Thermodynamically effective molecular surfaces for more efficient study of condensed-phase thermodynamics

Evaluation of molecular surfaces plays the key role in a wide range of cutting-edge scientific fields and technologies, due to the well-characterized dependency between molecular surfaces and condensed phase thermodynamics. Numerous methods to evaluate molecular surfaces such as van-der-Waals and solvent accessible surface areas and various parameterizations for each one, have been proposed in the literature and typically yield quite diverse estimations of molecular surfaces. Despite this diversity, numerous successful applications have been reported for each one, which has become possible via ad-hoc modifications and parametrizations employed to accommodate inappropriately defined molecular surfaces. The main aim of the present study is to propose “thermodynamically effective” molecular surface which unlike the conventionally accepted molecular surfaces, can be defined only uniquely, can be measured experimentally for each molecule directly and straightforwardly, is defined based on a well-characterized theoretically described dependency between molecular surfaces and solution thermodynamics, and is highly accurate in evaluating various thermodynamics quantities in solution for a wide temperature range and different types of molecules, without requiring any ad-hoc modification.

Properties and interactions – melting point of tribromobenzene isomers

Single crystals of isomeric 1,2,3-tribromobenzene (123TBB), 1,2,4-tribromobenzene (124TBB) and 1,3,5-tribromobenzene (135TBB) have been grown from different solvents and their structures determined by X-ray diffraction at 100, 200 and 270 K. The melting-point differences of ca 40 K between 135TBB, 123TBB and 124TBB have been correlated with the molecular symmetry and packing preferences in the crystal, as well as with the main types of intermolecular halogen interactions, i.e. Br...Br, Br...C (Br...π) and Br...H. The relationship between symmetry and melting point in Carnelley's rule has been extended to the accessibility of terminal atoms for the formation of intermolecular interactions, their occurrences and distribution, and the close packing. The electrostatic potential mapped on molecular surfaces demonstrates that in more symmetric molecules the more evenly distributed substituents are more accessible and form more optimum intermolecular interactions.