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

The 77 Se NMR chemical shifts (δ obsd (Se)) of p -YC 6 H 4 SeMe ( 1: Y = H ( a ), OMe ( b ), Me ( c ), Cl ( d ), Br ( e ), COOR ( f ), and NO 2 ( g )) and p -YC 6 H 4 SePh ( 2 ) were determined or redetermined in chloroform- d . The δ obsd (Se) values of 2, p -YC 6 H 4 SeR (R = CN ( 3 ), Bz ( 4 ), H ( 5 ), Br ( 6 ), Et ( 7 ), C 6 H 4 Y- p ( 8 ), CH CH 2 ( 9 ), CH CHCl- t ( 10 ), and CHCH 2 CCl 2 - cyclo ( 11 )), 1,1‘-[8-( p -YC 6 H 4 Se)C 10 H 6 Se] 2 ( 12 ), and 1-(MeSe)-8-( p -YC 6 H 4 Se)C 10 H 6 ( 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 δ obsd (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 δ obsd (Se) values of 1 − 13 uniformly: δ calcd (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 δ calcd (Se:θ B ) = (1 − sin θ B )δ calcd (Se) 14 + sin (θ B )δ calcd (Se) 15 . The δ obsd (Se) values of 1 − 13 correlated well with the new parameters, δ calcd (Se:θ B ), which gave the best-fitted θ B values. The structures of 1 − 13 in solutions were explained uniformly by the evaluated θ B values. The observed ratios of the slopes for g( m ) versus those of g( n ) were also correlated with the θ B values.