R. J. W. Le Fèvre

Molecular Refractivity and Polarizability

Publisher Summary This chapter discusses molecular refractivity and polarizability. Refractive indices ( n ) of pure substances are more accurately measurable than any other optical properties. The refractive index of a substance varies with its physical state, temperature t , and wave-length λ of the light by which n is observed. The first two of these effects are attributed to the density d . The specific refraction r of a substance multiplied by the molecular weight is referred as molecular refraction. Polarizability of a particle is defined as the dipole moment induced by an electric field of unit intensity. The polarizabilities of monatomic ions and molecules are generally independent of field direction. The principal polarizabilities of molecules can be analyzed in terms of anisotropic bond polarizabilities. Polarizabilities are also responsible for the birefringence, which appears whenever the orientations of anisotropic molecules in an assemblage are derandomized or changed by any disturbing force.

Molecular polarisability. Carbon–carbon bond polarisabilities in relation to bond lengths

The suggestion is made and examined that ‘vertical’ polarisabilities of C–C bonds in planar polynuclear aromatic hydrocarbons can be computed from known C–C inter-centre distances, and that molecular polarisabilities, b3, perpendicular to the ring-planes, can be obtained therefrom by appropriate summations. In-plane polarisabilities, b1 and b2, can then be extracted from observed molar Kerr constants and refractions. A related proposition for longitudinal bond polarisabilities is also considered.

Molecular polarisability. The dipole moments and molar Kerr constants of some para-substituted anisoles

Dipole moments and molar Kerr constants are reported for the molecules p-XC6H4·OMe (X = H, Me, Cl, Br, l, NO2, or CN) examined as solutes in carbon tetrachloride and benzene. The experimental data are analysed to indicate that (a) the Car–O–C group is virtually coplanar with the aromatic ring if X = NO2 or CN; for X = H, Me, or halogen, the apparent dihedral angle between the planes is ca. 20°(b) stereospecific benzene–solute complex formation occurs in the aromatic solvent.

Molecular polarisability. The conformations of some 9-substituted anthracenes as solutes

Dipole moments and Molar Kerr constants are recorded for anthracene and 9-chloro-, 9-bromo-, 9-acetyl-, 9-formyl-, 9-vinyl-, and 9-nitro-anthracene as solutes in carbon tetrachloride at 25°. Anisotropic polarisabilities for anthracene (b1= 24·46, b2= 35·90, b3= 15·88) derived from data on anthracene, 9-chloroanthracene, and 9-bromoanthracene are used to deduce the apparent conformations of the other anthracene derivatives.

Molecular polarisability. The anisotropic electron polarisability of aliphatic C–C and C–H bonds

Previous determinations of the anisotropic polarisability of aliphatic C–C and C–H bonds are reviewed. Kerr-effect measurements of several alicyclic hydrocarbons in solution and as pure liquids are reported, and previous measurements are reconsidered. The parameter ΓCC≡(bLCC–bTCC)–2(bLCH–bTCH) is evaluated from eight independent determinations in solution as 0·71 ± 0·3 A3, and from five determinations for liquids as 0·80 ± 0·15 A3, in good agreement with recent light-scattering results. Methods of obtaining C–C and C–H bond polarisabilities separately are discussed, and consideration given to the effect of small deviations from tetrahedral bond angles on the determinable polarisability parameters of aliphatic molecules. The conformation of the cyclooctane ring is investigated, but results are inconclusive.

Molecular polarisability. The conformations of diphenyl ketone, dimesityl ketone, and mesityl phenyl ketone as solutes

Dipole moments and molar Kerr constants (×1012) are recorded for diphenyl ketone (2·98 D, +126), dimesityl ketone (2·84 D, –202), and mesityl phenyl ketone (2·97 D, –165) as solutes in carbon tetrachloride at 25°. For diphenyl ketone and dimesityl ketone the experimental results are consistent with the presence of antisymmetric (C2) solute species in which the effective conformational angles between the planes of the aromatic rings and the C–CO–C plane are 42 and 61°, respectively. Possible conformations of mesityl phenyl ketone are discussed.

The configurations of some weak complexes of benzene

The geometries of benzene complexes of a number of simple polar solutes have been studied through the solvent-dependences of Molar Kerr constants and 1H n.m.r. spectra. Further evidence for the association of benzene with these polar solutes is provided by molecular relaxation times.

Molecular polarisability. The dipole moments and Molar Kerr constants of the dinitrobenzenes

Dipole moments and Molar Kerr constants at 25° are reported for 1,2- and 1,3-dinitrobenzene as solutes in carbon tetrachloride, cyclohexane, benzene, and mesitylene, and for 1,4-dinitrobenzene in benzene. The results provide information concerning the configurations of these molecules in the dissolved state. In 1,2-dinitrobenzene the nitro-groups are substantially rotated from coplanarity with the benzene ring. The data for 1,3- and 1,4-dinitrobenzene are compatible with the presence in solution of the resonance-stabilised planar configurations. The stereochemistry of complexes formed by the dinitrobenzenes with benzene and mesitylene is investigated.

Molecular polarisability. The conformations of pyridine-2-aldehyde, 2-acetylpyridine, and α-pyridil as solutes in benzene

Pyridine-2-aldehyde, 2-acetylpyridine, and α-pyridil have dipole moments of 3·35, 2·87, and 4·43 D, respectively, and molar Kerr constants of 578, 396, and –268 × 10–12. Data for the aldehyde and methyl ketone are consistent with mixtures of planar s-trans- and s-cis-conformers, with the trans-species predominating. The effective conformation of α-pyridil is a skew structure in which the dihedral angle between the C5H4N·CO planes is 80–83°; each C5H4N·CO moiety is assumed to adopt a flat s-trans arrangement.

Molecular polarisability. The dipole moments, molar Kerr constants, and conformations as solutes of thioanisole and some para-substituted thioanisoles

Dipole moments and molar Kerr constants are reported for the molecules p-X·C6H4·SMe (X = H, Cl, Br, or NO2) examined as solutes in carbon tetrachloride and in benzene. The experimental data are analysed to provide information on the preferred solute conformations and on the incidence and stereospecificity of benzene–solute complex formation.