Photodissociation and Infrared Spectroscopy of Uranium-Nitrogen Cation Complexes.

Laser vaporization of uranium in a pulsed supersonic expansion of nitrogen is used to produce complexes of the form U+(N2)n (n = 1-8). These ions are mass selected in a reflectron time-of-flight spectrometer and studied with visible and UV laser fixed-frequency photodissociation and with tunable infrared laser photodissociation spectroscopy. The dissociation patterns and spectroscopy of U+(N2)n indicate that N2 ligands are intact molecules and that there is no insertion chemistry resulting in UN+ or NUN+. Fixed frequency photodissociation at 532 and 355 nm indicate that the U+-N2 bond dissociation energy varies little with changing coordination. The photon energy and the number of ligands eliminated allow an estimate of the average U+-N2 dissociation energy of 12 kcal/mol. Infrared bands are observed for these complexes near the N-N stretch vibration via elimination of N2 molecules. These resonances are observed to be shifted about 130 cm-1 to the red from the free-N2 frequency for complexes with n = 3-8. Density functional theory indicates that U+ is most stable in the sextet state in these complexes and that N2 molecules bind in end-on configurations. The fully coordinated complex is predicted to be U+(N2)8, which has a cubic structure. The vibrational frequencies predicted by theory are consistently lower than those in the experiment, independent of the isomeric structure or spin state of the complexes. Despite its failure to reproduce the infrared spectra, theory provides an average U+-N2 dissociation energy of 11.8 ± 0.5 kcal/mol, in good agreement with the value from the experiments.

Infrared spectroscopy and anharmonic theory of H3 +Ar2,3 complexes: The role of symmetry in solvation.

The vibrational spectra of H3 +Ar2,3 and D3 +Ar2,3 are investigated in the 2000 cm-1 to 4500 cm-1 region through a combination of mass-selected infrared laser photodissociation spectroscopy and computational work including the effects of anharmonicity. In the reduced symmetry of the di-argon complex, vibrational activity is detected in the regions of both the symmetric and antisymmetric hydrogen stretching modes of H3 +. The tri-argon complex restores the D3h symmetry of the H3 + ion, with a concomitant reduction in the vibrational activity that is limited to the region of the antisymmetric stretch. Throughout these spectra, additional bands are detected beyond those predicted with harmonic vibrational theory. Anharmonic theory is able to reproduce some of the additional bands, with varying degrees of success.

Photodissociation of Manganese Oxide Cluster Cations.

Manganese oxide cluster cations are produced by laser vaporization in a pulsed nozzle source and detected with time-of-flight mass spectrometry. The mass spectrum contains intense peaks for stoichiometries corresponding to (MnO) n+. Multiphoton photodissociation of these clusters yields smaller ions with the same stoichiometric ratio, either by sequential elimination of MnO units or by various fission processes with roughly equal efficiencies. Fragmentation of clusters containing excess oxygen also yields (MnO) n+ fragments. These apparently stable fragments are investigated further using density functional theory to determine their likely structures. The lowest energy structure for Mn2O2+ is found to be a planar ring, and those for Mn4O4+ and Mn6O6+ are cuboids. Mn3O3+ is predicted to have a six-membered ring structure and Mn5O5+ has a fused cube/ring configuration similar to the structure of the oxygen evolving center of Photosystem II. Open-shell, high-spin configurations on individual manganese atoms couple antiferromagnetically and ferromagnetically to produce low-spin and high-spin configurations on different sized clusters.

Laser Synthesis and Spectroscopy of Ligand-Coated Chromium Oxide Nanoclusters.

Ultra-small chromium oxide nanoclusters produced by laser vaporization in a fast flow tube reactor are ligand-coated by gas-phase reactions with acetonitrile vapor and then captured in a cold trap and transferred to solution. The resulting clusters are characterized with mass spectrometry, UV-visible absorption and emission spectroscopy, infrared spectroscopy, and surface-enhanced Raman spectroscopy. According to mass spectrometry, clusters of the form Cr xO y(MeCN) z are produced in the size range of x ≤ 10 and y < 25. The ligand-coated clusters in solution exhibit a limited number of prominent sizes, with the same preferences for specific stoichiometries seen in earlier gas-phase studies of ligand-free clusters. Computational studies provide structures and predicted spectra for these systems. The intrinsic stability of these clusters is confirmed by their production under different laser ablation conditions and by their significant shelf lives (several months) without aggregation or decomposition. UV-visible spectra indicate that these clusters contain highly oxidized chromium. Theory and previous experiments indicate that compact cages are favored for ligand-free clusters. However, infrared and Raman spectra suggest that ring and chainlike structures become prominent for ligand-coated clusters. Consistent with these observations, theory also indicates that these more open structures are energetically favored for ligated clusters. Apparently, ligand binding induces a structural transformation of the compact oxide core clusters, producing more extended ring and chain structures.

