Vibrational spectra are measured for Fe2+(CH4)n (n = 1–3) in the C–H stretching region (2650–3100 cm−1) using photofragment spectroscopy, by monitoring the loss of CH4. All of the spectra exhibit an intense peak corresponding to the symmetric C–H stretch around 2800 cm−1. The presence of a single peak suggests a nearly equivalent interaction between the iron dimer and the methane ligands. The peak becomes slightly blue shifted as the number of methane ligands increases. Density functional theory calculations, B3LYP and BPW91, are used to identify possible structures and predict the spectra. Results suggest that the methane(s) bind in a terminal configuration and the complexes are in the octet spin state.

Vibrational spectra are measured for Cu+(CH4)(Ar)2, Cu+(CH4)2(Ar), Cu+(CH4)n (n = 3–6), and Ag+(CH4)n (n = 1–6) in the C–H stretching region (2500–3100 cm–1) using photofragment spectroscopy. Spectra are obtained by monitoring loss of Ar or CH4. Interaction with the metal ion produces substantial red shifts in the C–H stretches of proximate hydrogens. The magnitude of the shift reflects the metal–methane distance and the coordination to the metal ion of the methane hydrogens (η2 or η3). The structures of the complexes are determined by comparing the measured spectra with spectra calculated for candidate geometries using the B3LYP and CAM-B3LYP density functionals with 6-311++G(3df,3pd) and aug-cc-pVTZ-PP basis sets. Because of the d10 electronic configuration of the metal ions, the complexes are expected to adopt symmetric structures, which is confirmed by the experiments. All of the complexes have η2 hydrogen coordination in the first shell, in accord with theoretical predictions; second-shell ligands sometimes show η3 hydrogen coordination. The vibrational spectrum of Cu+(CH4)(Ar)2 shows extensive structure due to Fermi resonance between the lowest-frequency C–H stretch and overtones of the H–C–H bends. The Cu+(CH4) cluster has a smaller red shift in the lowest-frequency C–H stretch than M+(CH4), M+ = Co+ (d8) and Ni+ (d9). Although all three ions have similar binding energies, the metal–ligand electrostatic interaction is largest for Cu+, while the contribution from covalent interactions is largest for Co+. The larger ionic radius of Ag+ leads to a larger metal–ligand distance and weaker interaction, resulting in substantially smaller red shifts than in the Cu+ complexes. The Cu+(CH4)2 and Ag+(CH4)2 clusters have symmetrical structures, with the methanes on opposite sides of the metal, while Cu+(CH4)3 and Ag+(CH4)3 adopt symmetrical, trigonal planar structures with all M–C distances equal. For Cu+(CH4)4, the tetrahedral structure dominates the observed spectrum, although a trigonal pyramidal structure may contribute; however, only the tetrahedral structure is observed for Ag+(CH4)4. The structures of Cu+(CH4)n and Ag+(CH4)n differ for clusters with n > 4. For copper complexes, these are primarily formed by adding outer-shell methane ligand(s) to the tetrahedral n = 4 core. The observed spectra of the larger Ag+ clusters are dominated by symmetrical structures in which all of the Ag–C distances are similar: Ag+(CH4)5 has a trigonal bipyramidal geometry and Ag+(CH4)6 is octahedral.

