Effects of a remote binding partner on the electric field and electric field gradient at an atom in a weakly bound trimer

Microwave spectra are reported for the C_3v symmetric complexes Kr-SO₃ and Kr-SO₃-CO. The S-C distance in the trimer, 2.871(9) Å, is the same as that previously determined for SO ₃-CO to within the estimated uncertainties. The Kr-S distances are 3.438(3) Å and 3.488(6) Å in Kr-SO₃ and Kr-SO ₃-CO, respectively, indicating that the addition of CO to Kr-SO ₃ increases the Kr-S distance by 0.050(9) Å. Measurements of the ⁸³Kr nuclear quadrupole coupling constants provide direct probes of the electric field gradient at the Kr nucleus, and a comparison between the two systems reflects the degree to which the CO influences the electronic structure of the krypton atom. Although the Kr and CO in the trimer are on opposite sides of the SO₃ and thus are not in direct contact, the addition of CO to Kr-SO₃ reduces the electric field gradient at the Kr nucleus by 18%. Calculations using the block localized wavefunction decomposition method are performed to understand the physical origins of this change. While the magnitudes of both the electric field and the electric field gradient at the Kr nucleus decrease upon addition of the CO to Kr-SO ₃, the changes are shown to arise from rather complex combinations of geometrical distortion, electrostatic, polarization, and electron transfer effects. For the electric field, the electrostatic term accounts for the largest portion of the reduction, while for the electric field gradient, polarization and structural change of the Kr-SO₃ moiety make the primary contributions. Despite significant changes in the electronic environment at the Kr nucleus, calculated binding energies indicate that the interactions are largely additive, with the binding energy of the trimer very nearly equal to the sum of the Kr-SO₃ and SO₃-CO binding energies.