The experimental equilibrium structure of acetylene.

The empirical equilibrium structure of acetylene has been derived by exploiting the very precise experimental rotational constants available in the literature for the 10 isotopologues relative to all the possible combinations of H, D, 12C and 13C atoms. The geometry obtained when data for all species are fitted together is: re(CH) = 106.167(14) pm and re(CC) = 120.2866(72) pm. This determination shows some systematic residuals due to the singly D-substituted isotopologues. If we exclude such species from the fit, we obtain our most precise evaluation: re(CH) = 106.1689(23) pm and re(CC) = 120.2817(12) pm. The possibility of a breakdown of the Born–Oppenheimer approximation has also been tested.