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 ?. 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 cm21 above the ground state. A
recommended ab initio equilibrium structure for SiC8 has also been established.