Orientational Effect of Aryl Groups on 77 Se NMR Chemical Shifts: Experimental and Theoretical Investigations

The orientational effect of p ‐YC 6 H 4 (Ar) on δ (Se) is elucidated for ArSeR, based on experimental and theoretical investigations. The effect is examined in the cases in which SeC R in ArSeR is either in the Ar plane (pl) or is perpendicular to the plane (pd). 9‐(Arylselanyl)anthracenes ( 1 ) and 1‐(arylselanyl)anthraquionones ( 2 ) are employed to establish the effect in pl and pd, respectively. Large upfield shifts are observed for Y=NMe 2 , OMe, and Me, and large downfield shifts for Y=COOEt, CN, and NO 2 in 1 , relative to Y=H, as is expected. Large upfield shifts are brought by Y=NMe 2 , OMe, Me, F, Cl, and Br, and downfield shifts by Y=CN and NO 2 in 2 , relative to Y=H, with a negligible shift by Y=COOEt. Absolute magnetic shielding tensors of Se ( σ (Se)) are calculated for ArSeR (R=H, Me, and Ph), assuming pl and pd, based on the DFT‐GIAO method. Observed characters are well explained by the total σ (Se). Paramagnetic terms ( σ p (Se)) are governed by ( σ p (Se) xx + σ p (Se) yy ), in which the direction of n p (Se) (constructed by 4p z (Se)) is set to the z axis. The main interaction in pl is the n p (Se)–π(C 6 H 4 )–p z (Y) type. The Y dependence in pl occurs through admixtures of 4p z (Se) in π(SeC 6 H 4 Y) and π*(SeC 6 H 4 Y), modified by the conjugation, with 4p x (Se) and 4p y (Se) in σ(CSeX) and σ*(CSeX) (X=H or C) under a magnetic field. The main interaction in pd is the σ(CSeX)–π(C 6 H 4 )–p x (Y) type, in which SeX is nearly on the x axis. The Y dependence in pd mainly arises from admixtures of 4p z (Se) in n p (Se) with 4p x (Se) and 4p y (Se) in modified σ*(CSeX), since n p (Se) is filled with electrons. It is demonstrated that the effect of Y on σ p (Se) in the pl conformation is the same regardless of whether Y is an electron‐donor or electron‐acceptor, whereas for pd conformations the effect is greater when Y is an electron donor, as observed in 1 and 2 , respectively. Contributions of each molecular orbital and each transition on σ p (Se) are evaluated, which enables us to recognize and visualize the effect clearly.