Intramolecular vibrational relaxation in aromatic molecules II: an experimental and computational study of pyrrole and triazine near the IVR threshold

The threshold region of vibrational energy redistribution (IVR) presents a great experimental and computational challenge for organic molecules with more than 10 degrees of freedom. The density of states is sufficiently high to require very high resolution measurements to cover all relevant time scales experimentally. Yet it is sufficiently low so IVR quantities, such as the initial relaxation time IVR or the number of participating states Neff, are very sensitive to the coupling structure. To extend a previous study of benzene [A. Callegariet al., J. Chem. Phys. (2000)], we have measured overtone spectra of pyrrole (C4H4NH) and 1,2,3-triazine (C3N3H3) by eigenstate-resolved double resonance spectroscopy, and performed large scale computations of IVR dynamics by using analytically fitted fourth order ab-initio force fields and filter diagonalization. The modeled Neff and τIVR agree with the experimental quantities within a factor of 2 to 3, reasonable for a rate theory in the threshold regime. The models also correctly predict the experimentally observed trends of IVR and Neff for the two molecules, and provide insight into the highly off-resonant coupling mechanism, which nonetheless yields very sharp linewidths. The series benzene-pyrrole-triazine highlights the competing effects of molecular symmetry on the accessible density of states and mode localization. † Present addresses AC: École Polytechnique Fédérale, Lausanne, Switzerland. RP: Columbia University, Dept. of Chemistry. PE: