Theoretical and experimental evaluation of the radial function for electric dipole moment of hydrogen iodide

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From relativistic quantum-chemical calculations of the molecular electronic structure of hydrogen iodide HI in electronic state X 1Σ+ or 0+ using the Dirac–Coulomb coupled-cluster method with singles, doubles and non-iterative triples (DC-CCSD(T)), we have evaluated the electric dipole moment p at 17 values of internuclear distance R. On this basis we have calculated the pure vibrational expectation value in the vibrational ground state and matrix elements of p(R) for transitions from that ground state to the first seven vibrationally excited states within the electronic ground state. For comparison with these theoretical results, we have undertaken a re-analysis of all experimental data of intensities of vibration-rotational transitions in infrared spectra, combined with a value of the expectation value of p(R) in the ground state from the Stark effect, to generate a radial function for electric dipole moment. The agreement between calculated and experimental values of vibrational matrix elements of the electric dipole moment is satisfactory, resolving outstanding questions about experimental and computational accuracy in the literature. We predict matrix elements for intensities of vibration-rotational bands 6–0 and 7–0, not yet measured.