Gas-phase energy differences between the Z and E rotamers and the rotational barrier heights of methyl formate and trifluromethyl formate: an ab initio study

Methyl formate and its mono and difluoro derivatives: conformational manifolds, basicity, and interaction with HF theoretical investigation

The conformational manifolds, scenarios of protonation, and hydrogen bond propensity of methyl formate and its mono and difluoro derivatives, which possess two oxygen atoms with different basicities, are studied at the B3LYP/6-311++G(3df,3pd) computational level. The optimized geometries of the title molecules, their energetics, and relevant harmonic vibrational frequencies, mainly of the ν(CH) mode of the H-C═O group, are of a primary focus. The Natural Bond Orbital analysis is invoked to obtain the second-order intra- or intermolecular hyperconjugation energies, occupations of antibonding orbitals, and hybridization of the carbon atoms. It is demonstrated that the Z conformers (and their rotamers) of the three title molecules are characterized by a higher stability compared to the E ones. The stabilities depend on the intramolecular hyperconjugative interaction and on the attraction or repulsion nonbonded interaction. The proton affinity of the carbonyl oxygen exceeds, by 15-20 kcal·mol(-1), that of the methoxy oxygen. Fluorine substitution causes a moderate lowering of the proton affinity of the oxygens. Protonation on the oxygen atoms yields a contraction of the C-H bond and large concomitant blue shift of the ν(CH) vibration. These changes are mainly determined by a lowering of the occupation of the corresponding σ*(CH) orbitals. The esters under consideration are probed on the interaction with the HF molecule. The complexes that are formed under this interaction on the oxygen of the H-C═O group are stronger than those formed on the oxygen belonging to the methoxy one. It is deduced that the hydrogen bond energies show a linear dependence on the proton affinities of the corresponding oxygen atoms. Hydrogen-bonded complexes of moderate strength are also formed, while HF interacts with the fluorine atoms of the fluorinated esters.