Ab Initio and DFT Study of the 29Si NMR Chemical Shifts in RSiSiR
Miriam Karni and Yitzhak Apeloig, Nozomi Takagi and Shigeru Nagase
The syntheses of the first two disilynes were reported recently: the first by Wiberg and co-workers, who synthesized RSiSiR (1; R = SiMe(Si-t-Bu3)2), and the second by Sekiguchi and co-workers, who synthesized RSiSiR (2; R = Si-i-Pr[CH(SiMe3)2]2), which was also characterized by X-ray crystallography and NMR spectroscopy. We report the first detailed quantum-mechanical study of the 29Si NMR chemical shifts of disilynes, RSiSiR, in particular those with R = H, CH3, SiH3, SiMe(SiH3)2, SiMe(SiMe3)2, SiMe(Si-t-Bu3)2 (1), Si-i-Pr[CH(SiMe3)2]2 (2). The main conclusions are as follows: (1) Small changes in geometry (i.e., in r(SiSi), in the RSiSi bond angle, and in the RSiSiR torsion angle) strongly affect the chemical shift. (2) (29Si) values of the triply bonded silicon atoms in RSiSiR (R = H, SiH3) are -26 and 68 ppm, respectively (at MP2/6-311G(3d)//B3LYP/6-31G(d,p)), reflecting a significant effect of the silyl substituent. (3) (29Si) values calculated using the HCTH407 GGA functional are in excellent agreement with those calculated at the MP2 and CCSD levels of theory for model disilynes and with experimental chemical shifts measured for disilenes. This is therefore our recommended method for calculating (29Si) values of disilynes, especially with large substituents. A poorer agreement is observed when applying the commonly used hybrid B3LYP functional. (4) The calculated chemical shift of the triply bonded silicon atoms in 1 is in the range of 88 ± 5 ppm, in good agreement with the observed chemical shift of the product obtained by Wiberg, supporting his assignment. (5) The calculated (29Si) value in 2 is ca. 60 ppm, considerably upfield from the experimental chemical shift of 89.9 ppm (in solution), raising the possibility that the structure of 2 in solution is slightly different from that in the solid state.
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