NMR Studies of ¹³C-Iodomethane:  Different Behavior in Thermotropic and Lyotropic Liquid Crystals

Abstract

High-resolution NMR spectra of 13C-iodomethane dissolved in thermotropic and lyotropic liquid crystalline solvents have been used to measure 1H−H and 13C−1H dipolar couplings. The ratio of these two couplings, which is a function of the H−C−H bond angle in 13C-iodomethane, is, in general, different from that expected from the known molecular structure; solvent−solute interactions in liquid crystalline solutions are responsible for this difference. In thermotropic liquid crystalline solutions, the apparent bond angle deviation (Δθa) increases with decreasing molecular ordering. In contrast, in lyotropic liquid crystals, no significant spectral aberration has been observed. These results indicate a fundamental physicochemical difference between the intermolecular interactions that prevail in thermotropic and lyotropic liquid crystals.

Assessment of solvent effects: do weak alignment media affect the structure of the solute?

“…Indeed, only small variations in RDCs were observed earlier in these weakly aligning SDS-based systems, compared to large deviations in thermotropic liquid crystals. 10,30,40 However, unusual behavior was also observed by other groups in strongly aligned lyotropic systems when high concentrations of SDS and low concentrations of alcohol were employed. 42,43 Although SDS-based systems are not wide-spread systems for aligning biomolecules, SDS is frequently used as an additional component in other media, either as a stabilizing agent, 44,45 a compound for modulating the tensor orientation of the biomolecule or for altering the charge of a micelle.…”

“…Moreover, we previously showed that solute structural parameters determined in thermotropic liquid crystals exhibited the largest deviations for the most weakly aligned molecules. 30 We also noted a correlation between the apparent structural deviations and the physical characteristics of the thermotropic liquid crystals, such as dielectric and diamagnetic anisotropies.…”

“…In addition, earlier studies had revealed that methyl halides, furan, and thiophene exhibited no significant solvent effects in lyotropic systems, whereas large effects were noted in thermotropic liquid crystalline media. 13,30,40 Differences in structural parameters of acetonitrile were also observed in several lyotropic systems, such as amphotropic ionic liquid crystals, indicating dual thermotropic-lyotropic behavior. 41 For all model compounds investigated here, high symmetry and rigidity allowed the detection of solvent effects by measuring dipolar coupling ratios.…”

“…31,32 They also work well for investigating 40 benzene, furan and thiophene 13 over the wide range of temperature (3-75°C) and concentrations of surfactants (2-15% w/w.). Complete molecular structures for furan and thiophene were determined in several liquid crystalline solutions using dipolar couplings 13 and no significant structural differences for all these model compounds compared to expectation values were noted in these systems ( …”

Obtaining the structure and bond rotational potential of a substituted ethane by NMR spectroscopy of solutions in nematic liquid-crystalline solvents

“…Comparisons of these two methods, and also with structures obtained from rotational spectroscopy, are usually within much closer agreement. 20 There are cases, however, involving small molecules, such as methyl fluoride 21 and methyl iodide, 22 when very large, and temperature-and solvent-dependent discrepancies are observed. These cases are most probably because of multiple site-specific interactions between the solute and solvent molecules.…”