Crystal (structural) physics of mixed stack π–π*molecular compounds
Crystal (structural) physics of mixed stack π–π*molecular compounds
Thermodynamic measurements for a limited sample of crystalline mixed-stack 1:1 π-molecular compounds show that most are enthalpy-stabilized, some entropy-stabilized, and a few both enthalpy and entropy-stabilized. Correlation of these thermodynamic results with crystal structures remains a task for the future. Combination of optical spectroscopic methods at very low temperatures and electron spin resonance measurements have provided proof of Mulliken’s theory also for the solid state. The room temperature crystal structures of 1:1 π-molecular compounds are not necessarily representative of the entire range of pressure-temperature behaviour of these materials. There are often hints of disorder (usually of the donor) in the room temperature structures, and these have been correlated for a few systems with disorder-to-order transitions (thermodynamically second-order, following Ehrenfest) that occur on cooling. These have been studied by a combination of calorimetric, diffraction, and resonance techniques. Despite overall similarities, each system surveyed has its own individual characteristics. A number of 1:1 π-molecular compounds have been shown to transform to quasi-plastic phases on heating. A small number of mixed stack 1:1 π-molecular compounds with neutral ground states have been shown to transform to ionic structures on cooling or application of pressure.
Keywords: thermodynamic measurements, Mulliken theory, low-temperature optical spectroscopy, disorder phase transitions, second-order phase transitions, neutral ground state, ionic structures
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