The protonated water trimer is a prototype “proton wire” that can transport two protons nearly concertedly. This “proton transfer mode” (PTM) is an important contributor to the infrared spectrum of the isolated gas-phase cluster. We have simulated its infrared spectrum for both the hydrated and deuterated isotopologues, using vibrational 2nd order perturbation theory (VPT2) and ab initio molecular dynamics (AIMD) trajectories. VPT2 calculations explain quantitatively the experimental spectra at both high and low frequencies, provided that high-level quantum chemistry is utilized. In the D₂-tagged hydrated cluster, the PTM undergoes giant Fermi resonances (FR’s) with two combination bands. In the deuterated analogue, one observes a single FR of “normal” intensity, manifested as the doublet recently reported experimentally. We provide band assignment for both isotopologues, with and without the D₂ tag applied in experiment. We discuss possible manifestations of the giant resonance on proton transfer through water wires.

质子化水三聚体是原型“质子丝”，可以几乎一致地运输两个质子。这种“质子传递模式”（PTM）是孤立的气相簇的红外光谱的重要贡献者。我们使用振动二阶微扰理论（VPT2）和从头算分子动力学（AIMD）轨迹，模拟了其水合和氘代同位素的红外光谱。如果利用了高级量子化学，VPT2的计算将定量地解释高频和低频下的实验光谱。在带有D ₂标签的水合簇中，PTM经历了巨大的费米共振（FR），其中两个组合乐队。在氘代类似物中，人们观察到一个“正常”强度的FR，表现为最近实验报道的双峰。我们提供了两种同位素异构体的带分配，无论是否在实验中应用了D ₂标签。我们讨论了通过水线传递质子的巨大共振的可能表现。