Normal mode analysis of vibrations of jet-cooled acridine dimer

The analysis of normal modes of vibrations was performed for acridine dimer , which has been recently observed and identified by laser-induced emission spectroscopy under conditions of jet-cooling in a supersonic helium expans ion. The frequencies and the forms of normal vibrations, computed with the use of the semi-empirical PM3 method are classifying all vibrations of acri dine dimer into two distinct categories of inter- and intramolecular vibrat ions. There are six low-frequency intermolecular normal vibrations which ar e characterizing relative movements of two acridine moieties in the dimer a nd at least two of them have vibrational frequencies which are close to the frequencies of vibronic bands observed in the fluorescence excitation spec trum of the dimer. The intramolecular normal vibrations of the dimer very s trictly correspond to the normal vibrations of acridine molecule (with a mi nor modification of the vibrational frequencies due to the splitting in the dimer). Furthermore, the intramolecular vibrations of acridine dimer, as w ell as the vibrations of acridine molecule are in good agreement with avail able experimental data (either IR and Raman spectra of acridine or fluoresc ence excitation spectrum of acridine dimer in supersonic jet). The results of the present analysis ate very clearly supporting the previous conclusion s concerning the ground-state equilibrium structure of acridine dimer forme d under jet-cooling conditions.