Group frequencies carboxylate ions

The aliphatic members of the amino acids exhibit no absorption in the ultraviolet region above 220 nm, while the aromatic amino acids, HIS, PHE, TRY and TYR, show characteristic maxima above 250 nm. The zwitteronic character of the a-amino acids shows up clearly in their infrared spectra. No absorption due to the normal NH stretching frequency at 3300-3500 cm" is observed, indicating the absence of an NH2 group. Instead, a few of the peaks near 3070 cm", 1600 cm", 1500 cm" due to the NHJ are seen (except in PRO, the NHJ group absorbing at 2900 cm" ). The carbonyl absorption of the unionized carboxyl group at 1700-1750 cm" is likewise replaced by carboxylate ion absorption at 1560-1600 cm" [6]. 

If the reacting molecules are already in contact, the rate is expected to be similar to that found for the recombination of H30+ and OH in ice, i.e. similar to the rate of vibration of a proton in a hydroxyl group. Under certain specialised conditions, the rates of recombination of H30 + and A in contact may be calculated from the width of lines in the Raman spectrum of a mixture of the acid and its conjugate base [15]. The trifluoracetate ion has a fairly narrow Raman peak at 1435 cm 1, attributed to the symmetrical carboxylate stretching frequency, which does not appear in acid solutions of trifluoracetic acid. Trifluoracetate— trifluoracetic acid buffers show a lower, wider peak than that for the trifluoracetate ion, and the width of this peak is inversely proportional to the time a proton spends on the carboxylate group. The variation of the width of the line with acid and anion concentrations gives a measure of the rate of transfer of a proton from an adjacent H30 + to the anion, and a rate coefficient of about 101 2 lmole 1 sec 1 has been calculated. 

A considerable amount of research has been concerned with the nature of the electrophiles that are involved in Friedel-Crafts acylation reactions. We will summarize the main points. Acyl halides and carboxylic acid anhydrides have been known, for many years, to form stable complexes with a variety of acid catalysts. A well-defined product is formed between acetyl fluoride and boron trifluoride at low temperatures. Analytical and conductivity data characterized the material as acetylium tetrafluoroborate, and this was further confirmed by IR measurements. In the system acetyl chloride-aluminum chloride the acetylium ion can be differentiated from the donor-acceptor complex involving the carbonyl group by means of their IR carbonyl stetching frequencies. A number of other acyl fluorides have been shown to form well-defined acylium salts by interaction with a number of metal fluorides. Acylium salts can also be prepared from acyl chlorides by means of metathetical reactions involving anhydrous salts such as silver hexafluoroantimonate. As well as characterization by means of IR spectroscopy, acylium salts have been studied in non-nucleophilic solvents by NMR spectroscopy. The NMR data for the ben-... 

In these and other papers by Anslow and coworkers the frequency at which absorption is complete, is used to calculate the energy required for the presumed related dissociation process (e.g. of a proton from a carboxyl group, fission of the S—S bond in cystine), using an equation relating the mass of the ion produced to the frequency of complete absorption (see also Anslow, 1932a, b). As the frequency at which complete dissociation commences has been obtained with complete disregard for the well-established selective absorption of saturated carboxylic acids and aminocarboxylic acids in the vacuum ultraviolet below 2000 A. (see Platt and Klevens, 1944, for a review of the available data), and the more easily accessible short wave bands of the aromatic amino acids (see Section III, 1), and as the theoretical basis of the equation concerned is open to serious criticism, this particular aspect of the work will not be considered further. 

Figure 7.2 shows the dependence of e" upon the applied frequency for the above investigated systems. These measurements for CMC-metal complexes were found to be lower than CMC. This was confirmed the previously elucidated structures [11-13], to the formation of chelated bonds between the unsubstituted hydroxyl groups and/or carboxyl groups of CMC with metal ions, which leads to a decrease in the freedom of moment of carboxymethyl groups and portions of cellulose molecules. 

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