Contribution of Hydrogen Bonds to Equilibrium αβ Transition of Resorcinol

Abstract

The equilibrium αβ transition temperature, T αβ, between the α crystalline phase and the β crystalline phase of resorcinol has been determined when both supercooling and superheating effects vanished. The latent volume and dT αβ/dP have been determined by use of a precision pycnometer, and the thermodynamic characteristics of the αβ transition are presented. The P − V − T relations of each phase have also been obtained. All of the thermal expansion coefficient, the isothermal compressibility, and the internal pressure for the higher-temperature, higher-density polymorph, the β crystal, are much larger than those for the lower-temperature, lower-density polymorph, the α crystal. Contributions of the hydrogen bonds and the van der Waals energy to the internal energy have semiquantitatively been analyzed by use of a simple potential model. When the α crystal transforms to the β crystal, the energy of hydrogen bonds decreases due to the breakdown of the hydrogen bonds, whereas the van der Waals energy increases with the contraction in volume. Since the latter cancels the most of the former, we observe the small latent heat. The breakdown of the hydrogen bonds induces a significant change in the potential depths of the covalent structure O−H···O and ionic, proton-transferred structure O-···H−O+ in each hydrogen bond. In consequence, the distribution of the protons between the covalent structure and the ionic structure changes with the transition. The increase in the entropy produced from the redistribution of the protons is the same order of magnitude as the latent entropy.

Capturing the Role of Temperature and Entropy in Crystal Polymorphs Through Molecular Simulations

“…Structures that can be isolated at one temperature may become kinetically unstable at higher temperatures. Furthermore, the thermodynamically most stable structure can also change with the temperature [18,46,[92][93][94][95][96][97][98][99][100][101][102][103] or pressure [76][77][78][79][80][81] leading to new observable forms.…”

“…Relative entropies between of polymorphs can be computed by measuring the temperature and enthalpy change at the transition between forms with DSC [93,98,101,102]. Comparison between estimated and measured entropies and free energies are presented in the systems in Chapter 3.…”

“…A complicating factor in the search for commercially viable solid phases is that the stable forms of a material can change based on external factors such as temperature [18,46,[92][93][94][95][96][97][98][99][100][101][102][103], pressure [76][77][78][79][80][81] and humidity [47,82,83,200]. A complete experimental mapping of all possible solid forms for a given material would therefore require screening at every condition the material will encounter.…”

“…Temperature-mediated polymorphic transformations involve a crystal structure that is globally stable at low temperatures becoming metastable at higher temperatures and subsequently restructuring to a more stable form. Polymorphs that reversibly change stability order as a function of temperature in this manner are referred to as enantiotropically related forms and have been observed experimentally for a number of real organic molecules [18,46,[92][93][94][95][96][97][98][99][100][101][102][103].…”

“…Structures that can be isolated at one temperature may become kinetically unstable at higher temperatures. Furthermore, the thermodynamically most stable structure can also change with the temperature [18,46,[92][93][94][95][96][97][98][99][100][101][102][103] or pressure [76][77][78][79][80][81] leading to new observable forms.The complete set of all 'observable polymorphs' for a given material therefore consists of all structures that can be produced from any crystallization method in any thermodynamic state of practical interest to the material. The challenge for experimental polymorph screening procedures is to cover a large enough range within this multidimensional space to achieve high confidence that another undiscovered form, which could compromise the performance of the product, will not be observed.…”