Spectra infrared, of aqueous solutions

Infrared studies of aqueous solutions provide a valuable tool for elucidating the structures of complex ions in equilibria. To observe the infrared spectrum of an aqOeous solution, it is necessary to use window materials such as AgCl and BaF2, which are insoluble in water, and a thin spacer (0.02-0.01 mm) to 

Type B, a-ammonium carboxylate (R2N H--CH2COO ), 1630-1620 cm Type C, -aminocarboxylate (R2N—CHjCOO ), 1585-1575 cm  

As stated in Sec. 111-8, the coordinated (ionized) COO group absorbs at 1650-1620cm for Cr(lll) and Co(III), and at 1610-1590cm for Cu(Il) and Zn(ll). Thus it is possible to distinguish the coordinated COO group from those of Types B and C if a proper metal ion is selected. 

1625-cm band becomes weaker, as the pD increases. It was concluded that this change is due mainly to a shift of the following equilibrium in the direction of complex formation  

By plotting the intensity of these two bands as a function of pD, the stability constant of the complex ion was calculated to be 5.24. This value is in good agreement with that obtained from potentiometric titration (5.41). 

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The tiny absorbance values associated with quantitative near-infrared spectroscopy render these measurements extremely sensitive to slight variations in spectrometer alignment. Indeed, slight differences in incident radiant power between the sample and reference spectra create small positive or negative offsets along the absorbance axis. Such offsets are commonly observed for near-infrared spectra of aqueous solutions, as is apparent in the spectra presented in Figure 13.3. 

Another consequence of the strong absorption properties of water is the spectral impact of the displacement of water by dissolved solutes. Generally, in absorption spectroscopy, the solvent is selected not to absorb over the wavelength range of interest. When the absorption properties of the solvent are negligible, any displacement of solvent molecules from the optical path by the dissolution of solute molecules has a negligible effect on the measured spectrum. For near-infrared spectra of aqueous solutions, however, the absorption spectrum depends heavily on the degree of water displacement by solutes in the sample. 

CXE2C1, C1E3C1, and CXE4C1 were commercially supplied, while CxEsCx and CxEgCx were prepared by the Williamson ether synthesis. The infrared spectra of aqueous solutions of these compounds were measured for various concentrations at room temperature by using a horizontal ATR accessory with a ZnSe prism. Tibe lowest concentration studied was a mole fraction of approximately 0.005. The spectra were recorded on a JASCO FT/IR-7300 spectrometer. Each of the spectra was obtained by the coaddition of 200 scans at a resolution of 4... 

Methods for measuring the infrared spectra of aqueous solutions or suspensions of humic substances are discussed later in this chapter. 

Bunzl KW (1967) Near-infrared spectra of aqueous solutions of some tetra-n-alkylammonium bromides. J Phys Chem 71 1358-1363... 

Even though the number of the normal vibrations in a molecule is 3N-6, where N is the number of atoms in a molecule, the nucleic acids give no more than 50 well-defined absorption bands in the mid-infrared region 400-4000 cm . Vibrational spectroscopy is sensitive to conformational changes in biopolymers brought about by metal ions and it can be used to obtain information about the structure in both the solid and solution states. The spectra obtained with a Fourier transform infrared spectrometer are studied with differential techniques and the absorption of the H2O can be digitally subtracted from the solution spectra. Infrared spectra of aqueous solutions can be examined now with FT-IR techniques as opposed to the old prism and grating instruments. The spectra obtained with an FT-IR are handled with differential techniques and the data are treated with a computer. 

M.J.D. Low and R.T. Yang, "The Measurement of Infrared Spectra of Aqueous Solutions Using Fourier Transform Spectroscopy". Spectrochimica Acta, 29A, 1761 (1973). 

Jones and Penneman " made an extensive infrared study of the equilibria of cyano complexes in aqueous solution. (For aqueous infrared spectroscopy, see Sec. 111-9.) Figure 111-35 shows the infrared spectra of aqueous silver cyano complexes obtained by changing the ratio of Ag to CN" ions. Table 111-35 lists the frequencies and extinction coefficients from which equilibrium constants can be calculated. Chantiy and Plane studied the same equilibria using Raman spectroscopy. 

