Visible absorption and magnetic-rotation spectroscopy of 1CH2: Analysis of the 1A1 state and the 1A1–3B1 coupling

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Abstract
In methylene, the rotational levels of the ã 1A1 (0,0,0) and (0,1,0) vibronic states are so heavily perturbed by nearby rovibrational levels of the ground triplet state (X̃ 3B1) that standard rotation–vibration Hamiltonians do not provide a satisfactory fit or any indication as to which levels are strongly perturbed and which are less perturbed. Recent spectroscopic and theoretical work gives triplet state term values and the singlet–triplet energy difference with an accuracy of a few tens of wave numbers. Using these term values and ab initio spin‐orbit matrix elements it is shown that all Ka=1, 3, and 4 levels of 1A1 (0,0,0) and all Ka=1 levels of 1A1 (0,1,0) are strongly perturbed by 3B1 (0,v2,0) levels with 2≤v2≤4. Individual levels in the other Ka stacks are also perturbed but most can be fit satisfactorily with a Watson Hamiltonian. The shifts between the observed term values and those calculated from the Watson Hamiltonian are reproduced in each Ka stack by a spin‐orbit matrix element value only 5%–30% larger than the ab initio value. Over 60% of the measured term values show shifts greater than 0.2 cm−1. Shifts of the 322 and 000 levels can only be explained by interaction with levels in either (1,0,0) or (0,0,1) states; possible values for the vibrational frequencies of ν1 and ν3 of 3B1 are given. Molecular constants for 1A1 (0,0,0) and (0,1,0) are derived and quantitative estimates of spin‐orbit mixing for individual levels of 1A1 (0,0,0) and (0,1,0) are given. From a chemical point of view singlet methylene is never in a pure spin state and always has some triplet character in its wave function. These data provide a basis for proper modeling of the kinetics of chemical reactions of ‘‘singlet’’ and ‘‘triplet’’ methylene species and their interconversion by ‘‘intersystem crossing.’’