Enthalpy of formation and anharmonic force field of diacetylene

The enthalpy of formation of diacetylene ( C 4 H 2 ) is pinpointed using state-of-the-art theoretical methods, accounting for high-order electron correlation, relativistic effects, non-Born-Oppenheimer corrections, and vibrational anharmonicity. Molecular energies are determined from coupled cluster theory with single and double excitations (CCSD), perturbative triples [CCSD(T)], full triples (CCSDT), and perturbative quadruples [CCSDT(Q)], in concert with correlation-consistent basis sets ( cc-pV X Z , X = D , T, Q, 5, 6) that facilitate extrapolations to the complete basis set limit. The first full quartic force field of diacetylene is determined at the highly accurate all-electron CCSD(T) level with a cc-pCVQZ basis, which includes tight functions for core correlation. Application of second-order vibrational perturbation theory to our anharmonic force field yields fundamental frequencies with a mean absolute difference of only 3.9 cm − 1 relative to the experimental band origins, without the use of any empirical scale factors. By a focal point approach, we converge on an enthalpy change for the isogyric reaction 2 H - C ≡ C - H → H - C ≡ C - C ≡ C - H + H 2 of ( + 0.03 , + 0.81 ) kcal mol − 1 at (0, 298.15) K. With the precisely established Δ f H ° of acetylene, we thus obtain Δ f H ° ( C 4 H 2 ) = ( 109.4 , 109.7 ) ± 0.3 kcal mol − 1 at (0, 298.15) K. Previous estimates of the diacetylene enthalpy of formation range from 102 to 120 kcal mol − 1 .