Ortho-para interconversion in cation-water complexes: The case of V+(H2O) and Nb+(H2O) clusters.

Vanadium and niobium cation-water complexes, V+(H2O) and Nb+(H2O), are produced by laser vaporization in a pulsed supersonic expansion, mass selected in a time-of-flight spectrometer, and studied with infrared photodissociation spectroscopy using rare gas atom (Ar, Ne) complex predissociation. The vibrational bands measured in the O-H stretching region contain K-type rotational sub-band structure, which provides insight into the structures of these complexes. However, rotational sub-bands do not exhibit the simple patterns seen previously for other metal ion-water complexes. The A rotational constants are smaller than expected and the normal 3:1 intensity ratios for K = odd:even levels for independent ortho:para nuclear spin states are missing for some complexes. We relied on highly correlated internally contracted multi-reference configuration interaction and Coupled Cluster [CCSD(T)] electronic structure calculations of those complexes with and without the rare gas atoms to investigate these anomalies. Rare gas atoms were found to bind via asymmetric motifs to the hydrated complexes undergoing large amplitude motions that vibrationally average to the quasi-C2v symmetry with a significant probability off the C2 axis, thus explaining the reduced A values. Both vanadium and niobium cations exhibit unusually strong nuclear spin coupling to the hydrogen atoms of water, the values of which vary with their electronic state. This catalyzes ortho-para interconversion in some complexes and explains the rotational patterns. The rate of ortho-para relaxation in the equilibrated complexes must therefore be greater than the collisional cooling rate in the supersonic expansion (about 106 s-1).

Distinctive Coordination of CO vs N2 to Rhodium Cations: An Infrared and Computational Study.

Carbonyl and nitrogen complexes with Rh+ are produced in a molecular beam using laser ablation and a pulsed-nozzle source. Mass-selected ions of the form Rh(CO)n+ and Rh(N2)n+ are investigated via infrared laser photodissociation spectroscopy. The fragmentation patterns and infrared spectra provide information on the coordination and geometries of these complexes. The shifts in vibrational frequencies relative to the uncoordinated ligands give insight into the nature of the bonding interactions involved. Experimental band positions and intensities are compared to those predicted by density functional theory (DFT). Rh+ coordinates only four nitrogen molecules, whereas it can accommodate five carbonyl ligands. The fifth CO ligand resides in an axial site with bonding intermediate between coordination and solvation. The carbonyl stretch in Rh(CO)4+ (2160 cm-1) is blue-shifted with respect to the molecular CO vibration (2143 cm-1). Conversely, the N-N stretch in Rh(N2)4+ (2297 cm-1) is red-shifted with respect to the free N2 vibration (2330 cm-1). The opposite directions of these frequency shifts is explained by a combination of σ donation and electrostatic ligand polarization.

Photodissociation of Cerium Oxide Nanocluster Cations.

Cerium oxide cluster cations, CexOy(+), are produced via laser vaporization in a pulsed nozzle source and detected with time-of-flight mass spectrometry. The mass spectrum displays a strongly preferred oxide stoichiometry for each cluster with a specific number of metal atoms x, with x ≤ y. Specifically, the most prominent clusters correspond to the formula CeO(CeO2)n(+). The cluster cations are mass selected and photodissociated with a Nd:YAG laser at either 532 or 355 nm. The prominent clusters dissociate to produce smaller species also having a similar CeO(CeO2)n(+) formula, always with apparent leaving groups of (CeO2). The production of CeO(CeO2)n(+) from the dissociation of many cluster sizes establishes the relative stability of these clusters. Furthermore, the consistent loss of neutral CeO2 shows that the smallest neutral clusters adopt the same oxidation state (IV) as the most common form of bulk cerium oxide. Clusters with higher oxygen content than the CeO(CeO2)n(+) masses are present with much lower abundance. These species dissociate by the loss of O2, leaving surviving clusters with the CeO(CeO2)n(+) formula. Density functional theory calculations on these clusters suggest structures composed of stable CeO(CeO2)n(+) cores with excess oxygen bound to the surface as a superoxide unit (O2(-)).

Infrared spectroscopy and theory of the formaldehyde cation and its hydroxymethylene isomer.

