Infrared Photodissociation Spectroscopic Studies On Heteronuclear Transition Metal Carbonyl Cluster Anions

Infrared photodissociation spectroscopy(IRPD) is a powerful method to study the ions in the gas phase, which can provide the vibrational information of mass selected ions in the gas phase with high sensitivity. In this paper, we explicated the home-made collinear tandem time-of-flight mass spectrometer, which is designed and installed for the measurement of infrared photodissociation spectroscopy of mass-selected ions in the gas phase. In combination with quantum chemical calculation, the heterotrinuclear iron-copper carbonyl anions and the heterodinuclear iron-chromium carbonyl anions have been studied in the gas phase both experimentally and theoretically. The main work is as follows:(1)The design and construction of the home-made infrared photodissociation spectrometer. This apparatus have three main parts, including the laser vaporization cluster source, linear time-of-flight mass spectrometer and tandem infrared photodissociation mass spectrometer. The ions from a pulsed laser vaporization supersonic cluster source are analyzed and seperated by the linear time-of-flight mass spectrometer. The species of interest is mass selected and dissociated by a tunable IR laser. The fragment and parent ions are mass analyzed by tandem infrared photodissociation mass spectrometer. The infrared photodissociation spectrum is obtained by monitoring the yield of the fragment ions as a function of the dissociation IR laser wavelength and normalizing to parent ion signal.(2) Mass-selected heteronuclear iron-chromium carbonyl cluster anions Cr Fe(CO)n-(n=4-8) are studied by infrared photodissociation spectroscopy in the carbonyl stretching frequency region in the gas phase. Their geometric structures are determined by comparison of the experimental spectra with those calculated by density functional theory. The experimentally observed Cr Fe(CO)n-(n=4-8) cluster anions are characterized to have(OC)4Fe-Cr(CO)n-4 structures. Bonding analysis indicates that the Fe-Cr bond in the Cr Fe(CO)n-(n=4-8) cluster anions is an σ type single bond with the iron center possessing the most favored 18-electron configuration. In these anions, the Fe-Cr bonding orbital is formed between a Fe 3dz2, 4s, 4p hybrid orbital and a Cr 4s, 4p hybrid orbital. There calculated dissociation energies of Cr Fe(CO)n-(n=4-8) are higher than the IR photon energies in the CO stretching frequency region and multiphoton processes happened during the fragmentations. The Fe-Cr bond length decreased with the number of CO ligands bonded to the Cr center, except for Cr Fe(CO)8- which has the longest Fe-Cr bond length due to the steric crowding. The Fe(CO)4 is keep C3 v or C2 v structure and act as a stabilizing group in these gas phase ions.(3) Mass-selected heteronuclear iron-copper carbonyl cluster anions Cu2Fe(CO)n-(n=6-8) are studied by infrared photodissociation spectroscopy in combination with theoretical calculation. The experimentally observed Cu2Fe(CO)n-(n=6-8) cluster anions each involves a triangle metal cores and are characterized to have Fe(OC)4 and Cu(CO)m(m=0-2) moistures. Bonding analysis indicates that the iron center possessing the most favored 18-electron configuration. The Cu-Cu bond length increased with the number of CO ligands bonded to the Cu center. The Fe-Cu(CO) bond length is relative short, indicating a strong bond between the two units. While The Fe-Cu(CO)2 bond length is relative longer, indicating a weak interaction between the Fe(CO)4 and Cu(CO)2 units. The Fe(CO)4 is keep slighted distorted C3 v or C2 v structure and act as a μ-2 group in these gas phase anions. The Cu2Fe(CO)n-(n=6-8) system can also characterized to be a triangle metal core stabilized by interacting with the CO ligands. The results provide important new insight into the structure and bonding of hetronuclear transition metal carbonyl cluster anions.