Origin of ⁷⁷Se NMR Chemical Shifts Revealed for Pre-α, α, β, and γ Effects

Abstract

The origin of δ (Se) is revealed based on the MO theory. The pre-α effect in H ₂ Se, defined in the text, is the generation of double σ(Se─H) and σ*(Se─H) through the protonation to spherical Se ²⁻ . The extension of HOMO-n (n = 0, 1, and 2) over the whole molecule of Me ₂ Se is mainly responsible for the α effect. The β effect originates not from the occupied-to-unoccupied transitions but from the occupied-to-occupied transitions. The γ effect of the upfield shifts is the results of the subtle balance between two types of transitions. Contributions from each MO (ψ _i ) and each ψ _i → ψ _a transition are evaluated separately, using a utility program (NMRANAL-NH03G).

Keywords:

• Ab initio calculations
• αβγ effects
• GIAO
• origin of ⁷⁷Se NMR chemical shifts
• paramagnetic shielding constants
• pre-α effect

Notes

^a In the σ (Se) scale;

^b values and the corresponding effects are shown;

^c observed values (see, ref. 1);

^d in the δ (Se) scale;

^e in DMF;

^f δ (Se) = −225.5 in neat;

^g gas phase;

^h δ (Se) = −115 in CDCl₃;

ⁱ in CDCl₃;

^j δ (Se) = 13.1 in gas phase.