Vibrational spectra of M+(CH4)m(Ar)3–m and M+(CH4)n (M = Co, Ni; m = 1, 2; n = 3, 4) in the C–H stretching region (2500–3100 cm–1) are measured using photofragment spectroscopy, monitoring the loss of argon or methane. Interaction with the metal leads to large red shifts in the C–H stretches for proximate hydrogens. The extent of this shift is sensitive to the coordination (η2 vs η3) and to the metal–methane distance. The structures of the complexes are determined by comparing measured spectra with those calculated for candidate structures at the B3LYP/6-311++G(3df,3pd) level. Binding energies are also computed using the CAM-B3LYP functional. In all cases, CH4 shows η2 coordination to the metal. The m = 1 complexes show very large red shifts of 370 cm–1 (for M = Co) and 320 cm–1 (for M = Ni) in the lowest C–H stretch, relative to the symmetric stretch of free CH4. They adopt a C2v structure with the heavy atoms and proximate hydrogen atoms coplanar. The m = 2 complexes have slightly reduced red shifts, and Tee-shaped structures. Both Tee-shaped and equilateral (or quasi-equilateral) structures are observed for the n = 3 complexes. The measured photodissociation onset and significantly reduced intensity for low-frequency C–H stretches imply a value of 2650 ± 50 cm–1 for the binding energy of Ni+(CH4)2–CH4. The Co+(CH4)4 complexes have two low-lying structures, quasi-tetrahedral and distorted square-planar, which contribute to the rich spectrum. In contrast, the symmetrical, square-planar Ni+(CH4)4 complex is characterized by a very simple vibrational spectrum.

The microsolvation of cobalt and nickel dications by acetonitrile and water is studied by measuring photofragment spectra at 355, 532 and 560–660 nm. Ions are produced by electrospray, thermalized in an ion trap and mass selected by time of flight. The photodissociation yield, products and their branching ratios depend on the metal, cluster size and composition. Proton transfer is only observed in water-containing clusters and is enhanced with increasing water content. Also, nickel-containing clusters are more likely to undergo charge reduction than those with cobalt. The homogeneous clusters with acetonitrile M2+(CH3CN)n (n = 3 and 4) dissociate by simple solvent loss; n = 2 clusters dissociate by electron transfer. Mixed acetonitrile/water clusters display more interesting dissociation dynamics. Again, larger clusters (n = 3 and 4) show simple solvent loss. Water loss is substantially favored over acetonitrile loss, which is understandable because acetonitrile is a stronger ligand due to its higher dipole moment and polarizability. Proton transfer, forming H+(CH3CN), is observed as a minor channel for M2+(CH3CN)2(H2O)2 and M2+(CH3CN)2(H2O) but is not seen in M2+(CH3CN)3(H2O). Studies of deuterated clusters confirm that water acts as the proton donor. We previously observed proton loss as the major channel for photolysis of M2+(H2O)4. Measurements of the photodissociation yield reveal that four-coordinate Co2+ clusters dissociate more readily than Ni2+ clusters whereas for the three-coordinate clusters, dissociation is more efficient for Ni2+ clusters. For the two-coordinate clusters, dissociation is viaelectron transfer and the yield is low for both metals. Calculations of reaction energetics, dissociation barriers, and the positions of excited electronic states complement the experimental work. Proton transfer in photolysis of Co2+(CH3CN)2(H2O) is calculated to occur via a (CH3CN)Co2+–OH−–H+(NCCH3) salt-bridge transition state, reducing kinetic energy release in the dissociation.

Gas-phase Ag+(CH3OH) molecules are produced in a laser ablation source, cooled in a supersonic expansion and their vibrational spectrum is measured by photofragment spectroscopy in a reflectron time-of-flight mass spectrometer. Three techniques – infrared multiple photon dissociation (IRMPD), argon-tagging, and infrared laser-assisted photodissociation spectroscopy (IRLAPS) – are used to measure the vibrational spectra of ions produced under identical conditions. The sharpest spectrum is obtained using IRLAPS, a two-color scheme in which a tunable OPO/OPA infrared laser excites the O–H stretch and a TEA-CO2 laser dissociates the vibrationally excited ions via absorbing multiple C–O stretch photons. The O–H stretch is observed at 3660 cm−1. Monitoring loss of argon from Ag+(CH3OH)(Ar) gives a slightly broader peak, with no significant shift. The vibrational spectrum obtained using IRMPD is shifted to 3635 cm−1, is substantially broader, and is asymmetrical, tailing to the red. Analysis of the experimental results is aided by comparison with hybrid density functional theory computed harmonic and anharmonic frequencies.Three techniques – IRMPD, argon-tagging, and infrared laser-assisted photodissociation spectroscopy (IRLAPS) – are used to measure the vibrational spectra of Ag+(CH3OH) produced under identical conditions.

