RAIR Spectra Research Articles

Homoleptic complexes of isocyano- and diisocyanobiazulenes with a 12-electron, ligand-based redox capacity.

Oligo- and polyazulenes are attractive π-conjugated building blocks in designing advanced functional materials. Herein, we demonstrate that anchoring one or both isocyanide termini of the redox non-innocent 2,2'-diisocyano-6,6'-biazulenic π-linker (1) to the redox-active [Cr(CO)5] moiety provided a convenient intramolecular redox reference for unambiguously establishing that the 6,6'-biazulenic scaffold undergoes a reversible one-step 2e- reduction governed by reduction potential compression/inversion. Treatment of bis(η6-naphthalene)chromium(0) with six equiv. of 2-isocyano-1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene (6) or [(OC)5Cr(η1-2,2'-diisocyano-1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene)] (11) afforded homoleptic Cr(0) complexes 13 and 14 with a 12e- (per molecule) ligand-based reduction capacity at mild E1/2 of -1.29 V and -1.15 V vs. Cp2Fe0/+, respectively. The overall reversible redox capacity varies from 15e- for the mononuclear complex 13 to 21e- for the heptanuclear complex 14. The latter "nanocomplex" has a diameter of ca. 5 nm and features seven Cr(0) centers interlinked with six 2,2'-diisocyano-6,6'-biazulenic bridges. The X-ray structure of [(OC)5Cr(2-isocyano-1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene)] (7) indicated a 43.5° interplanar angle between the two azulenic moieties. Self-assembly of 11 on a Au(111) substrate afforded an organometallic monolayer film of 11 featuring approximately upright orientation of the 2,2'-diisocyano-6,6'-biazulenic linkers, as evidenced by ellipsometric measurements and the RAIR signature of the C4v-symmetric [(-NC)Cr(CO)5] infrared reporter within 11. Remarkably, comparing the FTIR spectrum of 11 in solution with the RAIR spectrum of 11 adsorbed on Au(111) suggested electronic coupling at a ca. 2 nm distance between the Cr(0) and Au atoms linked by the 2,2'-diisocyano-6,6'-biazulene bridge.

Methylidyne Adsorption on Pt(211) Probed by Reflection Absorption Infrared Spectroscopy (RAIRS).

Methylidyne, CH(ads), adsorbed on a Pt(211) surface and its interaction with chemisorbed hydrogen atoms was studied by reflection absorption infrared spectroscopy (RAIRS). Methylidyne was formed on Pt(211) by methane dissociation from a molecular beam followed by thermal decomposition of the methane dissociation products. CH(ads) was detected by RAIRS via its symmetric C-H stretch vibration resulting in three discrete absorption peaks in the region of 2950-2970 cm-1. While the frequencies of the three C-H stretch peaks remain fixed, their relative intensities depend on the H(ads) co-coverage. This differs markedly from what was observed previously for the RAIR spectra of CH(ads) adsorbed on Pt(111) by the group of Trenary,1 who observed a single C-H stretch peak, which showed a continuous blue shift with increasing H(ads) coverage. Based on our experimental results and density functional theory (DFT) calculations, we propose that the three discrete absorption peaks on Pt(211) are due to the adsorption of methylidyne on the steps of Pt(211) forming one-dimensional rows of adsorbates. Depending on the H(ads) coverage, the CH(ads) species on the step sites can have either zero, one, or two neighboring H(ads) atoms, leading to three different vibrational C-H stretch frequencies and a reversible shift in relative peak intensity depending on the H(ads) coverage.

Acetone–Water Interactions in Crystalline and Amorphous Ice Environments

We present research that systematically examines acetone interacting with various D2O ices of terrestrial and astrophysical interest using time-resolved, in situ reflection absorption infrared spectroscopy (RAIRS). We examine acetone deposited on top of different D2O ice films: high-density, nonporous amorphous (np-ASW), and crystalline (CI) films as well as porous amorphous (p-ASW) with various pore morphologies. Analysis of RAIR spectra changes after acetone exposure, and we find that more hydrogen bonding occurs between acetone and p-ASW ices as compared to acetone and np-ASW or CI ices. Hydrogen bonding quantification occurred by two independent RAIR spectral changes: a greater relative intensity of the 1703 cm-1 feature at low acetone coverage as part of a 14 cm-1 shift in the C═O region and an ∼30% integrated dangling bond area reduction after acetone exposure. Interestingly, when changing the water structure to be more porous (deposited at 70° compared to 30°), there is a further reduction in the amount of hydrogen bonding that occurs. This suggests that there is a lack of access to surface sites with dangling bonds in the pores as initial layers of acetone block the pores and acetone is unable to diffuse within the structure at low temperatures. In general, these results offer a clearer picture of the mechanisms that can occur when small organic hydrocarbons interact with various icy interfaces; a quantitative understanding of these interactions is essential for the accurate modeling of many astrophysical processes occurring on the surface of icy dust particles.

