Calculated forbidden rotational spectra of H3(+)

The forbidden rotational spectra of the H₃⁺ molecular ion have been calculated in the ground state and in the v₂ state, anticipating the observation of such spectra in space and in the laboratory. The small moments of inertia and the large vibrational amplitude of H₃⁺ make the former spectrum unusually strong. The Watson coefficient θxxx for the centrifugal distortion induced dipole moment and the Mizushima-Venkateswarlu dipole moment have been calculated to be 1.1 × 10⁻³ and 5.2 × 10⁻³ debye, respectively. The former is larger by a factor of ∼ 50 than that of CH₄, for which the forbidden rotational spectrum has been observed in the laboratory. The calculated spectral pattern for laboratory observations is given, and the intensities of spontaneous emissions from dense molecular clouds are also discussed.