Hydrogen bond acetic acid, dimer

One after the other, examine methanol dimer and acetic acid dimer. Do the hydrogen-bond lengths in these systems differ significantly from the optimum distance in water dimer Are the hydrogen-bond angles in these compounds significantly different from those in water dimer Rationalize your results. 

Supramolecular self-assembly can be as simple as acetic acid dimerization through mutually complementary hydrogen bonds or as complex as supramolecule formation from multiple components as demonstrated by numerous examples of resorcin[4]arene and calix[4]arene capsules. One of the more interesting directions to have been taken has been the design and synthesis of self-complementary... 

In addition to these ab initio calculations, CNDO calculations continue to be made on hydrogen-bonded systems. The influence of substituents on acetic acid dimers, for example, has been investigated at this level of approximation.121... 

The hydrogen-bonded dimers with biologically interesting functional groups that have been studied by these methods are the OwH Ow bond in the water dimer and the OH -0=C bond in the formic and acetic acids dimers. They are important because they refer more directly to the intrinsic properties of a particular bond and provide experimental data for comparison with the ab-initio molecular orbital calculations on simple systems described in Chapter 4. 

Borisenko, K. B., Bock, C. W., and Hargittai, I., Geometrical consequences of intermolecular hydrogen bond formation in the formic acid and acetic acid dimers from ab initio MO calculations, J. Mol. Struct. (Theochem) 332, 161-169 (1995). 

FIGURE 15.2. Hydrogen bonding, represented by broken lines, in some simple organic molecules, (a) o-hydroxybenzaldehyde, (b) m-hydroxybenzaldehyde, and (c) acetic acid dimers. Note the hydrogen bonds in (a) and (c), but not in (b). 

The explanation was provided by the theory of the hydrogen bond. According to this theory, two acetic acid molecules with the generally accepted structural formula could combine by forming two hydrogen bonds with one another, to produce a double molecule (called acetic acid dimer), as shown in the adjacent drawing. 

The first question, which is raised by this simple analogy, concerns the very possibility of exciting slow mode wave packets in a hydrogen bond at all. Taking a different perspective it touches the very issue of interpretation of the notoriously complex IR spectra [9]. In the condensed phase much of this complexity is hidden under bands broadened by the solvent interaction. Hence it was only recently that coherent wave packet motion of a 100 cm i hydrogen bond mode could be observed after OH-stretch excitation, although in a system which has only a single minimum potential [10]. Meanwhile coherent low-frequency dynamics has also been observed in a double minimum system (acetic acid dimer) [11]. With this... 

Fig. 5-3. (a) The structure of acetic acid dimer which has an enthalpy of dissociation of 57.9 kJ mol-1 (A. D. H. Ciague and H. J. Bernstein, Spectrochim. Acta, 1959, 25A, 593). (b) Hydrogen bonding between base pairs such as adenine-thymine or cytosine-guanine. 

FIGURE 9.3 Typical hydrogen bonds (a) methyl alcohol dimer and (b) acetic acid dimer. 

The energy components of the acetic acid dimer, shown in Figure 5, show clearly the contribution that the hydrogen bond shape function makes to the total dimer... 

As a second example, we shall assume that the side-chain hydrogen bonds are all equivalent homologous double bonds of the acetic acid dimer... 

Fig. 54. Contribution of a homologous double hydrogen bond to AFg as a function of pH at 300 K., computed from Eq. (IV-36) for a carboxyl-carboxyl, acetic acid dimer type hydrogen bond, with the parameters of Fig. 47 (Scheraga, 1960d).

It is possible that some of the hydrogen bonds broken are of a more complicated type than the heterologous single bonds just considered. As an example of such a complex situation we. shall consider a homologous double bond, the carboxyl-carboxyl acetic acid dimer type bond. For such a bond xim is given by Eq. (IV-35). Therefore, on n lecting the term 2 K2/IH ],... 

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