Extinction spectra in the fundamental region of hexafluoroethane at different condensed phases

RAIR spectra of liquid and crystalline hexafluoroethane were recorded in the temperature range 100–200 K. The ν5 and ν10 reflection bands had been normalized and used to obtain their extinction profiles. New normalizing method outlined in this paper. The integral absorption coefficients had been determined at each phase and compared with literature data. The contours obtained have asymmetrical and broad profiles with enhanced low frequency branch (TO-branch). The structures of these contours were explained in framework of resonance dipole-dipole (RDD) interaction. The mathematical apparatus of spectral moments were used to describe profiles of the bands studied.

Propane and propane-water interactions: a study at cryogenic temperatures.

The phase transition of solid propane and a propane-water mixture under ultrahigh vacuum has been investigated using reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption mass spectrometry (TPD-MS). Here, the investigation is divided into two sections: the phase transition of pure propane and the interaction of propane with water. RAIR spectra of pure propane reveal an unknown crystalline phase at 50 K (phase I), which gradually converts to a known crystalline phase (phase II) at higher temperature. This conversion is associated with certain kinetics. Co-deposition of water and propane restricts the amorphous to crystalline phase transition, while sequential deposition (H2O@C3H8; propane over predeposited water) does not hinder it. For an alternative sequential deposition (C3H8@H2O; water over predeposited propane), the phase transition is hindered due to diffusional mixing within the given experimental time, which is attributed to the reason behind the restricted phase transition.

Selective Hydrogenation of Acetylene to Ethylene in the Presence of a Carbonaceous Surface Layer on a Pd/Cu(111) Single-Atom Alloy

Reflection absorption infrared spectroscopy (RAIRS) was used to simultaneously monitor gas-phase and surface species in the presence of ambient pressures of acetylene and hydrogen over a single-atom alloy (SAA). The alloy consisted of isolated Pd atoms at surface coverages in the range of 0.0028–0.085 ML in a Cu(111) surface. When C2H2(g) is present, but not H2(g), the RAIR spectra are similar for Cu(111) with and without Pd, independent of C2H2(g) pressure for temperatures between 180 and 500 K. The addition of H2(g) leads to different RAIR spectra depending on the presence of Pd. With a C2H2:H2 ratio of 1:100 and a SAA-Pd/Cu(111) surface with less than 1% Pd, C2H2(g) is converted to C2H4(g) at 400 K at total pressures up to 10 Torr. From the rate of change in the gas-phase IR peaks, a range of initial turnover frequencies was estimated, which depend on which sites are assumed to be active for hydrogenation. Postreaction surface analysis with Auger electron spectroscopy (AES) showed a significant carbon ...

Spectroscopic Identification of Surface Intermediates in the Decomposition of Methylamine on Ru(001)

The thermal decomposition of methylamine on Ru(001) was studied with reflection absorption infrared spectroscopy (RAIRS) and temperature programmed reaction spectroscopy (TPRS). After the multilayer methylamine desorbs at 150 K, the RAIR spectra of the remaining monolayer methylamine undergo small changes due to structural rearrangements but do not indicate any chemical changes until 250 K. The results are in agreement with recent theoretical investigations indicating that CH3 dehydrogenation to produce HxCNH2 species occurs before N–H and C–N bond scission. Experimental spectra of 13C- and 15N-substituted methylamine combined with DFT calculations of HxCNHy fragments attached to a Ru19 cluster to simulate the RAIR spectra, including isotopic shifts, clarified the spectral assignments.

Coexistence of ice clusters and liquid-like water clusters on the Ru(0001) surface.

