A new treatment is presented to account for the extreme anomalous centrifugal distortion displayed by the open-shell methylene radical. This new treatment is based on a four-dimensional approach in which both the overall rotation and the large amplitude bending mode are treated simultaneously. It accounts for the spin–rotation and spin–spin fine couplings, assumed to depend on the large amplitude bending coordinate, as well as for the hyperfine coupling. The new treatment is tested analyzing the available high-resolution data. 336 transitions, involving the ground and first excited vibrational states of the bending mode, are reproduced with a unitless standard deviation of 1.3, using 42 molecular constants. Compared to a previous analysis [S. Brünken et al., J. Chem. Phys. 123, 164315 (2005)], the present analysis is more satisfactory as it accounts for a larger dataset and the ratio of the number of data to the number of varied constants is larger. The present theoretical treatment also allows us to retrieve the bending potential and the main kinetic energy term.

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