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Abstract Effects of the dispersion interactions on the parameters of the water dimer equilibrium configuration and IR spectra are analyzed in the paper. Dimer equilibrium geometry was calculated using a series of the density functionals, both with (B3LYP-D3, B2PLYPD, B2PLYP-D3, mPW2PLYPD, wB97XD) and without (B3LYP, B2PLYP, mPW2PLYP, wB97X) accounting of the dispersion interactions and using density functional M-06-2X that include the dispersion interaction effects. For the determined equilibrium configurations of the water dimer, IR spectra in harmonic and anharmonic (VPT2) approximations were calculated at the corresponding levels of theory. Additional analysis of the donor hydroxyl group vibrations was performed using the 3D potential energy surfaces, calculated at B3LYP/cc-pVTZ, B3LYP-D3/cc-pVTZ, B2PLYP/cc-pVTZ, B2PLYPD/cc-pVTZ, M06-2X/cc-pVTZ, wB97XD/cc-pVTZ levels of theory. Moreover, double anharmonic couplings of the analyzed modes with other vibrational modes were accounted using the "hybrid" method. Comparison of the calculated results with the experimental and theoretical data from the literature was performed. Highlights • Anharmonic calculations of WD configurations and IR spectra were performed at different levels of theory. • Effects of the dispersion interactions on the parameters of WD equilibrium configuration and IR spectra were analyzed. • Analysis of the donor hydroxyl group vibrations was also performed using the 3D PES, calculated in different approximations. • Double anharmonic interactions of donor OH group vibrations with other vibrational modes were taken into account. [ABSTRACT FROM AUTHOR]

The structure and frequencies of the protonated water dimer normal vibrations were calculated at the MP4/Aug-cc-pVTZ level of theory. Shared proton stretching vibrations were analyzed by constructing 4D potential energy surface using normal coordinates Q 6 , Q 7 , Q 8 , Q 9 , which describe intermolecular vibration of water monomers, stretching and bending vibrations of the shared proton, respectively. 1–4 dimensional vibrational Schrödinger equations were solved using the discrete variable representation method. Values of some anharmonic constants, responsible for the triple and quadruple modes coupling with the shared proton stretching vibrations were obtained. The efficiency of the hybrid method in the determination of the frequency of shared proton stretching vibrations was demonstrated during the solution of the vibrational problem with nD reduced dimension for all used values of n (n = 1–4). [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

The structure and IR absorption spectrum of the formic acid dimer (FAD) were calculated using the B3LYP DFT functional and cc-pVTZ basis set. Matrices of force and anharmonic constants were calculated in order to find the fundamental frequencies more accurately. The 1D and 2D potential energy surfaces (PES) were built using symmetry coordinates and stretching coordinates of С Н and O H bonds. For O H stretching coordinates, the 2D PES were built with and without optimization of the remaining geometric parameters. It was shown that in the second case besides the global minimum, an additional local minimum on the unrelaxed PES is formed with the energy more than 4000 cm −1 higher than the energy of the ground state. A numerical solution of Schrödinger equations gives the values of O H and C H stretching vibrations. Comparison of the obtained theoretical results with the experimental data presented in the literature allowed us to make new assignments for IR and Raman spectral bands related to hydroxyl groups stretching vibrations in FAD. [ABSTRACT FROM AUTHOR]