Vibrational frequency hydrogen transfer

In addition, the frequency cooo, as well as the tunneUng distance can also be extracted from the same empirical data. Thus all the information needed to construct a PES is available. Of course, this PES is a rather crude approximation, since all the skeleton vibrations are replaced by a single mode with effective frequency cooo and coupling parameter C. From the experimental data it is known that the strong hydrogen bond (roo < 2.6 A) is usually typical of intramolecular hydrogen transfer. 

We now consider hydrogen transfer reactions between the excited impurity molecules and the neighboring host molecules in crystals. Prass et al. [1988, 1989] and Steidl et al. [1988] studied the abstraction of an hydrogen atom from fluorene by an impurity acridine molecule in its lowest triplet state. The fluorene molecule is oriented in a favorable position for the transfer (Figure 6.18). The radical pair thus formed is deactivated by the reverse transition. H atom abstraction by acridine molecules competes with the radiative deactivation (phosphorescence) of the 3T state, and the temperature dependence of transfer rate constant is inferred from the kinetic measurements in the range 33-143 K. Below 72 K, k(T) is described by Eq. (2.30) with n = 1, while at T>70K the Arrhenius law holds with the apparent activation energy of 0.33 kcal/mol (120 cm-1). The value of a corresponds to the thermal excitation of the symmetric vibration that is observed in the Raman spectrum of the host crystal. The shift in its frequency after deuteration shows that this is a libration i.e., the tunneling is enhanced by hindered molecular rotation in crystal. 

In contrast to the subsystem representation, the adiabatic basis depends on the environmental coordinates. As such, one obtains a physically intuitive description in terms of classical trajectories along Born-Oppenheimer surfaces. A variety of systems have been studied using QCL dynamics in this basis. These include the reaction rate and the kinetic isotope effect of proton transfer in a polar condensed phase solvent and a cluster [29-33], vibrational energy relaxation of a hydrogen bonded complex in a polar liquid [34], photodissociation of F2 [35], dynamical analysis of vibrational frequency shifts in a Xe fluid [36], and the spin-boson model [37,38], which is of particular importance as exact quantum results are available for comparison. 

Abstract The problem of the low-barrier hydrogen bond in protonated naphthalene proton sponges is reviewed. Experimental data related to the infra-red and NMR spectra are presented, and the isotope effects are discussed. An unusual potential for the proton motion that leads to a reverse anharmonicity was shown The potential energy curve becomes much steeper than in the case of the harmonic potential. The isotopic ratio, i.e., vH/VD (v-stretching vibration frequency), reaches values above 2. The MP2 calculations reproduce the potential energy curve and the vibrational H/D levels quite well. A critical review of contemporary theoretical approaches to the barrier height for the proton transfer in the simplest homoconjugated ions is also presented. 

All theoretical studies on benzoic acid dimer underlined the need for a multidimensional potential surface. These studies have investigated the temperature dependence of the transfer process They included a density matrix model for hydrogen transfer in the benzoic acid dimer, where bath induced vibrational relaxation and dephasing processes are taken into account [25]. Sakun et al. [26] have calculated the temperature dependence of the spin-lattice relaxation time in powdered benzoic acid dimer and shown that low frequency modes assist the proton transfer. At high temperatures the activation energy was found to be... 

In this chapter, coherent low-frequency motions and their role in hydrogen bond dynamics and hydrogen transfer are discussed. In Section 15.2, the basic vibrational excitations and couplings in a hydrogen bond are introduced. Recent results on coherent low-frequency motions of intra- and intermolecular hydrogen bonds in the electronic ground state are presented in Section 15.3. The role of low-frequency motions in excited state intramolecular hydrogen transfer is addressed in Section 15.4, followed by some conclusions (Section 15.5). 

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