Relative induced shift data

The reaction mechanism of any shift reagent-substrate interaction is extremely d iffic u lt to determine and consequently certain assumptions regarding a given stoichiometry have been made in the 4-parameter data treatment method presented. For the majority of substrates shown in this thesis, i t has been found that the two-step 2:1 reaction mechanism gives better results, when measured in terms of the agreement factor outlined, than the results of a 1:1 reaction mechanism. Again, a comparison of the results obtained from the 4-parameter data treatment method with the results obtained by simple means, may afford information

concerning the true nature of the equilibrium occurring in solution. Hence an analysis of the relative induced shift ratio was attempted, where this ratio is defined as the ratio of the induced sh ift of one proton relative to that of another proton.

I f both ES and ES2 complexes exist in solution, then the lanthanide induced shift is given by

1.2

The relative induced shift ratio for protons i and j within the same substrate molecule w ill therefore be given by

. . . 2.33

where

— and

X

In a 1:1 stoichiometry, y = o and the relative induced shift reduces to

This ratio w ill therefore be independent of the substrate and shift reagent concentrations and w ill be constant throughout the experiment. Alternatively, i f for a 2:1 stoichiometry, delta max. 1 is the same as delta max. 2, or indeed i f a simple mathematical relationship exists between delta max. 1 and delta max. 2, i.e . a constant lim iting incre­ mental shift ratio exists, then equation 2.33 reduces to

. . . 2.34

. . . 2.35

On the other hand the observation of varying relative induced s h ift ratios is a definite indication of a 2:1 stoichiometry; in tu itively i t can be expected that the presence of induced shifts resulting from the contact shift mechanism may also lead to different relative induced shift ratios for different protons in the same substrate. However, with substrates that are considered to be free from contact shifts, e.g. saturated ketones, the observation of varying relative induced sh ift ratios not only indicates a 2:1 reaction mechanism between shift reagent and substrate but also that there is no simple mathematical relationship between the values of delta max. 1 and delta max. 2.

Table 2.18 lis ts some of the relative induced shift ratios calculated for several substrate ketone molecules, obtained from the experimental induced shifts measured as a function of the substrate concentration whilst the shift reagent concentration is kept constant. In the case of pentan-2-one, the relative induced shift ratios obtained are approx­ imately constant throughout the experiment. These results could

indicate a 1:1 stoichiometry, but in view of the results of the 4- parameter data treatment method are more likely to represent a 2:1 stoichiometry with a simple mathematical relationship between the values of delta max. 1 and delta max. 2, i.e . a constant lim iting incremental shift ratio. This is shown to be so from the constant limiting incremental shift ratios obtained for pentan-2-one and shown in Table 2.11.

The varying relative induced shift ratios obtained for the other

substrates in Table 2.18 indicates the presence of a 2:1 stoichiometry and that no simple relationship exists between delta max. 1 and delta max. 2. Also since the adduct geometry of the substrate-shift reagent

Table 2.18

/ a

\

1

a) Complexes of Eu(fod)~ in CDC1~ obtained at constant sh ift reagent concentrations.

co, o <_) ro , o CO co CL o o c_> oo CO < r _in o C_3 _{C_>} CO _CO CO 3=1 oID O o_CO CO CO CO CO _CO, o _o 'C_J _{o in} CO CO_{o o} CO Q . CO CO, o _CO o CO _{o CO CO CM CVJ} o C_3 CO, _oCO, o r^» t^s c *. 3=| in_I CO o c_> CO, o o C_) Cl 4-> c_> 4-> _o o CD CO o CO, o o CO, CJ>CM co cm c o in c o co CO CO 0 0 0 0 CO CO CO _o o CO CO CM CM o o o CO o_o o CO o o _{o o} 113

complex changes when the stoichiometry changes, then the relative

induced shift ratios w ill change as the substrate concentration changes whilst the shift reagent concentration is kept constant. At Tow substrate concentrations, where the 1:1 adduct predominates, the relative induced shift ratio w ill approach the value of the corresponding ratio of the delta max. 1 values obtained for the protons under investigation. Sim ilarly, when the substrate concentration increases, favouring 2:1 complex formation, then the relative induced shift ratio w ill approach the value of the ratio obtained from the corresponding delta max. 2 values. Highlighting this behaviour are the relative induced shift

values obtained for the C^pHg. and C^Hg proton groups of the camphor- Eu(fod)g complex which range between 2.45 - 2.14 as the substrate

concentration is increased. These values are very close to the ratio

respectively obtained from the corresponding limiting incremental sh ift values shown in Table 2.11.

2.4.6 Conclusions

A method has been outlined which enables intrinsic parameters to be determined. This method involves a comparison between experimental data and theoretically predicted data obtained from known intrinsic parameters. The results for both 1:1 and 2:1 stoichiometries have been recorded, but i t is shown that for the majority of substrates studied, the 2:1 stoichiometry gives better results, when measured in terms of a.percentage standard deviation, than the results of a 1:1 stoichiometry. Also, results obtained but not shown, for a one-

step 2:1 reaction mechanism were very much inferior to those results