A Solid-State NMR and Theoretical Study of the 17O Electric Field Gradient and Chemical Shielding Tensors of the Oxonium Ion in p-Toluenesulfonic Acid Monohydrate

We report a solid-state 17 O NMR study of the 17 O electric field gradient (EFG) and chemical shielding (CS) tensors for the oxonium ion, H 3 O +, in p -toluenesulfonic acid monohydrate (TAM). Both the 17 O EFG and CS tensors of the H 3 O + ion are axially symmetric within the experimental errors. The 17 O quadrupole coupling constant (QCC) is found to be 7.05 ± 0.02 MHz, and the 17 O chemical shift anisotropy (CSA) is 87 ± 5 ppm. Experimental results are compared with extensive quantum chemical calculations using restricted Hartree−Fock approach (RHF), second-order Møller−Plesset perturbation theory (MP2), and density functional theory (DFT). The calculations showed that the strong hydrogen-bonding environment around the H 3 O + ion in TAM is responsible for a reduction of approximately 3 MHz in the 17 O QCC compared to that of an isolated H 3 O + ion. The effective 17 O quadrupole moment is calibrated at the B3LYP/cc-pVTZ level, Q = −2.400 fm 2 . Using this value, we obtained the best calculated 17 O QCC for the “bound” H 3 O + ion, +7.382 MHz, which is in reasonably good agreement with the observed value. The 17 O chemical shielding tensor is also calculated using the GIAO (gauge-including atomic orbital) approach. Although the calculated isotropic 17 O chemical shifts are in excellent agreement with the experimental data, the calculations with all the basis sets employed in the present study invariably underestimated 17 O CSAs by approximately 20 ppm.