Photoelectron velocity map imaging spectroscopy of group 14 elements and iron tetracarbonyl anionic clusters MFe(CO)4- (M = Si, Ge, Sn).

The heteronuclear group 14 M-iron tetracarbonyl clusters MFe(CO)4- (M = Si, Ge, Sn) anions have been generated in the gas phase by laser ablation of M-Fe alloys and detected by mass and photoelectron spectroscopy. With the support of quantum chemical calculations, the geometric and electronic structures of MFe(CO)4- (M = Si, Ge, Sn) are elucidated, which shows that all the MFe(CO)4- clusters have the M-Fe bonded, iron-centered, and carbonyl-terminal M-Fe(CO)4 structure with the C2v symmetry and a 2B2 ground state. The M-Fe bond can be considered a double bond, which includes one σ electron sharing bond and one π dative bond. The C-O bonds in those anionic clusters are calculated to be elongated to different extents, and in particular, the C-O bonds in SiFe(CO)4- are elongated more. The Si-Fe alloy thus turns out to be a better collocation to activate the C-O bonds in the gas phase among group 14. The present findings have important implications for the rational development of high-performance catalysts with isolated metal atoms/clusters dispersed on supports.

Copper-Catalyzed, Regioselective, Unsymmetrical Homocoupling of Quinoxalin-2(1H)-ones to Form C-N Homodimers.

An environmentally benign and efficient method for the synthesis of unsymmetrical diquinoxalin-2(1H)-ones with potential axial chirality via inexpensive copper-catalyzed, low-toxicity, and stable PIFA oxidation, rarely assisted by PhSeSePh, regioselective homocoupling of quinoxalin-2(1H)-ones under mild conditions is developed. This practical scheme is compatible with a variety of functional groups and allows the preparation of functionalized unsymmetrical dimeric quinoxalin-2(1H)-ones from readily available and safe starting materials, providing new ideas for the sustainable development of methodological studies of quinoxalin-2(1H)-ones.

Electrospray ionization photoelectron spectroscopy of cryogenic [EDTA·M(ii)]2- complexes (M = Ca, V-Zn): electronic structures and intrinsic redox properties.

We report here a systematic photoelectron spectroscopy (PES) and theoretical study of divalent transition metal (TM) EDTA complexes [EDTA·TM(ii)]2- (TM = V-Zn), along with the Ca(ii) species for comparison. Gaseous TM dianions (TM = Ca, Mn, Co, Ni, Cu and Zn) were successfully generated via electrospray ionization, and their PE spectra, with 157, 193, and 266 nm photons, were obtained at 20 K. The spectrum of each TM complex shows an extra peak at the lowest electron binding energy (eBE), compared to that of [EDTA·Ca(ii)]2-. DFT calculations indicate a hexacoordinated metal-EDTA binding motif for all complexes, from which the vertical detachment energies (VDEs) are calculated and these agree well with the experimental values. The calculations further predict negative or very small VDEs for TM(ii) = V, Cr, and Fe, providing a rational explanation for why these three dianionic species are not observed in the gas phase. Direct spectral comparison, electron spin density differences, and MO analyses indicate that the least bound electrons are derived from TM d electrons with appreciable ligand contributions, in contrast to [EDTA·Ca(ii)]2-, in which the detachment is entirely derived from the ligand. The extent of ligand modulation, i.e. non-innocence of EDTA ligands in the oxidation process, is found to vary across the 3rd row of TMs. Comparing the gas-phase VDEs of [EDTA·TM(ii)]2- with the 3rd ionization potentials of TMs and solution phase oxidation potentials reveals intrinsic correlations among these three quantities, with deviations being largely modulated by the ligand participation. The detailed microscopic information about the intrinsic electronic structures and bonding motifs of these complexes obtained in this work will help better understand the rich redox chemistries of these ubiquitous species under diverse environments. The present work, along with our previous studies, indicates that PES coupled with electrospray ionization is a unique ion spectroscopic tool that not only provides intrinsic electronic structure and bonding information about redox species, but also can predict the related electron transfer chemistries with quantitative capability.

Structures and Infrared Spectra of [M(CO2)7]+ (M = V, Cr, and Mn) Complexes.

