Carboxylic acids dimerization

It is also possible to measure microwave spectra of some more strongly bound Van der Waals complexes in a gas cell ratlier tlian a molecular beam. Indeed, tire first microwave studies on molecular clusters were of this type, on carboxylic acid dimers [jd]. The resolution tliat can be achieved is not as high as in a molecular beam, but bulk gas studies have tire advantage tliat vibrational satellites, due to pure rotational transitions in complexes witli intennolecular bending and stretching modes excited, can often be identified. The frequencies of tire vibrational satellites contain infonnation on how the vibrationally averaged stmcture changes in tire excited states, while their intensities allow tire vibrational frequencies to be estimated. 

Probably the best example of the manifestation of self-assembly processes in dendritic systems via H-bonds is seen in the recent work of Zimmerman et al. [156]. Dendritic wedges possessing tetraacid moieties (67) self-assembling into a hexameric, disc-like framework (68) was confirmed by SEC and -NMR studies. The tetraacid unit (69) is known to form cyclic as well as linear aggregates in solution via carboxylic acid dimerization (Fig. 30). However, with incorporation of larger dendritic wedges on 69, the hexamer form is preferred. 

Aromatic alcohol clusters have been well-studied, also for methodical reasons. The UV chromophore can be exploited for sensitive detection of the IR spectrum [35, 36, 120, 179]. Time-domain experiments become possible [21], which show that the initial energy flow out of the O—H stretching mode occurs primarily via C—H stretching and bending doorway states. Like in the case of carboxylic acid dimers [245], the role of the hydrogen bond is to shift the O—H stretching mode closer to these doorway states and thus to accelerate the initial energy flow. 

T. Haber, U. Schmitt, C. Emmeluth, and M. A. Suhm, Ragout jet FTIR spectroscopy of cluster isomerism and cluster dynamics From carboxylic acid dimers to N2O nanoparticles. Faraday... 

C. Emmeluth, M. A. Suhm, and D. Luckhaus, A monomers in dimers model for carboxylic acid dimers. J. Chem. Phys. 118, 2242 2255 (2003). 

Figure 2.4 Carboxylic acid dimer in the potential energy minima with local vibrational states. OV represents the correlation time for a thermally activated proton transfer, and TU, the correlation time for tunneling transfer. (Reproduced with permission from ref. 29.)...

In. a number of cases sub-maxima associated with vXH bands have been interpreted in this fashion and in the case of the carboxylic acid dimers this question has been investigated in some detail [4]. A prominent satellite band accompanying the main vOH bands has been assigned to an overtone of the <5QH vibration, and it has been possible to explain formally most of the multiplicity of peaks in the rOH band of formic acid in Fermi resonance terms. Although it is possible that some of these peaks correspond to Stepanov-type sub-bands, no convincing series of this type can be picked out. There seems little doubt that in many cases a considerable number of sub-bands in the rXH region are to be interpreted in terms of Fermi resonance [5, 43,... 

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

Fermi resonances of the v0H=l state with over- and combination tones of modes in the fingerprint region are considered a key element determining the spectral envelope of the O-H stretching bands of cyclic carboxylic acid dimers. In principle, such Fermi resonances open up channels for population relaxation of the O-H stretching modes through the O-H bending... 

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... 

Equilibration of carboxylic acid dimers and monomers in the sample solution depends on the extent to which the carboxy group is involved in hydrogen bonding with the solvent ... 

The very strongly H-bound carboxylic acid dimer has a very broad absorption at 2500 cm 1 for the OH bond. Because H bonding is dependent on concentration and on the polarity and H-bonding properties of the solvent, frequency shifts due to H bonding are quite variable. 

Two-Proton Transfer in Carboxylic Acid Dimer Crystals... 

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. 

A nice example of grinding-induced crystal engineering is the formation of a hydrogen bonded cocrystal between ferrocene dicarboxylic acid and 1,4-diazabicyclo [2.2.2] octane (DABCO). The ferrocene dicarboxylic acid exists in the solid as a hydrogen bonded dimer based on two repeats of the carboxylic acid dimer synthon (solid A). Grinding with DABCO (solid B) gives rise to a new solid, C, in which the carboxylic acid dimers have been broken and replaced by an acid-amine synthon, Figure 8.25.30... 

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