X-ray and thermal studies on the crystalline phases of normal alkanethiols n-CnH2n+1SH (n=18, 19, 22, 23, 24)

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Crystal structures, polymorphism, and phase transitions of five n-aklanethiols (n-CnH2n+1SH: CnSH), C18SH, C19SH, C22SH, C23SH, and C24HSH have been investigated by means of differential scanning calorimetry and X-ray diffraction. Normal alkanethiols with odd carbon number have only one crystalline form, the E2 form. While, n-alkanethiols with even carbon number have two low-temperature crystalline forms, the most stable E1 form and the meta-stable E1′ form. When even n-alkanethiols are crystallized from the solution (solution growth crystal: SGC), they crystallize into the E1 form. When they are cooled from the melt (melt growth crystal: MGC), on the other hand, the E1′ form appears. In all crystalline phases, two n-alkanethiol molecules are connected at SH end mutually by hydrogen bond resulting in forming a dimer in both forms. The SH groups and the CH3 groups are arranged on the respectively different layer surfaces resulting in the bilayer structure in the E1 form. In the E1′ form, on the other hand, although the molecules form dimers locally, the SH and methyl groups do not form the one layer surface, respectively, but are arranged disorderly. At temperature just below the melting point, characteristic high-temperature phase, the rotator phase, is observed in even n-alkanethiols as well as in n-alkane system.
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