Thursday, April 20, 2006

Chem. Eur. J., 77Se chemical shifts, experiment and theory

Andre, please check out....

Chemistry - A European Journal

Volume 12, Issue 14 , Pages 3829 - 3846

Orientational Effect of Aryl Groups on 77Se NMR Chemical Shifts: Experimental and Theoretical Investigations
Waro Nakanishi, Prof. 1 *, Satoko Hayashi, Dr. 1, Daisuke Shimizu 1, Masahiko Hada, Prof. 2
1Department of Material Science and Chemistry, Faculty of Systems Engineering, Wakayama University, 930 Sakaedani, Wakayama 640-8510, Japan, Fax: (+81) 73-457-8253
2Department of Chemistry, Faculty of Graduate School of Science, Tokyo Metropolitan University, 1-1 Minami-osawa, Hachioji-shi, Tokyo 192-0397, Japan

ab initio calculations • aryl selenides • NMR spectroscopy • orientational effects • selenium • substituent effects

The orientational effect of p-YC6H4 (Ar) on (Se) is elucidated for ArSeR, based on experimental and theoretical investigations. The effect is examined in the cases in which SeCR 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=NMe2, OMe, and Me, and large downfield shifts for Y=COOEt, CN, and NO2 in 1, relative to Y=H, as is expected. Large upfield shifts are brought by Y=NMe2, OMe, Me, F, Cl, and Br, and downfield shifts by Y=CN and NO2 in 2, relative to Y=H, with a negligible shift by Y=COOEt. Absolute magnetic shielding tensors of Se ((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 (Se). Paramagnetic terms (p(Se)) are governed by (p(Se)xx+p(Se)yy), in which the direction of np(Se) (constructed by 4pz(Se)) is set to the z axis. The main interaction in pl is the np(Se)-(C6H4)-pz(Y) type. The Y dependence in pl occurs through admixtures of 4pz(Se) in (SeC6H4Y) and *(SeC6H4Y), modified by the conjugation, with 4px(Se) and 4py(Se) in (CSeX) and *(CSeX) (X=H or C) under a magnetic field. The main interaction in pd is the (CSeX)-(C6H4)-px(Y) type, in which SeX is nearly on the x axis. The Y dependence in pd mainly arises from admixtures of 4pz(Se) in np(Se) with 4px(Se) and 4py(Se) in modified *(CSeX), since np(Se) is filled with electrons. It is demonstrated that the effect of Y on 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 p(Se) are evaluated, which enables us to recognize and visualize the effect clearly.

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