How 77 Se NMR Chemical Shifts Originate from Pre‐α, α, β, and γ Effects: Interpretation Based on Molecular Orbital Theory

Plain rules founded in a theoretical background are presented that can be used to determine the structure of selenium compounds on the basis of δ (Se) data and to predict δ (Se) data from a given structure with satisfactory accuracy. As a first step to establish such rules, the origin of δ (Se) is elucidated on the basis of MO theory. The Se 2− ion was chosen as the standard for the analysis. The concept of the pre‐α effect is proposed, which is defined as the downfield shift due to protonation of a lone‐pair orbital of Se. The pre‐α effect of two protons in H 2 Se is explained by the generation of double σ(SeH) and σ*(SeH) through protonation of the spherical Se 2− ion. The orbitals, together with n p (Se), result in effective transitions for the pre‐α effect. The α effect is the downfield shift caused by the replacement of SeH by SeMe. The extension of HOMO−2 [4p y (Se)], HOMO−1 [4p x (Se)], and HOMO [4p z (Se)] over the whole Me 2 Se molecule is mainly responsible for the α effect. The β effect originates not from the occupied‐to‐unoccupied (ψ i →ψ a ) transitions but from the occupied‐to‐occupied (ψ i →ψ j ) transitions. Although ψ i →ψ j transitions contribute to upfield shifts in Me 2 Se, the magnitudes become smaller as the methyl protons are substituted by Me groups one after another. The γ effect of upfield shifts is also analyzed, although complex. The effect of p(Se)–π(CC) conjugation is analyzed in relation to the orientational effect. Contributions from each MO (ψ i ) and each ψ i →ψ a transition are evaluated separately, by using a utility program derived from the Gaussian 03 program suite (NMRANAL‐NH03G). The treatment enables us to visualize and understand the origin of 77 Se NMR chemical shifts.