Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

148 EXPERIMENTAL METHODS FOR FAST REACTIONSFig. 16. Proton resonance spectrum of pure dry ethanol (cf. ref. 64).species present in water (e.g., OH-, H,O). The exchange lifetime has been determinedfrom the temperature dependence of the c~alescence~~.The addition of a small amount of hydrochloric acid or caustic soda to pureethanol leads also to a sharpening of the OH-peak and the corresponding simplificationin the methylene band. Again, exchange of the hydroxyl proton is the causebut this time several reactions of the typeEtOH +A Z? EtO- -k AH +are involved. A comparison of the shape of the CH,-band with the theoretical curveshas led to the evaluation of the overall first order rate coefficient for the exchange byall of these reactiod4. This is divided into two components, associated with thosereactions involving water and those involving only the derivatives of ethanol. Thefrrst of these may be evaluated from the simple H2 O/EtOH exchange, and, by followingthe variations of the component first order rate coefficients with c,+ or cOH-,the second order rate coefficients for many of the individual steps have been evaluated.A third interesting observation is that in pure dry ethanol the chemical shift ofthe hydroxyl proton relative to the methylene group decreases as the temperatureis raked. This has been interpreted in the following way. In ethanol it is known thatthe hydroxyl protons are involved in hydrogen-bonding to the oxygen atoms ofother ethanol moleculesand it is reasonable that an H-bonded and a non-H-bondedproton will be subject to different magnetic shielding. If the exchange between thesetwo states is fast, the proton resonance corresponds to the average shielding forthe two states. As the temperature is altered, the proportions of ethanol moleculeshydrogen-bonded will also change, and so the resonance frequency will be temperature-dependent.This explanation is confirmed by the observation of a similareffect when a non-H-bonding solvent (e.g., chloroform) is added. Many similar“hydrogen-bond shifts” have been observed, and it is sometimes possible to obtainthe lifetime of the H-bonded state, or at least to indicate its limits6’.2. One of the surest ways of reducing the nuclear magnetic relaxation time for aspecies is to introduce aparamagnetic ion into its vicinity. The NMR spectra of aqueous