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Methyl cation, radical, and anion

Although we have focused so far on neutral closed-shell molecules, eFF should in principle be able to optimize cationic and anionic species and radicals as well. To calculate accurate bond dissociation energies, it is especially important to have a well-balanced description of radicals and closed shell species.

Consider the series of molecules $ \mathrm{CH_{3}^{+}}$, $ \mathrm{CH_{3}}$ radical, and $ \mathrm{CH_{3}^{-}}$. Both $ \mathrm{CH_{3}^{+}}$ and $ \mathrm{CH_{3}}$ radical are expected to be planar and have similar bond length. Our force field reproduces bond lengths well (for $ \mathrm{CH_{3}^{+}}$, 1.095 $ \mathrm{\AA}$ versus 1.087 $ \mathrm{\AA}$ exact; for $ \mathrm{\cdot CH_{3}}$, 1.091 $ \mathrm{\AA}$ versus 1.079 $ \mathrm{\AA}$ exact).

Figure 4.10: In eFF, methyl cation and radical are stable but methyl anion is unbound

eFF makes methyl radical less stable than it should be (adiabatic ionization potential is 64 kcal/mol versus 226.8 kcal/mol exact), as it is not capable of properly describing the radical electron, which should reside in a $ p$ orbital. As in the case of multiple bonds, our force field compensates for its lack of $ p$ functions by making the radical electron very diffuse and placing it above the molecular plane.

eFF makes methyl carbanion unbound relative to methyl radical. This is not a surprising result, since in reality methyl carbanion is only marginally more stable than methyl radical (1.8 kcal/mol energy difference [28]); high-level theoretical calculations [29] (1.6 kcal/mol energy difference found) with large basis sets and correlation included are necessary to show that methyl carbanion is a stable species relative to methyl radical.


next up previous contents
Next: Homolytic versus heterolytic bond Up: Validation against ground state Previous: Conformational analysis of hydrocarbon   Contents
Julius 2008-04-29