Gas-phase infrared photodissociation spectra of [V(CO2) n]+ complexes revealed three new vibrational bands at 1140, 1800, and 3008 cm-1 at n = 7, the features of which are retained in the larger clusters (Ricks, A. M.; Brathwaite, A. D.; Duncan, M. A. J. Phys. Chem. A 2013, 117, 11490-11498). However, structural assignment of this intriguing feature remains open. Herein, quantum chemical calculations on [V(CO2)7]+ were carried out to identify the structure of the low-lying isomers and to assign the observed spectral features. The comparison of calculated infrared spectra of [V(CO2)7]+ with experimental infrared spectra identified the formation of a bent CO2- species, suggesting the ligand-induced activation of CO2 by the vanadium cation. The structures and infrared spectra of [Cr(CO2)7]+ and [Mn(CO2)7]+ were also predicted and discussed.

Infrared Spectroscopy of Hydrogen-Bonding Interactions in Neutral Dimethylamine-Methanol Complexes.

Infrared spectra of the neutral dimethylamine-methanol cluster, DMA-CH3OH, were measured in the spectral range of 2800-3900 cm-1 using an infrared-vacuum ultraviolet (IR-VUV) scheme. Quantum chemical calculations and ab initio molecular dynamic (AIMD) simulations were carried out to understand the experimental spectral features. Experimental and theoretical results reveal the coexistence of N···HO and O···HN hydrogen-bonded structures. AIMD simulations show that the methyl group in methanol internally rotates around the N···O axis, addressing the dynamic effect of the fluctuation of hydrogen bonds on the vibrational features. The bonding analysis was performed to elucidate the nature of the intermolecular interaction between DMA and CH3OH. The present work provides the fundamental understanding of hydrogen-bonding networks in the amine-alcohol complexes.

Infrared spectra of neutral dimethylamine clusters: An infrared-vacuum ultraviolet spectroscopic and anharmonic vibrational calculation study.

Infrared-vacuum ultraviolet (IR-VUV) spectra of neutral dimethylamine clusters, (DMA)n (n = 2-5), were measured in the spectral range of 2600-3700 cm-1. The experimental IR-VUV spectra show NH stretch modes gradually redshift to 3200-3250 cm-1 with the increase in the cluster size and complex Fermi Resonance (FR) pattern of the CH3 group in the 2800-3000 cm-1 region. Ab initio anharmonic vibrational calculations were performed on low-energy conformers of (DMA)2 and (DMA)3 to examine vibrational coupling among CH/NH and to understand the Fermi resonance pattern in the observed spectra features. We found that the redshift of NH stretching mode with the size of DMA cluster is moderate, and the overtone of NH bending modes is expected to overlap in frequency with the CH stretching fundamental modes. The FR in CH3 groups is originated from the strong coupling between CH stretching fundamental and bending overtone within a CH3 group. Well-resolved experimental spectra also enable us to compare the performance of ab initio anharmonic algorithms at different levels.

Solvation effects on the N-O and O-H stretching modes in hydrated NO3-(H2O)n clusters.

NO3-(H2O)n clusters are a molecular model used to understand the solvation interaction between water and nitrate, an important anion in nature, industrial processes and biology. We demonstrate by ab initio molecular dynamics simulations that among the many isomeric structures at each cluster size with n = 1-6, thermal stability is an important consideration. The vibrational profile at a particular size, probed previously by infrared multiple photon dissociation (IRMPD) spectroscopy, can be accounted for by the isomers, which are both energetically and dynamically stable. Conversion and broadening due to the fluctuation of hydrogen bonds are important not only for the O-H stretching modes but also for the N-O stretching modes. Distinct patterns for the O-H stretching modes are predicted for the various solvation motifs. We also predict a surface structure for NO3-(H2O)n as n increases beyond 6, which can be verified by an early onset of strong libration bands for H2O in the IRMPD spectra and a flattening of the vertical detachment energy in the photoelectron spectra.

Solvation effects on the vibrational modes in hydrated bicarbonate clusters.

