The first spectroscopic evidence of a double-bridged silicon molecule is reported. The submillimeterwave rotational spectrum of the disilyne S12H2 has been observed in a low-power silane plasma cooled at liquid-nitrogen temperature, with argon as buffer gas. These measurements led to the determination of the ro structure, which is in good agreement with ab initio calculations. This nonclassical structure is confirmed by the observation of the 29 Si and 30 Si monosubstituted forms.PACS numbers: 33.10.Ev, 33.20.Bx, 35.20.Dp, 35.20.Pa There is presently a growing interest in the study of small unsaturated silicon compounds, related to the vapor deposition of amorphous silicon films. 1 " 3 Detection of some monosilicon intermediate species [SiH" (Ref. 4); SiH" + (Ref. 5), n<3] by high-resolution spectroscopy has already led to the development of nonintrusive diagnostic techniques of silane plasmas. 2,3,6 In contrast, experimental data on unsaturated disilicon hydrides are very scarce, although recent mass-spectrometry investigations 7 have shown that Si2H2 and Si2H4 can be abundant in SiH4 plasmas and therefore can play a key role in deposition processes (Refs. 1-3, and 8, and references therein).The study of these highly reactive species also has its own interest, for a fundamental comparison between carbon and silicon bonding properties. For example, ab initio calculations have shown that the simplest unsaturated hydrides SiiFh and Si2H4 exhibit geometries very different from the analogous C-containing molecules, and that the smaller basis sets are not adequate to properly describe the structures and the relative stabilities of the various isomers. 9 " 15 In particular, it has been shown that addition of Si d functions and of electron correlation makes the nonclassical bridged structure of the disilyne Si(H2)Si in its singlet ground state more stable than the silasilene F^SiSi classical structure, which is the minimum-energy structure at the HF/3-21G level. 14 The energy difference between the two structures is rather small, in the range of 7-13 kcal/mol. 9 " 12,14 In fact, bridged structures are well known in molecules involving electron-deficient atoms such as boron. 16 Nevertheless, very little is experimentally known regarding the existence of such three-center bonds in the group-IV compounds. Protonated acetylene, recently investigated by ir spectroscopy 17 and Coulomb-explosion experiments, 18 is the only example of such a situation. The nonclassical bridged form HC(H)CH + is calculated to be lower in energy than the classical form F^CCH* by about 4-7 kcal/mol, 19,20 which is consistent with the experiments. However, no complete structural information on C2H3 -1 " can be deduced from these experiments at the moment.In this Letter, we present the first spectroscopic evidence of the disilyne Si2H2, and the first determination of a bridged structure for a silicon-containing molecule.In the course of an investigation of a silane plasma by submillimeter-wave spectroscopy, some unidentified lines were observed a...

Raman spectra of aqueous solutions containing up to 40 mol% of formaldehyde were recorded in the 120-4000 cm-1 spectral range. The water vibrational modes are not too heavily modified by the presence of the polymerized species in the aqueous solutions. The intensity of the bands corresponding to the C-O and C-H vibrational modes depends on the water content. Two bands are found for the C-O symmetric and antisymmetric stretching modes. The first is related to the C-O-C groups formed within the oligomer chains as the condensation increases and the second to the C-OH groups located at the end of the oligomer chains. A change in the intensity evolution of the bands corresponding to the stretching modes of the C-O-C groups occurs above 15 mol% formaldehyde concentration, corresponding to the formation of the HO(CH2O)nH oligomers with n3. The intensity of the stretching modes due to the C-OH groups varies linearly with concentration. A linear variation of the band intensity of the CH modes as a function of formaldehyde content is also observed. Copyright © 2003 John Wiley & Sons, Ltd

A monobridged isomer of the disilyne Si2H2, the planar bridged-2 form Si(H)SiH, has been detected through its millimeter- and submillimeter-wave rotational spectrum. The molecules are produced in a low pressure, low power plasma in a mixture of SiH4 and Ar cooled at liquid nitrogen temperature. From the analysis of a total of 148 lines, a set of accurate rotational constants have been derived. Using the geometry predicted by ab initio calculations (see the preceding paper), the spectrum of Si(D)SiD has been predicted, observed, and analyzed. From the two sets of rotational constants, a preliminary molecular structure has been derived by fixing the Si1–Si2–bridged H angle to the ab initio value of 52.5°: Si1–Si2=2.119 Å; Si2–bridged H=1.629 Å; Si2–terminal H=1.474 Å; Si1–Si2–terminal H=157.5°.

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