Acidity measurement chemical shift

Case58 investigated the effect of ring currents on NMR shielding constants by means of the DFT calculations. The studied rings included the ones commonly found in proteins and nucleic acids. The shielding constants were calculated for methane molecule placed in several positions relative to the ring. The calculations provided data needed to derive structural parameters from measured chemical shifts in proteins and nucleic acids. 

Fiala R, Sklenar V. 13C-detected NMR experiments for measuring chemical shifts and coupling constants in nucleic acid bases. 75. I. Biomol. NMR. 2007 39 153-163. 

The nature of the acyl residues follows from the proton n.m.r. spectrum. INDOR measurements were used, wherein the methyl signals at 5 1.83,1.90, and 2.18 were chosen as monitoring lines. The methyl signal at d 1.83 is coupled with the olefinic proton at S 6.82, while the two others couple with that at 5 5.67, indicating exclusively allylic couplings. The former spin-system corresponds to the dicarboxylic acid. The chemical shifts fit mesaconic acid (5 6.69 and 5.76, respectively). The second spin-system is congruent with senecioic acid (5 1.87,... 

Katritzky et a/.511 have measured rate coefficients for deuteration of 3,5-dimethylphenol and heterocyclic analogues. As in all of the deuteration work of this group, rates of exchange were measured by the nmr method, which is useful for following exchanges at more than one position in the molecule but is, of course, much less accurate than detritiation techniques. In this study, the chemical shift for the ortho and para protons for the parent compound was too small to allow separate integration, but it was apparent that rates of exchange at these two positions did not differ by a factor > 4. From the rate-acidity profile (Table 149) reaction clearly occurs on the neutral species at pD < 3.5 (the log kl versus pD slope was 0.96) and upon the anion at pD > 3.5 (slope zero), and the reactivity of the anion to the neutral molecule was estimated as 107-8, close to the value of 107 noted above. 

A method for determining the basicities of phosphoryl compounds has been decribed which is based on 31P n.m.r. chemical shift measurement and a two phase system consisting of an organic solvent and 12H sulphuric acid.245 The gas-phase protonation of aliphatic phosphine oxides have been determined by cyclotron resonance. There was a good correlation of pK4 with Kabachnik constants and HO... 

The basicity of cyclopropenones was determined by examining the disappearance of typical IR-absorptions (see p. 38) on protonation or the change in the -NMR chemical shifts of three-ring substituents as a function of H0 in solutions of various acidity. From these measurements half-protonation was found to occur as follows ... 

The measurement of pA bh+ and m values is sometimes complicated by the fact that the spectroscopic methods used (generally H and 13C NMR spectroscopy, and UV-vis) are subject to medium effects. Water and (say) 80 wt% H2SO4 are very different media, and it is not very surprising that spectral peaks for the same species in the two media can occur at different wavelengths or different chemical shifts. Several methods have been devised for handling this problem 24,99,100 the excess acidity method lends itself to dealing with medium effects quite well.101... 

It is well known23 that ionization ratios / can be determined from spectra using the expression /=( > — DB)/ D — D), where D is the measured UV-vis optical density or NMR chemical shift and DA and Z>B refer to the values for the pure acid form (i.e. BH+) and pure base form (i.e. B), respectively. Recasting this in terms of D gives equation (27) ... 

The experimental 13C NMR chemical shift of the simplest member of the acylium ions, the formyl cation [HCO]+ was reported as S = 139.5 (measured under CO pressure of 85 atm) which compares well with the GIAO-MP2 calculated shift of S — 136ppm.116,117 The analogous fluoroformyl cation [FCO]+ and protonated fluoroformic acid [FC(OH)2]+ were characterized by 13C NMR spectroscopy, experimentally118 ((5=117.5 and 157.8 ppm, respectively) as well as computationally (S = 118.6 and 170.6ppm, respectively).119... 

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