Zhi Zhao

Reactions of Copper and Silver Cations with Carbon Dioxide: An Infrared Photodissociation Spectroscopic and Theoretical Study

The reaction of copper and silver cations with carbon dioxide was studied by mass-selected infrared photodissociation spectroscopy. Quantum chemical calculations were performed on these products, which aided the experimental assignments of the infrared spectra and helped to elucidate the geometrical and electronic structures. The Cu+ and Ag+ cations bind to an oxygen atom of CO2 in an end-on configuration via a charge-quadrupole electrostatic interaction in the [M(CO2)n]+ complexes. The formation of oxide-carbonyl and carbonyl-carbonate structures is not favored for the interaction of CO2 with Cu+ and Ag+. For n = 3 and 4, the n + 0 structure is preferred. [Note on the nomenclature: Using i + j, i denotes the number of CO2 molecules in the first coordination shell, and j denotes the number of CO2 molecules in the second coordination shell.] The two nearly energy-identical n + 0 and (n - 1) + 1 structures coexist in n = 5 and 6. While the six-coordinated structure is favored for [Cu(CO2)n=7,8]+, the n + 0 configuration is dominated in [Ag(CO2)n=7,8]+. The reaction of CO2 with the cationic metal atoms has been compared to that with the neutral and anionic metal atoms, which would have important implications for understanding the interaction of CO2 with reduction catalysts and rationally designing catalysts for CO2 reduction based on cost-effective transition metals.

Infrared-Vacuum Ultraviolet Spectroscopic and Theoretical Study of Neutral Methylamine Dimer

The methylamine dimer, (CH3NH2)2, is a model system to study the CH3 and NH2 spectral patterns in the neutral microsolvated systems relevant to chemical biology, atmospheric chemistry, and catalysis. We report infrared-vacuum ultraviolet spectroscopic measurements to probe the neutral (CH3NH2)2. Quantum chemical calculations and ab initio molecular dynamics simulations were performed to understand the observed spectral features. Experimental and theoretical results indicate the likely coexistence of both cis and trans structures. A salient feature of this work is that the peak widths are not significantly affected by the structural transformation and the fluctuation of hydrogen bond distance, allowing the stretching modes to be clearly resolved.

The Formation of Surface Lithium-Iron Ternary Hydride and its Function on Catalytic Ammonia Synthesis at Low Temperatures

Lithium hydride (LiH) has a strong effect on iron leading to an approximately 3 orders of magnitude increase in catalytic ammonia synthesis. The existence of lithium-iron ternary hydride species at the surface/interface of the catalyst were identified and characterized for the first time by gas-phase optical spectroscopy coupled with mass spectrometry and quantum chemical calculations. The ternary hydride species may serve as centers that readily activate and hydrogenate dinitrogen, forming Fe-(NH2 )-Li and LiNH2 moieties-possibly through a redox reaction of dinitrogen and hydridic hydrogen (LiH) that is mediated by iron-showing distinct differences from ammonia formation mediated by conventional iron or ruthenium-based catalysts. Hydrogen-associated activation and conversion of dinitrogen are discussed.

Observing the Transition from Equatorial to Axial CO Chemisorption: Infrared Photodissociation Spectroscopy of Yttrium Oxide-Carbonyls.

A series of yttrium oxide-carbonyls are prepared via a laser vaporization supersonic cluster source in the gas phase and identified by mass-selected infrared photodissociation (IRPD) spectroscopy in the C-O stretching region and by comparing the observed IR spectra with those from quantum chemical calculations. For YO(CO)4(+), all four CO ligands prefer to occupy the equatorial site of the YO(+) unit, leading to a quadrangular pyramid with C4v symmetry. Two energetically nearly degenerate isomers are responsible for YO(CO)5(+), in which the fifth CO ligand is either inserted into the equatorial plane of YO(CO)4(+) or coordinated opposite the oxygen on the C4 axis. YO(CO)6(+) has a pentagonal bipyramidal structure with C5v symmetry, which includes five equatorial CO ligands and one axial CO ligand. The present IRPD spectroscopic and theoretical study of YO(CO)n(+) extends the first shell coordination number of CO ligands in metal monoxide carbonyls to six. The transition from equatorial to axial CO chemisorption in these yttrium oxide-carbonyls is fortunately observed at n = 5, providing new insight into ligand interactions and coordination for the transition metal oxides.

Interaction of Metal Ions with the His13-His14 Sequence Relevant to Alzheimer’s Disease

The interaction of a series of metal ions (i.e., groups 1 and 2, first-row transition metals, and groups 11-14) with the His13-His14 sequence relevant to Alzheimers disease has been studied using quantum chemical calculations. Metal ions prefer to occupy three coordination sites at two Nδ of the imidazole rings and one carbonyl oxygen. Simulated IR spectra reveal that vibrational frequency of C-O stretch affords a sensitive probe for understanding the interaction of His13-His14 with metal ions. The relative strength of the interaction of His13-His14 with the representative metal ions follows the order of K(+) < Ca(2+) < Zn(2+) < Cu(2+) < Fe(3+) < Al(3+), which is closely correlated with the available experimental results, providing a vivid physical picture about how metal ions bind to amyloid β-peptide. IR spectra of the [M·(His13-His14)](n+) complexes could be measured by infrared photodissociation spectroscopic technique and thus afford useful information for the understanding of structure-function relationship and the design of suitable drugs.

Temperature-Dependent Infrared Photodissociation Spectroscopy of (CO2)3+ Cation

Infrared photodissociation spectra of He-buffer-gas-cooled (CO2)3+ were measured at ion trap temperatures of 15, 50, 150, and 280 K. Electronic structure calculations at the mPW2PLYPD/aug-cc-pVDZ level were performed to identify the structures of the low-lying isomers and to assign the observed spectral features. The experimental and calculated infrared spectra show that the (CO2)3+ cations formed in the source are primarily dominated by the charge partially delocalized C2O4+ motif, in which the positive charge is partially delocalized over the two CO2 molecules. Thermal heating at elevated internal temperature supplies sufficient energy to overcome the isomerization barriers and gives access to the charge completely delocalized (CO2) n+ ( n = 3) motif, in which the positive charge is almost completely delocalized over all of the constituent CO2 molecules.