NMR shielding constants in SeH₂ and TeH₂

Abstract

Ab initio values of nuclear magnetic resonance shielding constants in SeH₂ and TeH₂ are calculated. We analyse the role of electron correlation, the magnitude of relativistic effects, the dependence of these results on the basis set and, in addition, the rovibrational effects. The relativistic effects on the isotropic shielding constants calculated with the Dirac–Coulomb Hamiltonian at the Hartree–Fock and Spin-Density Functional Theory levels are very similar, and the electron correlation effects are much smaller. For both molecules, however, the electron correlation contribution to the anisotropy of the heavy atom shielding is much more significant than for the isotropic constant. The total shielding constants derived by adding to the non-relativistic coupled-cluster values the Dirac–Hartree–Fock values of the relativistic effects, σ^300 K _iso(Se) = 2447 ppm and σ^300 K _iso(Te) = 4809 ppm, may be applied to define the absolute shielding scales for these heavy nuclei.

Keywords:

• ab initio calculations
• nuclear magnetic resonance
• shielding constants
• relativistic effects

Acknowledgements

We are indebted to Trond Saue for reading the manuscript and for the valuable comments. We acknowledge a grant of computer time from the Norwegian Supercomputing Program (Notur) and partial support by the National Science Centre (Poland) grant, according to the decision No. DEC-2011/01/B/ST4/06588.

Dedicated to Trygve Helgaker on the occasion of his 60th birthday.

Notes

^aUncontracted aug-cc-pVTZ basis is used for hydrogen atoms.

^bThe same basis set is used for hydrogen atoms.

^cUncontracted aug-cc-pVQZ basis is used for hydrogen atoms.

^aDyall.acv4z/aug-cc-pVTZ basis set. DFT values obtained with Lévy–Leblond Hamiltonian and point nucleus model.

^aT and Q denote aug-cc-pVTZ and aug-cc-pVQZ hydrogen basis sets, respectively.

^aThis work; SeH₂ - Dyall.acv4z/aug-cc-pVTZ, TeH₂ - Dyall.acv4z/aug-cc-pVQZ basis set. KT2 functional was applied in DFT and SDFT calculations.

^bThe quoted non-relativistic reference values were obtained at the Hartree–Fock level, unless stated otherwise.

^c4-component Dirac–Coulomb Hartree–Fock calculations with common gauge origin at the heavy atom.

^dA sum of non-relativistic terms and relativistic corrections, calculated with the Breit-Pauli perturbation theory [74].

^eSecond-order Douglas–Kroll–Hess Coupled Hartree–Fock (CHF) method; common gauge origin at the heavy atom; σ^rel _iso(Te) = 4657.6 ppm and σ^rel _iso(H) = 41.49 ppm in TeH₂ in another approximation.

^fBarysz–Sadlej–Snijders CHF method; common gauge origin at the heavy atom.

^gRelativistic RPA approach, Restricted Kinetic Balance (RKB) results. The corresponding Unrestricted Kinetic Balance (UKB) values are 2415.27 ppm for Se and 4725.05 ppm for Te.

^hQuasi-relativistic generalised unrestricted Hartree–Fock (QR-GUHF) calculations.

ⁱMP2 and quasi-relativistic generalised unrestricted MP2 (QR-GUMP2) calculations.