Deuterated compounds hydrogen bonds

The method has been applied to various hydrogen-bonded ferro- and antifer-roelectrics (see Chap 2 in this volume). The first results have, however, been obtained for the antiferroelectric compound H2C4O4 (squaric acid) and its deuterated analogue D2C4O4. 

Based on three-bond carbon-hydrogen coupling constants, specific selective proton decoupling experiments and investigations of specifically deuterated compounds, the resonances of C-6 and C-8 in 5,7-dihydroxyflavonoids appear in the range of 90 to 100 ppm and C-8 is always more shielded compared to C-6. The chemical shift differences found are small for flavanones ( 1 ppm) and larger for flavones and flavonols ( 5 ppm). 

The results of studies on the primary NMR isotope effect performed with a set of protonated DMAN-s are illustrated in Fig. 19.4 which presents the relationships between the isotope effect and the XH chemical shift of non-deuterated compounds. The experimental points (full circles) come mainly from paper [23] and a few of them from [31]. The results collected for DMAN-s are compared with the corresponding relationship for OHO hydrogen-bonded systems [32]. The majority of the experimental points are in the region of A<5 0.6-0.7 ppm and r> 17.3— 18.8 ppm, i.e., close to data for non-substituted DMAN H+, for which 8= 18.65 ppm and A<5 = 0.66 ppm. One should mention here that asymmetry of the cation does not markedly affect either the (5 or A8 values. This is most probably due to similar electron density distributions on both nitrogen atoms and, consequently, similar populations of the two potential energy minima. Therefore, in our case, we can omit the contribution of the equilibrium effect on A8 value. It seems that this effect is below 0.1 ppm. 

The halonium structure 17 was also attributed107 to protonated bromo- and chloromethane, in agreement with theoretical predictions105,106. However, in these two ions an additional interaction (stronger for the bromo than for the chloro compound) is present between the hydrogen bonded to the halogen and the carbon atom107. Protonated iodomethane, on the contrary, structurally resembles protonated methane with the proton bonded to the carbon atom. These conclusions are supported by both MI and CID data for protonated and deuterated halomethanes and by thermochemical data. 

The kinetic isotope effect was introduced in Chapter 19. If a bond to deuterium is formed or broken in the rate-determining step of a reaction, the deuterated compound will react more slowly, usually by a factor of about 2-7. This effect is particularly valuable when C-H bonds are being formed or broken. In Chapter 22 we told you that the rate-determining step in the nitration of benzene was the attack of the electrophile on the benzene ring. This is easily verified by replacing the hydrogen atoms round the benzene ring with deuteriums. The rate of the reaction stays the same. 

Isotope Effects. If the hydrogen ion departs before the arrival of the electrophile (Sgl mechanism) or if the arrival and departure are simultaneous, there should be a substantial isotope effect (i.e., deuterated substrates should undergo substitution more slowly than non-deuterated compounds) because, in each case, the C—H bond is broken in the rate-determining step. However, in the arenium ion mechanism, the C—H bond is not broken in the ratedetermining step, so no isotope effect should be found. Many such studies have been carried out and, in most cases, especially in the case of nitrations, there is no isotope effect. This result is incompatible with either the SeI or the simultaneous mechanism. 

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