Orientational Melting and Reorientational Motion in a Cubane Molecular Crystal: A Molecular Simulation Study

Abstract

Detailed molecular simulations are carried out to investigate the effect of temperature on orientational order in cubane molecular crystal. We report a transition from an orientationally ordered to an orientationally disordered plastic crystalline phase in the temperature range 425-450 K. This is similar to the experimentally reported transition at 395 K. The nature of this transition is first order and is associated with a 4.8% increase in unit cell volume that is comparable to the experimentally reported unit cell volume change of 5.4% (Phys. Rev. Lett. 1997, 78, 4938). An orientational order parameter, eta(T), has been defined in terms of average angle of libration of a molecular 3-fold axis and the orientational melting has been characterized by using eta(T). The orientational melting is associated with an anomaly in specific heat at constant pressure (C(P)) and compressibility (kappa). The enthalpy of transition and entropy of transition associated with this orientational melting are 20.8 J mol(-1) and 0.046 J mol(-1) K(-1), respectively. The structure of crystalline as well as plastic crystalline phases is characterized by using various radial distribution functions and orientational distribution functions. The coefficient of thermal expansion of the plastic crystalline phase is more than twice that of the crystalline phase.

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“…5), it can be seen that the plastic crystalline phases (phase II and phase I) have larger thermal expansion coefficients than the crystalline phase (phase III), which should be attributed to the orientational or rotational disorder in the plastic crystalline phases weakening the interactions between cations and anions and hence allowing a larger thermal expansion. 54…”

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“…Instead, MC algorithms incorporating changes in unit cell size and shape have been used to probe dynamic, orientationally disordered phases ('plastic phases') and the static distributions that can be obtained by cooling them. [37][38][39] These techniques are entirely suitable for the dynamically disordered systems or static systems with low barriers to rearrangement. Higher barriers to switching between components will prevent these approaches from exploring the configuration space, so they cannot be used for studying static, binary systems, excluding a whole class of molecular crystals from consideration.…”

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“…For this reason, the transition temperature from simulations is overestimated than the experimental values. 41 Moreover, the transition temperature also strongly depends on the force-field employed. 41 Since our aim is not to study this aspect in detail, we have studied the system in two different well-separated temperatures.…”

“…41 Moreover, the transition temperature also strongly depends on the force-field employed. 41 Since our aim is not to study this aspect in detail, we have studied the system in two different well-separated temperatures. The structures for the input geometry and the representative structures for low and high temperature PNIPAm polymer structures are shown in Fig.…”