摘要
We employed temperature-controlled Raman spectroscopy to obtain the vibrational structures of the linear tricobalt metal-string complex Co 3 (dpa) 4 Cl 2 (dpa = di(2-pyridyl) amide) and to identify its low-lying electronic states. The density functional theory (DFT) method B3LYP*-D3 was used to obtain the molecular structures, vibrational frequencies, and electronic levels of varied spin states. Co 3 (dpa) 4 Cl 2 has sym- and unsym-forms of Co–Co metal bonding. The Raman intensities of pyridyl breathing bands appeared to be sensitive to the coordinated Co atom and hence could be used to identify both forms. In s-Co 3 crystal, the split pyridyl Raman breathing bands indicated both symmetric and unsymmetric forms. This is explained that the molecule populated both the 2 A 2 (the ground state) and possibly 2 B (reduced symmetry of 2 E) states even at 77 K. The 2 B state is coupled to the 4 B state, which has an unsymmetric Co–Co bond. As the temperature increased to 423 K, the intensity of the low-wavenumber pyridyl breathing band further increased and was attributed to the 4 B state. At 77–323 K, the Raman spectra of u-Co 3 crystal populated mostly in the 4 B state. From the SERS measurements, the samples were prepared in solution phase, and u-Co 3 displayed spectral behavior similar to that of s-Co 3 , implying that the global minimal geometry is a symmetric form.