The Trigonal Bipyramidal MN_3M Species:A New Kind of Aromatic Complex Containing a Multiple-Fold Aromatic N_3 Subunit

摘要

A new kind of aromatic trigonal bipyramidal MN_3M (M = Be,B,Mg,Al,and Ca) species,with all real frequencies,is obtained at the MP2/6-311 + G(3d) level.The nucleus-independent chemical shift values are -102.16 ppm for the N_3~(3-) ring,and -74.09,-79.39,-65.06,-74.44,and -62.33 ppm (at the geometrical center of the trigonal bipyramid) for BeN_Be,BN_3B,MgN_3Mg,AIN_3Al,and CaN_3Ca,respectively.Molecular orbital analysis indicates that the regular triangular N_3~(3-) ring and each MN_3M species have three aromatic six-electron systems (pi,sigma_(p'),and sigma_s) and exhibit threefold aromaticity.The CaN_3Ca species has a very low vertical ionization energy of 3.64 eV at the CCSD(T)/6-311 +G(3d) level,which is even lower than the ionization energy (3.9 eV) of the Cs atom.Therefore,CaN_3Ca can be considered as a new su-peralkali species.A further study on the CaN_3CaCI molecule confirms the superalkali characteristics of CaN_3Ca.Two interesting phenomena are explored in the MN_3M species:the delocalized electron cloud of the N_3 subunit is elongated by two M cations,and the electron clouds of two M cations are distended by the N_3~(3-) ring.