Competition between linear and cyclic structures in monochromium carbide clusters CrCn- and CrCn (n = 2-8): a photoelectron spectroscopy and density functional study.

Photoelectron spectroscopy (PES) is combined with density functional theory (DFT) to study the monochromium carbide clusters CrCn- and CrCn (n = 2-8). Well-resolved PES spectra were obtained, yielding structural, electronic, and vibrational information about both the anionic and neutral clusters. Experimental evidence was observed for the coexistence of two isomers for CrC2-, CrC3-, CrC4-, and CrC6-. Sharp and well-resolved PES spectra were observed for CrCn- (n = 4,6,8), whereas broad spectra were observed for CrC5- and CrC7-. Extensive DFT calculations using the generalized gradient approximation were carried out for the ground and low-lying excited states of all the CrCn- and CrCn species, as well as coupled-cluster calculations for CrC2- and CrC2. Theoretical electron affinities and vertical detachment energies were calculated and compared with the experimental data to help the assignment of the ground states and obtain structural information. We found that CrC2- and CrC3- each possess a close-lying cyclic and linear structure, which were both populated experimentally. For the larger CrCn- clusters with n = 4, 6, 8, linear structures are the overwhelming favorite, giving rise to the sharp PES spectral features. CrC7- was found to have a cyclic structure. The broad PES spectra of CrC5- suggested a cyclic structure, whereas the DFT results predicted a linear one.