Vibrational overtone spectroscopy and intramolecular dynamics of C-H stretches in pyrrole

Room-temperature photoacoustic spectra and jet-cooled action spectra of the regions of the first and second C-H stretch overtones of pyrrole were measured with the goal of gaining new insight on the vibrational patterns and the intramolecular energy flow out of the initially excited vibrational states. The rotational cooling of the action spectra helped in observing hitherto unresolved features, assisting determination of the existing multiple bands and their positions in each region. These bands were analyzed by building vibrational Hamiltonian matrices related to a simplified joint local-mode∕normal-mode (LM∕NM) model, accounting for two types of C-H stretches and their Fermi resonances with the CCH deformation modes. The diagonalization of the LM∕NM vibrational Hamiltonians and the fitting of the eigenvalues to the band positions revealed model parameters, enabling assignment of the observed bands. The time dependences of the survival probabilities of the C-H stretches in the region of the first and second overtones, deduced from the vibrational Hamiltonian, show quantum beats due to the couplings to the deformations and decays driven by weaker interactions to the bath states. The C-H stretches, although somewhat lower in energy, show stronger coupling than the N-H stretches.