Pyridine N-oxide/trichloroacetic acid complex in acetonitrile: FTIR spectra, anharmonic calculations and computations of 1–3D potential surfaces of O–H vibrations

Anharmonic analysis of CH and OH stretching vibrations of the formic acid dimer

“…The wave function was expressed as one-dimensional or two-dimensional The frequency values of the 1 ν and 17 ν vibrations found in such a way were specified by taking into account the anharmonic interaction of these modes with other normal modes within the PT2 approximation. Such a hybrid approach to the estimation of an anharmonicity influence, that takes into account all vibrational degrees of freedom, was successfully employed earlier for intermolecular H-bonds [53,59,60] and for intramolecular H-bond in malonaldehyde molecule [61]. In frame of the PT2 approximation [54], the frequency value of the fundamental vibration f i νɶ can be found if its harmonic frequency ( h i νɶ ) is known and the matrix of the anharmonicity constants ij χ is determined:…”

“…According to the procedure described in [53], we have estimated the "quantum chemical" amplitudes of atoms parallel to the q i ; q j plane, were projected to the planes q i ; q j , and they are presented in Figure 5.…”

“…The choice of the basis set as well as the method that allows taking into account exchange effects and electron correlation were tested in our previous studies of organic molecules [49,50] and of molecular complexes with the hydrogen bond [51][52][53]. In the process of geometry optimization, the fact that a local minimum on the PES was reached has been supported by the absence of imaginary frequencies in the calculated vibrational spectra.…”

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

“…The peculiarity of the PES of the PWD with the C 2 configuration is that accounting for anharmonicity effects in the frame of the standard model leads to a blue-shift of the frequency of the stretching vibration and to a red-shift of the frequencies of the bending vibrations. In many works, however, it is noted that accounting for anharmonicity effects in the standard PT2 model leads to overestimating both stretching and bending frequencies. , At the same time, usually the true value of the frequency of the fundamental vibration is between the calculated harmonic and anharmonic values. Taking into account this remark and the values of the stretching and bending vibrations obtained in the harmonic and anharmonic approximations (for the B3LYP/acc-pVQZ level of theory), one can expect that the stretching vibrations of the central proton should be lower than the bending ones.…”

“…However, this was applied only to the stretching vibrations of the O–H bonds of peripheral water molecules. It is commonly understood − that in the case of very anharmonic PES (like for the PWD) the PT2 anharmonic approximation could be insufficient to correctly estimate vibrational frequencies. In such cases, calculations of PES with restricted set of the vibrational coordinates are needed. − The 1D and 2D PES for the stretching vibrations of the central proton and the stretching O···O vibrations of the hydrogen bridge were for the first time calculated in ref .…”