Novel Substituent Effect on (77)Se NMR Chemical Shifts Caused by 4c-6e versus 2c-4e and 3c-4e in Naphthalene Peri Positions: Spectroscopic and Theoretical Study.

delta((8)Se) values for 1-[8-(p-YC(6)H(4)Se)C(10)H(6)]SeSe[C(10)H(6)(SeC(6)H(4)Y-p)-8']-1' (1: Y = H, OMe, Me, Cl, Br, COOEt, and NO(2)) showed a good correlation with those of 1-(MeSe)-8-(p-YC(6)H(4)Se)C(10)H(6) (2). While the delta((1)Se) values correlated well with delta((8)Se) in 2 with a positive proportionality constant of 0.252 (regular correlation), a similar correlation for 1 gave a negative proportionality constant of -0.282 (inverse correlation). To clarify the mechanism associated with the inverse correlation in 1, together with the regular correlation in 2, ab initio MO calculations, containing the GIAO magnetic shielding tensor for the Se nucleus (sigma(Se)), were performed on p-YC(6)H(4)(A)SeH- - -(B)SeH-(B)SeH- - -H(A)SeC(6)H(4)Y-p (3: model of Se(4) 4c-6e for 1) and on p-YC(6)H(4)(A)SeH- - -(B)SeH(2) (4 and 5: models of Se(2) pi type 2c-4e and (A)Se- - -(B)Se-H 3c-4e for 2, respectively) with the 6-311+G(2d,p) basis sets at B3LYP and/or HF levels. The characteristic nature of the substituent effects on atomic charges and delta(Se) values in 3 is demonstrated to be Y(delta-)<--C(6)H(4)-Se(delta)(+)- - -Se(delta)(+)-Se(delta)(+)- - -Se(delta)(+)-C(6)H(4)-->Y(delta-) and Y(delta-)<--C(6)H(4)-Se(down)- - -Se(up)-Se(up)- - -Se(down)-C(6)H(4)-->Y(delta-), respectively, (Y = electron-withdrawing) and in 5 is Y(delta-)<--C(6)H(4)-Se(delta)(+)- - -Se(delta)(-)-H(delta)(+) and Y(delta-)<--C(6)H(4)-Se(down)- - -Se(down)-H(down), respectively. In the case of 4, a substantial contribution through the naphthylidene group is suggested. These results indicate that the nature of the interaction between the linear four Se atoms in 1 is of the 4c-6e type and that between the two Se atoms in 2 is pi type 2c-4e and/or 3c-4e according to the conformations around the Se atoms. The observed NMR parameters are well explained by model calculations on 3-5. Plots of (4)J((1)Se,(8)Se) versus delta((8)Se) of 1 and 2 gave good correlations with negative proportionality constants, which indicates that the J values become larger as the electron density on the (8)Se atoms increases.