Pulsed discharges in supersonic expansions containing the vapor of different precursors (formaldehyde, methanol) produce the m/z = 30 cations with formula [H2,C,O]+. The corresponding [H2,C,O]+ Ar complexes are produced under similar conditions with argon added to the expansion gas. These ions are mass selected in a time-of-flight spectrometer and studied with infrared laser photodissociation spectroscopy. Spectra in the 2300-3000 cm-1 region produce very different vibrational patterns for the ions made from different precursors. Computational studies with harmonic methods and various forms of anharmonic theory allow detailed assignment of these spectra to two isomeric species. Discharges containing formaldehyde produce primarily the corresponding formaldehyde radical cation, CH2O+, whereas those with methanol produce exclusively the cis- and trans-hydroxymethylene cations, HCOH+. The implications for the interstellar chemistry of these cations are discussed.

Communication: Trapping a proton in argon: Spectroscopy and theory of the proton-bound argon dimer and its solvation.

Ion-molecule complexes of the form H+Arn are produced in pulsed-discharge supersonic expansions containing hydrogen and argon. These ions are analyzed and mass-selected in a reflectron spectrometer and studied with infrared laser photodissociation spectroscopy. Infrared spectra for the n = 3-7 complexes are characterized by a series of strong bands in the 900-2200 cm-1 region. Computational studies at the MP2/aug-cc-pVTZ level examine the structures, binding energies, and infrared spectra for these systems. The core ion responsible for the infrared bands is the proton-bound argon dimer, Ar-H+-Ar, which is progressively solvated by the excess argon. Anharmonic vibrational theory is able to reproduce the vibrational structure, identifying it as arising from the asymmetric proton stretch in combination with multiple quanta of the symmetric argon stretch. Successive addition of argon shifts the proton vibration to lower frequencies, as the charge is delocalized over more ligands. The Ar-H+-Ar core ion has a first solvation sphere of five argons.

Infrared spectroscopy of the methanol cation and its methylene-oxonium isomer.

The carbenium ion with nominal formula [C,H4,O](+) is produced from methanol or ethylene glycol in a pulsed-discharge supersonic expansion source. The ion is mass selected, and its infrared spectrum is measured from 2000 to 4000 cm(-1) using laser photodissociation spectroscopy and the method of rare gas atom tagging. Computational chemistry predicts two isomers, the methanol and methylene-oxonium cations. Predicted vibrational spectra based on scaled harmonic and reduced dimensional treatments are compared to the experimental spectra. The methanol cation is the only isomer produced when methanol is used as a precursor. When ethylene glycol is used as the precursor, methylene-oxonium is produced in addition to the methanol cation. Theoretical results at the CCSD(T)/cc-pVTZ level show that methylene-oxonium is lower in energy than methanol cation by 6.4 kcal/mol, and is in fact the global minimum isomer on the [C,H4,O](+) potential surface. Methanol cation is trapped behind an isomerization barrier in our source, providing a convenient method to produce and characterize this transient species. Analysis of the spectrum of the methanol cation provides evidence for strong CH stretch vibration/torsion coupling in this molecular ion.

Infrared spectroscopy of the acetyl cation and its protonated ketene isomer.

[C2,H3,O](+) ions are generated with a pulsed discharge in a supersonic expansion containing methyl acetate or acetone. These ions are mass selected and their infrared spectra are recorded via laser photodissociation and the method of argon tagging. Computational chemistry is employed to investigate structural isomers and their spectra. The acetyl cation (CH3CO(+)) is the global minimum and protonated ketene (CH2COH(+)) is the next lowest energy isomer (+176.2 kJ/mol). When methyl acetate is employed as the precursor, the infrared spectrum reveals that only the acetyl cation is formed. Partially resolved rotational structure reveals rotation about the C3 axis. When acetone is used as the precursor, acetyl is still the most abundant cation, but there is also a minor component of protonated ketene. Computations reveal a significant barrier to interconversion between the two isomers (+221 kJ/mol), indicating that protonated ketene must be obtained via kinetic trapping. Both isomers may be present in interstellar environments, and their implications for astrochemistry are discussed.

Infrared photodissociation spectroscopy of saturated group IV (Ti, Zr, Hf) metal carbonyl cations.

Group IV metal carbonyl cations of the form M(CO)n(+) (M = Ti, Zr, Hf; n = 6-8) are produced in a supersonic molecular beam via laser vaporization in a pulsed nozzle source. The ions are mass selected in a reflectron time-of-flight spectrometer and studied with infrared laser photodissociation spectroscopy in the carbonyl stretching region. The number of infrared active bands, their relative intensities, and their frequency positions provide insight into the structure and bonding of these complexes. Density functional theory calculations are employed to aid in the analysis of the experimental spectra. The n = 6 species is found to be the fully coordinated complex for each metal, and all analogues have a D3d structure. This symmetric structure and the resulting simple spectra facilitate the investigation of trends in the bonding and infrared band positions of these complexes. The carbonyl stretching frequencies of the M(CO)6(+) species are all red-shifted with respect to the gas phase CO vibration at 2143 cm(-1), occurring at 2110, 2094, and 2075 cm(-1) for titanium, zirconium and hafnium. The magnitude of the red shift increases systematically going from titanium to hafnium.

