Benzene electron diffraction

In the case of polyatomic molecules the radial distribution curve as deduced from electron-diffraction gas experiments may also be considered as a kind of a weighted sum of p ip curves for the internal motion in the molecule, but here all intemuclear distances are inseparably mixed together in a one-dimensional representation. For a rigid molecule, such as carbon tetrachloride or benzene, electron diffraction may produce quite accurate information as to the geometry of the molecule. As to the internal motion of the molecule, vibrational amplitudes may be deduced and compared with the corresponding data, differently but usually considerably more accurately, obtained by spectroscopic methods. How this is actually done in practice is perhaps most elegantly described by S. Cyvin4). 

Structural parameters and interatomic distances derived from electron diffraction (7) (77JST(42)l2i) and X-ray diffraction (8) studies (76AX(B)3178) provide unequivocal evidence that pyrazine is planar with >2a symmetry. There is an increased localization of electron density in the carbon-nitrogen bonds, with carbon-carbon bonds being similar in length to those in benzene. ... 

An electron-diffraction study of benzene, p-xylene, mesitylene, and hexamethylbenzene has been made recently by Jones, 0 who, on the basis of measurements extending to To = 14, reported the values Ca,. —Caj = 1.50 = = 0.01 A. and Car — Car = 1.40 = = 0.01 A. Jones t0 values are about 1.3% greater than ours, and comparison of them with our calculated values of s.4 leads to... 

The Electron Diffraction Investigation of the Structure of Benzene, Pyridine, Pyrazine, Butadiene-1,3, Cyclopentadiene, Furan, Pyrrole, and Thiophene... 

The results of the electron diffraction investigation reported in this paper are collected in Table X. The re-investigation of benzene confirms the value 1.39 A. for the C-C distance and provides a rough experimental value for the C-H distance. In pyridine and pyrazine the C-N distance is greater than expected for Kekuld resonance the effect is attributed to extra reso-... 

Low-energy electron diffraction has been used by Pitkethly and others to investigate the chemisorption of benzene, at pressures up to 10-7 Torr and at temperatures ranging from ambient to about 500°C, on the (100) (22), (110) (23,24) and (111) (22,24) faces of nickel single crystals. 

The deformation of the benzene ring in substituted benzenes is a sensitive indicator of substituent effects. Extensive experimental evidence accumulated over the past two decades, mainly from X-ray diffraction studies of solid state samples However, the first report of a ring distortion in a benzene derivative was done by Keidel and Bauer in their pioneering (1956) gas-phase electron diffraction study of the molecular structure of phenylsilane Recently a... 

Originally the electron diffraction data on toluene have been analysed assuming D tt syn " metry for the benzene ring ). Later this constraint was removed and the carbon ring was allowed to assume C2v symmetry ). The results of this later work are cited here... 

The deformation of the benzene ring has also been determined in the electron diffraction study of o-phenylene-sulfite by Schultz et al. °. This seemingly com-... 

Other studies, such as infrared and Raman spectra of gaseous benzene, neutron diffraction studies of crystalline benzene, and electron diffraction and microwave spectral studies, are equally incapable, according to critical analysis [87AG(E)782], of resolving unanimously the Dih—Deh structural dilemma of the benzene molecule. Furthermore, no decisive conclusion could be drawn from photoelectron spectra or H—NMR spectrum measurements of benzene molecules in a liquid crystal environment. The latter experiments merely indicate that the average lifetime of a Dih structure (if it appears on the PES) is less than 10 4 sec corresponding to the energy barrier of the Dih- >D6h-+D h interconversion of approximately 12 kcal/mol. 

Schomaker, V., and L. Pauling The electron diffraction investigation of the structure of benzene, pyridine, pyrazine, butadiene-1,3, cyclopenta-diene, furan, p5rrrole, and thiophene. J. Amer. chem. Soc. 61,1769 (1939). 

The situation is further complicated by the fact that under different physical conditions identical conformations are not always found for the same molecules. This is exemplified by the molecule biphenyl, the two benzene rings of which have been indicated by electron diffraction in the gas phase (Almenningen and Bastiansen, 1958 Bastiansen, 1949 Karle and Brockway, 1944) to make an angle of 45° with one another, whereas in the crystal the two phenyl groups are parallel by crystal symmetry (Dhar, 1932 Robertson, 1961). 

Of the dihalogenobenzenes, the 1,2-dibromo- and dichloro-isomers have been studied in the gas phase by electron diffraction by Bastiansen and Hassel (1947). The diffraction patterns are interpreted in terms of a puckered molecule with the C—Cl bonds bent out of the benzene plane by about 18°. 

The X-ray crystal structure of benzene, proving the equivalence of the six C-C bonds, appeared in 1929 and 1932, and Pauling reported its electron-diffraction data in 1931. Note that several of the structures proposed in the 19th century, such as Dewar benzene (non-planar) and Ladenburg s prismane, which are valence isomers of benzene, have now actually been prepared from benzene derivatives photochemically. They are kinetically stabilized, since they do not spontaneously revert to benzene or its derivatives [17-20]. 

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