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

Zero-point vibrationally averaged (rg(0)) structures were computed at the PBE0/SDD/6-31G* level for [Pt(35)Cl6](2-) and [Pt(37)Cl6](2-), for the [Pt(35)Cln(37)Cl5-n(H2O)](-) (n = 0-5), cis-Pt(35)Cln(37)Cl(4-n)(H2O)2 (n = 0-4), and fac-[Pt(35)Cln(37)Cl(3-n)(H2O)3](+) (n = 0-3) isotopologues and isotopomers. Magnetic (195)Pt shielding constants, computed at the ZORA-SO/PW91/QZ4P/TZ2P level, were used to evaluate the corresponding (35/37)Cl isotope shifts in the experimental (195)Pt 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.