中央研究院 原子與分子科學研究所

Complex vibrational features of solvated hydronium ion, H₃O⁺, in 3 μm enable us to look into the vibrational coupling among O-H stretching modes and other degrees of freedom. Two anharmonic coupling schemes have often been engaged to explain observed spectra: coupling with OH bending overtone, known as Fermi resonance (FR), has been proposed to account for the splitting of the OH stretch band at ~3300 cm^-1 in H₃O⁺…Ar₃, but an additional peak in H₃O⁺…(N₂)₃ at the similar frequency region has been assigned to a combination band (CB) with the low-frequency intermolecular stretches. While even stronger vibrational coupling is expected in H₃O⁺…(H₂O)₃, such pronounced peaks are absent. In the present study, vibrational spectra of H₃O⁺…Kr₃ and H₃O⁺…(CO)₃ are measured to complement the existing spectra. Using ab initio anharmonic algorithms, we are able to assign the observed complex spectral features, to resolve seemingly contradictory notions in the interpretations, and to reveal simple pictures of the interplay between FR and CB.

Experimental infrared spectra between 2600 to 3800 cm^-1 for a series of asymmetric proton bound dimers with protonated trimethylamine (TMA–H⁺) as the proton donor were recorded and analyzed. Based on conventional wisdom, the frequency of the N-H⁺ stretching mode is expected to red shift as the proton affinity of proton acceptors (Ar, N₂, CO, C₂H₂, H₂O, CH₃OH, and C₂H₅OH) increases. The observed band, however, shows a peculiar splitting of ≈300 cm^-1 with the intensity shifting pattern resembling a two-level system. Theoretical investigation based on ab initio anharmonic algorithms reveals that the observed band splitting and its extraordinarily large gap of ≈300 cm^-1 is a result of strong coupling between fundamental of the proton stretching mode and overtone states of the two proton bending modes, that is commonly known as Fermi resonance (FR). We also provide a simple and general theoretical model to link the strong FR coupling to the quasi-two-level system behavior in the observed band intensity. Since the model does not depend on the molecular specification of TMA–H⁺, the strong coupling we observed here is an intrinsic property associated with proton motions in a wide range of molecular systems.