Infrared vibrational autodetachment spectroscopy of microsolvated benzonitrile radical anions

Vibrational spectra of microsolvated benzonitrile radical anions (C6H5CN- -S; S = H2O and CH3OH) were measured by probing the electron detachment efficiency in the 3 microm region, representing resonance bands of autodetachment via OH stretching vibrations of the solvent molecules. The hydrogen-bonded OH band for both the cluster anions exhibited a large shift to the lower energy side with approximately 300 cm-1 compared to those for the corresponding neutral clusters. The solvent molecules are bound collinearly to the edge of the CN group of the benzonitrile anion in the cluster structures optimized with the density functional theory, in which the simulated vibrational energies are in good agreement with the observed band positions. Natural population analyses were performed for a qualitative implication in changes of solvent orientation upon electron attachment. Asymmetric band shapes depending on the vibrational modes are discussed with respect to dynamics of the autodetachment process from a theoretical aspect incorporated with density functional calculations.     

Gas phase infrared spectroscopy of mono- and divanadium oxide cluster cations

The vibrational spectroscopy of the mono- and divanadium oxide cluster cations VO(1-3)+ and V2O(2-6)+ is studied in the region from 600 to 1600 wave numbers by infrared photodissociation of the corresponding cluster cation-helium atom complexes. The comparison of the experimental depletion spectra with the results of density functional calculations on bare vanadium oxide cluster cations allows for an unambiguous identification of the cluster geometry in most cases and, for VO(1-3)+ and V2O(5,6)+, also of the electronic ground state. A common structural motif of all the studied divanadium cluster cations is a four-membered V-O-V-O ring, with three characteristic absorption bands in the 550-900 wave number region. For the V-O-V and V=O stretch modes the relationship between vibrational frequencies and V-O bond distances follows the Badger rule.     

Characterization of thin metastable vanadium oxide films by Raman spectroscopy

Thin films are potentiodynamically generated on vanadium in Ba²⁺/acetate electrolyte systems at high voltages. The influence of the anodic potential up to 400 V on the composition and structure of the about 500 nm thin anodic conversion films are investigated. Raman spectroscopy indicates that different film types depend on the electrochemical process parameters. The relationship between the Raman laser excitation power and the amorphous or microcrystalline film structure is also discussed. Beside metastable disordered structures the films contain crystalline phases of V₂O₅, V₄O₉ and barium vanadate, respectively. Received: 15 July 1997 / Revised: 16 February 1998 / Accepted: 21 February 1998     

Characterization of thin metastable vanadium oxide films by Raman spectroscopy

Thin films are potentiodynamically generated on vanadium in Ba²⁺/acetate electrolyte systems at high voltages. The influence of the anodic potential up to 400 V on the composition and structure of the about 500 nm thin anodic conversion films are investigated. Raman spectroscopy indicates that different film types depend on the electrochemical process parameters. The relationship between the Raman laser excitation power and the amorphous or microcrystalline film structure is also discussed. Beside metastable disordered structures the films contain crystalline phases of V₂O₅, V₄O₉ and barium vanadate, respectively.     

Electronic spectroscopy of CoNe+ via mass-selected photodissociation

The CoNe(+) diatomic cation is produced by laser vaporization in a pulsed-nozzle source and studied with photodissociation spectroscopy at visible wavelengths. Vibronic structure is assigned to the (3)Π(2) ← (3)Δ(3) band system correlating to the Co(+)((3)P(2) ← (3)F(4)) + Ne asymptote. The origin band (13,529 cm(-1)) and a progression of 14 other vibrational bands are detected ending in the dissociation limit at 14,191 cm(-1). The excited state dissociation energy is therefore D(0)(') = 662 cm(-1), and an energetic cycle using this, the origin band energy, and the atomic transition produces a ground state dissociation energy of D(0)(") = 930 cm(-1). The excited state vibrational frequency is 116.1 cm(-1). A rotationally resolved study of the origin band confirms the electronic transition assignment and provides the bond distance of r(0)(") = 2.36 ?. The properties of CoNe(+) are compared to those of other CoRG(+) and MNe(+) complexes studied previously.     

