Investigation of internal and external potentials acting on benzyl halides dissolved in different nematic solvents

The conformational behaviour of naproxen and flurbiprofen in solution by NMR spectroscopy

The conformational equilibrium of common anti-inflammatory drugs has been studied experimentally in solution by NMR in weakly ordered PBLG phases.

Smectic order parameters via liquid crystal NMR spectroscopy: Application to a partial bilayer smectic A phase

Solute molecules were dissolved in the liquid crystal 4-cyano-4'-n-octyloxybiphenyl (8OCB), known to form a partial bilayer smectic-A phase. Through measurement of solutes' and solvent's orientational order parameters via nuclear magnetic resonance spectroscopy, and their analysis via a statistical thermodynamic density functional theory, values of the solvent's positional order parameters and solutes' positional-orientational distribution functions were obtained. Near to the transition to the nematic phase, the main positional order parameter of the smectic liquid crystal turned out to be comprised in the interval 0.4-0.6, though the quality of the fittings assuming the phase as nematic all across the temperature range investigated was only slightly worse. This may be ascribed to the looseness of the partial bilayer smectic structure. Solutes were found to preferentially lie in those regions where liquid crystal molecule terminal chains are located.

Vibrational spectra of partially oriented molecules having two conformers in nematic and isotropic solutions: furfural and 2-chlorobenzaldehyde

Vibrational analysis of the two conformers of furfural and 2-chlorobenzaldehyde has been carried out on the basis of their IR and Raman spectra measured in isotropic and anisotropic (nematic liquid crystalline) solvent. The average orientation of the individual conformers in the nematic solvent has been determined by means of a recently developed approach for low symmetry planar molecules using DFT calculations of the vibrational transitions moments. The complex shape of the carbonyl band additionally split into several components is interpreted as an effect of Fermi resonance.

The NMR spectra of samples dissolved in liquid-crystalline phases: automatic analysis with the aid of multiple quantum spectra--the case of flexible molecules

Homonuclear N(S) = 0 and heteronuclear N(S) not equal 0 multiple quantum spectra, involving changes in the magnetic number m(I) by (N(I)-1), (N(I) - 2), and (N(I)-3), with N(I) and N(S) the number of interacting nuclei of magnetogyric ratio gamma(I) and gamma(S), are used for the automatic analysis of (1)H NMR spectra of flexible molecules dissolved in liquid-crystalline phases. The automatic procedure has been applied to study molecules of general formula Ph-CH(2)-X starting from a parameter set having all the spectral parameters set to zero. The results of such an analysis are then used as starting parameters for analysis of the single quantum spectrum. The method was first tested when X = Br and X = H in order to compare strategies differing for the types of parameters used and was then applied to the analysis of 3-phenylprop-1-yne.

Conformational dynamics of thyroid hormones by variable temperature nuclear magnetic resonance: the role of side chain rotations and cisoid/transoid interconversions

1H NMR spectra of the thyroid hormone thyroxine recorded at low temperature and high field show splitting into two peaks of the resonance due to the H2,6 protons of the inner (tyrosyl) ring. A single resonance is observed in 600 MHz spectra at temperatures above 185 K. An analysis of the line shape as a function of temperature shows that the coalescence phenomenon is due to an exchange process with a barrier of 37 kJ mol-1. This is identical to the barrier for coalescence of the H2',6' protons of the outer (phenolic) ring reported previously for the thyroid hormones and their analogues. It is proposed that the separate peaks at low temperature are due to resonances for H2,6 in cisoid and transoid conformers which are populated in approximately equal populations. These two peaks are averaged resonances for the individual H2 and H6 protons. Conversion of cisoid to transoid forms can occur via rotation of either the alanyl side chain or the outer ring, from one face of the inner ring to the other. It is proposed that the latter process is the one responsible for the observed coalescence phenomenon. The barrier to rotation of the alanyl side chain is > or = 37 kJ mol-1, which is significantly larger than has previously been reported for Csp2-Csp3 bonds in other Ph-CH2-X systems. The recent crystal structure of a hormone agonist bound to the ligand-binding domain of the rat thyroid hormone receptor (Wagner et al. Nature 1995, 378, 690-697) shows the transoid form to be the bound conformation. The significant energy barrier to cisoid/transoid interconversion determined in the current study combined with the tight fit of the hormone to its receptor suggests that interconversion between the forms cannot occur at the receptor site but that selection for the preferred bound form occurs from the 50% population of the transoid form in solution.