Gas phase FeO+ can convert methane to methanol under thermal conditions. Two key intermediates of this reaction are the [HO−Fe−CH3]+ insertion intermediate and Fe+(CH3OH) exit channel complex. These intermediates are selectively formed by reaction of laser-ablated Fe+ with organic precursors under specific source conditions and are cooled in a supersonic expansion. Vibrational spectra of the sextet and quartet states of the intermediates in the O−H and C−H stretching regions are measured by infrared multiple photon dissociation of Fe+(CH3OH) and [HO−Fe−CH3]+ and by monitoring argon atom loss following irradiation of Fe+(CH3OH)(Ar) and [HO−Fe−CH3]+(Ar)n (n = 1, 2). Analysis of the experimental results is aided by comparison with hybrid density functional theory computed frequencies. Also, an improved potential energy surface for the FeO+ + CH4 reaction is developed based on CCSD(T) and CBS-QB3 calculations for the reactants, intermediates, transition states, and products.

Gas-phase FeO+ can convert benzene to phenol under thermal conditions. Two key intermediates of this reaction are the [HO-Fe-C6H5]+ insertion intermediate and Fe+(C6H5OH) exit channel complex. These intermediates are selectively formed by reaction of laser ablated Fe+ with specific organic precursors and are cooled in a supersonic expansion. Vibrational spectra of the sextet and quartet states of the intermediates in the O-H stretching region are measured by infrared multiphoton dissociation (IRMPD). For Fe+(C6H5OH), the O-H stretch is observed at 3598 cm−1. Photodissociation primarily produces Fe++C6H5OH; Fe+(C6H4)+H2O is also observed. IRMPD of [HO-Fe-C6H5]+ mainly produces FeOH++C6H5 and the O-H stretch spectrum consists of a peak at ∼3700 cm−1 with a shoulder at ∼3670 cm−1. Analysis of the experimental results is aided by comparison with hybrid density functional theory computed frequencies. Also, an improved potential energy surface for the FeO++C6H6 reaction is developed based on CBS-QB3 calculations for the reactants, intermediates, transition states, and products.

Vibrational spectra are measured for Fe+(CH4)n (n = 1−4) in the C−H stretching region (2500−3200 cm−1) using photofragment spectroscopy. Spectra are obtained by monitoring CH4 fragment loss following absorption of one photon (for n = 3, 4) or sequential absorption of multiple photons (for n = 1, 2). The spectra have a band near the position of the antisymmetric C−H stretch in isolated methane (3019 cm−1), along with bands extending >250 cm−1 to the red of the symmetric C−H stretch in methane (2917 cm−1). The spectra are sensitive to the ligand configuration (η2 vs η3) and to the Fe−C distance. Hybrid density functional theory calculations are used to identify possible structures and predict their vibrational spectra. The IR photodissociation spectrum shows that the Fe+(CH4) complex is a quartet, with an η3 configuration. There is also a small contribution to the spectrum from the metastable sextet η3 complex. The Fe+(CH4)2 complex is also a quartet with both CH4 in an η3 configuration. For the larger clusters, the configuration switches from η3 to η2. In Fe+(CH4)3, the methane ligands are not equivalent. Rather, there is one short and two long Fe−C bonds, and each methane is bound to the metal in an η2 configuration. For Fe+(CH4)4, the calculations predict three low-lying structures, all with η2 binding of methane and very similar Fe−C bond lengths. No single structure reproduces the observed spectrum. The approximately tetrahedral C1 (4A) structure contributes to the spectrum; the nearly square-planar D2d (4B2) and the approximately tetrahedral C2 (4A) structure may contribute as well.

