Intermolecular potential between an atom and a diatomic molecule: The structure of ArCIF

The radio‐frequency and microwave spectrum of Ar³⁵ClF has been measured by molecular beam electric resonance spectroscopy. The molecular constants are: $$\begin{array}{ccc}& \underset{¯}{{\mathrm{Ar}}^{35}\mathrm{ClF}}& \underset{¯}{{\mathrm{Ar}}^{37}\mathrm{ClF}}\\ B& \text{1327.113(5)}\mathrm{MHz}& \text{1319.650(5)}\mathrm{MHz}\\ D_J& \text{4.66(20)}\mathrm{kHz}& \text{4.72(20)}\mathrm{kHz}\\ {\mu }_a& \text{1.053(3)}\mathrm{D}& \\ {\mathrm{eqQ}}_a& \text{−140.869(15)}\mathrm{MHz}& \text{−111.053(15)}\mathrm{MHz}\end{array}$$ The atomic arrangement is Ar–Cl–F and the vibrationally averaged argon‐chlorine distances are 3.3301(1) and 3.3290(1) Å in Ar³⁵ClF and Ar³⁷ClF, respectively. The Ar–ClF bond stretching frequency is 47 cm⁻¹. From the value of eqQ_a the equilibrium structure is linear with an angle bending frequency of 41 cm⁻¹. The linear structure is striking evidence against the validity of an additive pair potential for an atom and a diatomic molecule. The observed structure is well predicted by Walsh's rule for triatomic molecules.