Structures of the linear silicon carbides SiC4 and SiC6: Isotopic substitution and Ab Initio theory

The structures of two linear silicon carbides, SiC4 and SiC6, have been det ermined by a combination of isotopic substitution and large-scale coupled-c luster ab initio calculations, following detection of all of the singly sub stituted isotopic species in a supersonic molecular beam with a Fourier tra nsform microwave spectrometer. Rotational constants obtained by least-squar es fitting transition frequencies were used to derive experimental structur es; except for those nearest the center of mass, individual bond lengths fo r both chains have an error of less than 0.008 Angstrom. Accurate equilibri um structures were derived by converting the experimental rotational consta nts to equilibrium constants using the vibration-rotation coupling constant s from coupled-cluster calculations, including connected triple substitutio ns. Equilibrium dipole moments and harmonic vibrational frequencies were al so calculated for both chains. On the basis of the calculated vibration-rot ation and l-type doubling constants, weak rotational satellites from a low- lying vibrational state of SiC4 were assigned to v(6), a bending mode calcu lated to lie about 205 cm(-1) above the ground state. A recommended ab init io equilibrium structure for SiC8 has also been established. (C) 2000 Ameri can Institute of Physics. [S0021-9606(00)01537-3].