Structural Study of Aryl Selenides in Solution Based on 77Se NMR Chemical Shifts: Application of the GIAO Magnetic Shielding Tensor of the 77Se Nucleus

The 77Se NMR chemical shifts (δobsd(Se)) of p-YC6H4SeMe (1: Y ) H (a), OMe (b), Me (c), Cl (d), Br (e), COOR (f), and NO2 (g)) and p-YC6H4SePh (2) were determined or redetermined in chloroform-d. The δobsd(Se) values of 2, p-YC6H4SeR (R ) CN (3), Bz (4), H (5), Br (6), Et (7), C6H4Y-p (8), CHdCH2 (9), CHd CHCl-t (10), and CHCH2CCl2-cyclo (11)), 1,1′-[8-(p-YC6H4Se)C10H6Se]2 (12), and 1-(MeSe)-8-(p-YC6H4Se)C10H6 (13) were plotted against those of 1. The plots were analyzed as two correlations. For example, the points corresponding to a-c make a group (g(m)), and those of d-g belong to another one (g(n)). This must be a reflection of the differences in the dihedral angles between the aryl rings and the Se-R bonds, which should result in the different contributions of the inductive and mesomeric effects of the substituents Y on the δobsd(Se) values. After reexamination of the applicability of the GIAO magnetic shielding tensor for the selenium nucleus (σ(Se)) in selenium compounds of various structures, σ(Se) was calculated for the model compounds, 5, with the B3LYP/6-311+G(d,p) method, to explain the δobsd(Se) values of 1-13 uniformly: δcalcd(Se) was defined as -(σ(Se) σ(Se)MeSeMe). Each selenol was optimized to be the planar structure (14) or the perpendicular one (15). New parameters were devised such as δcalcd(Se:θB) ) (1 sin θB)δcalcd(Se)14 + sin (θB)δcalcd(Se)15. The δobsd(Se) values of 1-13 correlated well with the new parameters, δcalcd(Se:θB), which gave the best-fitted θB values. The structures of 1-13 in solutions were explained uniformly by the evaluated θB values. The observed ratios of the slopes for g(m) versus those of g(n) were also correlated with the θB values.