The role of aromatic interactions in the structure and energetics of benzyl ketones

A qualitative and quantitative energetic and structural study of dibenzyl ketone (DBK) and benzyl ethyl ketone (BEK) was carried out in order to obtain insights into the type and magnitude of aromatic interactions that these systems present in their different phases. The crystal structure of DBK was obtained by X-ray crystallography, and it shows that the conformation adopted in the crystalline state is governed by the intermolecular interactions. The standard (p0 = 105 Pa) molar enthalpy of formation in the gaseous state at T = 298.15 K was derived by Calvet and combustion calorimetry. Using a homodesmic reaction scheme, the first calorimetric evaluation of the interaction enthalpy between two stacked phenyl rings is presented. A stabilizing enthalpic effect of (12.9 ± 4.9) kJ mol−1 associated with the intramolecular π–π interaction in DBK was found. The gas phase intramolecular π⋯π interaction in DBK is in agreement with quantum chemical calculations at B3LYP/6-311++G(d,p) and MP2 with various basis-sets. An intramolecular π⋯π interaction in DBK and a weak C–H⋯π interaction in BEK were found by variable-temperature 1H-NMR spectroscopy in MeOD. These observations are consistent with a hindered rotor interpretation, supported by ab initio calculations for the gas phase at the MP2/cc-pVDZ level. The global results indicate a distinct molecular structure on going from crystalline DBK to liquid, gas, and solution phases, ruled by the overall contribution of the intra- and intermolecular interactions.