Exchangeability of OH proton

Exchange of nuclei between different environments affects spin-spin couplings. An example is ethanol. When a small amount of aqueous HC1 is added to pure CH3CH2OH, the OH triplet becomes a single line and the CH2 multiplet becomes a quartet. This removal of the coupling between OH and CH2 protons results from the rapid, acid-catalyzed exchange of OH protons between different ethanol molecules. 

Yakerson et al (14)examined the infrared spectra of pyridine adsorbed on Zn-ZSM-5 with different amounts of Zn. The number of Bronsted acid sites decreases linearly with zinc content upto 2 wt% Zn. while the number of Lewis acid sites increases linearly with zinc content. These facts indicate that the decrease in number of Bronsted acid sites by introduction of Zn results in exchange of OH protons by Zn ions, which are the source of Lewis acid sites. 

Figure 3.42. (A) Slow rate of exchange of —OH proton results in coupling between hydroxyl (i) and methylene (ii) protons in ethanol. (B) Fast rate of exchange of —OH proton.

The exchange of aromatic protons can be effected in the absence of any -OH or —NH2 activating group during the course of a Clemmensen reduction in deuteriochloric and deuterioacetic acid mixture (see section Ill-D). This reaction has been carried out with various tricyclic diterpenes and is best illustrated by the conversion of dehydroabietic acid into its 12,14-d2-labeled analog (40 -+ 41).Amalgamated zinc is reportedly necessary for the exchange reaction since the results are less satisfactory when a zinc chloride-mercuric chloride mixture is used. 

The sequential reactions 4.1 and 4.2 represent the self-dissociation of water as the exchange of a proton between water molecules, where hydration of the proton according to reaction 4.2 is the driving force for its separation (reaction 4.1) although the proton hydration is not limited to one H20 (hydration number 1), nor is the occurrence of unhydrated OH ion realistic, the overall reaction 4.3 is generally written as the simplest form to show the principle of proton acidity. 

Labile protons can always be positively identified by in situ exchange with D2O. In practice, a normal H NMR spectmm is recorded then deuterium exchange of labile protons is achieved by simply adding a drop of deuterated water (D2O) to the NMR sample. Labile protons in -OH, -COOH, -NH2 and -SH groups exchange rapidly for deuterons in D2O and the H NMR is recorded again. Since deuterium is invisible in the iH NMR spectmm, labile protons disappear from the NMR spectmm and can be readily identified by comparison of the spectra before and after D2O is addition. 

KI and KSCN are dissolved by 5.12 but without complexation of the boron atom. The stabilisation of the K+ ion by the crown ether moiety is apparently sufficient in these cases.12 Similarly, the 18-membered phenolic crown ether analogue can be metalated (= exchange of a proton for a metal ion in this case the OH proton) with trimethylaluminium to give 5.13 which forms a ditopic complex with LiCl in solution and in the solid state. 

On the other hand, the bands at 490 and 570 nm can be attributed to the radiative decay process of two types of photoexcited Cu(I) ion, a and // respectively, formed upon reduction of Cu(II) ions introduced by exchange of the protons of bridging hydroxyl groups (Si-OH-Al). Cu(I) ions of type j> are assumed to be coordinated by -Si0(A10P0A10) Si-groups with relatively large values of n as compared to Cu(I) ions of type a. 

As previously described for aromatic carboxylic acids, measurements of the line-widths of the CH3 resonance in methanol give the rate of exchange of hydroxylic protons on methanol, and measurements of the line-widths of the OH resonance give the rate of exchange of protons on phenol. Values of ft, k2 and k3 have been determined for p-bromo-phenol and p-nitrophenol [67], and are tabulated in Table 11. 

In dimethyl sulfoxide, exchange of hydroxyl protons is slow enough that hydroxyl protons are observed separately coupled to adjacent CH protons in the NMR spectra of carbohydrates [237,238,239]. Hydroxyl protons involved in intramolecular hydrogen bonds are shielded in comparison to those involved in intermolecular hydrogen bonds to dimethyl sulfoxide [240,241]. The coupling constants can be used in conjunction with Karplus-type relationships [242,243] to identify the orientation of the OH groups. The J values observed for... 

The chemical shift of the —OH hydrogen is variable, its position depending on concentration, solvent, temperature, and presence of water or of acidic or basic impurities. This peak can be found anywhere in the range of 0.5-5.0 ppm. The variability of this absorption is dependant on the rates of —OH proton exchange and the amount of hydrogen bonding in the solution (Section 6.1). 

The first reaction is an exchange of OH and Cl, the second, fourth and fifth reactions are fragmentation reactions leading to an olefin and either carbonyl compound and HCl or an addition product from the latter two compounds. The product of the third reaction is obtained by a step involving the protonation of ethylene which occurs as an intermediate. This step is unrealistic under basic conditions. 

The simplest technique is pre-saturation in which the solvent is subjected to CW irradiation for a short time at a level sufficient to saturate the solvent without affecting other solute peaks. The saturating field is removed immediately before the observation pulse so that the solvent peak has no opportunity to relax. Several solvent peaks can be saturated if rapid frequency switching can be performed during the saturation period. If proton exchange between solvent and solute occurs, for example exchange of OH and NH with H2O, the solvent saturation can be transferred to the solute. However, this is rarely a problem in synthetic polymers, and because of its ease of application the pre-saturation method is the preferred method. 

Comparison of spectra la and 2a provided evidence for the complete exchange of the protons of the acidic OH groups as indicated by the disappearance of the band at 3610 cm The decrease in the intensity of the band at 3750 cm shows also that a fraction of the silanol groups have reacted with CaCl2. Pyridine adsorption confirmed that, after solid-state ion exchange of... 

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