On the origin of 35/37Cl isotope effects on 195Pt 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 [Pt 35Cl 6] 2- and [Pt 37Cl 6] 2-, for the [Pt 35Cl n 37Cl 5-n(H 2O)] - (n = 0-5), cis-Pt 35Cl n 37Cl (4-n)(H 2O) 2 (n = 0-4), and fac-[Pt 35Cl n 37Cl (3-n)(H 2O) 3] + (n = 0-3) isotopologues and isotopomers. Magnetic 195Pt shielding constants, computed at the ZORA-SO/PW91/QZ4P/TZ2P level, were used to evaluate the corresponding 35/37Cl isotope shifts in the experimental 195Pt 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. © 2012 American Chemical Society.