An examination by the x-ray diffraction ionization method was made of benzene, toluene, $o$-, $m$- and $p$-xylene, mesitylene, ethyl benzene and isopropyl benzene; cyclohexane, methyl cyclohexane, $o$-, $m$ and $p$-dimethyl cyclohexane; phenol, aniline, cyclohexanol and cyclohexanone; 2-hydroxy-1,3-dimethyl benzene and three isomers, $o$-, $m$- and $p$-toluidine, and $o$-, $m$- and $p$-cresyl methyl ether. The diffraction peaks are taken to be caused by the semi-orderly space arrangement of the molecules in the liquid, or cybotactic condition, and the distances of separation of the planes containing diffraction centers are computed by Braggs' diffraction law. Both assumptions have been previously proved justifiable.

Structure and dimensions of benzene and cyclohexane rings.—The structure of the benzene and cyclohexane rings are shown to be distinctly flat, having a thickness of 4.70A.u. and 5.10A.u. respectively. The proof of the flatness rests upon the effect of substitutions and the relative prominence of diffraction peaks. The general dimension in a plane perpendicular to the thickness is indicated by a diameter of approximately 6A.u., as indicated by an area of ${31.4(\mathrm{A}.\mathrm{u}.\mathrm{})\mathrm{}}^2$ for benzene and ${35.2(\mathrm{A}.\mathrm{u}.\mathrm{})\mathrm{}}^2$ for cyclohexane. These dimensions of the rings are based upon direct measurements and are limited in reliability only by the correctness of the theory of the x-ray measurements. The evidence is preponderately in favor of the maintenance of the general shape of the benzene and cyclohexane rings as units of structure. The similarity of the thickness to the diameter of $n$-paraffin and $n$-alcohol and monobasic $n$-fatty acids chains indicates the general correctness of the ring conception.

Effect of substituents on the thickness of the rings.—It is shown that the thicknesses of the benzene and cyclohexane "rings" depend upon the relative positions of the substituents. Four ortho, meta and para series, one with derivatives of cyclohexane and three with derivatives of benzene, show that the para substitutions give distinctly the least "thickness" of ring and that usually the meta gives less thickness than the ortho position. In a case of three substituents, variations caused by position were distinctly noticeable.

Other effects of substituents on the diffraction patterns.—Six compounds having the substituent OH, with one a cyclohexane derivative, were found to show "double" molecules produced by the juxtaposition of the two OH groups. None of the others showed this type of orientation. This evidence was obtained by a second semi-orderly arrangement of molecules with diffraction centers in planes separated by a distance of the magnitude of two molecular lengths. Ten compounds gave also an additional set of planes, caused more importantly by the positions than by the nature of the substituents. Of these ten, three, the hydroxy-dimethyl benzenes, showed the three sets of planes; namely, one corresponding to the thickness of the ring, one to an arrangement in a perpendicular direction caused by the substituents, and one in a third direction perpendicular to the other two sets and corresponding to the double molecule of the polar groups.

The above conclusions are in harmony with current investigations from other viewpoints and strengthen the theory of semi-orderly and temporary space arrangements of the molecules in a liquid, or the cybotactic condition.

• Received 20 March 1929

DOI:https://doi.org/10.1103/PhysRev.33.889