Rotamerism and intramolecular interactions of n-propyl halides and their partially and fully fluorinated derivatives. Electronic structure, topology and vibrational spectroscopy

Abstract

n-Propyl halides (C3H7X; X = Cl, Br, I) and their partially and fully fluorinated analogues were investigated through quantum mechanical calculations and vibrational spectroscopy. Two energetic minima, corresponding to the rotational isomers (s-trans and s-gauche) were identified for all the species. The Bader quantum theory of Atoms-In-Molecules (AIM) analysis identified some bond and ring critical points for CF3-type gauche compounds involving F atoms on the CF3 group and X and F atoms on the CXF2 group. Such non-covalent intramolecular interactions are found not only responsible for the physical properties of n-propyl halides, but also the features of 19F NMR of n-perfluoropropyl iodide. The comparison between experimental vibrational spectra of some compounds and the calculated ones in both harmonic and anharmonic approximation indicated the frozen phase of these compounds is dominated with s-trans isomers, whereas the liquid phase is a mixture of both isomers. Rovibrational coupling was hypothesized to be responsible for the anharmonicity reflected in the vibrational spectra of C3H7X (X = Br and I). Based on the maps of Molecular Electrostatic Potential (MEP) surfaces, a σ-hole was featured for all 24 compounds in both isomers. The thermodynamics of isomerisation and fluorination was thoroughly analysed and well supported by the Highest Occupied Molecular Orbitals (HOMOs) and MEP.