Two-bond isotope effects

Figure 7 The 0 isotopomers in the 94.7 MHz NMR spectrum of [V(C0)j n(C 0)J" for n = 0 to 5. The 0 isotopomeric pattern is repeated in the satellite doublet. The one-bond isotope effect due to naturally abundant CO clearly exceeds that of the two-bond isotope effect originating from...

In cases like 22, the two-bond isotope effects can be really small as the double bond order is very low due to double bond fixation. 

The two-bond isotope effects can be related to OH chemical shifts , and to other parameters such as five-bond isotope effects, A OD isotope effects. ... 

Two-bond isotope effects on C chemical shifts are a good measure of hydrogen bond strength . A simple example is seen for 33 and 34. The phenol provides a common scaffold so that o-hydroxy-substituted phenols form a suitable way of ranking acceptor... 

X (D)], where n is the number of intervening bonds between the deuterium and the observed nucleus, X . The most studied of these effects is the two-bond isotope effect on chemical shifts, AC(XD), produced upon H/D exchange at XH. This isotope effect has been correlated with d(XH) and, when XH takes part in an intramolecular hydrogen bond, to the hydrogen-bond enthalpy The magnitude of... 

Li et al. [5] used a combination of 2D-NMR methods to study a similar perflu-oropolyether. They examined carboxyl-terminated low MW oligomers of PHFPO which served as a model for Dupont s Krytox fluoropolymers. They used spectra from selective one- and two-bond Fj C HSQC experiments, similar to the data shown in Figure 24.8, to identify the atomic connectivities within each polyether unit. Note that the correlations to F resonances A3 and Aj occur at slightly different chemical shifts in the one-bond experiment (top spectra) compared to those in the two-bond spectra (bottom spectra). This is due to a one-bond C isotope effect which shifts resonances of attached fluorines up-field by approximately 0.1 ppm compared to the resonances of fluorines attached to C. The two-bond isotope effect is 10-fold smaller and is too small to notice in these 2D-NMR spectra. Similar sets of correlations were observed for each monomer unit in the structure. 

AC(XD), also called two-hond isotope effects, are large and depends on, for example, hydrogen-bond strength. As the isotope effects are transmitted via the bonds, the larger the bond order, the larger the isotope effect Furthermore, steric strain plays a role [22]. In compounds such as 7 (see later), AC(OD) is as large as 720 ppb [23]. Two-bond isotope effects may also be correlated to other parameters such as OH chemical shifts [23]. For isotope effects on chemical shifts, see Section 6.4. 

Isotope effects on both the carbon and hydrogen of the breaking C-H bond have been measured. However, for this reaction both forward and reverse commitments are sizable so the three equations corresponding to Equation 11.48 have four unknowns the forward and reverse commitments and two intrinsic isotope effects. Measurements of the secondary deuterium kinetic isotope effect (at position 4 of nicotinamide ring of NADP+) and the carbon kinetic isotope effect with the secondary position deuterated introduce two additional equations, but only one more unknown ... 

There are two geometric isotope effects A/ (D — H), which refers to the change in bond length / . .. g of a hydrogen bond AHB on deuteration, and 8/ , which is defined as the change in the distance between the two minima of the potential energy well of the hydrogen bond when it is deuterated. The isotope effect, A/ (D—H), was dealt with previously (see p. 263). 

Christofides J C, Davies D B 1983 A method to assign C-NMR signals of compounds with exchangeable hydrogen atoms using two and three bond isotope effects cellobiose. J Chem Soc Chem Commun 324-326... 

The intrinsic one-bond isotope effects on CS are rather small as seen from typical examples given in Figure 6.13 [35]. For a review, see [36]. Effects over two... 

Considerable isotope effects on Se resonances can be encountered. Enormous one-bond deuterium effects have been reported to exist for H2Se and its deuterated derivatives HDSe and D2Se, and nearly equal shielding effects of 133 Hz (7 ppm at 19.0 MHz) for each H/D exchange were found (50). The two-bond deuterium effect is smaller but still remarkable. H/D exchange of H-2 in selenophene shields the selenium nucleus by 32 Hz (1.7 ppm at 19.1 MHz) over two bonds (78Jak). Ph-Se-CH3 and Ph-Se-CD3 have Se chemical shifts of S= 201 and S= 195 at 47.7 MHz (93Dud). 

In the particular reaction used as an illustration, there are one primary and two secondary isotope effects and a good deal of ancillary kinetic evidence to fall back on. Generally, however, the burden of uncertainty is much too great for a minute and not always a very precisely determined secondary isotope effect to bear. Small wonder then that many workers in the field have preferred to lash themselves firmly to the formal framework of isotope effect theory After a model for the transition state is assumed, bond lengths and angles can be altered, force constants increased and decreased ad libitum, until the experimental result is more or less comfortably accommodated in the Procrustean bed of theory. How much useful mechanistic information is derived by this procedure is open to question. 

A kinetic isotope effect that results when the bond to the isotopic atom is not broken is called a secondary isotope effect. Here are two examples ... 

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