Spectroscopic properties of novel aromatic metal clusters:NaM_4 (M=Al,Ga,In) and their cations and anions

摘要

The ground- and several excited states of metal aromatic clusters,namely NaM4 and NaM_4~(+-) (M=Al,Ga,In) clusters have been investigated by employing complete active-space self-consistent-field followed by multireference singles and doubles configuration interaction computations that included up to 10 million configurations and other methods.The ground states NaM_4~- of aromatic anions are found to be symmetric C_(4v) (~1A_1) electronic states with ideal square pyramid geometries.While the ground state of NaIn_4 is also predicted to be a symmetric C_(4v) (~2A_l) square pyramid,the ground state of the NaAl_4 cluster is found to have a C_(2v) (~2A_1) pyramid with a rhombus base,and the ground state of NaGa_4 possesses a C_(2v)(~2A_1) pyramid with a rectangle base.In general,these structures exhibit two competing geometries,viz.,an ideal C_(4v) structure and a distorted rhomboidal or rectangular pyramid structure (C_(2v)).All of the ground states of the NaM_4~+ (M=Al,Ga,In) cations are computed to be C_(2v) (~3A_2) pyramids with rhombus bases.The equilibrium geometries,vibrational frequencies,dissociation energies,adiabatic ionization potentials,adiabatic electron affinities for the electronic states of NaM_4 (M=Al,Ga,In),and their ions are computed and compared with experimental results and other theoretical calculations.On the basis of our computed excited states energy separations,we have tentatively suggested assignments to the observed X and A states in the anion photoelectron spectra of Al_4Na~- reported by Li et al [X.Li,A.E.Kuznetov,H.F.Zheng,A.I.Boldyrev,and L.S.Wang,Science 291,859 (2001)].The X state can be assigned to a C_(2v) (~2A_1) rhomboidal pyramid.The A state observed in the anion spectrum is assigned to the first excited state (~2B_1) of the neutral NaAl_4 with the C_(4v) symmetry.The assignments of the excited states are consistent with the experimental excitation energies and the previous Green's function-based methods for the vertical transition energy separations between the X and A bands.