• ab initio calculations;
  • millimeterwave spectroscopy;
  • silicon compounds;
  • structure elucidation

139 rotational lines of the 28Si isotopomer of free silaethene H2C[DOUBLE BOND]SiH2 (1) have been observed in the gas phase by millimeter wave spectroscopy. The search was guided by ab initio calculations at the MP2, CCSD, and CCSD(T) levels with TZ2Pf basis sets. The rotational constants determined experimentally and by ab initio methods for 1 with a planar C2v-symmetric structure agree within about 0.5%. Compound 1 was best produced by vacuum pyrolysis of 5,6-bis(trifluoromethyl)-2-silabicyclo[2.2.2]octa-5,7-diene in an Ar stream; 1 decomposes exponentially with a 1/e lifetime at ambient temperature of 30±2 ms.