State-state transitions for CCl2(X1A1, A1B1, a3B1) radical and collisional quenching of CCl2(A1B1 and a3B1) by O2, N2, NO, N2O, NH3, and various aminated molecules

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 (CCl(2)) has been a point of controversy in the recent literature. In this study, we report single vibronic level emission spectra of the A(1)B(1)-->X[combining tilde](1)A(1) system of the carbene C(35)Cl(2), which probes the vibrational structure of the X[combining tilde](1)A(1) state up to approximately 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 approximately 5,000 cm(-1) belong to levels of the ã(3)B(1) state. The K(a)'-sorting method discriminates between singlet and triplet levels via the (A''-B[combining macron]'') rotational constant, which is significantly larger for pure triplet levels due to the larger equilibrium bond angle. In the region between 3,500 and 9,000 cm(-1) above the vibrationless level of the X[combining tilde](1)A(1) state, we find only a very modest increase in (A''-B[combining macron]''), and approximately 86% of the lines observed between 5,000 and 9,000 cm(-1) can be assigned to X[combining tilde](1)A(1) 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 C(35)Cl(2) isotopomer, and the X[combining tilde](1)A(1) state term energies up to 4,000 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.