Hyperfine Structure in the Rotational Spectrum of GaF: A Comparison of Experimental and Calculated Spin–Rotation and Electric Field Gradient Tensors

Abstract

The high-resolution pure rotational spectrum of GaF has been measured using a Balle–Flygare-type Fourier transform spectrometer. Improved nuclear quadrupolar coupling constants and rotational constants have been obtained along with the first reported fluorine spin–rotation constant for gallium fluoride, CI (69Ga19F, v = 0) = +32.0(21) kHz. Accurate spin–rotation tensors from microwave or molecular beam spectroscopy are particularly important to NMR spectroscopists and theoreticians because these data provide information about anisotropic nuclear magnetic shielding in the absence of intermolecular effects. For quadrupolar nuclei such as gallium, the quadrupolar interaction is sufficiently large that it is very difficult to characterize shielding tensors directly via NMR spectroscopy. The experimentally determined nuclear quadrupolar coupling constants and spin–rotation constants for GaF are compared with the results of a series of high-level ab initio calculations carried out at various levels of theory with a range of basis sets. Further calculations on BF and AlF, supplemented with available experimental data for InF and TlF, allow for the investigation of trends in nuclear magnetic shielding, spin–rotation, and electric field gradient tensors in the group-13 fluorides. Calculations at the MP2/6-311++G** and MP2/6-311G(2df, 2pd) levels provide the most consistently satisfactory results in comparison with the experimental data.