Spectroscopic characterization of the lowest Π and Σ bending states of ArHCN

Abstract

The lowest excited bending states, Σ1 and Π1, of the ArHCN complex have been measured by millimeter-wave electric resonance optothermal spectroscopy. The principal molecular constants determined for the Σ1 state are ν0=164 890.790(12) MHz; B=1958.8571(37) MHz; D=−0.075 23(29) MHz; eqaaQ=0.825(27) MHz; and μa=−0.521(30) D. For the Π1 state, the constants are ν0=181 984.4126(47) MHz; B=2031.3624(17) MHz; D=0.153 35(16) MHz; eqaaQ=0.904(11) MHz; and μa=0.273 02(63) D. The leading Σ1–Π1 coupling constants are the Coriolis coefficient β0=1016.998(13) MHz and the transition dipole moment μb=2.2535(57) D. The rotational constants for the two bending states indicate that the average separation between the argon and the HCN center of mass contracts by roughly 0.5 Å compared to the linear ground state. This is consistent with the nearly T-shaped average geometry for each state established by analysis of the dipole moments and quadrupole coupling constants. Agreement between this work and prior theory confirms attribution of the anomalous distortion and isotope effects in the ground state to extreme angular–radial coupling. The relative sign of the dipole moments for the Σ1 and Π1 states is resolved in this work, allowing an unambiguous interpretation of the angular information. Assuming Laguerre angular distributions, we obtain that the Σ1 state wave function has a maximum at an angle of 108° with a halfwidth of 49°, and that the Π1 state maximum is at 80° with a halfwidth of 37°. This estimate for the Σ1 state angular distribution indicates that although the state is not antilinear (ArNCH), as was expected, it does approach this configuration. The Π1 state is nearly a free rotor eigenstate, showing that the angular part of the potential surface near 90° is extremely flat. The combined data from the ground, Σ1, and Π1 states reflect virtually the entire angular coordinate along the radial minimum of the potential, and should provide a reliable benchmark for ab initio potential energy surfaces for ArHCN near the bottom of the well. We compare the data to predictions from available models.