Zero-point vibrational contributions to fluorine shieldings in organic molecules

Abstract

Zero-point vibrationally averaged nuclear shielding constants have been calculated for 24 fluoro-containing organic compounds ranging in size from fluoromethane to difluorobenzene. A perturbation theory approach in which the contribution from the anharmonicity of the potential surface is included by shifting the geometry of the molecule has been used for calculating zero-point vibrationally averaged nuclear shieldings. In addition, the leading contribution arising from the curvature of the shielding surface is included. Results are presented for all isotropic nuclear shieldings with special attention paid to the fluorine shieldings. The zero-point vibrational contribution to the fluorine shieldings varies from −0.6 ppm for m-difluorobenzene to −35 ppm for 2-fluorobutane and it can therefore not be neglected in comparison to experiment, even for relative chemical shifts. In contrast to proton shieldings, for which transferable atom-type zero-point vibrational contributions have been established, it seems difficult to determine transferable zero-point vibrational contributions for the fluorine shieldings. For the molecules included here, the range of the zero-point vibrational contribution corresponds to approximately 18% of the range of the electronic contribution for both the fluorine and proton shieldings.