At the molecular level, basically all photon modes associated with the thermal energy (or lower) will be already thermally occupied. E = hf = k_bT/2. This frequency at room temperature is about 30THz. So on the microscopic level, any frequencies under 30THz are constantly irradiated by thermal fields anyway.
Edit: Furthermore, the Gibb's free energy of any molecular process determines the reversibility of the process at a given temperature. Any molecular process with Gibb's free energy that is lower than the thermal mean energy is going to be essentially a reversible equilibrium process, and stimulating it with radiation will only shift the equilibrium very slightly I believe. I think it's for this reason that we don't see radio catalysed reactions in chemistry, unlike photocatalysed reactions.
Edit: Furthermore, the Gibb's free energy of any molecular process determines the reversibility of the process at a given temperature. Any molecular process with Gibb's free energy that is lower than the thermal mean energy is going to be essentially a reversible equilibrium process, and stimulating it with radiation will only shift the equilibrium very slightly I believe. I think it's for this reason that we don't see radio catalysed reactions in chemistry, unlike photocatalysed reactions.