Free energy barrier

We are trying to calculate the free energy barriers for some dissociative reactions. As a model, we scan the B-N bondlength in H3B-NH3from the equilibrium distance to three times of that, then the frequency analyses were performed for each optimized points. Surprisingly, the free energies oscilate when the distance is longer than about twice of the equilibrium distance. So, we cannot find the free energy barrier by this way.
Is there any other way to calculate activation free energy for dissociative processes? Or some program available for this goal?
 Thanks in advance,
 Yubo Fan
 Chemistry Department at TAMU