Theoretical and experimental approaches to the effects of solvation on the small internal rotational potential of benzal fluoride

The internal rotational potential for benzal fluoride is computed at various levels of molecular orbital theory, including correlation-gradient, MP2 (frozen core) methods. The perturbations of the potential caused by solvents are calculated with the Onsager model (ellipsoidal cavity with l = 6 in the multipole expansion) as well as with the self-consistent isodensity – polarizable continuum model (SCI–PCM). Analysis of the 1H and 19F nuclear magnetic resonance spectra in cyclohexane-d12 and acetone-d6 solutions yields long-range spin–spin coupling constants from which the expectation values of [Formula: see text] can be derived. These expectation values can be compared with those calculated from the theoretical internal rotational potential. Reasonable agreement is found for potentials obtained from MP2/6-31G* approaches in both solvent models. Long-range coupling constants between 19F and 13C nuclei are also reported and provide very rough checks of the [Formula: see text] values. For the isolated molecule an additivity scheme based on the potential for benzyl fluoride reproduces much of the potential for benzal fluoride except for a deviation caused by the rather larger relative magnitude of the fourfold component in the latter. The minimum in the potential for benzal fluoride occurs for a torsional angle, [Formula: see text] of 90° corresponding to a conformation in which the C—H bond of the side chain lies in a plane perpendicular to the phenyl plane and is rationalized on the basis of electrostatic forces. The conformations of minimum energy for the benzyl and benzal fluorides and chlorides are compared and contrasted. The magnitudes of the internal potentials of the fluorides are only a little larger than thermal energies at 300 K and can become smaller than the latter in soludon. Key words: NMR spectroscopy, of benzal fluoride; spin–spin coupling constants, long range in benzal fluoride; solvent effects, on internal rotational potential in benzal fluoride; molecular orbital computations, structure, internal rotational potential, and its solvent perturbations in benzal fluoride; benzal fluoride, 1H, 19F, and 13C NMR on, internal rotational potential, MO computations.

Experimental and theoretical assessments of the substituent and medium dependence of the internal rotational potentials in benzyl fluoride. 3,5-Difluorobenzyl fluoride and 4-fluorobenzyl fluoride

The 1H, 19F, and 13C {H} nuclear magnetic resonance spectra at 300 K of 4-fluoro- and 3,5-difluorobenzyl fluoride, dissolved in CS2–C6D12 and acetone-d6, are fully analyzed. Spin–spin coupling constants over four, five, and six formal bonds are used to derive expectation values of sin2θ and [Formula: see text] and the apparent twofold internal rotational potentials; [Formula: see text] is the angle by which the α C—F(C—H) bond twists out of the ring plane. The conformation of lowest energy has [Formula: see text] for the 3,5-difluoro compound in the polar and nonpolar solutions, whereas it has [Formula: see text] for the 4-fluoro derivative. The magnitudes of the potentials lie between 2 and 4 kJ/mol, that is, comparable to thermal energies. These data are compared with previous results for the parent compound and its 3,5-dichloro derivative. Geometry-optimized molecular orbital computations, including some correlation-gradient procedures, for benzyl fluoride and the two fluoro derivatives have [Formula: see text] for the conformations of highest energy of the free molecules. However, geometry-optimized SCFRF calculations of the solvent perturbations of the potential (dipole terms are insufficient) are in semiquantitative agreement with experiment in the sense that both solvents are predicted to destabilize the conformation with [Formula: see text] For example, the predominant twofold component in the computed (6-31G*) potential is 3.4 (free), −0.7 (CS2), and −3.3 (acetone-d6) kJ/mol for benzyl fluoride, a negative number indicating [Formula: see text] for the stable conformer; the experimental values are −0.8(2) (CS2) and −2.7(2) (acetone-d6) kJ/mol. The agreement between experiment and theory is of a similar quality for the fluoro derivatives. The stabilization of the conformer with [Formula: see text] for the 4-fluoro derivative is tentatively attributed to hyperconjugative electron acceptance by the α C—F bond, enhanced by the π-electron donor at the para position. A number of coupling constants are discussed in terms of the possible mechanisms of their transmission. Future experiments are indicated. Keywords: 1H, 19F, 13C NMR of 4-fluorobenzyl fluoride and 3,5-difluorobenzyl fluoride; MO calculations and internal rotational potentials in benzyl fluoride, 3,5-difluorobenzyl fluoride, and 4-fluorobenzyl fluoride; solvent effects and experimental and theoretical approaches to internal rotational potentials in 3,5-difluorobenzyl fluoride and 4-fluorobenzyl fluoride.