Solvolysis theoretical calculations

In addition to these probes, theoretical calculations are being made in efforts to decide upon the inherently most stable stmcture of the ion. These will be discussed along with the conclusions which have been derived from the study of isotope effects in solvolysis and substituent effects in stabilizing media. 

This mechanism also accounts for the very large 5-deuterium isotope effect observed for the solvolysis of (4-D)bicyclo[2.2.2]oct-l-yl mesylate. This mechanism is also supported by theoretical calculations of 4-substituted bicyclo[2.2.2]oct-l-yl cations74. 

Theoretical calculations support the existence of discrete ion pair intermediates in solvolysis reactions. Figure 8.11 shows a reaction coordinate diagram, and Figure 8.12 shows calculated structures for the solvolysis of f-butyl chloride in water. The (CH3)3C CD contact ion pair and the solvent-separated ion pair are distinct species, with calculated energies indicating that each is a local minimum. The more widely separated ion pair Figure 8.12(c) is similar in energy to the solvent-separated ion pair. ... 

Kinetic studies showing that the reaction is first order in acid, and second order overall, a solvent isotope effect ( h2S04 AD2SO4) of 0.91, a Hammett p value of -6.5, a AS of -37.2 Jmol K and theoretical calculations at the B3LYP/TZVP level of theory, have shown that the acid-mediated solvolysis of a-methylene-j0-hydroxy- 0-phenyl esters to allylic acetamides in acetonitrile occur by a rate-determining formation of a benzylic carbenium ion intermediate, that is, by an 5n 1 mechanism, after a rapid, reversible protonation by the acid catalyst. " ... 

It has been suggested that, regardless of the transition state structure during solvolysis, the norbomyl ion might have yet another structure under the stabilizing influence of strong acid media. Before considering those systems it is appropriate to discuss the current theoretical status as provided by molecular orbital calculations. 

The characteristic change of the r value in the solvolysis reaction of benzylic precursors and for the corresponding carbocations should provide important information concerning the solvolysis transition state. The r value, reflecting the TT-delocalization within the cationic species, appears to remain essentially the same in solution as in the gas phase, and the charge delocalization in the transition state of the solvolytic ionization should be close to that in the carbocation intermediate. Advanced ab initio molecular orbital calculations can be used to And the underlying relationship between quantum chemical quantities and experimental r values, and the relation between r values and theoretical indices provides a basis for the physical meaning of the r parameter (Nakata ei ai, 1996, 1998, 1999). 

In further theoretical studies, Schleyer and coworkers found" that the nonclassical 2-norbornyl cation (126) is more stable by 13.6kcalmoE than the 127 classical structure, and 126 is the only stable form in the gas phase and in solution, and the classical form (127) is unlikely to be involved in solvolysis reactions [MP4(fc)SDQ/6-31G /MP2(full)/6-31G -p ZPVE level]. They also performed a rigorous ab initio calculation and defined the classical 127 structure as an artifact and found that it is a transition state in the rearrangement of 126 into the bridged 2-norpinyl cation. The computed MP2-GIAO 0 chemical shifts for 126 are close to the experimental values, whereas those calculated by Schindler (IGLO) for 127 deviate considerably. The comparison of the chemical shifts for the two structures with the experimental data proves once again the symmetrical nonclassical structure. 

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