Origin of 77Se NMR chemical shifts revealed for pre-α, α, β, and γ effects

The origin of delta(Se) is revealed based on the MO theory. The pre-alpha effect in H2Se, defined in the text, is the generation of double (Se-H) and sigma*(Se-H) through the protonation to spherical Se2-. The extension of HOMO-n (n = 0, 1, and 2) over the whole molecule of Me2Se is mainly responsible for the alpha effect. The beta effect originates not from the occupied-to-unoccupied transitions but from the occupied-to-occupied transitions. The gamma effect of the upfield shifts is the results of the subtle balance between two types of transitions. Contributions from each MO (psi(i)) and each psi(i) --> psi(a) transition are evaluated separately, using a utility program (NMRANAL-NH03G).