An accurate gas-phase structure determination of cis-hex-3-ene-1,5-diyne (1), the parent molecule for the Bergman cyclization reaction, has been achieved through a combination of Fourier transform microwave spectroscopy and computational quantum chemistry. The microwave spectra of seven isotopes of enediyne 1 have been observed using a pulsed-nozzle Fourier transform spectrometer. The ground-state rotational constants were corrected for vibrational effects and the equilibrium structure (R e ) was deduced. The equilibrium structure displays excellent agreement with that obtained from high-level ab initio calculations (CCSD(T)/cc-pVTZ). Both approaches confirm that the molecule has C 2V symmetry with slight deviations of the alkyne units (C-CtC-H) from linearity. These groups distort away from each other in contra-distinction to their distortion during the Bergman cyclization reaction. The C1-C6 distance (4.32 Å), which has been viewed as a critical parameter governing reactivity in the Bergman cyclization, is notably longer than an earlier, widely publicized value (4.12 Å). An important role of the macrocyclic ring in enediyne antibiotics is to overcome an intrinsic distortion of the enediyne moiety in the direction opposite that of the reaction coordinate for Bergman cyclization.

The rotational spectra of nine isotopes of the mixed trimer, OCS-͑CO 2 ͒ 2 , have been assigned using pulsed nozzle FTMW spectroscopy techniques. The structure resembles a distorted triangular cylinder. It can be thought of as the slipped (CO 2) 2 dimer with the OCS above the dimer and crossed about 23°to the axis of each CO 2. The distance between the carbon atoms on the CO 2 is 3.68͑5͒ Å. The distance between the carbon on each CO 2 and the carbon on the OCS is 3.59͑5͒ and 3.66͑5͒ Å, respectively. The axes of the linear molecules are tilted 30°-35°from perpendicular relative to the edges of the CC -C plane. The dipole moment components for the trimer are a ϭ0.63(2) D, b ϭ0.16(10) D and c ϭ0.21(2) D. The structure and dipole moment components are consistent with an interaction model, which includes a distributed multipole moment electrostatic analysis and atom-atom terms describing dispersion and repulsion. The structure is compared to other related dimers and trimers containing CO 2 , OCS, and N 2 O.

No abstract