HCO3-(H2O)n clusters provide a model system to understand the solvation interaction between the bicarbonate ion and water. Based on harmonic analysis, ab initio molecular dynamics simulations, and comparison with infrared multiple photon dissociation spectra and with previous results on H2PO4-(H2O)n, the solvation effects on the vibrational modes of HCO3-(H2O)n are analyzed. Hydrogen bond interactions have a significant impact on the vibration, especially when a hydrogen atom is directly involved in a particular mode. The COH bending mode is flattened, when the COH group is solvated by water molecules. The emergence of broad water libration modes indicates the aggregation of water molecules and the formation of a surface structure with bicarbonate on the surface.

Photoelectron spectroscopic and computational studies of [EDTA·M(iii)]- complexes (M = H3, Al, Sc, V-Co).

Metal-EDTA complexes commonly exist as biological redox reagents. We have generated a series of such complexes, [EDTA·M(iii)]- (M = Al, Sc, V-Co), via electrospray ionization and characterized them by cryogenic mass-selected negative ion photoelectron spectroscopy (NIPES) and quantum chemical computations. Experiments clearly revealed one more spectral band at low electron binding energy for transition metal complexes with d electrons (M = V-Co) compared to those without d electrons (M = Al and Sc). Quantum chemical calculations suggested that all of the metal complexes possess hexacoordinated metal-ligand binding motifs, from which the calculated adiabatic/vertical detachment energy (ADE/VDE) and band gaps are in good agreement with experimental values. Direct spectrum and electronic structure analyses indicted that [EDTA·V(iii)]- can be easily oxidized to [EDTA·V(iv)] with the smallest ADE/VDE of 3.95/4.40 eV among these metal complexes, but further oxidation is hindered by the existence of a 2.30 eV band gap, a fact that accords with the special redox behavior of vanadium-containing species in biological cells. Spin density and molecular orbital analyses reveal that [EDTA·V(iii)]- was overwhelmingly detached from the vanadium atom, in stark contrast to [EDTA·Sc(iii)/Al(iii)]-, where the detachment occurred from the EDTA ligand. For all other metal complex anions, from M = Cr to Co, the detachment process is derived from contributions from both the metal and ligand. The intrinsic electronic and geometric structures of these complexes, obtained in this work, provide a molecular foundation to better understand their redox chemistries and specific metal bindings in condensed phases and biological cells.

Infrared photodissociation spectroscopy of ion-radical networks in cationic dimethylamine complexes.

Infrared photodissociation spectroscopy was employed to establish the general trends in the stepwise growth motif of cationic dimethylamine (DMA)n+ (n = 4-13) complexes. Electronic structure calculations were performed to identify the structure of the low-lying isomers and to assign the observed spectral features. The results showed the preference of the formation of the proton-transferred (CH3)2NH2+ ion core. The (CH3)2NH2+-[(CH3)2N] ion-radical pair contact and the ion-radical separated pair could coexist at n = 4. The [(CH3)2N] radical is separated from the (CH3)2NH2+ ion core by one DMA molecule at n = 4-6 and by two or more DMA molecules in the larger clusters. This suggests that the (CH3)2NH2+-[(CH3)2N] ion-radical contact pair is not stable in the subsequent radiation-induced processes of DMA, and the [(CH3)2N] radical is released from the charged site in the cationic DMA networks.

Infrared photodissociation spectroscopy of cold cationic trimethylamine complexes.

Cryogenic ion-trap infrared photodissociation spectroscopy combined with a dielectric barrier discharge source was constructed to establish the general trends in the stepwise growth motif of trimethylamine (TMA)n+ complexes. The results showed a strong preference for the formation of a stable charge-shared NN type (TMA)2+ ion core over the proton-transferred CHN type ion core, evidencing that the source condition has a remarkable effect on the kinetic stability of isomers. A maximum of four TMA molecules are located perpendicularly to the NN axis of the charge-shared (TMA)2+ ion core. In the n = 7 and 8 clusters, the subsequent two TMA molecules are located at each end of the NN axis of the (TMA)2+ ion core, completing the first coordination shell. Starting at n = 9, the additional TMA molecules form a second solvation shell, and the cluster spectra show similarities to the solution phase spectrum of aqueous TMA.

Coordination-induced CO2 fixation into carbonate by metal oxides.