The RAIRS spectra of water adsorbed on Ru(0001) at 85 K are recorded from 600 cm-1 to 4000 cm-1. Measured at water coverages from 0.13 ML to 2.0 ML, the RAIRS spectra suggest that chemisorption of water on Ru(0001) depends on coverage. Water adsorbs on a clean Ru surface as chemisorbed ice-like clusters (likely through an O-Ru bond) up to 0.33 ML. Above this coverage, the chemisorbed layer saturates. Upon more exposure, water adsorbs as a liquid-like H-bonded layer without bonding to the Ru substrate. The chemisorbed water absorbs 7 times less IR per molecule than the liquid-like structure, which indicates that the orientation of the chemisorbed water is more parallel to the surface. Additionally, the influence of water-Ru bonding on H-bonding is reflected in the OH symmetric stretching mode. Under perturbation from water-Ru bonding, a large red shift (40 cm-1) in the free OH stretching frequency is observed in the chemisorbed clusters. By deconvoluting the main H-bonded OH stretching peak into five Gaussian sub-bands at 2945 ± 5 cm-1, 3210 ± 5 cm-1, 3300 ± 15 cm-1, 3430 ± 5 cm-1 and 3570 ± 10 cm-1, changes in the H-bonding network are rationalized in terms of H-bonding motifs. The donor-acceptor-acceptor motif is significant only in the chemisorbed clusters. On the other hand, the donor-acceptor motif dominates in the liquid-like structure, which increases the disorder present in the adlayer. Although chemisorption is suppressed above 0.33 ML, no structural changes in the ice-like clusters are observed up to multilayer coverage. Therefore, ice-like and liquid-like water coexist in a meta-stable state at 85 K.

In-situ spectroscopic monitoring of the ambient pressure hydrogenation of C2 to ethane on Pt(111)

The hydrogenation of C2 molecules formed on the Pt(111) surface through acetylene exposure at 750K was monitored in-situ with reflection absorption infrared spectroscopy (RAIRS) in the presence of up to 10Torr of H2. The coverage of post-reaction surface carbon was measured with Auger electron spectroscopy. The RAIR spectra show that C2 is hydrogenated to an ethylidyne intermediate. The hydrogenation of ethylidyne was also monitored at 400K for H2(g) pressures of 1.0×10−2 to 10Torr. At H2(g) pressures greater than 1.0Torr, ethylidyne is completely hydrogenated. In an attempt to probe the nature of the C2 adsorption sites, RAIR spectra of coadsorbed CO were obtained. It is found that while C2 does not block CO adsorption, the spectra indicate that the surface carbon is free of hydrogen. In contrast, ethylidyne blocks CO adsorption sites. In the presence of coadsorbed CO, complete hydrogenation of ethylidyne occurs at 450K versus 400K in the absence of CO.

Molecular adsorption of NO on PdO(101)

We investigated the adsorption of NO on the PdO(101) surface using temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations. From TPD, we find that NO reaches a saturation coverage of ~0.54 monolayer (ML) on PdO(101) at 90K, and that desorption from the saturated layer produces a complex TPD trace with five pronounced maxima between 100 and 550K. We observe negligible quantities of desorbing products other than NO during TPD, indicating that NO does not react on PdO(101) under the conditions studied. For low coverages, RAIR spectra reveal that NO adsorption at 90K produces both flat-lying and atop configurations of NO on the coordinatively unsaturated (cus) Pd atoms of PdO(101), but that these configurations are meta-stable and transform to bridge-NO species at higher temperature. RAIRS measurements further show that NO binding on atop-Pdcus sites becomes preferred with increasing coverage above ~25% of the Pdcus density, and provide evidence of a weakly-bound state at coverages above 100% (0.35 ML). Consistent with our experimental findings, DFT predicts that NO preferentially adsorbs on bridge-Pdcus sites at low coverage and that the site preference changes to atop-Pdcus sites with increasing NO coverage, resulting in mixtures of atop-bridge NO configurations as well as an all-atop configuration. DFT also predicts that NO can adopt a flat-lying configuration on the Pdcus row at low coverage, and can also achieve stable bonding on the fourfold Pd sites of PdO(101) at high NO coverages. Both the computational and experimental results show that NO binds on PdO(101) in multiple, local configurations even at low coverage.

Spontaneous electric fields in solid carbon monoxide.

Reflection-absorption infrared spectroscopy (RAIRS) is shown to provide a means of observing the spontelectric phase of matter, the defining characteristic of which is the occurrence of a spontaneous and powerful static electric field within a film of material. The presence of such a field is demonstrated here through the study of longitudinal-transverse optical splitting in RAIR spectra in films of carbon monoxide, based upon the deposition temperature dependence of this splitting. Analysis of spectral data, in terms of the vibrational Stark effect, allows the measurement of the polarization of spontelectric films, showing for example that solid carbon monoxide at 20 K may maintain a spontelectric field of 3.78 × 10(7) V m(-1), representing a polarization of 3.34 × 10(-4) cm(-2). We comment on the astrophysical implications of polarized carbon monoxide ices, on the surface of cosmic grains in star-forming regions.

