Structures of the linear silicon carbides SiC 4 and SiC 6 : Isotopic substitution and Ab Initio theory

The structures of two linear silicon carbides, SiC4 and SiC6, have been determined by a combination of isotopic substitution and large-scale coupled-cluster ab initio calculations, following detection of all of the singly substituted isotopic species in a supersonic molecular beam with a Fourier transform microwave spectrometer. Rotational constants obtained by least-squares fitting transition frequencies were used to derive experimental structures; except for those nearest the center of mass, individual bond lengths for both chains have an error of less than 0.008 Å. Accurate equilibrium structures were derived by converting the experimental rotational constants to equilibrium constants using the vibration–rotation coupling constants from coupled-cluster calculations, including connected triple substitutions. Equilibrium dipole moments and harmonic vibrational frequencies were also calculated for both chains. On the basis of the calculated vibration–rotation and l-type doubling constants, weak rotational satellites from a low-lying vibrational state of SiC4 were assigned to v6 , a bending mode calculated to lie about 205 cm -1 above the ground state. A recommended ab initio equilibrium structure for SiC8 has also been established. © 2000 American Institute of Physics. @S0021-9606~00!01537-3#