Carboxylic acid dimers dimeric structures

The structure of carboxylic acid dimers results by time-resolved femtosecond degenerate four-wave mixing spectroscopy... 

In concentrated solution 7-methylisatin-4-carboxvlic acid exhibits normal carboxylic acid dimerization, but in the solid state the lactol form 58 is present.228 Infrared studies indicate that hydrogen bonding as shown in 59 best represents the structure and mode of association of... 

The structures of three cocrystals of caffeine having a 1 1 stoichiometry with various hydroxy-2-naphthoic acids have been reported [62], The anticipated imidazole-carboxylic acid supramolecular synthon was observed in caffeine cocrystals containing l-hydroxy-2-naphthoic acid and 3-hydroxy-2-naphthoic acid, while a hydrogen-bonded carboxylic acid dimer (and no hydroxyl-caffeine heterosynthon) was observed in the caffeine cocrystal with 6-hydroxy-2-naphthoic acid. 

Atom-atom potential calculations for carboxylic acid dimers [167] suggested that, while the activation energy is lower for the double proton transfer, both mechanisms can be energetically plausible depending on the structure of the system under investigation. 

It is interesting to note that, for malonic acid (which is structurally related to DMMA), the activation energy measured from XH NMR Tx measurements [170] is 5.6 kj mol-1, which is significantly lower than in DMMA and is assigned to proton jumps between the two minima of an asymmetric double well potential. This emphasises the importance of the effect of the crystal packing on the asymmetry of the potential function, which defines the mechanism of the proton dynamics in carboxylic acid dimers. 

The archetypal self-complementary supramolecular synthons are carboxylic acid dimers, which form i (8) rings (Fig. la). Extension from the discrete zerodimensional dimer into one-, two- and three-dimensional structures can be facilitated by the incorporation of more than one carboxylic acid group into a molecule. Hence simple dicarboxylic acids such as terephthalic acid and isophthalic acid typically exhibit tape structures whereas tricarboxylic acids such as trimesic acid form sheet structures (Fig. 2). 

Bifurcated hydrogen bonds between dipyridinium cations and coordinated dithiooxalates have been used to assemble tapes [130] with similar geometry to those in which coordinated halides act as acceptors [15]. Replacement of dipyridinium with 4-carboxypiperidinium gave a structure in which the anions are linked by carboxylic acid dimer assembled dications. Similar bifurcated hydrogen bonds were observed in the structures of bipyridinium salts of anilic acids [131]. 

Fig. 12 Assembly of azobenzenes via carboxylic acid dimers, a TEM-micrograph of elongated aggregates (b, c) models for rod- and fiber-type structures. Reprinted with permission from [74]...

Because of the structural arrangement, the carboxylic acid dimer is unusual. The O—H bonds of the two acid units of the dimer must vibrate in unison, in-phase or out-of-phase. If the cyclic dimers are planar or close to planar, only the out-of-phase mode of v, will be IR-active. This has particular significance in reference to the anharmonicity. By synunetry the potential function for the out-of-phase vibration cannot involve odd powers of the displacements. The absence of odd power... 

Band Width. The structure of has been discussed at length in Chapter 3 (Section 3.3.8). As indicated there, one of the currently popular theories is based on the assumption of extreme mechanical anharmonicity of Vi (268, 724, 1). The theory is well described (268), and will not be reproduced here in view of the experimental evidence concerning the anharmonicity presented in Section 3.3.9. Of particular interest are the remarks in that section referring to the absence of odd power terms in the potential function for the IR-active Vg mode of a cyclic carboxylic acid dimer. The implication which can be drawn is that the anharmonicity of this vibration may be quite low. Since an explanation of the extreme band-widths must encompass the carboxylic acids (see Fig. 3-20), anharmonicity does not seem to be a likely one. The weight of evidence suggests that Vg is not unusually anharmonic. 

S. Bratoi, D. Hadii, and N. Sheppard. Spectrochim. Acta 8, 249-61 (1956). IR combination tones and structure of carboxylic acid dimers. 

Fig. 1 X-ray crystal structure of the mixed carboxylic acid dimer (a-cyclooctyl-4-car-boxypropiophenone) (acetic acid).

Desiraju has reported a crystalline supramolecular wheel-and-axle compound with a structure based on a carboxylic acid dimer.33 Specifically, the group predicted that 4-(triphenylmethyl)benzoic acid would self-assemble to give a homodimer. The dimer was expected, owing to an inability to efficiently pack, to form inclusion compounds that host solvent molecules as guests. Such inclusion would be reminiscent of structurally similar organic molecules that serve as wheel-and-axle compounds in the solid state. The homodimer would, thus, circumvent a covalent synthesis. As predicted, the carboxylic acid formed a homodimer that produced solids that exhibited solvent inclusion (Fig. 15). The packing was dominated by... 

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