The empirical equilibrium structure of diacetylene ⋆

High-level quantum-chemical calculations are reported at the MP2 and CCSD(T) levels of theory for the equilibrium structure and the harmonic and anharmonic force fields of diacetylene, H−C ≡ C−C ≡ C−H. The calculations were performed employing Dunning’s hierarchy of correlation-consistent basis sets cc-pVXZ, ccpCVXZ, and cc-pwCVXZ, as well as the ANO2 basis set of Almlöf and Taylor. An empirical equilibrium structure based on experimental rotational constants for thirteen isotopic species of diacetylene and computed zero-point vibrational corrections is determined (r e : rC−H = 1.0615 Å, rC≡C = 1.2085 Å, rC−C = 1.3727 Å) and in good agreement with the best theoretical structure (CCSD(T)/cc-pCV5Z: rC−H = 1.0617 Å, rC≡C = 1.2083 Å, rC−C = 1.3737 Å). In addition, the computed fundamental vibrational frequencies are compared with the available experimental data and found in satisfactory agreement.