Gas phase hydrogen/deuterium exchange reactions of fluorophenyl anions

Hydrogen/deuterium (H/D) exchange reactions of fluorophenyl and difluorophenyl anions (C₆H₄F^?, o-C₆H₃F ₂ ^? , m-C₆H₃F ₂ ^? , p-C₆H₃F ₂ ^? ) have been studied using the flowing afterglow-selected ion flow tube technique. The C₆H₄F^? anion exchanges all hydrogens for deuterium upon reaction with D₂O. The difluorophenyl anions o-, m-, and p-C₆H₃F ₂ ^? exchange three, two, and one hydrogen, respectively, with D₂O, whereas they undergo one, two, and three H/D exchanges, respectively, with CH₃OD. The structures of the anions and the isotope exchange dynamics within the intermediate ion-dipole complexes are discussed using ab initio molecular orbital calculations. Calculated values for the proton affinities of the most stable anions are 385.2, 378.0, 371.9, and 378.2 kcal/mol for C₆H₄F^?, o-C₆H₃F ₂ ^? , m-C₆H₃F ₂ ^? , and p-C₆H₃F ₂ ^? , respectively, in excellent agreement (within 2 kcal/mol) with the previous experimental values for the acidities of the corresponding fluorobenzenes. The H/D exchange results are explained by the energy differences of the intermediate DO^? and CH₃O^? species within the ion-dipole complexes; CH₃O^? is mobile within the “hot” intermediate complex, whereas DO^? is nearly “frozen” within the complex and cannot migrate across the barriers caused by the fluorine atoms or by the π electrons.     

Identification, structure, and spectroscopy of neutral vanadium oxide clusters

Neutral vanadium oxide clusters are studied by photoionization time-of-flight (TOF) mass spectroscopy, electronic spectroscopy, and density functional theory (DFT) calculations. Mass spectra of vanadium oxide clusters are observed by photoionization with lasers of three different wavelengths: 118, 193, and 355 nm. Mechanisms of 118 nm single photon ionization and 193 and 355 nm multiphoton ionization/fragmentation of vanadium oxide clusters are discussed on the basis of observed mass spectral patterns and line widths of the mass spectral features. Only the 118 nm laser light can ionize vanadium oxide neutral species by single photon ionization without fragmentation. The stable vanadium oxide neutral clusters under saturated oxygen growth conditions are found to be of the form (VO2)x(V2O5)y. Structures of the first few members of this series of clusters are determined through high level DFT calculations. Fragmentation of this series of clusters through 355 and 193 nm multiphoton ionization processes is discussed in light of these calculated structures. The B(2)B2 <-- X(2)A1 transition is observed for the VO2 neutral species, and nu1 and nu2 vibrations are assigned for both electronic states. From this spectrum, the VO2 rotational and vibrational temperatures are found to be approximately 50 and approximately 700 K, respectively.     

Structures and vibrational spectroscopy of partially reduced gas-phase cerium oxide clusters

This work demonstrates that the most stable structures of even small gas-phase aggregates of cerium oxide with 2-5 cerium atoms show structural motifs reminiscent of the bulk ceria. This is different from main group and transition metal oxide clusters, which often display structural features that are distinctly different from the bulk structure. The structures of Ce(2)O(2)(+), Ce(3)O(4)(+), and (CeO(2))(m)CeO(+) clusters (m = 0-4) are unambiguously determined by a combination of global structure optimizations at the density functional theory level and infrared vibrational predissociation spectroscopy of the cluster-rare gas atom complexes. The structures of Ce(2)O(2)(+) and Ce(2)O(3)(+) exhibit a Ce-O-Ce-O four-membered ring with characteristic absorptions between 430 and 680 cm(-1). Larger clusters have common structural features containing fused Ce-O-Ce-O four-membered rings which lead to intense absorption bands at around 500 and 650 cm(-1). Clusters containing a terminal Ce=O bond show a characteristic absorption band between 800 and 840 cm(-1). For some cluster sizes multiple isomers are observed. Their individual infrared signatures are identified by tuning their relative population through the choice of He, Ne or Ar messenger atoms. The present results allow us to benchmark different density functionals which yield different degrees of localization of unpaired electrons in Ce 4f states.     

