Knowledge Bank

Higher-order vibration-rotation perturbation calculations are unusually simple for the equilateral triangular $X3$ molecule because the main Coriolis terms $H21$ are diagonal in a harmonic representation, so that the corresponding contact transformation, function $S21$ vanishes Formulas are obtained for several higher-order molecular parameters. Including the vibrational dependence of the centrifugal and Coriolis constants. The attempted application of these formulas to date for $H3+$ and $D3+$ will be discussed.