Structures Of Ion-molecule Complexes Studied With Photoelectron Spectroscopy And Infrared Photodissociation Spectroscopy

Determination of the geometric structures of ion-molecule complexes and unravelling intermolecular interactions within complexes in the gas phase are key steps for gaining deep insights into chemical reactions and intrinsic mechanisms.Particularly for one of the focal research areas in chemical transformation,catalysis,it is essential to scrutinize gas-phase ion-molecule complexes concerning homogeneous and heterogeneous catalytic reactions as ideal molecular models for elucidation of catalytic activities and reaction mechanisms.Significant efforts have been undertaken aiming to obtain a molecular-level understanding of phenomena in the condensed phase for further rational design of novel catalysts and improvement over traditional chemical synthesis routes.In this thesis,several powerful spectroscopic techniques ranging from negative ion photoelectron spectroscopy(NIPES)to infrared photodissociation spectroscopy(IRPD)are employed to systematically investigate well-defined model reacting systems composed of catalysts and reactant molecules.The combination of experimental measurements and high-level quantum chemical calculations provides crucial information on geometric and electronic structures along with the nature of intermolecular interactions between components,lending support to the interpretation of catalytic reaction mechanisms.Additionally,the design and construction of a hybrid instrument equipped with mass spectrometer and infrared photodissociation spectrometer while featuring an electrospray ionization(ESI)source for ion production,which shows great potential as a high-performance platform for structural characterization,are described in detail.Key findings are as follows:1.A dominant direction-specific effect in the course of complexation.Sizeselective NIPES in conjunction with theoretical calculations is employed to investigate the geometric and electronic structures of a protype system in catalytic olefin epoxidation research,i.e.,deprotonated hexafluoroisopropanol([HFIP-H]-)complexed with hydrogen peroxide(H2O2).Spectral assignments and molecular electrostatic surface analyses unveil a surprising prevalent existence of a high-lying isomer with asymmetric dual hydrogen-bonding configuration that is preferably formed driven by influential direction-specific electrostatic interactions upon H2O2 approaching[HFIPH]-anion.This work conspicuously illustrates the importance of directionality encoded in intermolecular interactions involving asymmetric and complex molecules,while the produced hydroperoxyl radical HOO· offers a possible new pathway in olefin epoxidation chemistry.2.An effective approach proposed for the generation of singlet oxygen(1O2).The ability of 1O2 for selective degradation of dyes in environmental remediation of wastewater has attracted extensive attention.In this part,we showcased that 1O2 can be effectively generated from an anion complex composed of[HFIP-H]-with hydroperoxyl radical via ultraviolet(UV)photodetachment.Electronic structure calculations and cryogenic NIPES unveil critical proton transfer upon complex formation and electron ejection,effectively photoconverting prevalent triplet ground state 3O2 to long-lived excited 1O2,stabilized by nearby HFIP.Inspired by this spectroscopic study,a novel"photogeneration" strategy is proposed to produce 1O2 with the incorporation of atmospheric O2 and HFIP,acting as a catalyst.Conceptually,the designed catalytic cycle upon UV irradiation and electron injection is able to achieve different degradations of dye molecules in a controllable fashion from decolorization to complete mineralization,shedding new light on potential water purification.3.Two-state reactivity(TSR)as a responsible paradigm for catalytic reduction of CO2.The isomeric structures of gas-phase[TaC4O8]+ and[TaO3(CO2)n]+(n=2-5)ion-molecule complexes formed in a laser ablation source have been investigated using IRPD combined with quantum chemical calculations,providing basic information for heterogeneous catalytic activation of CO2.Based on the great agreements between vibrational characteristics of experimental and simulated infrared spectra,various activation modes of CO2 by the cationic tantalum metal and cationic tantalum oxides are identified.The strong absorption in the carbonyl stretching region of[TaC4O8]+provides solid evidences for the insertion reactions by Ta+into the C=O bonds of CO2,where spin conversion between different spin states plays a crucial role.Moreover,the most favorable cluster growth pathway for[TaO3(CO2)n]+(n=2-5)is verified to proceed in a similar way with a TSR scenario where the energetically favored triplet transition state cross over the singlet ground state to form the triplet core ion structure.4.Development of an infrared photodissociation spectrometer using an ESI source and a cryogenic ion trap.Electrospray ion production and mass spectrometry technique with high sensitivity and resolution allow creation and selection of ionmolecule complexes of continuously varying sizes for gas-phase studies.Targeted ionmolecule complexes strongly correlated to solution phase reactions are accumulated in a three-dimensional Paul trap and subsequently interrogated by IRPD with enhanced sensitivity.Direct comparison of experimental results with the computed absorption spectra deduces structural information concerning functional groups and hydrogen bonding patterns,enabling the discrimination of different isomers and to resolve intermolecular interactions.Moreover,the construction of online Venturi sonic-spray ionization source without the assistance of electrical pumping offers opportunity to observe transient features of reactive intermediates involved in various processes.The design of ion temperature control to be accomplished with an ion trap attached to the cold head of a closed cycle helium refrigerator is expected to overcome limitations originated from poor spectral resolution,rendering this apparatus capable of assessing interactions involving weakly binding species.