Internal modes of iodopentamethylbenzene in the disordered phase by vibrational spectroscopy

Infrared and Raman spectra of oriented and non-oriented single crystal of iodopentamethylbenzene were measured at room temperature over the 4000-400 cm⁻¹ region. The dichroism of the IR bands is discussed. An assignment of the fundamental vibrations of IPMB is presented and based on a comparison with the spectra of some similar molecules.     

Simultaneous measurement of magnitude and phase in interferometric sum-frequency vibrational spectroscopy

We present a visible-infrared sum-frequency spectroscopic technique that is capable of simultaneously determining the magnitude and phase of the sample response from a single set of experimental conditions. This is especially valuable in cases where the phase stability is high, as in collinear beam geometries, as it enables multiple experiments to be performed without re-measuring the local oscillator phase or the reference phase. After illustrating the phase stability achievable with such a geometry, we provide a technique for quantitatively determining the magnitude and phase from a single set of two-dimensional spectral-temporal interference fringes. A complete demonstration is provided for the C-H stretching frequency region at the surface of an octadecyltricholosilane film.     

Infrared multiphoton dissociation spectroscopy of gas-phase mass-selected hydrocarbon-Fe+ complexes

Infrared spectra in the mid-infrared region (800-1600 cm(-1)) of highly unsaturated Fe(+)-hydrocarbon complexes isolated in the gas phase are presented. These organometallic complexes were selectively prepared by ion-molecule reactions in a Fourier transform ion cycloton mass spectrometer (FTICR-MS). The infrared multiphoton dissociation (IRMPD) technique has been employed using the free electron laser facility CLIO (Orsay, France) to record the infrared spectra of the mass selected complexes. The experimental IRMPD spectra present the main features of the corresponding IR absorption spectra calculated ab initio. As predicted by these calculations, the experimental spectra of three selectively prepared isomers of Fe+(butene) present differences in the 800-1100 cm(-1) range. On the basis of the comparison with calculated IR spectra, the IRMPD spectrum of Fe(butadiene)(+) suggests that the ligand presents the s-trans isomeric form. This study further confirms the potentialities of IRMPD spectroscopy for the structural characterization of organometallic ionic highly reactive intermediates in the gas phase. In conjunction with soft ionization techniques such as electrospray, this opens the door to the gas-phase characterization of reactive intermediates associated with condensed phase catalysts.     

Gas phase fullerene anions hydrogenation by methanol followed by IRMPA dehydrogenation

The characterization in the gas phase of the mechanisms responsible for hydride formation can contribute to the development of new materials for hydrogen storage. The present work provides evidence of a hydrogenation-dehydrogenation catalytic cycle for C₆₀^•− anions in the gas phase using methanol vapor at room temperature as hydrogen donor. The involvement of methanol in the reaction is confirmed by experiments using CD₃OD and CD₃OH. C₆₀ hydride anions with up to 11 hydrogen atoms are identified via elemental composition analysis using FT-ICR mass spectrometry. For the longer reaction times, partial conversion of the C₆₀ hydride ions into oxygen containing ion products occurs. Dehydrogenation using infrared multiphoton activation with a CO₂ laser restores the C₆₀^•− anions.     

