Nonplanar Conformers and the Phase Behavior of Solid n-Alkanes

Solid-solid phase transitions of the odd n-alkanes n-C₁₇H₃₆ through n-C₂₉H₆₀ were studied by using differential scanning calorimetry (DSC) and infrared (IR) spectroscopy. Two phase transitions were found in C₂₅, C₂₇, and C₂₉ in addition to the previously reported highest-temperature solid-solid transition (the so-called “rotator” transition). IR spectra revealed that, as the temperature is raised, the concentration of nonplanar conformers successively increases through each phase transition. Three types of nonplanar defects have been identified in the highest-temperature phase of all the n-alkanes: “end-gauche” (gt…), “kink” (…gtg′…), and “double-gauche” (…gg…). The end-gauche defect was observed in the lower-temperature phases as well. Kink conformers, however, were observed only in the highest-temperature phase while double-gauche defects were found in measurable concentrations only within a few degrees of the melting point. The concentrations of nonplanar conformers in the highest-temperature phases increase with increasing chain length. For example, roughly 70% of C₂₉ molcules are nonplanar prior to melting, in contrast to 5–10% for C₁₇. The relationship between the existence of nonplanar conformers and the various distinct solid phases presents a major puzzle.