The permanent electric dipole moment of CaOH

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Abstract
Theoretical calculations yield electric dipole moments ( μ) of 0.98, 0.49, and 0.11 Debye at the computed equilibrium geometries of the X 2Σ+, A 2Π, and B 2Σ+ states of CaOH. Thus the pure rotational spectra in these electronic states are expected to be weaker than the corresponding alkaline monohalides. This is consistent with our failure to observe the pure rotational spectrum of the X 2Σ+ state by microwave‐optical double resonance spectroscopy, and is in agreement with our experimental determination of μA 2Π3/2 state. However, the vibrational–rotational spectrum is quite strong, since dμ/dr is quite large in these ionic molecules. Using the free‐ion polarizabilities, the modified Rittner model predicts a μ value of about −8 D for the X 2Σ+ state, which is in error by about 9 D. The theoretical values for total dipole moment, as well as contributions from Ca+ and OH−, can be reproduced using effective polarizabilities. However, this analysis shows that the effective polarizability of OH− is a much smaller fraction of the free‐ion value than for Ca+, because Pauli repulsion restricts the OH− polarization toward the Ca+. Some account of this effect seems to be a prerequisite for the successful application of either Rittner or ligand field models to ionic systems.