Here, we have investigated how coordination induces CO2 fixation into a carbonate using a cationic yttrium oxide model catalyst. The infrared spectra show that the first three CO2 molecules are weakly bound to the metal. Subsequent coordination of CO2 ligands leads to the formation of a carbonate complex and results in a core ion transition. The conversion of Y = O and CO2 to carbonate is achieved by the donation of electrons from the ligands to the metal. Systematic analyses of the effects of different ligands and metals on the coordination-induced CO2 fixation demonstrate that the present system serves as an efficient and rational model for adjusting CO2 fixation and CO2 emission.

Ligand-Enhanced CO Activation by the Early Lanthanide-Nickel Heterodimers: Photoelectron Velocity-Map Imaging Spectroscopy of LnNi(CO) n- (Ln = La, Ce).

Heterobimetallic lanthanum-nickel and cerium-nickel carbonyls, LnNi(CO) n- (Ln = La, Ce; n = 2-5), were generated using a pulsed laser vaporization/supersonic expansion ion source. These compounds were characterized by photoelectron velocity-map imaging spectroscopy and quantum chemical calculations. The binding motif in the most stable isomers of the n = 2 and 3 clusters consists of one side-on-bonded carbonyl. A new building block of two side-on-bonded carbonyls is favored at n = 4, which is retained at n = 5, evidencing the increase of the number of extremely activated CO molecule in the larger clusters. The experimental and theoretical results demonstrate the ligand-enhanced CO activation by the early lanthanide-nickel heterodimers, which would have important implications for the design of alloy catalysts for activation of a molecular ligand.

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.

Photoelectron Velocity-Map Imaging and Theoretical Studies of Heterotrinuclear Metal Carbonyls V2Ni(CO)n- (n = 6-10).

Photoelectron velocity-map imaging spectroscopy was conducted for the heterotrinuclear metal carbonyls V2Ni(CO)n- (n = 6-10). Electronic structure calculations were performed to understand the experimental spectral features. The binding motif of a V-V-Ni chain with two side-on-bonded carbonyls and two bridging carbonyls is favored in the n = 6-9 clusters. A V2Ni triangle core structure is formed at n = 10 with the involvement of two carbonyls with the carbon atom triply coordinated to metal atoms, three bridging carbonyls, and five terminal carbonyls, in which CO bonding configurations mirror the adsorption features in the three-fold hollow, bridging, and atop sites on the closely packed surface, respectively. The present study provides a stepwise picture for molecular level understanding of CO bonding on heteronuclear metal clusters, which is directly relevant to the elementary processes of CO on the alloy surfaces/interfaces.

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.

Probing the early stages of solvation of cis-pinate dianions by water, acetonitrile, and methanol: a photoelectron spectroscopy and theoretical study.

cis-Pinic acid is one of the most important oxidation products of α-pinene--a key monoterpene compound in biogenic emission processes. Molecular level understanding of its interaction with water in cluster formation is an important and necessary prerequisite for ascertaining its role in the aerosol formation processes. In this work, we studied the structures and energetics of the solvated clusters of cis-pinate (cis-PA(2-)), the doubly deprotonated dicarboxylate of cis-pinic acid, with H2O, CH3OH, and CH3CN by negative ion photoelectron spectroscopy and ab initio theoretical calculations. We found that cis-PA(2-) prefers being solvated alternately on the two -CO2(-) groups with increase of solvent coverage, a well-known solvation pattern that has been observed in microhydrated linear dicarboxylate dianion (DCn(2-)) clusters. Experiments and calculations further reveal an intriguing feature for the existence of the asymmetric type isomers for cis-PA(2-)(H2O)2 and cis-PA(2-)(CH3OH)2, in which both solvent molecules interact with only one of the -CO2(-) groups, a phenomenon that has not been observed in DCn(2-) water clusters and exhibits that the subtle effect of the rigid four-membered carbon ring brought on the cis-PA(2-) solvation. The dominant interactions between cis-PA(2-) and solvent molecules form bidentate O(-)...H-O H-bonds for H2O, O(-)...H-O and O(-)...H-C H-bonds for CH3OH, and tridentate O(-)...H-C H-bonds for CH3CN. The formation of inter-solvent H-bonds between H2O and CH3CN is found to be favorable in mixed solvent clusters, different from that between H2O and CH3OH. These findings have important implications for understanding the mechanism of cluster growth and the formation of atmospheric organic aerosols, as well as for rationalizing the nature of structure-function relationship of proteins containing carboxylate groups in various solvent environments.