# Why is the methyl radical planar?

Why is the methyl radical planar? The VSEPR theory would predict an angle between 120 and 109.5 degrees, while it is actually 120 degrees.

• VSEPR is about "electron PAIR repulsion". I would say it does not predict anything for radicals. Or at least be very careful to not make definite statements. ;-) – Karl Aug 12 '15 at 17:55
• – user7951 Aug 12 '15 at 19:37

This model works well in some cases. However, when quantum mechanics comes into play, it falls pretty flat, as it totally neglects (the really important) quantum effects. A better way of finding the molecular geometry is to find the geometry that minimizes the total energy of the molecular orbitals. In this case, quantum mechanical calculations find that a set of 3 sp$^2$ hybridized molecular orbitals arising from a D$_{3h}$ symmetry has a lower total energy than breaking the symmetry to form the pyramidal C$_{3v}$ geometry.
However, this does not mean that all CX$_3$ radicals follow this pattern. Put an extremely electronegative substituent on in place of H, eg in CF$_3$, and the situation changes. The lowest energy geometry is now the pyramidal one.