Briefly on the subject of deuteration: C-H stretching normal modes are often the most common accepting modes for aromatic hydrocarbons . Since it is most often mid-high energy normal modes with the largest Huang-Rhys factors, they are therefore more likely to participate in a given transition. Atomic deformations due to C-C modes, for example, while larger than those of C-H-type modes, are unable to combat the effect of a significantly smaller frequency. This ends up with large configurational indexes , thus smaller contributions are observed to the rate constant. In addition, nuclear deformation due to electronic excitation, for highly symmetric compounds like most aromatic hydrocarbons, in order to maintain symmetry are observed through stretching, breathing, and twisting normal modes. It is therefore interesting to highlight the effect of deuteration; the much smaller energy of C-D modes requires more quanta per normal mode . Another way to examine this result is to consider the mass of hydrogen Vs. deuterium. Huang-Rhys factors are sensitive to nuclear mass; since deuterium is twice as heavy as hydrogen, the energy of the mode can be expected to weaken by a factor of ∼ √ 2.