Spectroscopic Consequences of Localized Electronic Excitation in Anthranilic Acid Dimer

Abstract

The electronic and infrared spectroscopy of anthranilic acid (o-aminobenzoic acid) dimer has been studied in a supersonic jet. Fluorescence-dip infrared (FDIR) spectra have been obtained in both the ground and first excited electronic states. The ground-state FDIR spectrum shows a broad, highly shifted OH stretch absorption commensurate with a cyclic, doubly hydrogen bonded structure, as has been observed for other carboxylic acid dimers. Density functional theory calculations predict that the dimer retains the monomer propensity for the amino group to be adjacent to the carbonyl group of the carboxylic acid, producing a C2h ground-state geometry for the dimer. The presence of the amino group shuts off the double proton tunneling that is present in benzoic acid dimer. The excited-state FDIR spectrum shows NH stretch fundamentals that are the sum of the S0 and S1 FDIR spectra of anthranilic acid monomer, indicating that the electronic excitation is localized on one of the monomers in the excited electronic state. The ultraviolet spectrum of the dimer shows a strong Franck−Condon progression involving a 58 cm-1 vibration and many combination bands with this mode. Comparison with density functional theory calculations indicates that the 58 cm-1 mode involves the in-plane geared bend of the two monomer units, which has bu symmetry in the C2h ground state. While this non totally symmetric fundamental appears in the R2PI and IR−UV hole-burning spectra, the dispersed fluorescence spectra from the S1 origin, +58 cm-1 band, and +118 cm-1 band display intensity only in even members of the 58 cm-1 progression. This Franck−Condon activity is quantitatively fit by a model in which the excited-state vibrations are simple sums and differences of localized, shifted harmonic oscillator vibrational wave functions, producing unresolved ag/bu tunneling doublets associated with the large barrier that separates the two minima on the excited-state surface.