Adsorption and interaction of CO2 on rutile TiO2(110) surfaces: a combined UHV-FTIRS and theoretical simulation study.

CO2 adsorption and interaction on rutile TiO2(110) surfaces was studied by UHV-FTIRS combined with theoretical simulations. With increasing CO2 exposure, CO2 adsorbs in succession at the oxygen vacancy (Vo) sites, on the five-coordinated Ti cation (Ti5c) sites and the bridging oxygen (Obr) sites at low temperature. The coupling has occurred between neighboring CO2 adsorbed on Ti5c sites from rather low CO2 coverage (∼0.5 ML), leading the ν3(OCO) asymmetric stretching vibrations to split into two absorption bands in IR spectra. Two kinds of coupled geometries of adjacent CO2 on Ti5c sites are determined by theoretical simulations. For the higher CO2 coverage (∼1.5 ML), the horizontal adsorption configuration along the [11[combining macron]0] azimuth of CO2 adsorbed on Obr sites is identified for the first time using polarization- and azimuth-resolved RAIRS in experiments. The significant deviation of CO2 from the top of Obr sites demonstrates the strong coupling between CO2 adsorbed on Obr and Ti5c sites.

UV-induced photodesorption and photochemistry of O2 ice

In dense inter- and circumstellar clouds, molecules are maintained in the gas phase at temperatures well below their thermal accretion value, and the UV photodesorption of ice has been proposed as an efficient non-thermal desorption mechanism explaining this observation. The present work aims at studying the UV photodesorption yield of solid O2, a molecule considered as a starting point in the formation of water. For the infrared inactive O2 the standard spectroscopic procedure does not work, and a new experimental procedure is introduced that links quadrupole mass spectrometry of photodesorbed oxygen ice in the gas phase to a RAIR spectrum of a polar reference molecule in the solid state (in our case CO). The resulting photodesorption rate of pure O2 ice is found to be (6 ± 2) × 10 −4 molecules photon −1 upon irradiation with broad-band Lyα light. The main photodesorption product is O2; a smaller part photodesorbs as O-atoms. Within the accessible range, the photodesorption rate does not vary with ice temperature. Upon UV irradiation also ozone ice is formed and the present work allows in parallel to determine the photodesorption rate for O3 as (3 ± 1) × 10 −4 and (5 ± 2) × 10 −4 molecules photon −1 for 14 and 52 K, respectively. Ozone is found not to desorb directly, but via its fragments O2 and O; UV induced dissociation is concluded to be much more efficient than direct photodesorption.

Dissociation of trimethylgallium on the ZrB2(0001) surface

X-ray photoelectron spectroscopy and reflection absorption infrared spectroscopy (RAIRS) have been used to study the dissociative adsorption of trimethylgallium (TMG) on the ZrB2(0001) surface. Spectra were obtained as a function of annealing temperature following TMG exposure at temperatures of 95 and 300 K, and also as a function of TMG exposure for a surface temperature of 300 K. After annealing above 220 K, a significant decrease in the relative concentration of carbon and gallium occurred accompanied by a shift of ∼0.2 eV in the Ga 2p3/2 binding energy. The RAIR spectra show that after annealing to ∼220 K, only one CH3 deformation band at 1196 cm−1 remains, the intensity of which is considerably decreased indicating loss of at least one methyl group from TMG. Further annealing leads to the sequential loss of the other methyl groups. The first methyl desorbs while the last two dissociate to deposit two C atoms per TMG molecule onto the ZrB2 surface.

Surface science investigations of the role of CO 2 in astrophysical ices

We have recorded reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) data for a range of CO₂-bearing model astrophysical ices adsorbed on a graphitic dust grain analogue surface. Data have been recorded for pure CO₂, for CO₂ adsorbed on top of amorphous solid water, for mixed CO₂:H₂O ices and for CO₂ adsorbed on top of a mixed CH₃OH:H₂O ice. For the TPD data, kinetic parameters for desorption have been determined, and the trapping behaviour of the CO₂ in the H₂O (CH₃OH) ice has been determined. Data of these types are important as they can be used to model desorption in a range of astrophysical environments. RAIR spectra have also shown the interaction of the CO₂ with H₂O and CH₃OH and can be used to compare with astronomical observations, allowing the accurate assignment of spectra.