Very few of the infrared studies of proteins have been carried out on aqueous solutions of the proteins. Except for the work of Koenig and Tabb (1), the few aqueous IR studies have been on single proteins. Correspondingly, most of the assignments of the backbone vibrations (the so-called Amide I, II, III, etc. vibrations) have been based on either Raman spectra of aqueous solutions (2) or on infrared spectra of proteins in the solid state (3). Where infrared solution spectra have been obtained, it has mostly been on D2O solutions (4) - not H2O solutions. Since these Amide I, II, III, etc. vibrations involve motion of the protein backbone, they are sensitive to the secondary structure of the protein and thus valid assignments are necessary in order to use infrared spectroscopy for determining the conformations of proteins. 

Gamma Globulins. In experiments (14) similar to those previously described for albumin, adsorbed IgG films were exposed to saline and then to either methanol or ethylene glycol and infrared spectra were obtained. In addition, spectra of aqueous solutions of IgG at various pH s were obtained as well as spectra of IgG as it adsorbed onto the ATR crystal from a very dilute solution of IgG in ethylene glycol. In the spectra of the adsorbed IgG exposed to methanol or to ethylene glycol there was increased... 

Duval (1956) has studied the spectra of amino acids recorded from a single drop of aqueous solution. Parker and Kirschenbaum (1960) have recorded the spectra of several a- and non-a-amino acids in water. The spectra of aqueous solutions of glycine, N-methylglycine (sarcosine), yV,yV-dimethylglycine, and yV,yV,A(-trimethylglycine (betaine) in dilferent states of ionization have also been recorded (Kirschenbaum, 1963). Another solvent which has been suggested for infrared spectral determination of amino acids is antimony trichloride (Lacher et al., 1954). 

Milch (1964) obtained infrared spectra on aqueous solutions of some aliphatic and aromatic aldehydes. Saturated compounds which are capable of acting as interchain cross-linking agents for the polypeptide chains of either gelatin sols or native... 

Bunzl (1967) used the shift of the 0.97 pm band of water in the infrared spectra of aqueous tetraalkylammonium bromides at 10-70 °C to establish their structural temperatures. The differences AT = T — Tstr for 1 m solutions at 35 °C are 11-14 K, not showing a clear dependence on the nature of the cation, ranging from (CH3)4N+ to (C4H9)4N+. The temperature dependence of the differences does show cation size... 

Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

Substituted 2-amino-l-thia-3,4-diazoles have been studied by Testa et Comparison of ultraviolet and infrared spectra of the parent compound (197 198) with those of its four possible methylated derivatives indicated that 197 predominates in aqueous solution and... 

Iron(II) complexes are often included in studies when complexes are prepared from a large number of different metal ions. 2-formylpyridine thiosemicarbazone, 5, forms brown [Fe(5)2A2] (A = Cl, Br) when prepared in ethanol and [Fe(5-H)2] from aqueous alcohol solution [156], All of these complexes are diamagnetic. The resonance Raman and infrared spectra of [Fe(5-H)2] were examined in detail [130] and coordination occurs via the pyridyl nitrogen, azomethine nitrogen and thiol sulfur. There is appreciable d-d sulfur-to-iron(II) Jt-bonding. Solution studies of iron(II) complexes of some 5-substituted-2-formylpyridine thiosemicarbazones have been reported [157], but no solids... 

Tabb, D. L., Koenig, J. L. Infrared Spectra of Globular Proteins in Aqueous Solution, in Analytical Applications of FT-IR to Molecular and Biological Systems, Durig, J. R. (Ed.) D. Reidel 1980, p. 241... 

It is not possible to dissolve the asphaltenes in water by treatment of the halo derivatives with aqueous sodium hydroxide or with aqueous sodium sulfite (II). The hydrolyzed products remained insoluble even in a strongly alkaline solution. The decreased (H + Cl)/C ratios and the increased O/C ratios of the products relative to those of the parent halo-asphaltenes indicate that partial reaction occurs. The infrared spectra of the products showed a broad band centered at 3450 cm 1, a region assigned to the presence of hydroxyl groups in the products, but it was not possible to establish conclusively the presence of sulfonic acid group(s) in the product from the sodium sulfite reaction by assignment of infrared absorption bands to this particular group. 

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