Calculation of the effect of radiation damage on hindered rotation in a molecular crystal

A model is proposed to describe the observed distribution of the heights of barriers to rotation of the 4-methyl group in gamma -irradiated 4-methyl-2,6-di-tertiary butylphenol (MDBP). The intramolecular barrier height in undamaged MDBP is calculated using approximate molecular orbital methods and the results show the dependence of the molecular dipole moment on the orientation of the 4-methyl group. The intermolecular barrier height is calculated from the Buckingham potential between atom-atom pairs and the dipolar interaction between the molecules. The total calculated barrier is in good agreement with experiment. gamma -irradiation is simulated by substituting a free radical with its associated dipole moment, for an undamaged molecule. The change in barrier of adjacent methyl groups due to the change in the intermolecular potential is determined and a distribution of barrier heights is obtained. Experimental results are presented which agree with the general shape of the distribution and which confirm the weakening of the dipole perturbation with increasing distance from the radical.