temperature dependence of the inter-ring twist angle of biphenyl dissolved in uniaxial mesophases

Abstract

In the present work, the quite intriguing question of the temperature dependence of the inter-ring angle of biphenyl in fluid condensed phases has been investigated by the proton liquid crystal nuclear magnetic resonance technique. The spectra of the molecule dissolved in three different thermotropic uniaxial solvents (one of which shows also a smectic A phase) at different values of temperature have been analysed by standard procedures, and the resulting temperature-dependent dipolar coupling sets have been rationalised by the additive potential for the treatment of the ordering interactions method, combined with the direct probability description of the torsional curve, in order to obtain the distribution of the twist angle j for each temperature. The results emphasise a very slight but unequivocal and systematic increase of j M (the most probable value of the dihedral angle) with temperature, so qualitatively confirming what was previously found by a cruder hybrid approach (based on experimental quadrupolar splittings of perdeuteriated biphenyl combined with computer-simulated order parameters) carried out by two of the present authors (Celebre, G.; De Luca, G.; Mazzone, G. J. Mol. Struct. (THEOCHEM) 2005, 728, 209-214).

NMR of liquid crystals and micellar solutions

“…27 Concerning structural investigation on small molecules, the 1 H NMR spectra of biphenyl dissolved in three different thermotropic uniaxial solvents have been analyzed, and the resulting temperature-dependent dipolar coupling sets have been analyzed in order to obtain the distribution of the inter-ring angle. 28 3.2 Lyotropic liquid crystals A lyotropic L.C. consists of two or more components that exhibit liquidcrystalline properties in certain concentration ranges.…”

Order and Conformation of Biphenyl in Cyanobiphenyl Liquid Crystals: A Combined Atomistic Molecular Dynamics and 1H NMR Study

“…[34], we computed the average values of S ab for 2P, 5CB and 8CB as a function of the phenyl-phenyl dihedral angle f choosing a molecular frame, in which u z is parallel to the 2P long axis, u x is normal to u z and bisects angle f. Hence, for f = 0, the 2P unit is planar and u x lies on the molecular plane and u y is normal to the plane. The observable Saupe ordering matrix S ab is calculated as a time average of S ab (t j ).…”

“…[28,34] For example 2P, like many oligoaromatic compounds, is twisted with an inter-ring angle of around 448 in the gas phase, [35] with various quantum mechanics (QM) methods giving values between 38 and 468, [36] but is flat in the crystalline phase. [28,34] For example 2P, like many oligoaromatic compounds, is twisted with an inter-ring angle of around 448 in the gas phase, [35] with various quantum mechanics (QM) methods giving values between 38 and 468, [36] but is flat in the crystalline phase.…”

“…[28,34] For example 2P, like many oligoaromatic compounds, is twisted with an inter-ring angle of around 448 in the gas phase, [35] with various quantum mechanics (QM) methods giving values between 38 and 468, [36] but is flat in the crystalline phase. [28,[31][32][33][34] In the second part of the article, we also investigate the coupling between the phenyl-phenyl dihedral angle, both in the solute and the solvents, and the alignment and biaxiality of the corresponding conformers. [28,[31][32][33][34] In the second part of the article, we also investigate the coupling between the phenyl-phenyl dihedral angle, both in the solute and the solvents, and the alignment and biaxiality of the corresponding conformers.…”

Conformational and Orientational Determination from Dipolar Couplings in a Weakly Ordering Organic-Based Lyotropic and a Strongly Ordering Thermotropic Liquid Crystal

“…The main limitation is the complexity of the spectra, that are characterized by the presence of a very high number of distinct transitions evenly dispersed over a few 10 4 Hz frequency range and by strong second order spectral features. Such spectral complexity limited traditionally their use to highly symmetric molecules with no more than 10 to 12 spins [9][10][11][12][13][14][15]. Several strategies have been proposed to help in the spectral analysis and recently automated spectral fitting routines gained some interest [16][17].…”