Ab initio and density functional methods studies on the conformations and thermodynamic properties of propyl nitrate

The conformational analysis has been performed on propyl nitrate by using ab initio and density functional methods. Seven energy minima corresponding to seven different conformers have been found and fully optimized at the B3LYP/6-31G∗ and MP2/6-311G∗ levels. The rotational transition states have been optimized too at the B3LYP/6-31G∗ level. The geometries obtained by the B3LYP agree well with those obtained by the MP2, and both methods predict that the gauche conformer with the ∠CCCO (t3) being twisted by about 63° has the lowest energy and is 1.71 (MP2) and 0.53 (B3LYP) kJ mol−1 more stable than the planar conformer, but the stability order of different conformers obtained by them are somewhat different. The energy differences between various conformers and the barriers to internal rotation are all very low. Torsion around ∠CCON decreases while torsion around t3 increases the stability by about 3.2 and 1.5 kJ/mol, respectively, from MP2 calculations. Vibrational analyses have been performed on the stable conformers at the B3LYP/6-31G∗ level. The standard thermodynamic functions and the equilibrium mole fractions have been derived. The heats of formation of various conformers have been estimated too from the B3LYP total energies and an isodesmic reaction. The calculated thermodynamic properties and heat of formation of propyl nitrate are in reasonable agreement with the experimental results.