The vibrational potential function of the bent XY2 molecule has been expanded in terms of the internal coordinates to the fourth order and a general formulation which relates the coefficients of this expansion (second-, third-, and fourth-order potential constants) to the cubic and quartic constants in the normal-coordinate system has been derived. The third- and fourth-order constants in the internal-coordinate system for H2O, D2O, H2Se, D2Se, and SO2 have been determined by using the experimental α and x values taken from the literature. It has been shown that the third- and fourth-order constants which represent the anharmonic bond-stretching vibration, frrr and frrrr, are much larger than the other higher-order constants and that they are nearly equal (to within 12 per cent) to the anharmonic potential constants of the corresponding diatomic radicals. The method presented in Part I of this paper, which makes use of the “diatomic approximation” for estimating the anharmonic constants and the α and x values, has been applied to the bent XY2 molecules. A comparison of the results with currently available experimental data has shown that the signs and the orders of magnitude are in most cases correctly predicted by this approximation. This method has been further applied as an aid for the detection and analysis of the microwave absorption lines in the case of the SO2 and OF2 molecules in their excited vibrational states.

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