PH dependence chemical shifts

Table XVIII contains the 13C-n.m.r.-spectral data for the model compounds a,/ -Araf — Hyp and a,/ -Ara/— Hyp. The assignments of Qfi and Cs of Hyp were readily obtained19 by comparison with 13C data for L-Hyp in the free amino acid form and for L-Hyp residues of peptides.109 The chemical shift of C 5 was, as expected, 7-8 p.p.m. downfield from the values reported for nonglycosylated L-Hyp. The chemical shifts of C and Cy appeared in the same region as carbohydrate resonances and were therefore identified through their chemical shift-pH dependence.19 The chemical shift of the / -anomeric carbon atom in compound 60 is rather up-field, in agreement with a syn orientation for the C-1 - O-1 and...

The 13C chemical shift pH dependence for both the Mo(IV) and W(IV) systems is similar to that observed in Fig. 6 for the Re(V), with the exception that only one protonation step is observed in weak to mild basic solutions (there is a large difference in the pKal and pKa2 values for both the Mo(IV) and W(IV) systems, see Table II). The corresponding acid dissociation constants were similarly determined as in the case of Re(V) complexes. 

Multiplicities in parentheses. Signals of residual solvent protons. Chemical shift highly dependent on pH and temperature. (Courtesy of Cambridge Isotope Laboratories, Andover, MA) ... 

Phosphorus compounds give resonances in characteristic ranges depending primarily on the oxidation state and coordination munber of the phosphorus atoms present (Table 1). Chemical shifts also depend on the pH, concentration, and salt content of the solution, solvent effects, and the electronegativity of any substituents. Since the phosphorus atom is a comparatively reactive center, it can usually... 

Chemical shifts of pyridine and the diazines have been measured as a function of pH in aqueous solution and generally protonation at nitrogen results in deshielding of the carbon resonances by up to 10 p.p.m. (73T1145). The pH dependence follows classic titration curves whose inflexions yield pK values in good agreement with those obtained by other methods. 

If a sample contains groups that can take up or lose a proton, (N//, COO//), then one must expect the pH and the concentration to affect the chemical shift when the experiment is carried out in an acidic or alkaline medium to facilitate dissolution. The pH may affect the chemical shift of more distant, nonpolar groups, as shown by the amino acid alanine (38) in neutral (betaine form 38a) or alkaline solution (anion 38b). The dependence of shift on pH follows the path of titration curves it is possible to read off the pK value of the equilibrium from the point of inflection... 

Chemical shift in acidic media is defined as at pH <2.0. Chemical shift in basic media is defined as at pH >11.0 for more details, see Ref. 23. b C of N-terminal glycine exhibited pH-dependence at pH >7.0. C of C-terminal glycine exhibited pH-dependence at pH <7.0. 

On studying a series of ammonium 1,3,2,5-dioxaborataphosphorinane oxides (111), the dependence of the tautomeric equilibrium position on amine basicity was analyzed. The equilibrium position was estimated from chemical shift values of bis(oxymethyl)phenylphosphine oxide with 8 3IP of 35 ppm being used as a model of an acyclic form and 5-Ph-5-oxo-1,3,5-dioxaphosphorinane (107, R = H) with 8 3IP of 6 ppm used as a model of a cyclic compound. The chemical shift values (111, X = 0, R = H) and dissociation constants (pKa) of conjugate acids for amines are presented in Table V. 

Fig. 13. Observed and calculated pH dependence of 54.227 MHz 170 signal for the W(IV) complex (a) linewidth (oxo fast exchange ). Dashed lines 1 and 2 illustrate how the limiting value of kih was determined (6). (b) Chemical shift (oxo aqua/...

Fig. 14. The pH dependence of 170 spectra for the W(IV) system at 25.0°C with total complex concentration [W] = 0.2 m and /jl = 1.0-1.4 m (KN03), NMR chemical shift reference is the nitrate ion (<5(N03) = 413 ppm). The spurious signals have not been identified but are partially due to the dimeric species (6). (Adapted with permission from Roodt, A. Leipoldt, J. G. Helm, L. Merbach, A. E. Inorg. Chem. 1994, 33, 140-147. Copyright 1994 American Chemical Society.)...

The line-broadening data as a function of pH, typically shown for the W(IV) in Figs. 13 and 14, incorporating the known pKa values (Table II), were fitted in 5 X 5 Kubo-Sack matrices describing the exchange based on the above schemes (6, 57). The experimentally determined chemical shift and linewidth data in the absence of exchange for the aqua oxo, hydroxo oxo, and dioxo species and the pH-dependent species distribution as calculated from the acid dissociation constants for the four systems were all introduced in the different matrices and the spectra were computer simulated. For each set of chosen rate con-... 

---