Vibrational spectra and structures of silicon-carbon clusters

Abstract

Fourier transform studies of silicon-carbon clusters produced by vaporizing mixtures of silicon and carbon-12 or carbon-13 and quenching the products in argon, have enabled the identification of new vibrational spectra and structures. The far infrared $\nu\sb3(b\sb2)$ fundamental has been found at 160.4 cm$\sp{-1}$ for SiC$\sb2$. Its structure is now described as T-shaped with ionic bonding. In the case of Si$\sb2$C, a previously observed vibration at 1188.4 cm$\sp{-1}$ has been confirmed as the $\nu\sb3(b\sb2)$, antisymmetric Si-C stretching mode, and a new vibration at 839.5 cm$\sp{-1}$ has been identified as the $\nu\sb1(a\sb1)$, symmetric Si-Si stretching fundamental. The $\nu\sb2(a\sb1)$ bending mode has not been observed; however, the $\nu\sb2(a\sb1)$ + $\nu\sb3(b\sb2)$ combination band has been identified at 1354.8 cm$\sp{-1}$. The results of force constant adjustment calculations confirm the ground state geometry of Si$\sb2$C as a floppy, bent symmetrical structure. Five out of six fundamental modes have been identified for the Si$\sb3$C cluster which is observed here for the first time: the symmetric breathing vibration, $\nu\sb1(a\sb1)$ = 658.2 cm$\sp{-1}$; the Si$\sb\beta$-Si$\sb\alpha$-Si$\sb\beta$ symmetric deformation vibration, $\nu\sb2(a\sb1)$ = 511.8 cm$\sp{-1}$; the Si$\beta$-C-Si$\sb{\beta}$ symmetric deformation vibration, $\nu\sb3(a\sb1)$ = 309.5 cm$\sp{-1}$; the Si$\sb\beta$-C antisymmetric stretching vibration, $\nu\sb5(b\sb2)$ = 1101.4 cm$\sp{-1}$; and the Si$\sb\alpha$-Si$\sb\beta$ antisymmetric stretching vibration, $\nu\sb6(b\sb2)$ = 357.6 cm$\sp{-1}$. The fundamental frequencies, relative intensities, and isotopic shifts are in excellent agreement with the results of ab initio calculations predicting a rhomboidal structure of $C\sb{2v}$ symmetry for Si$\sb3$C, with carbon-silicon transannular bonding between the two equivalent Si atoms. Another new silicon-carbon cluster, rhombic Si$\sb2$C$\sb2$, has been identified and the $\nu\sb3(b\sb{\rm 1u})$ = 982.9 and $\nu\sb4(b\sb{\rm 2u})$ = 382.2 cm$\sp{-1}$ fundamentals have been assigned. These results are in excellent agreement with ab initio calculations. The first assignment of vibrational fundamentals for linear centrosymmetric Si$\sb2$C$\sb3$: the C=C stretching mode, $\nu\sb3(\sigma\sb{u})$ = 1955.2 cm$\sp{-1}$, and the Si-C stretching mode $\nu\sb4(\sigma\sb{u})$ = 898.9 cm$\sp{-1}$, has been made. These results are again in agreement with ab initio calculations. The discovery of new spectra and structures for a large number of silicon-carbon clusters has been achieved through comprehensive experimental measurements of isotopic shifts and relative intensities, and a productive interplay with ab initio theory.