Proton-rich isotopes can decay by electron capture. And release neutrino (to conserve lepton number).
But daughter isotope (just nucleus) must have smaller mass-energy, than parent isotope (nucleus), to conserve energy properly.
In the case of lone proton and electron, there is missing 0.782 MeV mass-energy.
Because free neutron has 939.565 MeV/c^2, proton has 938.272 MeV/c^2 and electron has 0.511 MeV/c^2 mass.
938.272 MeV/c^2 + 0.511 MeV/c^2 - 939.565 MeV/c^2 = -0.782 MeV/c^2
Negative means disallowed/missing energy. If it would be >0, such decay mode would be at least plausible.
Edited by Sensei, 21 August 2016 - 10:09 PM.