Experimental And Theoretical Investigations Of Molecular Structures Of Halide-Containing Cluster Ions

The molecular structures,bonding properties and reactive activation of small gas-phase cluster ions are significant in physical chemistry,to provide crucial clues for the interstellar chemical evolution and catalytic mechanisms.Especially,the spectroscopic studies on halide-containing cluster ions,not only provide basic data to study the interactions between molecules and halides and to explore essential factors of the functionalization of the related molecules in condensed phase,but also help to dig into the hydrogen bonds and electronic properties in relevant complicated systems.During the past decades,some combined methods of the ion trap and gas-phase ion spectroscopy like negative ion photoelectron spectroscopy(NIPES)and infrared photodissociation spectroscopy(IRPD)have rapidly developed,which lay a solid foundation for the detection and characterization of small gas-phase cluster ions.In this thesis,we have systemically investigated the structures and bonding properties of halide-containing cluster ions using a joint method of the low-temperature magnetic bottle photoelectron spectrometer and theoretical calculations.Moreover,we have contructed a home-made infrared photodissociation spectrometer coupled with an electrospray ionization(ESI)source and three-dimensional(3D)ion trap,in order to obtain vibrational assignments of target ions which cannot be well characterized by the NIPES due to insuffient vibrational resolution.(1)Stabilization mechanism of carbonic acid by halides in the gas phase.The holistic H2CO3·X-(X=F,Cl,Br,I)cluster anions have been successfully generated,detected and characterized using a joint NIPES and theoretical study,indicating that carbonic acid can be effectively stabilized by halides through the formation of a dual hydrogen bonds structure.Additionally,upon electron detachment,only marginal geometry changes are observed for the X=Cl,Br and I cluster.In contrast,significant geometric rearrangement associated with electron-detached hydrogen atom relocation from carbonic acid to fluoride is elucidated,manifested by the appearance of the extraordinary complexity in the H2CO3·F-spectrum.More importantly,we found a perfect consistency between the most stable hydrogen bonding structures and the pKa values of corresponding acids,which cannot be well-predicted by proton affinity,providing direct criteria for the hydrogen bonding structures and binding motifs during the stabilization of diprotic oxyacids by halide anions in the gas phase.(2)Interaction between amino acids and anions using the "Iodide-tagging"NIPES technique.We have proposed the temperature-dependent "iodide-tagging”photoelectron spectroscopy method to perform a case study on the Gly·I-cluster anion,combined with theoretical calculations.Five isomers with the computed CCSD(T)relative energies spanning 3.93 kcal/mol have been successfully trapped and characterized according to the excellent agreements between experimental and calculated spectra,revealing diverse binding motifs between glycine and iodide in the gas phase.Similarly,we have inspected glycine derivates and arginine to examine the capability of this method in probing zwitterionic forms of amino acids stabilized by other halide ions.Moreover,the similar "bromide-tagging" and "chloride-tagging"experiments were also performed to confirm the superiority of iodide in this field.(3)Development of infrared photodissociation spectrometer coupled with an ESI source and 3-D ion trap.IRPD is one of the central techniques to detect and characterize the molecular structure and vibrational information of mass-selected ions with extremely high sensitivity.The cluster cations,generated from an ESI source,are electronically transmitted and mass-selected before guided into a quadrupolar ion trap,where they are accumulated and photodissociated by IR photons.The fragments and residual precursors are then pulsedly pushed out and vertically introduced into the extraction zone of a Wiley-McLaren time-of-flight mass spectrometer at a repetition rate of 10 Hz.Through recording the fragment ion intensity vs.the IR photon energy,we have obtained the vibrational spectrum of[Trp+H]+cation,which agrees very well with the previous result.Thus,the reliability of this apparatus is verified for the future structural analysis on cluster ions.