MP4 Study of the Anharmonic Coupling of the Shared Proton Stretching Vibration of the Protonated Water Dimer in Equilibrium and Transition States

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 121; no. 10; pp. 2151 - 2165
• Main Authors: Pitsevich, G, Malevich, A, Kozlovskaya, E, Mahnach, E, Doroshenko, I, Pogorelov, V, Pettersson, Lars G. M, Sablinskas, V, Balevicius, V
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 16.03.2017
• Subjects: Anharmonicity; Bridging; Infrared spectroscopy; Mathematical analysis; Stretching; Symmetry; Vibration; Vibration analysis
• ISSN: 1089-5639; 1520-5215; 1520-5215
• DOI: 10.1021/acs.jpca.7b00536

The structure and harmonic and anharmonic IR spectra of the protonated water dimer (PWD) were calculated in C 1, C 2, and C s symmetry at the MP4/acc-pVTZ level of theory. We found that structure and IR spectra are practically identical in C 2 and C 1 symmetry, demonstrating that an equilibrium C 1 configuration of the PWD is not realized. Anharmonic coupling of the shared proton stretching vibration with all other modes in the PWD in C 2 and C s symmetry was the focus of this investigation. For this purpose, 28 two-dimensional potential energy surfaces (2D PES) were built at the MP4/acc-pVTZ level of theory and the corresponding vibrational Schrödinger equations were solved using the DVR method. Differences in the coupling of the investigated mode with other modes in the C 2 and C s configurations, along with some factors that determine the red- or blue-shift of the stretching vibration frequency, were analyzed. We obtained a rather reasonable value of the stretching frequency of the bridging proton (1058.4 cm–1) unperturbed by Fermi resonance. The Fermi resonance between the fundamental vibration ν7 and the combined vibration ν2 + ν6 of the same symmetry was analyzed through anharmonic second-order perturbation theory calculations, as well as by 3D PES constructed using Q 2, Q 6, and Q 7 as normal coordinates. A significant (up to 50%) transfer of intensity from the fundamental vibration to the combined one was found. We have estimated the frequency of the bridging proton stretching vibration in the C s configuration of the PWD based on calculations of the intrinsic anharmonicity and anharmonic double modes interactions at the MP4/acc-pVTZ level of theory (1261 cm–1).