Double proton transfer in formic acid dimer

4 Double proton transfer in formic acid dimer 

There are many molecules in nature that have multiple hydrogen bonding like DNA in a double helix structure. In those systems, multiple proton 

In this subsection, we study the dynamical electron mechanism of the elementary process (in gas phase) of double proton transfer in FAD with an electron wavepacket method. To begin with, we ask ourselves a question what is the basic mechanism of this double proton transfer (Fig. 7.18(a)), hydrogen-atom migration (Fig. 7.18(b)), hydride transfer (Fig. 7.18(c)), or else (We term the event of Fig. 7.18(a) proton transfer tentatively imtil we can determine the mechanism.) At a glance none of the above three mechanisms seems totally satisfactory. What is wrong with these classic mechanisms  

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P. R. L. Markwick, N. L. Doltsinis, and D. Marx (2005) Targeted Car-Parrinello molecular dynamics Elucidating double proton transfer in formic acid dimer. J. Chem. Phys. 122, 054112... 

The most recent calculations on the tunneling dynamics of double proton transfer in formic acid dimer and benzoic acid dimers have been reported by Smedar-china et al. [20]. They used direct dynamics calculations to predict the tunneling splittings of both carboxylic dimers. 

Figure 6.20. (a) Projection of a three-dimensional PES K(p,p2,p3) for two-proton transfer in formic acid dimer onto the (p, p,) and (p, p3) planes. In contrast with points A and B, in points C and D the potential along the p3 coordinate is a double well resulting in bifurcation of the reaction path [from Shida et al., 1991b]. (b) The contour lines correspond to equilibrium value of p3 and potential (6.37) when V(Q) = V0(Q4 - 2Q2), V0 = 21 kcal/ mol, C = 5.()9V0, A = 5.351/, Qn = 0.5. When Q > Qc, two-dimensional tunneling trajectories exist in the shaded region between curves 1 and 2. Curve 3 corresponds to synchronous transfer. 

S. Miura, M. Tuckerman, and M. Klein (1998) An ab initio path integral molecular dynamics study of double proton transfer in the formic acid dimer. J. Chem. Phys. 109, p. 5290... 

Figure 29.7 Stationary points representing the reactant (R), transition state (TS), product (P), and (where applicable) intermediate (Int) related to double proton transfer in analogs of the formic acid dimer (a) formamidine dimer [43] ...

The work on the formic acid dimer focused on the double-well potential for a highly symmetric system. An attempt to locate a double-well potential for a less symmetric system was made by Zielinski and Poirier (1984). They studied the formamide dimer and isolated a possible structure for the transition state for a double-proton transfer along the reaction path to the formimidic acid dimer (a dimer of the enol form of formamide) using the 3-21G basis set. The proposed transition state is only slightly less stable than the formimidic acid dimer. In other words, a very asymmetric double-well potential was found with a very shallow well on the formimidic acid dimer side of the reaction. It will be interesting to see the shape of the function for a double-proton transfer between formamide and amidine, which would more closely mimic the double-proton transfer that may be possible for the A-T pair. 

Early theoretical work on FAD was concerned with the dimer equilibrium geometry and the electronic structure (see for example Refs. [32-36]). Ah initio molecular orbital studies on the structure of formic acid dimer in 1984 agreed very well with the experimental structures as determined by electron diffraction [37]. Due to the importance of the double proton transfer of FAD as a key prototype for multiple proton transfer reactions several theoretical studies have been reviewed in the literature [38]. Rotational constants for formic acid dimer were obtained by high resolution spectroscopy of (DCOOH)2 [39] and by femtosecond degenerate four wave mixing experiments in the gas cell at room temperature and under supersonic jet experiments by Matylisky et al. [40]. 

Figure 29.6 Schematic representation of the synchronous double proton transfer process in the formic acid dimer (top) and benzoic acid dimer (bottom).

With one exception, these results are based solely on quantum-chemical calculations of the potential energy surface. Theoretical evaluation of the transfer dynamics has been attempted only for the formic acid dimer, for which two general level splittings have been observed and assigned to synchronous double proton tunneling in the ground state and a vibrational excited state, respectively. 

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