Great Amplitude Motions in Isolated H-Bonds

How can we approach the molecular mechanisms of such reactions We have already seen in Chs. 4 and 5, devoted to the description and analysis of the specific properties H-bonds exhibit in IR spectra, that the stretching vibrations of the H-atoms that establish H-bonds are strongly anharmonic, which is due to having unusually large ampUmdes. The analysis of IR spectra has shown that the coupling between v, described by coordinate q, with the relative stretching vibration of the two molecular parts X H and Y when they are H-bonded and form a molecular complex X-H Y, is at the origin of most of the exceptional properties H-bonds display in IR spectroscopy. We have seen (in eqs. (5.2) and 

The 03 dependence of the force constant k(Q) = rnoP iQ) creates an anharmonic coupling between both modes q and Q. It is at the origin of the exceptional width of bands of H-bonds, and of their shifts towards lower wavenumbers when compared to bands of the same X H molecules, when they do not establish H-bonds. These most intense bands are 0 1 transitions, or transitions between the ground vibrational state of q and its first excited state. The other possible transitions, 0 n with n 1, have intensities equal to 0 in the case of an harmonic oscillator in q (no terms in q, (f, etc. in V q, Q)). Overtones of v, particularly those corresponding to 0 2 transitions in the 6000-6500 cm region, however often 

We have already encountered this type of potential in eq. (4.6) where it governed Q, intermonomer modes. It tends towards the harmonic potential of eq. (6.1) when the parameter 8, characteristic of this ID anharmonicity in q, tends towards 0. The vibrational potential experienced by the slow mode when the rapid mode q is in its nth excited state, is then equal to 

Although the existence of such a ID anharmonicity looks quite general, it may be considered in most cases a small correction, with no real importance, in the same way as we have considered Fermi resonances in the 0 1 transition in mid-IR spectra. This correction is 

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