Quantitative Estimate of the Resonance Effects in Some Unsaturated, Monocyclic, and Aromatic Hydrocarbons Based on the Renewed Optical Exaltations

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The present review discusses a new viewpoint on refractometry as the oldest experimental physical method, whose scientific potential in the estimation of structural effects in organic chemistry has been missed so far. The author demonstrates that upon certain adjustment and redesign of refractometry, this potential can be tapped and successfully used to determine a type of π-electron interaction, delocalization degree of π-electrons in organic compounds, and to perform quantitative estimates of resonance effects in unsaturated, (polycyclic) aromatic, and other polyconjugated systems (e.g., fullerenes). The method for accurate separation of molar refraction into additive and constitutive components was suggested; the method is based on the specially developed additive scheme. It was revealed that the negative deviations from additivity for cycloalkanes depend linearly on the number of carbon atoms in the ring. Excellent linear correlations between renewed optical exaltations, the number of π-electrons in a conjugated system, and experimentally found resonance energy (determined from hydrogenation heat values) were demonstrated. Angular coefficients of the correlation series (ρ-constants) are considered as a criterion of classification, which characterizes the degree of mobility of π-electrons in the conjugated system of a given type. It is emphasized that the development of methods for precise measurement of the constitutive components of molar refraction may become a useful additional source of information about resonance and other effects in organic and polymer chemistry.

The crystal and molecular structure of perylene

Crystals of perylene, C 20 H 12 , are monoclinic with four molecules in a unit cell of dimensions a = 11.27 7 , b = 10.82 6 , c = 10.26 3 Å, B = 100.55°, space group P 2 1 /a The intensities of 1135 reflexions were measured with a scintillation counter and Mo K a radiation, and the 436 strongest reflexions were used in the structure refinement. The gross features of the structure previously determined from two projections, have been confirmed by the new threedimensional data, the positional and thermal parameters of the carbon atoms have been refined by least-squares and differential syntheses, and the hydrogen atoms have been approximately located. There are small, but significant deviations from a completely planar arrangement ; this minor distortion of the molecule is the result of slight intermolecular steric effects, and does not appear to be associated with any possible overcrowding in the 1, 12 and 6, 7 positions. The mean bond distances agree fairly closely with the values predicted by valence-bond and molecular-orbital calculations; the peri -bond lengths are 1.471 + 0.006 Å. All the intermolecular separations correspond to normal van der Waals interactions; the perpendicular distance between mean molecular planes is 3.46 Å.

The crystal and molecular structure of perylene

Perylene, C 20 H 12 , crystallizes in the monoclinic system, space group P 2 1 / a , with four molecules in the unit cell. The structure resembles that of pyrene and 1:12-benzperylene, and has been determined by trial and error and Fourier series methods. In the principal electron-density projection eleven of the twenty carbon atoms in the molecule can be resolved, and the positions of the others can be estimated. The accuracy is not very high, but after averaging in accordance with the expected molecular symmetry the bond lengths can be determined to within about ± 0·04 Å. The results agree with molecular orbital calculations, and in particular it is found that a pair of unusually long bonds ( ~ 1·50 Å) connect the two naphthalene nuclei. The structure of the molecule is discussed in relation to other physical and chemical evidence. The carbon skeleton is at least approximately planar, but the 1,12 and 6,7 hydrogen atoms may deviate from the mean molecular plane.