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Investigates the HC4N molecular system, which is calculated to possess several low-lying, highly polar carbene isomers. High-level coupled cluster calculation; Rotational spectrum of the bent HC4N singlet carbene; Use of Fourier transform microwave spectroscopy; Leading centrifugal distortion constants; Nitrogen hyperfine coupling constants; Large inertial defect.

The structures of two linear silicon carbides, SiC[sub 4] and SiC[sub 6], 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 SiC[sub 4] were assigned to v[sub 6], a bending mode calculated to lie about 205 cm-1 above the ground state. A recommended ab initio equilibrium structure for SiC[sub 8] has also been established. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]