X-Ray Diffraction in Liquids: A Comparison of Isomers of Normal Heptane and of Certain Carbon Chains

With the $K\alpha$ doublet of molybdenum, diffraction ionization curves are reported on synthetic decane, 2-2-4 trimethyl pentane, 2 methyl hexylene 2 and the following eight isomers of normal heptane obtained from the Ethyl Gasoline Corporation: 2-2-3 trimethyl butane, 3-3, 2-3, 2-2 and 2-4 dimethyl pentane, 3 ethyl pentane, 3 methyl hexane, and 2 methyl hexane. The experiments are a continuation of those of Stewart and Morrow, Stewart, and Stewart and Skinner. The diffraction peak of synthetic decane occurs at precisely the same angle as of eight of the other normal paraffins containing from five to fifteen carbon atoms so that the diameter of the normal paraffin chain throughout this range, as computed by Bragg's diffraction law, is 4.64A. The experience with normal decane was a repetition of that with normal pentane previously reported by the author. The length of the chain is in agreement with the formula, $L=(1.24n+2.70)\mathrm{}\times {10}^{-8\mathrm{}}$ cm if $n$ is the number of carbon atoms, and if the molecules may be regarded as longitudinally parallel and in square array in the plane normal to their lengths. The agreement is within less than one percent. With the exception of 2 methyl hexane wherein two diameters are found, and of 3 ethyl pentane which is a symmetrically branched structure, all the other heptane isomers, six in number, have diameters that are $1.04A$ larger than the normal paraffins. A consideration of this fact and of the alterations in density not in agreement with the formula just cited reach the conclusion that the molecular chains of the isomers are distorted either by bending or by another type of atom separation. A tentative conclusion is that the attachment of two C${\mathrm{H}}_3$ groups to the same atom may or may not increase the diameter by the same amount, this depending upon whether a third C${\mathrm{H}}_3$ branch is attached to an adjacent atom or to the next but one. The attachment of a C${\mathrm{H}}_3$ group on the next to end carbon atom seems, in the cases of 2 methyl hexane and 2 methyl hexylene 2, to produce two symmetrical branches of one C${\mathrm{H}}_3$ group each. Both this and the preceding conclusion are highly tentative but suggest the possibility of learning more of the nature of the chain by more extended experiments on isomers of normal paraffins. The foregoing experiments contribute not only to our knowledge of molecular structure but also to the evidence for the cybotactic state as one of importance in fundamental phenomena in liquids.