Electronic spectroscopy of jet-cooled 1,2 '-binaphthyl

The electronic spectroscopy of jet-cooled 1,2 ' -binaphthyl has been examin ed by laser-induced fluorescence spectroscopy. Many medium length progressi ons were observed in two active vibrations with frequencies nu (A ') = 35.1 cm(-1) and nu (B)' = 53.5 cm(-1) assigned as the torsion and out-of-plane wag between the two naphthalene moieties, respectively. Two other low-frequ ency vibrations were measured: nu (C)' = 60.6 cm(-1) and nu (D)' = 138.1 cm (-1). These are also likely to involve inter-ring motion, possibly an in-pl ane wag for ve and an antisymmetric inter-ring motion for nu (D), although a low-frequency naphthalene ring vibration is also possible. The electronic origin transition was not observed because of a very weak Franck-Condon fa ctor with the ground state. However, a Franck-Condon analysis provided an e stimate of the origin transition at T-00 = 30 828.2 cm(-1) Progressions in both nu (A) and nu (B) are harmonic up to 13 quanta for nu (A) and 5 quanta for nu (B). Therefore, the barrier for interconversion between the cis and trans conformers of 1,2 ' -binaphthyl must be much higher than the 500 cm( -1) of harmonic vibrations measured here. A diffuse band, about 800 cm(-1) higher in energy, was assigned to the S-2 <-- S-0 transition. Previous semi empirical calculations predicted the electronic energy of the SI state and the magnitude of the naphthalene dimer splitting quite well. However, the c alculations placed the interconversion barrier through 90 degrees to be as low as 300 cm(-1). The Franck-Condon analysis also provided a change in equ ilibrium conformation angle of 18 degrees upon electronic excitation, again in disagreement with calculations, which predict essentially no change in equilibrium torsional angle between the S-0, S-1, and S-2 states.