Formic acid, dimerization

A calculation of tunneling splitting in formic acid dimer has been undertaken by Makri and Miller [1989] for a model two-dimensional polynomial potential with antisymmetric coupling. The semiclassical approximation exploiting a version of the sudden approximation has given A = 0.9cm" while the numerically exact result is 1.8cm" Since this comparison was the main goal pursued by this model calculation, the asymmetry caused by the crystalline environment has not been taken into account. 

Chojnacki, H., J. Andzelm, D. T. Nguyen, and A. Sokalski. 1995. Preliminary Density Functional Calculations on the Formic Acid Dimer. Computers and Chemistry 19,181. 

P. Zielke and M. A. Suhm, Raman jet spectroscopy of formic acid dimers Low frequency vibrational dynamics and beyond. Phys. Chem. Chem. Phys. 9, 4528 4534 (2007). 

G. V. Mil nikov, O. Kuhn, and H. Nakamura, Ground state and vibrationally assisted tunneling in the formic acid dimer. J. Chem. Phys. 123, 074308 (2005). 

It is important to distinguish, in this discussion, between the H-bond as it occurs in a single isolated grouping (e.g. (IV) above, or systems such as XOH OMe2 in which there is no opportunity for an extensive continuous series of H-bonds) and as it occurs in multiple groupings (e.g. the formic acid dimer (V) or ice, where two or more such bonds are mutually dependent). In V for example, the motion of... 

Next let us consider a correlated system, typified by the formic acid dimer... 

Note, that in the marked region at a 124 ps a J-type recurrence of the formic acid dimer appears in the experimental spectrum. 

Fig.la shows the first 200ps of the fs DFWM spectrum of HCOOH vapor at room temperature. A complete fitting of the spectrum delivers values of rotational and CD constants and information on the diagonal elements of the PT for HCOOH. The rotational and CD constants are in good agreement with reference values from a Fourier-Transform microwave experiment. [7] The experimental spectrum is fitted very well by the simulation (Fig. 2b), except for the region 120-130ps (marked in Fig. 1). The spectral features in this region originate from the formic acid dimer (HCOOHh-...

Fig.2. Fs DFWM spectrum of the formic acid dimer (O-H...O/O...H-O type). Magnified regions around J-type recurrences obtained under supersonic jet conditions ( 64K). a) Experimental spectrum, b) Fitted simulation of the spectrum. Extracted constants are given in Table 1.

Fitting results for fs DFWM experiments of the formic acid dimer (ground state) rotational constants, CD constants, parametrized polarizability parameter cp, Ray s asymmetry parameter k and temperature T. ... 

Quantum-chemical calculations of the formic acid dimer structure were carried out by Graf et al. [1981] and Meier et al. [1982]. The bond lengths and angles in the ground and transition states are listed in Table 6.3. Hydrogen transfer in two OH----O fragments occurs when the distance... 

The structure of Formic Acid Dimer in Equilibrium and Transition States... 

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. 

As mentioned in section B. 1., only one rotational constant, (B + C), per molecule is available from the observed a-type spectrum (AK = 0, AJ = 1), and the results are given in Table 1. (B + C)/2 is essentially an effective diatomic molecule rotational constant, and therefore depends most strongly on the distance between the monomers of the complex. The rotational constants in Table 1 were best fitted with a model in which O. .. H—O distances were 2.67 A and O. .. H—N distances were 2.71 A. These O. .. H—O distances are somewhat shorter than the 2.73 A distance in formic acid dimer and the 2.76A distances in acetic acid dimer68. However, they are... 

Figure 21-15. Attachment of an excess tt electron to a cyclic hydrogen-bonded cluster facilitates inter-molecular proton transfer (a) formic acid dimer, (b) formamide dimer, and (c) formic acid-formamide (Figure 1 of ref. [51]. Reused with permission. Copyright 2005, American Institute of Physics)...

Bachorz RA, Haranczyk M, Dqbkowska I, Rak J, Gutowski M (2005). Anion of the formic acid dimer as a model for intermolecular proton transfer induced by a it excess electron. J Chem Phys 122 204304-1-7. 

Table 2.39 Optimized parameters (A and degs) of the geometry of the formic acid dimer T... 

From computations with basis sets of the polarized 6-3IG type, at SCF and MP2 levels, the change from formic acid dimer to acetic acid dimer has only a very minor influence upon the energetics of binding . This conclusion is indeed verified by resonant laser photoacoustic spectroscopic measurements i 79,181,182 which find further that trifluoroacetic and propionic acid dimers have a very similar binding enthalpy to formic dimer. One may extrapolate that the formic acid dimer Fl-bond energy is probably applicable for most carboxylic acids with alkyl chains replacing the CH group. 

Table 2.40 Energetics of binding (kcal/mol) in the formic acid dimer. Data corrected for BSSE .

Hayashi, S., Umemura, J., Kato, S., and Morokuma, K., Ab initio molecular orbital study of the formic acid dimer, J. Phys. Chem. 88, 1330-1334 (1984). 

Scheiner, S. and Kern, C. W., Molecular orbital investigations of multiply hydrogen bonded systems. Formic acid dimer and DNA base pairs, J. Am. Chem. Soc. 101,4081-4085 (1979). 

FIGURE 3-19 IR spectra of gaseous formic acid dimers. /> = 15 mm, T = 26 G. [From Millikan, Ph.D. Thesis, University of California (1956).]... 

Slutsky and Bauer (1893) repeated the calculations of Halford, but used what seems to be an unreasonably high estimate of the bending force constant of the H bond. They calculate 172 and 124 cm"" for the symmetric and asymmetric stretching modes, of the formic acid dimer. 

The formic ° and acetic acid decompositions are probably not unimolecular, intramolecular eliminations as reported. An. 4-factor lower than 10 sec (Table 38, HCOOH-> H2 + CO2) is quite impossible. In addition, decomposition of formic acid dimer, (HCOOH)2 - 2H2O + CO, is equally suspect. The authors... 

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 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. 

This isotopic difference is small and has been scarcely measured. A difference of -0.02 A for O- O distances in the H-bonds and D-bonds of formic acid dimers in the gas phase has been measured by electron diffraction (1) as covered in Ch. 3. These dimers are excellent models of H-bonds. Their strnctnre is drawn in Figures 1.6 and 4.4. From the analysis of the shape of their bands, developed in Ch. 5 and from which the equations developed in the appendix of this chapter are dednced, a value for the dimensionless parameter b that characterizes the anharmonic coupling between and intermonomer modes and is proportional to dcoldQ, following the eqnation... 

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