Figure 3.

Left: sections of the experimental spectra showing the c-type transition 5_3,3–4_2,3 of Phe–H₂O (a), Phe–H₂¹⁸O (b), and Phe–D₂O (c) and the c-type transition 5_1,4–4_0,4 of Phe–HDO (d). Right: sections of the experimental spectra showing the a-type transition 5₁₅–4₁₄ of Phe–H₂O (a), Phe–H₂¹⁸O (b), and Phe–D₂O (c) and the a-type transition 5_3,3–4_3,2 of Phe–HDO (d). Red dashed lines connect the two tunneling states generated by C₂ internal rotation of the water molecule, while the blue arrows connect the two tunneling states arising from water’s translation motion. The two transitions are labeled following the notation J_Ka,Kc ← J_K′a,K′c^′. For each component of the tunneling splitting, the lower and upper tunneling states are indicated as 0⁺, 0^–, 1⁺, and 1^–. The magnitude of the splittings is given in megahertz.