UV-Vis spectrophotometric titration

The originally reported pA a value of 41.2 for 139, which was in the vicinity of that for P3 and P4 phosphazene bases, was recently re-evaluated at 32.82 in acetonitrile by UV-Vis spectrophotometric titration, as shown in Table 2.13 [100]. This value is similar to the phosphazene base P2-Et (pAa = 32.74). The pATa values for 139-141 determined in acetonitrile by P NMR spectroscopy fall in the same range (32.84—33.63). X-ray-determined P-Nax bond distances of 141 and 142 are 2.037 and 1.958 A. An even larger pXa value for 143 was determined by P NMR spectroscopy in acetonitrile (34.49) [101], while the parent compound 138 has a pATa value of 29.6 in DMSO [102]. The greater stability and therefore weaker acidity of cation 138H+ compared with 139 (R = CH3) and 144... 

Elucidation of the solution phase anion complexation properties of receptors 40a and 40b (determined by UV-Vis spectrophotometric titrations in dichloromethane at 298 K) revealed that 40b bound only acetate with a stability constant of 13 900 whereas strong binding was observed for benzoate, acetate, N02" and CN" with receptor 40a (43 000,19 000,13 000 and 5600 M , respectively). 

Figure 15 UV -vis spectrophotometric titration of the dizinc(ll) porphyrin host (H) shown in Figure 14 with DABCO in toluene ...

The majority of the formation constants (equilibrium (18)) characterizing the formation of triple-stranded lanthanide helicates in solution have been determined by direct spectrophotometric titrations in the UV-Vis range (Tables 8 and 9). 

The metal-peptide stoichiometry of the dimeric Cd peptide was studied by UV-Vis spectroscopy (77) as an absorption band at 238 nm is observed upon addition of Cd(II) to the peptide which is assigned to the ligand-to-metal charge-transfer (LMCT) transition of the newly formed Cd-S bonds. A Job plot demonstrated that the complex consists of 2 peptides and 1 metal ion. These results were supported by spectrophotometric titrations analyzed according to the following equilibrium (1) to yield n = 2 and IQ = 0.65 0.08 pM. 

Figure 9-11. (A) Structure of 3-nitrotyrosine. Bond distances (A) are taken from the crystallographic structure of free NT [103]. (B-C) Spectrophotometric titration of Y3NT by UV/Vis absorption spectroscopy. Absorption spectra (B) and plot of the absorbance values at 430nm of Y3NT (2mL, 48(xM) as a function of pH (C). Measurements were carried out in 2-mM citrate-borate-phosphate buffer, at the indicated pH. Fitting data points yields a pKa value of 6.74 + 0.02 for NT.

Iron(II) has been used as a supramolecular template for the formation of a tris-imidazoUum receptor from Ugand 117 [79]. NMR studies and X-ray crystal structure determination were used to demonstrate the encapsulation of bromide in the cavity of the receptor, with the anion coordinated by six C-H fragments (Fig. 2). Spectrophotometric titrations in acetonitrile solution revealed that this receptor binds halides with selectivity for chloride > bromide > iodide, but has no affinity for dihydrogenphosphate or hydrogensulfate. Presumably the restricted size of the receptor cavity excludes the binding of these larger tetrahedral anions. The Hnear anions azide, cyanate and thiocyanate also produced a response in the UV/vis spectrum, and azide was found to bind preferentially to 117 in comparison to the non-symmetrical linear anions. 

Spectrophotometric measurements can also be used to determine the equivalent point in a titration when the analyte, the reagent or the product of the titration absorb in the UV-vis region. 

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