On the Origin of Cl-35/37 Isotope Effects on Pt-195 NMR Chemical Shifts. A Density Functional Study

Zero-point vibrationally averaged (r(g)(0)) structures were computed at the PBE0/SDD/6-31G* level for [(PtCl6)-Cl-35](2-) and [(PtCl6)-Cl-37](2-), for the [(Pt35ClnCl5-n)-Cl-37(H2O)](-) (n = 0-5), cis-(PtClnCl(4-n))-Cl-35-Cl-37(H2O)(2) (n = 0-4), and fac-[(PtClnCl(3-n))-Cl-35-Cl-37(H2O)(3)](+) (n = 0-3) isotopologues and isotopomers. Magnetic Pt-195 shielding constants, computed at the ZORA-SO/PW91/QZ4P/TZ2P level, were used to evaluate the corresponding Cl-35/37 isotope shifts in the experimental Pt-195 NMR spectra. While the observed effects are reproduced reasonably well computationally in terms of qualitative trends and the overall order of magnitude (ca. 1 ppm), quantitative agreement with experiment is not yet achieved. Only small changes in Pt-Cl and Pt-O bond lengths upon isotopic substitution, on the order of femtometers, are necessary to produce the observed isotope shifts.