The concepts of phonon assistance and chemical pressure have been advanced to better understand the collective nature of the energetics and dynamics of solid-state reactions. With the 2,5-distyrylpyrazine photoreaction, these hypotheses were examined by employing lattice dynamical calculations and Raman and piezomodulated Raman spectroscopy in the lattice mode region. Since modal motion and anharmonicity are important in substantiating these models, the analysis of this solid-state reaction focused on these aspects. The motions calculated by lattice dynamical methods are consistent with the results from the piezomodulated Raman spectra. The anharmonicity of the phonon modes was found to increase significantly following the initiation of the reaction. This was particularly evident for those modes having motions along the reaction coordinate. Chemical pressure is shown to be of great utility in explaining this solid-state reaction while phonon-phonon interactions are demonstrated to have no significant role.