NMR Indirect Spin–Spin Coupling Constants in a Modern Quasi-Relativistic Density Functional Framework

A quasi-relativistic
implementation of NMR indirect spin–spin
coupling constants is presented. The exact two-component (X2C) Hamiltonian
and its diagonal local approximation to the unitary decoupling transformation
(DLU) are utilized together with the (modified) screened nuclear spin–orbit
approach. In a restricted kinetic balance, the finite nucleus model
is available for both the scalar and vector potentials. The implementation
supports density functionals up to the fourth rung of Jacob’s
ladder, i.e., (range-separated) hybrid and local hybrid functionals
based on a seminumerical ansatz. We assess the quality of our quasi-relativistic
X2C approach by comparison with “fully” relativistic
four-component results for small main-group molecules and alkynyl
compounds. The mean absolute error introduced by the DLU scheme is
less than 0.05 × 1019 T J–2 of the
reduced coupling constant for the small main-group molecules and 0.5
Hz for the alkynyl compounds. Thus, the error is significantly smaller
than finite nucleus size effects for heavy elements. The basis set
convergence and the impact of different density functional approximations
are further studied. We propose a simple scheme to develop segmented-contracted
relativistic all-electron basis sets for NMR spin–spin couplings.
Our implementation allows us to perform calculations of extended molecules
with reasonable computational effort, which is illustrated for the 1J(119Sn, 31P) coupling
constant of a low-valent tin phosphinidenide complex. The corresponding
results are in good agreement with the experimental findings.