Electron correlation and relativistic effects in the secondary NMR isotope shifts of CSe2

Abstract

Secondary isotope effects on nuclear shielding provide an experimentally well-defined reference point of quantum-chemical methodology. We carry out a quantum-mechanical treatment of thermal rovibrational motion in the linear CSe₂ molecule and combine it with relativistic modeling of ⁷⁷Se and ¹³C nuclear magnetic shieldings. The effects of electron correlation are studied at nonrelativistic (NR) ab initio and both NR and relativistic density functional theory (DFT) levels. Fully relativistic 4-component Dirac-DFT (D-DFT) as well as Breit–Pauli perturbation theory (BPPT) are used to produce the relativistic shielding surfaces. Quantitative agreement with the experimental secondary isotope effects can be obtained by a piecewise combination of a high-level ab initio NR shielding surface with the more approximate, albeit reasonable-quality relativistic corrections by DFT at either Dirac or BPPT levels of theory, when operating at the basis-set limit. Using a high-quality potential energy surface is also crucial, which further underlines the highly demanding nature of modeling of the isotope shifts.