JOURNAL OF MOLECULAR SPECTROSCOPY 180, 75–80 (1996) ARTICLE NO. 0225

The microwave rotational spectra of the J 1 R 0 and 2 R 1 transitions of six isotopic species of the singlet carbene butatrienylidene (H2CCCC, H213CCCC, H2C13CCC, H2CC13CC, H2CCC13C, and D2CCCC) were measured with a Fourier-transform microwave spectrometer and the r0 and rs structures were determined. Both structures are consistent with a cumulenic carbon-chain backbone. Our empirical structure is rs (HC(1) ) 1.079 { 0.002 A ˚ , rs (C(1)C(2) ) 1.306 { 0.004 A ˚ , rs (C(2)C(3) ) 1.294 { 0.004 A ˚ , rs (C(3)C(4) ) 1.278 { 0.004 A ˚ , and /sHC(1)H 119.4 { 0.47. Except for the CC bond closest to the carbene carbon, rs (C(3)C(4) ) , an equilibrium geometry determined in a high-level ab initio calculation (M. Oswald and P. Botschwina, J. Mol. Spectrosc. 169, 181 (1995)) is in very good agreement with the experimental structure. q 1996 Academic Press, Inc. I. INTRODUCTION a standard supersonic nozzle pass through a small electric discharge in the nozzle throat and into a high-Q Fabry– Butatrienylidene (H2CCCC; Fig. 1) —2 eV higher in en- Perot microwave cavity. A saturating pulse of microwave ergy (1) than diacetylene (HC4H) —is the third member of radiation, timed to coincide with the passage of the gas the cumulene carbene series (H2Cn) observed in the labora- pulse, is introduced into the cavity, producing a macroscopi