Quantum chemical determination of the equilibrium geometries and harmonic vibrational frequencies of 1,2′- and 2,2′-binaphthyl in their ground and excited (1La) electronic states

This work is concerned with the characterization of the lowest electronic excited states of 1,1'-, 1,2'- and 2,2'-binaphthyl, in particular with the determination of their equilibrium geometries, low-frequency vibrations, and torsional potentials, via ab initio and density functional (DFT) calculations. The methods employed are configuration interaction with singles (CIS) and time-dependent DFT, in conjunction with Hartree-Fock and DFT(B3LYP) theories for the ground states, using the 3-21G basis set. The strengths and weaknesses in these methods for these large systems were tested extensively by reference to higher level calculations performed on the benzene/biphenyl systems and on naphthalene itself In the case of 1,2'- and 2,2'-binaphthyl the theoretical predictions are consistent with a reassignment of the experimentally observed spectrum as S-3(L-1(a)) <-- S-0 rather than S-1(L-1(b)) <-- S-0 as previously assumed. With this reassignment it is possible to provide definitive assignment of a number of the observed low-frequency torsional and out-of-plane bending modes, as well as a more confident identification of the conformation of the observed excited states. For 1,1'-binaphthyl the theoretical results are, however, at variance with the prevailing interpretation of the spectral data. As expected on the basis of qualitative pi molecular orbital theory, pi-delocalization has been found to be considerably more important in the L-1(a) excited states of the binaphthyls than in the ground and L-1(b) excited states. The different degrees of pi delocalization in the ground and excited states of these biaryls provide an explanation as to why all these systems adopt nonplanar twisted geometries in their ground and L-1(b) excited states, while in their L-1(a) excited states they are computed to be either completely planar or considerably more so than in the ground state.