INFRARED SPECTRAL PROPERTIES OF THE NAPHTHALENE CATION - RADIATIVE COOLING KINETICS EXPERIMENTS AND DENSITY-FUNCTIONAL CALCULATIONS

Time-resolved photodissociation thermometry and density functional theory have been applied to the study of the infrared (IR) spectral properties of the naphthalene cation. The radiative cooling rate of isolated gasphase naphthalene ions was measured directly using a thermometric approach based on time-resolved photodissociation (TRPD). Two-photon excitation of the ion at a wavelength of 355 nm was used for TRPD rate measurements at various cooling times after formation of the ions (by multiphoton ionization at 193 nm). The observed dissociation rates were calibrated against a previous experimentally determined rate-energy curve as a thermometric probe, providing cooling curves for initially hot ions. Radiative cooling time constants were observed to be 0.9 s (C10H8+) and 1.9 s (C10D8+). For comparison with these experimental results, the IR absorption intensities were calculated using nonlocal density functional theory (DFT) implementing Becke's three-parameter functional for incorporating exchange and correlation effects. A Dunning/Huzinaga valence double-zeta basis set with a single set of d-polarization functions on the C atoms was employed. Both the observed cooling curves and the DFT calculations provide strong support for the absolute IR intensities reported from direct absorption measurements in Szczepanski, J.; Roser, D.; Personette, W.; Eyring, M.; Fellow, R.; Vala, M. J. Phys. Chem. 1992, 96, 7876. Increased accuracy of nonlocal DFT, even when employing moderate-sized basis sets, as compared with Hartree-Fock calculations is suggested by the better agreement with experimental observations of the absolute IR intensities for cationic naphthalene and for a sample series of organic molecules.