Infrared photodissociation spectroscopy of vanadium oxide-carbonyl cations.

Mass selected vanadium oxide-carbonyl cations of the form VO(m)(CO)(n)(+) (m = 0-3 and n = 3-6) are studied via infrared laser photodissociation spectroscopy in the 600-2300 cm(-1) region. Insight into the structure and bonding of these complexes is obtained from the number of infrared active bands, their relative intensities and their frequency positions. Density functional theory calculations are carried out in support of the experimental data. The effect of oxidation on the carbonyl stretching frequencies of VO(CO)(n)(+), VO2(CO)(n)(+), and VO3(CO)(n)(+) complexes is investigated. All of these oxide-carbonyl species have C-O stretch vibrations blue-shifted from those of the pure vanadium ion carbonyls. The V-O stretches of these complexes are also investigated, revealing the effects of CO coordination on these vibrations. The oxide-carbonyls all have a hexacoordinate core analogous to that of V(CO)6(+). The fully coordinated vanadium monoxide-carbonyl species is VO(CO)5(+), and those of the dioxide and trioxide are VO2(CO)4(+) and VO3(CO)3(+), respectively.

IR photodissociation spectroscopy of H7(+), H9(+), and their deuterated analogues.

Cluster ions of H7(+)/D7(+) and H9(+)/D9(+) produced in a supersonic molecular beam with a pulsed discharge source are mass selected and studied with infrared laser photodissociation spectroscopy. Photodissociation occurs by the loss of H2 (D2) from each cluster, producing resonances in the 2000-4500 cm(-1) region. Vibrational patterns indicate that these ions consist of an H3(+) (D3(+)) core ion solvated by H2 (D2) molecules. There is no evidence for the shared proton structure seen previously for H5(+). The H3(+) ion core vibrational bands are weakened and broadened significantly, presumably by enhanced rates of intramolecular vibrational relaxation. Computational studies at the DFT/B3LYP or MP2 levels of theory (including scaling) are adequate to reproduce qualitative details of the vibrational spectra, but neither provides quantitative agreement with vibrational frequencies.

Vibrational spectroscopy and theory of the protonated benzene dimer and trimer.

Protonated benzene cluster ions, H(C(6)H(6))(2)(+) and H(C(6)H(6))(3)(+), are produced in a pulsed electrical discharge source coupled to a supersonic expansion. Mass-selected complexes are investigated with infrared photodissociation spectroscopy in the 1000-3200 cm(-1) region using the method of argon tagging. The IR spectra of H(C(6)H(6))(2)(+)-Ar and H(C(6)H(6))(3)(+)-Ar contain broad bands in the high frequency region resulting from CH-π hydrogen bonds. Sharp peaks are observed in the fingerprint region arising from the ring modes of both the C(6)H(7)(+) and C(6)H(6) moieties. M06-2X calculations have been performed to investigate the structures and vibrational spectra of energetically low-lying configurations of these complexes. H(C(6)H(6))(2)(+) is predicted to have three nearly isoenergetic conformers: the parallel displaced (PD), T-shaped (TS), and canted (C) structures [Jaeger, H. M.; Schaefer, H. F.; Hohenstein, E. G.; Sherrill, C. D. Comput. Theor. Chem. 2011, 973, 47-52]. A comparison of the experimental dimer spectrum with those predicted for the three isomers suggests an average structure between the TS and PD conformers, which is consistent with the low energy barrier predicted to separate these two structures. No evidence is found for the C dimer even though it lies only 1.2 kcal/mol above the PD dimer. Although the trimer is also computed to have many low lying isomers, the IR spectrum limits the possible species present.

Infrared spectroscopy of the mass 31 cation: protonated formaldehyde vs methoxy.

Pulsed discharges containing methanol or ethanol produce ions having the nominal formula [C,H(3),O](+), i.e. m/z = 31. Similar ions resulting from electron impact ionization in mass spectrometers are long recognized to have either the CH(2)OH(+) protonated formaldehyde or CH(3)O(+) methoxy cation structures. The H(2)OCH(+) oxonio-methylene structure has also been suggested by computational chemistry. To investigate these structures, ions are expanded in a supersonic beam, mass-selected in a time-of-flight spectrometer, and studied with infrared laser photodissociation spectroscopy. Sharp bands in the O-H and C-H stretching and fingerprint regions are compared to computational predictions for the three isomeric structures and their vibrational spectra. Protonated formaldehyde is the most abundant isomer, but methoxy is also formed with significant abundance. The branching ratio of these two ion species varies with precursors and formation conditions.