Structural variability in transition metal oxide clusters: gas phase vibrational spectroscopy of V3O(6-8)+

We present gas phase vibrational spectra of the trinuclear vanadium oxide cations V(3)O(6)(+)·He(1-4), V(3)O(7)(+)·Ar(0,1), and V(3)O(8)(+)·Ar(0,2) between 350 and 1200 cm(-1). Cluster structures are assigned based on a comparison of the experimental and simulated IR spectra. The latter are derived from B3LYP/TZVP calculations on energetically low-lying isomers identified in a rigorous search of the respective configurational space, using higher level calculations when necessary. V(3)O(7)(+) has a cage-like structure of C(3v) symmetry. Removal or addition of an O-atom results in a substantial increase in the number of energetically low-lying structural isomers. V(3)O(8)(+) also exhibits the cage motif, but with an O(2) unit replacing one of the vanadyl oxygen atoms. A chain isomer is found to be most stable for V(3)O(6)(+). The binding of the rare gas atoms to V(3)O(6-8)(+) clusters is found to be strong, up to 55 kJ/mol for Ar, and markedly isomer-dependent, resulting in two interesting effects. First, for V(3)O(7)(+)·Ar and V(3)O(8)(+)·Ar an energetic reordering of the isomers compared to the bare ion is observed, making the ring motif the most stable one. Second, different isomers bind different number of rare gas atoms. We demonstrate how both effects can be exploited to isolate and assign the contributions from multiple isomers to the vibrational spectrum. The present results exemplify the structural variability of vanadium oxide clusters, in particular, the sensitivity of their structure on small perturbations in their environment.     

Vapour phase synthesis of isobutyraldehyde from methanol and ethanol over mixed oxide supported vanadium oxide catalysts

The vapour phase synthesis of isobutyraldehyde from methanol and ethanol in one step was investigated over titania-silica, titania-alumina, titania-zirconia, titania-silica-zirconia, and magnesia supported vanadium oxide catalysts at 623 K and under normal atmospheric pressure. Among various catalysts the titania-silica binary oxide supported vanadia provided higher yields than the other single or mixed oxide supported catalysts. The high conversion and product selectivity of V₂O₅/TiO₂-SiO₂ catalyst (20 wt% V₂O₅) was related to the better dispersion of vanadium oxide over titania-silica mixed oxide support in addition to other acid-base and redox characteristics. A reaction path for the formation of isobutyraldehyde from methanol and ethanol mixtures over these catalysts was described.     

Structure characterization of metal oxide clusters by vibrational spectroscopy: possibilities and prospects

This article summarizes the methodological progress that has been made in the vibrational spectroscopy of isolated polynuclear metal oxide clusters, with particular emphasis on free electron laser-based infrared action spectroscopy of gas phase clusters, over the last decade. The possibilities, limitations and prospects of the various experimental approaches are discussed using representative examples from pivotal studies in the field.     

Photoelectron spectroscopy of radical anions

The photoelectron spectroscopy of a number of radical anions has been investigated. We find the following electron affinities: EA(C₃) =1.981 ±0.020 eV, EA(C₃H) = 1.858 ±0.023 eV, EA(C₃H₂) = 1.794 ± 0.025 eV, EA(C₃O) = 1.34±0.15 eV, EA(C₃O₂) = 0.85±0.15 eV, EA(C₄O)= 2.05±0.15 eV, and EA(CS₂) = 0.895± 0.020 eV. The structure and bonding for each of these ions is discussed.     

Thermodynamics of phase transitions in langmuir monolayers observed by vibrational sum frequency spectroscopy

Vibrational sum-frequency spectroscopy (VSFS) was used to study gauche defects in octadecylamine (ODA) monolayers at the air/water interface. The VSFS spectra provide unique insights into phase transitions that occur as a result of changes in the structure of the monolayer's hydrophobic region. These changes can be attributed to the increased presence of gauche conformers in the ODA alkyl chains during the monolayer's transition from the solid to liquid phase. Temperature-dependent spectra from monolayers at several different pressures were used to assign the phase transition temperature based on the observed changes in microscopic structure. Through application of a two-dimensional form of the Clapeyron equation, the first in situ measurements of the entropy and enthalpy changes associated with gauche conformers in a monolayer were made.