Methyl cation affinities

These bimolecular reactions have provided accurate proton affinities (PAs) for many amines165,166. In addition, cation affinities are accessible, usually by combining the enthalpy of formation (AH[) of cationic species derived from PA measurements with similar data for the bare cation. Thus, the knowledge that the PA of CH3NH2 is 896166 kJmol-1 sets A//f(CH3NH3 + ) = 611 kJmor1. Since A//f(CH3+) = 1092 kJmol-1 and A//f(NH3) = —46 kJmol-1 9, the methyl cation affinity of NH3 may be deduced to be 1092 — 46 — 611 = 435 kJ mol-1. 

Knowledge of the gas-phase interactions between cations involving elements of group 14 and electron donor molecules has been used to create various basicity scales. In addition, the absolute methyl cation affinities (MCA) scale has been built by calculating the... 

Figure 4. Plot of intrinsic barrier to degenerate exchange (intrinsic barrier for X + CHSX) vs. methyl cation affinity for selected nucleophiles.

Correlation with Methyl Cation Affinity. Intrinsic barrier data for some of the other systems we have examined are shown in Table II and Figure 4. Note that the barriers vary widely with structure. Some insight into the mechanism of this variation is obtained by noting that the only thermodynamic parameter which correlates well with the purely kinetic intrinsic barriers is the methyl cation affinity (MCA) of the nucleophile... 

A plot of intrinsic barrier vs. methyl cation affinity for several systems examined appears in Figure 4. In contrast to the excellent correlation shown in this figure, correlations... 

Note that the intrinsic barrier increases with methyl cation affinity. Such a correlation between purely kinetic and purely thermodynamic properties is not surprising in view of theoretical studies (21, 22) which have shown there to be significant contributions to the S 2 transition state from resonance structures such as the following ... 

Methyl cation affinities of benzene and some substituted benzenes have been calculated. These follow a simple additivity rule and the value for benzene shows good agreement with the experimental estimate. Conclusive evidence is presented that these values are linearly related to the corresponding proton affinities. The competition between deuteriation and alkylation in the reaction of radiolytically formed perdeuterio ethyl cations with iV-methylpyrrole and with thiophene has been studied. Deuteriation, the Brpnsted acid pathway, predominates and intramolecular selectivities have been determined for each reaction. ... 

An attempt to establish a rate-equilibrium relationship shows that a plot of E X -I- CH3X) as a function of the methyl cation affinity (MCA) of X-yields the linear correlation (44), where 7P(CH3) represents the ionization... 

Fig. 9.7 Methyl cation affinity vs. proton affinity of a senra of anionic bases of varying hardness. The solid line is a least-squares lit of the data drawn by the original investigators. The dashed line has been arbitrarily drawn through F and OH with unit slope (see Fig. 9.6). Hard bases lie close to the dashed I me. soft bases lie further away from it. [From Brauman, J. 1. Han. C.-C. J. Ant. Ckctn. Soc. 1988. HO. 5611 Reproduced with... 

The study of gas-phase methyl cation transfer equilibria (Scheme 3) with a HPMS and an ICR spectrometer has provided data on methyl cation affinity (MCA), defined as the enthalpy change for the reaction of equation 7, for a large number of neutral bases, including many halo-substituted compounds915. 

A steric parameter based on the proton affinity Ah and methyl cation affinity Ach3, calculated by computational chemistry methods, for reaction at the nitrogen atom series of compounds in which the nitrogen atom was unhindered, e.g. pyridines in which the 2- and 6-positions are unsubstituted [Jenkins et al, 1994 Jenkins et al., 1995 Baxter et al, 1996]. Using these affinity values, a reference regression model was found which correlates the methyl cation affinity Achs to the proton affinity Ah for unhindered compounds ... 

For nitrogen compounds where the nucleophilic nitrogen atom is hindered, the methyl cation affinity values calculated are less than those estimated by the reference model. The difference between the two values is assumed as the steric parameter, i.e ... 

This is unfortunate, since there is a great deal of data available on the methyl cation affinities (MCA) of neutral molecules... 

Finally, a very interesting, nonobvious correlation discussed previously (3) exists. If our intrinsic nucleophilicites (AE0 ) are plotted for a series of nucleophiles, the best correlation is with the methyl cation affinity of the nucleophile. That is, those nucleophiles for which the energy for CH3Y — ... 

Table III. Highest Occupied Molecular Orbital (HOMO) Energies, Mulliken Charges, g0> and Calculated and Experimental Proton and Methyl Cation Affinities of Some Oxyanions...

Of course, the gas-phase thermochemistry of ions is not solely restricted to the measurement of the quantities described above a wide range of other ion affinities have been measured, including methyl cation affinities, hydride affinities and halide affinities . Further, such measurements can often be related to unknown neutral thermochemistry via the appropriate thermochemistry cycle. For example, the phosphorus-carbon double bond strength (the sum of the a and n bond contributions) in HP=CH2 was recently estimated via mass spectrometric measurements to be 101 7 kcaF (ref. 43). 

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