Enthalpy of formation of 6-phenyl-1,5-diazabicyclo[3.1.0]hexane by combustion calorimetry and theoretical approach for efficient prediction of thermochemistry of diaziridines.

The combustion energy and standard molar enthalpy of formation of crystalline 6-phenyl-1,5-diazabicyclo[3.1.0]hexane (PDABH) were determined using an isoperibolic calorimeter with a static bomb. PDABH is the first diaziridine for which the experimental value of the enthalpy of formation was obtained. This value was validated by the theoretical values of gas phase enthalpy of formation and enthalpy of sublimation. The gas phase enthalpy of formation was calculated using the DLPNO-CCSD(T1)/CBS method in conjunction with isodesmic-type reactions. This method was chosen in comparison to another high quality evaluative method (G4), which has been shown to provide unreliable results for cyclic nitrogen containing compounds. The descriptors of the molecular electrostatic potential (MEP) were used to estimate the enthalpy of sublimation of PDABH. The proposed MEP model is based on experimental enthalpies of sublimation for 75 compounds structurally similar to PDABH. The high-level ab initio calculations of gas phase enthalpies of formation combined with enthalpies of sublimations estimated using descriptors of MEP allow predicting the enthalpies of formation of diaziridines in the solid phase.

Low-temperature heat capacity and pseudorotation in 2-methyltetrahydrofuran.

The heat capacity and phase transitions of 2-methyltetrahydrofuran in the temperature range from 7 to 350 K were measured using adiabatic calorimetry. The smoothed molar thermodynamic functions in the condensed state were determined on the basis of these measurements. The thermodynamic functions of the formation were also calculated. The gas-phase entropy at 298.15 K was obtained using the entropy of crystal 2-methyltetrahydrofuran, entropy of fusion and entropy of vaporization. This entropy value was used to clarify some aspects of the discrepancies between the interpretations of pseudorotation in 2-methyltetrahydrofuran. An extended quantum chemical study of pseudorotation in 2-methyltetrahydrofuran was undertaken to provide additional insight into the conformational features of a molecule. The contribution of pseudorotation to the entropy was calculated for different pseudorotational potentials constructed using various theoretical models. The experimental entropy value is in best agreement with intramolecular conversion between two low energy conformers connected by the transition state with an energy of about 4 kJ mol-1. The standard thermodynamic properties of 2-methyltetrahydrofuran in the gaseous state (T = 100 K to 1500 K) were calculated using experimental and theoretical molecular parameters.

Enthalpy formation of fluorene: a challenging problem for theory or experiment?

The large discrepancy between the experimental enthalpy of formation of fluorene and theoretical value calculated by the G3(MP2) method was revealed more than ten years ago. Three years later, a new experimental study of this compound was undertaken to ascertain whether there is any significant error in the thermochemical data. However, after this research, the agreement between theory and experiment was improved only slightly. In this work we decided to calculate the enthalpy of formation of fluorene using the high-level DLPNO-CCSD(T1)/CBS method which shows better results compared to Gn theories. To examine the accuracy of the available experimental data, the calculations were performed not only for fluorene but also for eleven fluorene derivatives. The discrepancy of about 9 kJ mol-1 between the experimental and theoretical enthalpies of formation of fluorene was confirmed by the present calculations, whereas good agreement was observed for the fluorene derivatives. It is highly unlikely that this discrepancy may disappear when using a higher-level theory. The possible reason for such inconsistency might be the experimental difficulty associated with the glass transition discovered in the stable crystalline state of fluorene. In this case, new experiments using the latest methods, such as differential scanning calorimetry combined with X-ray powder diffraction, are needed to gain deeper insight into the solid phase transformations of fluorene.