P-type doubling in the infrared spectrum of NO–HF

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Abstract
The HF stretching band of the NO–HF open‐shell complex has been recorded using a molecular‐beam optothermal spectrometer. The spectrum exhibits P‐type doubling indicative of an unpaired electron spin coupled to the rotational angular momentum of a bent complex with substantially quenched electron orbital angular momentum. From B̄‘=0.111 320(17) cm−1, and an off‐axis angle for the NO of 30°, the zero‐point center‐of‐mass separation is estimated to be 3.4396(3) Å. The HF frequency shift of 84 cm−1 indicates that the complex is hydrogen bonded, and the spectral intensities imply that the HF axis is aligned closely to the center‐of‐mass axis and the NO is off axis by 30±15°. The Renner–Teller‐like orbital quenching parameter is somewhat larger than the spin–orbit constant in the free NO molecule and increases substantially upon vibrational excitation. The transitions in this band exhibit vibrational predissociation broadening of 200±40 MHz (FWHM), similar to that observed for a number of closed‐shell hydrogen‐bonded HF complexes.