Ab initio calculations of quadrupole interactions and their isotope effects

Calculations are presented of nuclear quadrupole coupling constants in simple closed-shell molecules containing hydrogen, together with their muoniumsubstituted counterparts. Results are given for HCl and MuCl, N₂H⁺ and N₂Mu⁺, and NH₄⁺ and NH₃MU⁺. The quadrupole isotope effect proves to be small, even for the ammonium ion, where isotopic substitution breaks the tetrahedral symmetry. The calculated quadrupole interactions serve to test the quality of the calculations, and also as a signature for these diamagnetic or closed-shell species: they may be compared with values measured by muon level crossing resonance, where known, or used to predict the fields for resonance, where not.