IR spectroscopy of α- and ß-protonated pyrrole via argon complex photodissociation.

Protonated pyrrole cations are produced in a pulsed discharge/supersonic expansion source, mass-selected in a time-of-flight spectrometer, and studied with infrared photodissociation spectroscopy. Vibrational spectra in both the fingerprint and C-H/N-H stretching regions are obtained using the method of tagging with argon. Sharp vibrational structure is compared to IR spectra predicted by theory for the possible α-, ß-, and N-protonated structures. The spectral differences among these isomers are much larger than the frequency shifts due to argon attachment at alternative sites. Though α-protonation predominates thermodynamically, the kinetically favored ß-protonated species is also observed for the first time (in 3-4 times lower abundance under the conditions employed here). Theoretical investigations attribute the greater stability of α-protonated pyrrole to topological charge stabilization, rather than merely to the greater number of resonance contributors. The far-IR pattern of protonated pyrrole does not match the interstellar UIR bands.

Photodissociation of cobalt and nickel oxide cluster cations.

Cobalt and nickel oxide cluster cations, Co(x)O(y)(+) and Ni(x)O(y)(+), are produced by laser vaporization of metal rods in a pulsed nozzle cluster source and detected using time-of-flight mass spectrometry. The mass spectra show prominent stoichiometries of x = y for Co(x)O(y)(+) along with x = y and x = y - 1 for Ni(x)O(y)(+). The cluster cations are mass selected and multiphoton photodissociated using the third harmonic (355 nm) of a Nd:YAG laser. Although various channels are observed, photofragmentation exhibits two main forms of dissociation processes in each system. Co(x)O(y)(+) dissociates preferentially through the loss of O(2) and the formation of cobalt oxide clusters with a 1:1 stoichiometry. The Co(4)O(4)(+) cluster seems to be particularly stable. Ni(x)O(y)(+) fragments reveal a similar loss of O(2), although they are found to favor metal-rich fragments with stoichiometries of Ni(x)O(x-1). The Ni(2)O(+) fragment is produced from many parent ions. The patterns in fragmentation here are not nearly as strong as those seen for early or mid-period transition-metal oxides studied previously.

Infrared spectroscopy of Mn+ (H2O) and Mn2+ (H2O) via argon complex predissociation.

Singly and doubly charged manganese-water cations, and their mixed complexes with attached argon atoms, are produced by laser vaporization in a pulsed nozzle source. Complexes of the form Mn(+)(H(2)O)Ar(n) (n = 1-4) and Mn(2+)(H(2)O)Ar(4) are studied via mass-selected infrared photodissociation spectroscopy, detected in the mass channels corresponding to the elimination of argon. Sharp resonances are detected for all complexes in the region of the symmetric and asymmetric stretch vibrations of water. With the guidance of density functional theory computations, specific vibrational band resonances are assigned to complexes having different argon attachment configurations. In the small singly charged complexes, argon adds first to the metal ion site and later in larger clusters to the hydrogens of water. The doubly charged complex has argon only on the metal ion. Vibrations in all of these complexes are shifted to lower frequencies than those of the free water molecule. These shifts are greater when argon is attached to hydrogen and also greater for the dication compared to the singly charged species. Cation binding also causes the IR intensities for water vibrations to be much greater than those of the free water molecule, and the relative intensities are greater for the symmetric stretch than the asymmetric stretch. This latter effect is also enhanced for the dication complex.

Infrared photodissociation spectroscopy of copper carbonyl cations.

Copper carbonyl cations of the form Cu(CO)(n)(+) (n = 1-8) are produced in a molecular beam via laser vaporization in a pulsed nozzle source. Mass-selected infrared photodissociation spectroscopy in the carbonyl stretching region is used to study these ions and their argon "tagged" analogues. The geometries and electronic states of these complexes are determined by the number of infrared-active bands, their frequency positions, and their relative intensities compared to the predictions of theory. Cu(CO)(4)(+) has a completed coordination sphere, consistent with its expected 18-electron stability. It also has a tetrahedral structure similar to that of its neutral isoelectronic analog Ni(CO)(4). The carbonyl stretch in Cu(CO)(4)(+) (2198 cm(-1)) is blue-shifted with respect to the free CO vibration (2143 cm(-1)), providing evidence that this is a "non-classical" metal carbonyl.