P. Diehl

A study of the molecular structure and of the barrier to methyl rotation in o-chlorotoluene partially oriented in the nematic phase

The N.M.R. spectrum of o-chlorotoluene, dissolved in the nematic phase of 4-methoxybenzylidene-4-amino-α-methylcinnamic acid n-propylester is analysed and the shape of the proton skeleton determined. The position of lowest energy of the potential barrier to methyl rotation is found to be that in which the chlorine atom is staggered to the methyl group, and the height of the barrier is of the order of 5·0 kJ mol-1 (1200 cal/mole).

The structure of toluene as determined by NMR of oriented molecules

Abstract The proton magnetic resonance spectrum of toluene, dissolved in the nematic phase of n-(p-ethoxybenzylidene)-p′-n-butylaniline, has been analysed by use of a new computer program LEQUOR which takes into account total spin (see Appendix). The structure of the proton skeleton of toluene is determined, and small deviations of the ring-proton positions from benzene structure are detected. The height of the hindering potential to methyl rotation cannot be measured; however, the results indicate that there is a potential minimum when a CH bond is in a plane perpendicular to the benzene plane.

Vibrational corrections in NMR spectra of oriented molecules

Abstract Nuclear magnetic resonance of oriented molecules provides a convenient and precise technique for determination of molecular geometry. The precision is often high enough for the effect of the vibrational motions to be felt. In this paper the vibrational corrections are discussed in detail, and a novel formulation is presented which, apart from its simplicity, provides distinct advantages in numerical calculations. Although this formulation has not been published so far, a computer program based on it has been used extensively and with excellent results by the authors and their collaborators for several years.

The N.M.R. spectrum of pyridine oriented in the nematic phase

The N.M.R. spectrum of pyridine oriented in the nematic phase of anisole-azophenyl-n-capronate has been studied. The analysis has been carried out iteratively using a modified version of the computer programme LAOCOONII (re-named as LAOCOONOR). The details of the modification are discussed. The direct couplings are shown to be of negative sign. Ratios of the various inter-proton distances are determined irrespective of the orientation of the molecule. They agree within experimental error with the microwave results.

The rα structure of [1-13C]benzene by NMR of oriented molecules: A study of the possible precision and of solvent effects

Abstract Proton NMR spectra of benzene and [1-13C]benzene oriented in various types of liquid crystals (thermotropic nematic, lyotropic nematic, mixed cholesteric nematic, and smectic A) have been analyzed. Solvent effects on internuclear distance ratios of the order of 1% have been detected and attributed to molecular deformation which is correlated with the molecular orientation with respect to the liquid crystal solvent optic axis. A theoretical model for this solvent effect is developed and is found to reproduce the seven measured couplings within the experimental error. The solvent effects are practically independent of solute concentration and temperature. Also, there is no simple relationship between the benzene deformation in a liquid crystal and the [13C]methane deformation in the same solvent which was also measured in the present study. A relatively inert liquid crystal solvent is used to determine the “best” rα structure of benzene with the result of rα(CH)/rα(CC) = 0.77327 ± 0.00007. This novel nematic solvent, Merck ZLI 1167, orients with the optic axis perpendicular to the magnetic field and consequently allows sample rotation which leads to a linewidth smaller than 1 Hz.

The vibrationally averaged molecular structure: A comparison of NMR data for oriented benzene with results from electron diffraction and raman spectroscopy

Abstract The influence of vibrational motion on structural parameters is discussed, and the transformations from the observed effective NMR structures to average structures are presented. The average structures may be compared with corresponding ones from electron diffraction or from optical or microwave spectroscopy. The formalism is applied numerically to 1-13C-benzene, yielding a vibrational correction of 4% for the distance ratio r cH r cc . In addition, it is shown that only ratios containing small distances, as, e.g., the direct CH bond, are significantly affected, whereas the correction may usually be neglected for HH distance ratios. The fact that the average structures of NMR spectroscopy and electron diffraction agree with each other indicates that there is no detectable change in internuclear distances of benzene on changing from the gas to the liquid crystal solvent.

Effects of anharmonic vibrations on molecular properties

In consequence of recent advances made mainly in ab initio methods, relatively accurate anharmonic force fields are beginning to be available for an increasing number of molecules. This makes the calculation of anharmonic vibrational averaging effects on various molecular properties practical. In the present paper this calculation is discussed in detail, pointing out the simplifications which result when the effects of anharmonicity are specified by the conversion between the equilibrium and average geometries of the molecule. The different contributions to the vibrational averages of the internal coordinates of the molecules HCN, C2H2, CH4, C6H6 and NH3 are reported, considering their sensitivity to the details of the force fields used in the calculation. By combining the results with the experimental structures of these molecules their equilibrium or average geometries are derived.

The effects of the correlation between vibration and rotation of partially oriented molecules on the N.M.R. parameters

N.M.R. spectroscopy of partially oriented molecules provides a convenient technique for obtaining detailed information about molecular properties. However, the resulting data is sometimes internally inconsistent and markedly dependent on the conditions of the experiment, throwing some doubt on the general precision of the method. The probable reason for this is the correlation between the molecular vibrational and reorientational motion which is neglected in the conventional analysis. In this paper a theory for the effects of this correlation on the spectral parameters is elaborated. The resulting relation between molecular properties and spectral data is used to analyse the direct coupling constants of benzene measured in different liquid crystal solvents. The method solves the difficulties encountered in the conventional analysis: the molecular structure obtained proves to be internally consistent and solvent-independent. In addition to the structure the method yields also new information about the orie...

NMR spectra of oriented 13C-Acetylene: Redetermination of the shrinkage effects and analysis of the apparent temperature dependence of the molecular geometry

Abstract The NMR spectra of the 13C isomers of acetylene oriented in two nematic liquid crystals (EBBA and Phase IV) have been analyzed in order to determine the distance ratios and the shrinkage effects. In the case of the EBBA samples, the results agree with the IR data except for a small discrepancy in the distance ratios. In the Phase IV samples, we have found an unusually strong temperature dependence of the direct coupling constants and of the corresponding apparent distance ratios. In this case the discrepancies between NMR and IR are as large as 30%. We interpret this phenomenon on the basis of a two-site theory. The following assumptions have to be made in order to explain the apparent anomalies: (i) the enthalpy difference between the two sites is 200–400 cal/mole, (ii) the orientation parameters at the two sites have opposite signs, and (iii) at the site with the lower energy (complex?) the ratio r CH r CC is smaller by 0.5-1%. It is interesting that such modest conditions can lead, if neglected, to a 30% error in the distance ratios. With regard to this fact, it is probable that similar effects are not uncommon.

A general theory for the correlation between vibration and rotation of partially oriented molecules and for its effects on the NMR parameters

Abstract A general theory is developed which describes the interdependence of the small-amplitude internal motion and reorientation of a molecule in an anisotropic environment. The intramolecular motion is shown to be a superposition of two approximately independent components: orientation-dependent distortion and vibration. The effects of these motions on the NMR spectral parameters are calculated. The contributions originating from the molecular distortions are shown to explain all the apparent residual deviations of the observed benzene structure, corrected for harmonic vibrations, from a regular hexagon.