Higher-order effects in the vibration–torsion–rotation spectra of a molecule with an internal rotor

The sequential contact transformation technique is applied to the vibrational–torsional–rotational Hamiltonian for molecules containing a threefold symmetric internal rotor, which makes it possible to analyze various higher-order vibration–torsion–rotation interactions and their effects in the vibration–torsion–rotation spectra of the molecule. The formalism for the calculation of Hamiltonian coefficients is developed in terms of the fundamental molecular parameters. The resulting formulas are applicable to analyze the molecular structure. The theory presented is applied to the calculation of the quartic and sixtic centrifugal distortion constants of the molecule.