Benzoic acid dimer

A unique example of observation of tunneling splitting is given by Oppenlander et al. [1989]. Upon replacing the host benzoic acid dimer by a thioindigo molecule of nearly the same size, the resulting bias accidentally turns out to be small, of order of A. The 4x4 Hamiltonian of the complex of two dimers and the guest molecule is... 

Etter has shown that cocrystallization of mixed dimers of differently substituted benzoic acids can easily be achieved. We present here calculations that predict the relative stabilities of several possible mixed benzoic acid dimers that are variously substituted. 

Table IV. Interaction Stabilization Energies of Benzoic Acid Dimers (kcal /mole)...

Components of Substituted Benzoic acid Dimers Stabili-... 

Hydrogen bonded dimers of 4-RF-substituted benzoic acids [119, 120, 122, 126], /rcyclohexane carboxylic acids [120], and hydrogen bonded complexes of benzoic acids with 4,4 -bipyridine and stilbazole [166, 243-246] can be regarded as supramolecular rod-like mesogens with two fluorinated chains. Figures 33 and 34 show comparisons of selected benzoic acid dimers and their bipyridine and stilbazole complexes. Comparison of the benzoic acid dimers 122 and 123 with -alkyl chains and F-terminated -alkyl chains, respectively, indicates that a single F-atom at the end of the alkyl chain reduces the mesophase stability... 

Figure 1 Doubleproton transfer in benzoic acid dimers. When the 0-0 bond length becomes shorter, tunneling is enhanced.

Therefore, the concentration of benzoic acid dimers is given by ... 

Fig. 1 The two tautomers of the benzoic acid dimer. For each species the orientation of the vector connecting the two protons relative to the direction of the applied magnetic field (B0) is different [14]...

The deuterium isotope effect on the rate constant of the double-proton transfer in the solid state can be used as a criterion to judge whether the double-proton transfer proceeds in a simultaneous or in a stepwise manner. The difference between ratios of rate constants /chh/ hd and /chd/ dd depends on the mechanism of the double-proton transfer. If a double-proton transfer occurs in a stepwise manner, the slower step in the /chd process, that is, a deuteron transfer, becomes rate-determining. Consequently the ratio/chh/ hd should be large. On the other hand, since deuteron transfers are ratedetermining in both /chd and /cdd processes, the ratio /chd/ dd is expected to be small. By contrast, if a proton transfer and a deuteron transfer occur simultaneously, the ratios of /chh/ dd and /chd/ dd should be of the same order. The observed ratios for benzoic acid dimers in crystals were found to be /chh/ hd = 23, / hd/ dd = 10 at 15 K (Stbckli et ai, 1990). Accordingly, the double-proton transfer is considered to occur concertedly in benzoic acid dimers [16]. 

Here a is a dimensionless constant, 5p(R) is the density fluctuation of the medium at the position R (the center of symmetry of the benzoic acid dimer), 0)D is the Debye frequency, and N is the number of acoustic modes, cot = 7 sound k, (bk) is the Bose operator of creation (annihilation of a acoustic phonon with the wave vector k). In the localized representation we have... 

The first member on the right in expression (38) is the relaxation rate caused by dimmers of the pure material. The second member arises from dynamics of the benzoic acid dimers contained within the range of the thiondigo quest molecule 7j-1 (ffl) is stipulated by modulation of proton dipolar interaction induced by the proton transfer in the hydrogen bonds. 

Wojcik and coworkers [42] studied in particular the band shape for asymmetric OH stretching vibrations in benzoic acid dimer. The experimental spectra were compared with calculated ones. The calculations were based on... 

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. 

In the following we will present the results for two benchmark systems, the benzoic acid dimer and the formic acid dimer, which have been extensively studied in the past, both experimentally and theoretically. The study of these dimers in the gas phase allows a direct comparison between them. 

Benzoic acid dimer was the first carboxylic dimer to be investigated in detail spectroscopically. The first studies involved the measurements of temperature dependent relaxation times via NMR. Due to the presence of two inherent chromo-phores it can also be investigated by means of optical spectroscopy. 

The benzoic acid dimer is stabilized by two hydrogen bonds, with the two monomers being coplanar in the electronic ground state [21]. The proton transfer is shown to strongly couple to the heavy nuclei, implying that the whole benzoic acid frame rearranges itself slightly as the protons tunnel. 

X-ray structural studies showed that benzoic acid dimers crystallize in a monodin-ic cell with the hydrogens aligned close to the crystallographic c-axis [22]. Time of flight neutron diffraction measurements were carried out by Brougham et al. [23] and these yielded the crystal structure of a deuterated benzoic acid. The stmcture of the monomer units is assumed to be unchanged upon complexation. 

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... 

The benzoic acid dimer has been extensively studied in various condensed phase environments where it experiences an asymmetric local environment. By doping the crystal the inherent asymmetry in the crystal was reduced. However, in general, the crystal structure and the dopant can influence the observed tunneling matrix element. 

GHz for the benzoic acid dimer was deduced, yielding information on the proton transfer tunneling. 

In 2000 Remmers et al. [24] used the combination of molecular beam techniques and high resolution laser spectroscopy (inherent line width 3 MHz) to obtain the first information on proton transfer tunneling in any isolated gas phase carboxylic acid dimer. They reported the measurement of a high resolution ultraviolet spectrum of benzoic acid dimer in the gas phase. The spectra showed two... 

Figure 2.2 Potential energy surface of benzoic acid dimer.

However, based upon a direct dynamical calculation, which used a potential surface for benzoic acid dimer at a B3LYP/6-31 + G(d) level [20], Smedarchina et al. suggest that the hydrogen bonding is weaker in the excited state, in agreement with the experimentally observed increase in the intermolecular distance r [24]. 

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