Isotope shift silicon

According to the Born-Oppenheimer approximation, the potential function of a molecule is not influenced by isotopic substitution. Frequency shifts caused by isotopic substitution therefore provide experimental data in addition to the fundamentals which can yield information about the structure of a species. However, the half-widths of absorptions are too large to be resolved by the experimental techniques which are normally used, which is why these methods cannot reveal small isotopic shifts (some cm ). The half-widths of the bands are reduced drastically by applying the matrix-isolation technique (c.f. Sec. 4.4). The absorptions of many matrix-isolated species can therefore be characterized with the help of isotopic substitution, i.e., the molecular fragment which is involved in the vibration can be identified. The large - Si/" Si shift of the most intense IR absorption of matrix-isolated S=Si=S from 918 cm to 907 cm, for instance, demonstrates that silicon participates considerably in this vibration (Schnoeckel and Koeppe, 1989). The same vibration is shifted by 4 cm if only one atom is substituted by a atom. The band at 918 cm must be assigned to the antisymmetric stretching vibration, since the central A atom in an AB2 molecule with Doo/rsymmetry counts twice as much as the B atoms in the G-matrix (c.f. Wilson et al., 1955). 

This attribution is confirmed by the observation of an expected isotope component (unresolved) of the 1T7 line in a silicon sample doped with sulphur enriched with isotope 33S [206]. The isotope shifts observed for 1T7 and ir8 with respect to the strongest component, noted 0 in Fig. 6.18, are given in Table 6.17. 

Canaria C, Lees I, Wun A et al (2002) Characterization of the carbon-silicon stretch in methylated porous silicon - observation of an anomalous isotope shift in the FTIR spectrum. Inorg Chem Commun 5 560-564... 

Silicon only has one naturally occurring isotope ( Si) with a nonzero nuclear spin and Si NMR spectroscopy has become one of the most widely used techniques for the identification of silicon compounds. Unfortunately, the natural abundance of Si is only 4.7%, which combined with its long spin-lattice relaxation times, means that relatively long acquisition times may be needed to obtain high quality spectra. Si NMR spectroscopy has been the subject of a number of reviews and tables of chemical shift data and coupling constants are available. " ... 

Co-condensation of silicon atoms, generated from a resistively heated, doped silicon rod (1350 -1380 °C) with very diluted ammonia/argon gas mixtures (1 1000) at 10 K leads to the formation of a primary Si,NH3 n-adduct 1, which is stable under these conditions[2]. This compound can be identified by three prominent IR absorptions ( >013 3387.3, 1599.1, and 1185.6 cm ). The silicon-nitrogen stretching band is too low in intensity to be observed. These bands show the expected shifts when the co-deposition is repeated with isotopically labeled ammonia ( ND3 2525.4,1171.5, and 912.5 cm 3 3378.8, 1596.1, and 1180.0 cm ). 

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