How 77Se NMR Chemical Shifts Originate from Pre-α, α, β, and γ Effects: Interpretation Based on Molecular Orbital Theory
✍ Scribed by Waro Nakanishi; Satoko Hayashi; Masahiko Hada
- Publisher
- John Wiley and Sons
- Year
- 2007
- Tongue
- English
- Weight
- 378 KB
- Volume
- 13
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
Abstract
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.