Theoretical rotational-vibrational spectrum of H2S

The high resolution rovibrational spectrum of H₂S has been evaluated from three-dimensional ab initio potential energy and electric dipole moment functions and variational rovibrational eigenfunctions, which took full account of anharmonicity effects and rotation-vibration coupling. The quality of the near equilibrium theoretical potential energy function has been checked by comparisons with experimental equilibrium structure, empirical quartic force fields, vibrational band origins, centrifugal distortion constants, and rotational energy levels. All parameters agree well with the available experimental data. Vibrational band intensities for the ν₂, 2ν₂, ν₁, and ν₃ bands have been calculated from empirical and ab initio dipole moment functions and compared with experimental and theoretical integrated band intensities. The difficulties arising by the derivation of such data from the experimental intensities of H₂S are discussed. The theoretical results strongly suggest that higher than first derivatives are needed for a proper description of the dipole moment function. The room temperature absorption spectra have been evaluated ab initio for the pure rotational and the ν₂, 2ν₂, ν₁, and ν₃ transitions. The unusual intensity pattern of the P, Q, and R branches attributed to the rotational-vibrational coupling has been well reproduced. Absolute line intensities calculated previously by perturbation theory are compared with variational results. The purely theoretical line intensities agree satisfactorily with experiment for the bending transitions, however, the extremely flat regions of the dipole moment functions along the bond stretching displacements make the transition intensities very sensitive to the values of the dipole moment derivatives.