PERTURBATION OF SPIN-DENSITY DISTRIBUTION DUE TO DEUTERIUM SUBSTITUTION

The EPR analysis of the anion radicals of 1,3,6,8-tetradeuteriopyrene and 1,3,4,5,6,8,9,10-octadeuteriopyrene in THF with K+ at 173 K shows that the observed coupling constant for the protons in the 2,7 positio ns is decreased in both anion radicals by 25 mG (Delta a(H) = -25 mG) from its value in the anion radical of pyrene itself. This implies tha t there is only a very small, if any, contribution to this reduction i n the spin density at the 2,7 positions resulting from deuterium subst itution in the 4,5,9,10 positions. Ab initio calculations (6-31G(*) a nd 6-311G(*) basis sets) carried out considering the C-D bond to be s horter than the C-H bond by 0.01 Angstrom predict the effects of octad euteriation to be 21 and 18 mG, respectively, in good agreement with e xperiment. The calculations do, however, predict an approximately equa l contribution from the two tetradeuterio substitution patterns, with the effect of 1,3,6,8-deuteriation being slightly larger than that of 4,5,9,10-tetradeuteriation.