Ahmed, Peterka, and Suits [J. Chem. Phys. 110, 4248 (1999)] reported the first experimentally deduced value for singlet-triplet gap in acetylene in 1999. Using velocity map imaging (VELMI), they studied the photodissociation of the vinyl radical, C2H3. Using the measured translational energy of the dissociated H atoms, along with the known energy of the incoming photons and the heats of formation of vinyl radical and H atom, they determined the heat of formation of the observed acetylene fragments. The major product was singlet vinylidene, but a minor product with very slow H atoms was also observed and assigned to 3B2 acetylene based on previous theoretical work. However, the determined excitation energy T0 (3B2) of 28 900 cm-1 was not in good agreement with the previous TZ(2df,2p) CCSD(T) ab initio value of T0 = 30 500 +/- 1000 cm-1.
Challenged by this discrepancy, we considered the following to be the primary possible improvements to the previous theoretical work:
We adopted the following strategy to definitively determine the singlet-triplet gap (and the next few triplet excitation energies) in acetylene to ``chemical accuracy.''
Our full results are presented in C. D. Sherrill, E. F. C. Byrd, and M. Head-Gordon, J. Chem. Phys. 113, 1447-1454 (2000). A summary of the most pertinent results for the singlet-triplet gap appears below:
| TZ(2df,2p) CCSD(T)//DZP CISDb | 31 000 |
| cc-pVTZ CCSD(T) | 30 870 |
| Full T3 Correction | -316 |
| Core correlation | +260 |
| Basis Set Improvement TZ -> QZ | +410 |
| Basis Set Improvement QZ -> 5Z | +120 |
| Basis Set Improvement 5Z -> CBS est. | +120 |
| Relativistic correction | -3 |
| Final Te estimate | 31 460 |
| cc-pVTZ CCSD(T) ZPVE correction | -560 |
| Final T0 | 30 900 |
The previous theoretical result is surprisingly accurate and completely supported by the vastly improved results presented in the more recent publication. The discrepancy with experiment is attributed to internal energy in the vinyl radicals, a possibility suggested by Suits and co-workers.
