A second rhomboidal isomer of SiC3

A second low-lying cyclic isomer of SiC3 has been detected in a supersonic molecular beam by Fourier transform microwave spectroscopy. Calculated to l ie about 5 kcal above the ground state rhomboid, the new isomer is also a p lanar rhomboid with a transannular bond, C-2 upsilon symmetry, and a single t electronic ground state. The transannular bond, however, is between the S i and the opposite C, and the rotational spectrum as a result is that of an oblate, not a prolate, asymmetric rotor. Both rhomboidal isomers of SiC3 a re produced with comparable abundance under a wide range of experimental co nditions, which suggests that cyclic isomers of longer silicon carbides may now be observable with the present techniques. Oblate SiC3 is a plausible molecule for astronomical detection because it is calculated to be fairly p olar, and because radio emission lines of the ground state rhomboid have al ready been detected in the circumstellar shell of the evolved carbon star I RC + 10216. (C) 1999 American Institute of Physics. [S0021-9606(99)02840-8] .