Gas-phase FeO+ can convert benzene to phenol under thermal conditions. Two key intermediates of this reaction are the [HO-Fe-C6H5]+ insertion intermediate and Fe+(C6H5OH) exit channel complex. These intermediates are selectively formed by reaction of laser ablated Fe+ with specific organic precursors and are cooled in a supersonic expansion. Vibrational spectra of the sextet and quartet states of the intermediates in the O–H stretching region are measured by infrared multiphoton dissociation (IRMPD). For Fe+(C6H5OH), the O–H stretch is observed at 3598 cm−1. Photodissociation primarily produces Fe+ + C6H5OH; Fe+(C6H4) + H2O is also observed. IRMPD of [HO-Fe-C6H5]+ mainly produces FeOH+ + C6H5 and the O–H stretch spectrum consists of a peak at ∼3700 cm−1 with a shoulder at ∼3670 cm−1. Analysis of the experimental results is aided by comparison with hybrid density functional theory computed frequencies. Also, an improved potential energy surface for the FeO+ + C6H6 reaction is developed based on CBS-QB3 calculations for the reactants, intermediates, transition states, and products.FeO+ converts benzene to phenol. Vibrational spectra of [HO-Fe-C6H5]+ and Fe+(C6H5OH) intermediates in the O–H stretching region are measured by IRMPD, monitoring Fe+ and FeOH+.Download high-res image (94KB)Download full-size image

The microsolvation of cobalt and nickel dications by acetonitrile and water is studied by measuring photofragment spectra at 355, 532 and 560–660 nm. Ions are produced by electrospray, thermalized in an ion trap and mass selected by time of flight. The photodissociation yield, products and their branching ratios depend on the metal, cluster size and composition. Proton transfer is only observed in water-containing clusters and is enhanced with increasing water content. Also, nickel-containing clusters are more likely to undergo charge reduction than those with cobalt. The homogeneous clusters with acetonitrile M2+(CH3CN)n (n = 3 and 4) dissociate by simple solvent loss; n = 2 clusters dissociate by electron transfer. Mixed acetonitrile/water clusters display more interesting dissociation dynamics. Again, larger clusters (n = 3 and 4) show simple solvent loss. Water loss is substantially favored over acetonitrile loss, which is understandable because acetonitrile is a stronger ligand due to its higher dipole moment and polarizability. Proton transfer, forming H+(CH3CN), is observed as a minor channel for M2+(CH3CN)2(H2O)2 and M2+(CH3CN)2(H2O) but is not seen in M2+(CH3CN)3(H2O). Studies of deuterated clusters confirm that water acts as the proton donor. We previously observed proton loss as the major channel for photolysis of M2+(H2O)4. Measurements of the photodissociation yield reveal that four-coordinate Co2+ clusters dissociate more readily than Ni2+ clusters whereas for the three-coordinate clusters, dissociation is more efficient for Ni2+ clusters. For the two-coordinate clusters, dissociation is viaelectron transfer and the yield is low for both metals. Calculations of reaction energetics, dissociation barriers, and the positions of excited electronic states complement the experimental work. Proton transfer in photolysis of Co2+(CH3CN)2(H2O) is calculated to occur via a (CH3CN)Co2+–OH−–H+(NCCH3) salt-bridge transition state, reducing kinetic energy release in the dissociation.

Vibrational spectra are measured for Fe2+(CH4)n (n = 1–3) in the C–H stretching region (2650–3100 cm−1) using photofragment spectroscopy, by monitoring the loss of CH4. All of the spectra exhibit an intense peak corresponding to the symmetric C–H stretch around 2800 cm−1. The presence of a single peak suggests a nearly equivalent interaction between the iron dimer and the methane ligands. The peak becomes slightly blue shifted as the number of methane ligands increases. Density functional theory calculations, B3LYP and BPW91, are used to identify possible structures and predict the spectra. Results suggest that the methane(s) bind in a terminal configuration and the complexes are in the octet spin state.