Relativistic Spin–Orbit Heavy Atom on the Light Atom NMR Chemical Shifts: General Trends Across the Periodic Table Explained

Abstract

The importance of relativistic effects on the NMR parameters in heavy-atom (HA) compounds, particularly the SO-HALA (Spin-Orbit Heavy Atom on the Light Atom) effect on NMR chemical shifts, has been known for about 40 years. Yet, a general correlation between the electronic structure and SO-HALA effect has been missing. By analyzing H NMR chemical shifts of the sixth-period hydrides (Cs-At), we discovered general electronic-structure principles and mechanisms that dictate the size and sign of the SO-HALA NMR chemical shifts. In brief, partially occupied HA valence shells induce relativistic shielding at the light atom (LA) nuclei, while empty HA valence shells induce relativistic deshielding. In particular, the LA nucleus is relativistically shielded in 5d-5d and 6p HA hydrides and deshielded in 4f, 5d, 6s, and 6p HA hydrides. This general and intuitive concept explains periodic trends in the H NMR chemical shifts along the sixth-period hydrides (Cs-At) studied in this work. We present substantial evidence that the introduced principles have a general validity across the periodic table and can be extended to nonhydride LAs. The decades-old question of why compounds with occupied frontier π molecular orbitals (MOs) cause SO-HALA shielding at the LA nuclei, while the frontier σ MOs cause deshielding is answered. We further derive connection between the SO-HALA NMR chemical shifts and Spin-Orbit-induced Electron Deformation Density (SO-EDD), a property that can be obtained easily from differential electron densities and can be represented graphically. SO-EDD provides an intuitive understanding of the SO-HALA effect in terms of the depletion/concentration of the electron density at LA nuclei caused by spin-orbit coupling due to HA in the presence of a magnetic field. Using an analogy between the SO-EDD concept and arguments from classic NMR theory, the complex question of the SO-HALA NMR chemical shifts becomes easily understandable for a wide chemical audience.

Low‐Valent Group 14 Phosphinidenide Complexes [({SIDipp}P)2M] Exhibit P–M pπ–pπ Interaction (M=Ge, Sn, Pb)

“…This does not hold for compound 5 as for lead spin-orbit effects are important for the magnetic properties. [49,50] A similar behavior is observed for the 13 C NMR shifts. As expected, the Wiberg bond index (WBI) for the PÀM bonds is greater than one for both bonds in all three compounds, especially for compound 3 ( Table 2).…”

13C NMR Shifts as an Indicator of U–C Bond Covalency in Uranium(VI) Acetylide Complexes: An Experimental and Computational Study

“…10 We note in passing that this finding is consistent with previous theoretical studies on HALA (heavy-atom-effects on the lightatom) "spin-orbit" shifts in transition-metal and actinide complexes. 34,36,[50][51][52][53][54][55][56] Similarly, owing to the large hydrogen 1s-orbital contribution to the An-H bonding, as-yet elusive uranium(VI)…”

Four-component relativistic time-dependent density-functional theory using a stable noncollinear DFT ansatz applicable to both closed- and open-shell systems

“…18,19 Furthermore, the present authors have demonstrated that a four-component methodology is applicable to systems with up to 100 atoms, for calculation of nuclear magnetic resonance and electron paramagnetic resonance parameters. [20][21][22][23][24] Relativistic approaches to LR-TDDFT can be loosely classified using a combination of two criteria. The a) Electronic mail: stanislav.komorovsky@savba.sk first criterion is the method by which SOC and other relativistic effects are accounted for.…”