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

Abstract

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 ùB₁ → 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 K_a′ selectivity in excitation was possible, we measured K_a′-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 K_a′-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.