Analysis of CH2  a ̃ 1 A 1 (1,0,0) and (0,0,1) Coriolis-coupled states, a ̃ 1 A 1– X ̃ 3 B 1 spin–orbit coupling, and the equilibrium structure of CH2  a ̃ 1 A 1 state

The symmetric and antisymmetric stretch spectra of ã 1A1 CH2 are measured with Doppler-limited resolution by infrared flash-kinetic spectroscopy between 2600 and 3050 cm−1. The spectra are significantly perturbed by spin–orbit interactions between near-resonant levels of X̃ 3B1 in both lower and upper levels of the transitions, and by Coriolis interaction between symmetric and antisymmetric stretch states. The triplet character of the perturbed levels is detected by observing broadening of MJ structure in a magnetic field of about 1 kG. The levels that are likely to be perturbed by triplet methylene are identified using the experimentally observed and theoretically calculated 3B1 energy level structure. The term values of the remaining symmetric and antisymmetric stretch levels are simultaneously fit to Watson’s Hamiltonian including Coriolis coupling. This analysis provides the remaining information needed to determine the equilibrium rotational constants [Ae =19.8054(206), Be=11.2489(45), and Ce =7.2386(36) cm−1 ], and, therefore, the equilibrium structure (re=1.107 Å, and θe =102.4°) of this fundamentally important carbene. The experimental and theoretical structures are in good agreement.