Molecular Structure of o-Benzyne from Microwave Measurements

The o-benzyne molecule has been known for many years to be an important, but short-lived, reaction intermediate in substitution reactions and more recently in cyclization reactions of enediynes. Although there has been widespread interest in this transient molecule, previous experimental structural data were very limited. In the present work, rotational transitions for o-benzyne were measured with a pulsed-beam, Fourier transform microwave spectrometer for all unique, singly substituted ¹³C and single-D isotopomers. The o-benzyne was efficiently produced by flowing a dilute mixture of isotopically enriched benzene in neon through a pulsed-DC discharge beam source. The new data, combined with previous data for the normal isotopomer, provide a complete set of structural parameters for this molecule. The r _s substitution coordinates and the coordinates from a least-squares fit are reported and are in good agreement. When using the least-squares fit to obtain structural parameters, correction terms arising from harmonic terms in the vibrational averaging were subtracted from the measured rotational constants to obtain a better representation of the planar equilibrium structure. Further improvements in the fits were obtained by applying small, mass-dependent adjustments to the atom coordinates. Structural parameters obtained from the fit to these modified rotational constants are an acetylenic C ₁≡C₂ bond length of 1.264(3) Å, and the other bond lengths C₂-C₃3 = 1.390(3) Å, C ₃-C₄ = 1.403(3) Å, C₄C₅ = 1.404(3) Å, C₃-H₁ = 1.095(9) Å, and C ₄-H₂ = 1.099(4) Å. The C₁≡C ₂ bond is only 0.057 Å longer than the free acetylene bond. The other C-C bond lengths are within 0.01 Å of those of benzene C-C bonds. New spectral data for the single-D isotopomers were used to obtain better values for the deuterium quadrupole coupling. Bond-axis deuterium quadrupole coupling constants are eQq_zz(D₁) = 188(2) kHz, and eQq _zz(D₂) = 185(10) kHz, which agree well with the value for benzene-D₁. The new structural parameters are compared here with theoretical parameters and with an NMR measurement of the C_1--C ₂ bond length.