直鎖長鎖状第一臭化物の相転移 (第1報) : 偶数炭素数の1-ブロモ-<I>n</I>-アルカン (C<SUB>22</SUB>〜C<SUB>28</SUB>) のX線的研究
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The structures and transition behaviors of the linear long chain primary bromides of even carbon atoms from 1-bromodocosane to 1-bromooctacosane were studied by X-ray powder diffraction method at various temperatures from ordinary temperature to their melting points, and the solid state transition mechanism of the compounds was discussed.<BR>Crystal structures at room temperature of all the bromides studied here belong to monoclinic system. On the assumption that the <I>c</I>-axis of the unit cell is parallel to the chain axes of the molecules, the lattice constants <I>a</I> (5.51Å), <I>b</I> (7.44Å) and β (117°) are found to be the same within the experimental erorr for all the bromides. The values of the lattice constant <I>c</I> increase with the increasing carbon number of the bromides, 62.4Å for 1-bromodocosane, 67.7Å for 1-bromotetracosane, 72.4Å for 1-bromohexacosane and 78.0Å for 1-bromooctacosane. The <I>c</I> values indicate an arrangement of double layer of molecules.<BR>Crystal structures of 1-bromodocosane and 1-bromotetracosane at higher temperatures belong to hexagonal system. The basal planes have the same size in both compounds. The lattice constant <I>c</I> of 1-bromodocosane and 1-bromotetracosane are 63.5Å and 67.8Å, respectively. The structures of 1-bromohexacosane and 1-bromooctacosane belong to monoclinic system. Assuming that the crystal axis <I>c</I> is parallel to the molecular chain axes, the lattice constants <I>a</I>, <I>b</I>, <I>c</I> and β in these compounds can be determind as follows; <I>a</I>=4.92Å, <I>b</I>=8.20Å, <I>c</I>=72.3Å and β=98° for 1-bromohexacosane, <I>a</I>=5.12Å, <I>b</I>=8.22Å, <I>c</I>=77.6Å and β=104° for 1-bromooctacosane.<BR>The mechanism of the solid state phase transition of 1-bromodocosane and 1-bromotetracosane can be interpreted as follows. At their transition points the lattice constants <I>a</I> and <I>b</I> change and the <I>c</I> axis of the crystal is thought to become perpendicular to the basal plane. This transition phenomenon refer to the rotational transition as a molecular rotation around the chain axis. A similar mechanism of the transition could be applied for 1-bromohexacosane and 1-bromooctacosane.
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