Question

What do you mean by proton affinity?

Answer 1

The proton affinity for the reaction is defined as the negative of the reaction enthalpy at 298.15K, and hence (T: temperature, R: ideal gas constant) The energy of a nonlinear polyatomic molecule can be approximated as. Here ZPE stands for the zero point energy of the normal modes.
hope it helps you :)

Answer 2

The proton affinity (PA, Epa) of an anion or of a neutral atom or molecule is the negative of the enthalpy change in the reaction between above species and proton in the gas phase: A− + H+→ HA.