Energetics and Reactivity of Morpholine and Thiomorpholine: A Joint Experimental and Computational Study

The influence of the heteroatoms in the conformational, energetic, and reactivity trends exhibited by morpholine and thiomorpholine isosteres was obtained from computational and experimental thermochemical studies. For those purposes, the gas-phase standard (<i>p</i>° = 0.1 MPa) molar enthalpies of formation of the compounds, at <i>T</i> = 298.15 K, were determined from the experimental values of the standard molar enthalpies of formation, in the liquid phase, and of the standard molar enthalpies of vaporization, obtained by calorimetric techniques, and also from composite G3­(MP2)//B3LYP calculations making use of appropriate working reactions. A very good agreement was found between the calculated and the experimental gas-phase enthalpies of formation. The computational study was further extended to the calculation of other gas-phase thermodynamic properties of these compounds, namely, the N–H or C–H bond dissociation enthalpies, gas-phase acidities and basicities, proton affinities and adiabatic ionization enthalpies, and the energies and structures of the conformational stereoisomers of morpholine and thiomorpholine