Matrix-isolated species, isotopic substitution

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). 

A detailed assignment of the IR absorption frequencies of 1,2,4-trioxolane (1) in solid argon was achieved by comparison with the spectra of various isotopically substituted species <82JPC3154>. Similarly, for a series of substituted 1,2,4-trioxolanes, characteristic IR bands obtained via matrix isolation were assigned and compared with those of 1,2,4-trioxolane (1) (ethylene ozonide) (Table 7) <82JPC4548>. The spectra of cis- and trani-1,2,4-trioxolanes indicate that the cis isomer has characteristic absorptions in the range 820-855 cm with the trans isomer at 1320-1360 cm . 

The phosphorus sulfides PS, P2S, and PS2 have been made by allowing P4 and Sg vapors to react in an Ar discharge. Isotopic substitution studies show that P2S is linear like P2O and N2O, while PS2 is bent like PO2 and NO2. The species in the vapor above heated P4S10 have been the focus of some controversy. It has been claimed that P4S10 sublimes undissociated. However, matrix-isolation experiments reveal... 

Molecular Si02 is prepared by reacting matrix-isolated SiO with 0 atoms. Thus codeposition of SiO molecules with microwave-excited O2 and excess argon yields a species that shows a prominent IR band at 1416.5 cm T " Isotopic substitution experiments allow this feature to be assigned to the asymmetric stretch of a linear Si02 molecule. Force constant calculations show that the Si=0 bonds of Si02 are rather weaker than the C=0 bonds of CO2. Similarly, matrix-isolated Si2 O2 will react with O2 to yield (47), the dimer of Si02, whose structure has been determined by isotopic substitution with and the natural abundance of Si, Si, and °Si isotopes. 

By far the most important spectroscopic method for this purpose is IR spectroscopy. In combination with DFT or ab initio calculations matrix IR spectroscopy has become a very powerful tool for the reliable identification of reactive and unusual molecules. In addition, isotopic labeling with is frequently used to assign the IR spectra of oxidized species. However, a prerequisite for this technique is the availability of suitable photochemical or thermal precursor molecules of the reactive silicon species. During the last years, we have published details of the oxidation mechanism of alkyl-substituted silenes 2. °In this chapter, our mechanistic studies on the oxidation of silylenes 1 using the matrix-isolation technique are summarized. 

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