Knowledge Bank

A detailed study of the complex NO-HF has been carried out using the technique of Fourier Transform Microwave Spectroscopy. There have been in excess of a hundred hyperfine transitions recorded in the region from 7 GHz to 19GHz, originating from the J=5/2(f)-3/2(f), J=3/2(c)-1/2(e) and J=5/2(e)-3/2(e) rotational levles. A previous infrared study1, provided the rotational constants which facilitated the search in the microwave region. An assignment has been carried out using microwave double resonance to deduce connections between the levels. The hydrogen bond interaction is sufficiently strong to cause considerable quenching of the NO electronic orbital angular momentum giving rise to a Renner-Teller parameter $E2$ and a Zeeman interaction with the Earth#### magnetic field. A magnetic interaction has been observed between the unpaired electron on NO and both the hydrogen and fluorine nuclei in the HF. Initial analysis has shown that the Renner-Teller parameter $E2$ has a negative sign, indicating that the unpaired $\pi \ast$ electron prefers to be in the plane of the complex and as a consequence of an attractive interaction to the neighbouring HF. It is believed that this is the first such observation of a magnetic interaction induced by complexation. Such interactions are extremely sensitive to the nature of the forces between partners in a complex, and should be a common feature of microwave spectra of open-shell complexes involving magnetic nuclei such as hydrogen, nitrogen and fluorine.