Probing spin–orbit mixing and the singlet–triplet gap in dichloromethylene via Ka-sorted emission spectra

The magnitude of the singlet–triplet gap in dichloromethylene (CCl2) has been a point of controversy in the recent literature. In this study, we report single vibronic level emission spectra of the Ã1B1 → 1A1 system of the carbene C35Cl2, which probes the vibrational structure of the 1A1 state up to ≈10 000 cm−1 above the vibrationless level. By the careful selection of bands where complete isotope and Ka′ selectivity in excitation was possible, we measured Ka′-sorted emission spectra in order to test the previously established hypothesis [M.-L. Liu, C.-L. Lee, A. Bezant, G. Tarczay, R. J. Clark, T. A. Miller and B.-C. Chang, Phys. Chem. Chem. Phys., 2003, 5, 1352] that unassigned lines lying above ≈5000 cm−1 belong to levels of the ã3B1 state. The Ka′-sorting method discriminates between singlet and triplet levels via the (A″ − ″) rotational constant, which is significantly larger for pure triplet levels due to the larger equilibrium bond angle. In the region between 3500 and 9000 cm−1 above the vibrationless level of the 1A1 state, we find only a very modest increase in (A″ − ″), and ∼86% of the lines observed between 5000 and 9000 cm−1 can be assigned to 1A1 levels within 3 standard deviations of our Dunham expansion fit, which included more than 140 levels in total. A nearly complete set of Dunham parameters was determined for the C35Cl2 isotopomer, and the 1A1 state term energies up to 4000 cm−1 are in excellent agreement with recent variational calculations of Tarczay, et al. [G. Tarczay, T. A. Miller, G, Czakó and A. G. Császár, Phys. Chem. Chem. Phys., 2005, 7, 2881]. Finally, the implication of our results for the singlet–triplet gap in dichloromethylene is discussed.