Formation of Benzonitrile from the Reaction of Styrene with Nitrogen on the Pt(111) Surface

Temperature-programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS) were used to investigate the adsorption and reaction chemistry of styrene with clean and nitrogen covered Pt(111). The RAIR spectra indicate that the adsorption geometry of styrene is altered by both coverage effects and by the presence of nitrogen atoms on the surface. For an annealed monolayer of styrene on the clean Pt(111) surface, the most intense peaks correspond to out-of-plane bending vibrations indicating that the molecular plane is oriented parallel to the surface. For monolayer styrene on the nitrogen-precovered surface and for an unannealed monolayer of styrene, the in-plane and out-of-plane vibrations have comparable intensities indicating a more random orientation of the molecular plane. Upon heating, styrene reacts with the coadsorbed nitrogen atoms to form benzonitrile, which desorbs at 380 K. When benzonitrile is directly adsorbed on the Pt(111) surface, TPRS and RAIRS reveal that it desorbs at 350 K, indicating that its desorption at 380 K when formed from the reaction of styrene with nitrogen is reaction limited.

Adsorption and Dehydrogenation of Decaborane on the Pt(111) Surface

The adsorption and decomposition of decaborane (B10H14) on the Pt(111) surface was studied with reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). The molecule has a nido structure with 10 terminal B−H bonds and 4 bridging B−H−B bonds. Comparison of the experimental RAIR spectra with spectra calculated using density functional theory indicates that the molecule adsorbs at 85 K without dissociation and with an orientation in which the dipole moment is parallel to the surface. The TPD experiments show that the dissociation leads to H2 desorption up to ∼420 K but that no boron-containing species desorb. Dissociation occurs in stages via stable surface intermediates, as indicated by changes observed with RAIRS in the terminal B−H stretch region at ∼2600 cm−1. Boron, which is produced as a result of complete dissociation, is lost from the surface as indicated by XPS, most likely through dissolution into the bulk of the platinum crystal.

DFT and RAIRS Investigations of Methanol on Cu(110) and on Oxygen-Modified Cu(110)

The adsorption and subsequent reaction of methanol on Cu(110) and on the oxygen stripe phase on Cu(110) was investigated using reflection absorption infrared spectroscopy (RAIRS), temperature-programmed desorption (TPD) and density functional theory (DFT) calculations. It was shown that during high methanol exposures water can coadsorb and it is incorporated into the methanol rows formed in the chemisorbed layer. The RAIR spectra of adsorbed methanol are very similar with and without coadsorbed oxygen. This is due to the fact that the ν(CO) vibration is at the same frequency for both methanol and methoxy adsorbed in the more stable short-bridge site. However, methoxy adsorbed in the long-bridge site shows the ν(CO) vibration at lower wavenumbers and is found with increasing surface temperature. With coadsorbed oxygen the reaction products are formaldehyde, H2, and CO2. DFT and RAIRS results suggest that the intermediate leading to CO2 is an η2-formaldehyde and OH species on the surface, rather than formate.

The adsorption and desorption of ethanol ices from a model grain surface

Reflection absorption infrared spectroscopy (RAIRS) and temperature programed desorption (TPD) have been used to probe the adsorption and desorption of ethanol on highly ordered pyrolytic graphite (HOPG) at 98 K. RAIR spectra for ethanol show that it forms physisorbed multilayers on the surface at 98 K. Annealing multilayer ethanol ices (exposures >50 L) beyond 120 K gives rise to a change in morphology before crystallization within the ice occurs. TPD shows that ethanol adsorbs and desorbs molecularly on the HOPG surface and shows four different species in desorption. At low coverage, desorption of monolayer ethanol is observed and is described by first-order kinetics. With increasing coverage, a second TPD peak is observed at a lower temperature, which is assigned to an ethanol bilayer. When the coverage is further increased, a second multilayer, less strongly bound to the underlying ethanol ice film, is observed. This peak dominates the TPD spectra with increasing coverage and is characterized by fractional-order kinetics and a desorption energy of 56.3+/-1.7 kJ mol(-1). At exposures exceeding 50 L, formation of crystalline ethanol is also observed as a high temperature shoulder on the TPD spectrum at 160 K.

Influence of Modification Conditions on the Interaction of Methylacetoacetate with (R,R)-Tartaric Acid-Modified Ni{111}

Ni catalysts modified by the adsorption of the chiral molecule, (R,R)-tartaric acid, are known to be capable of catalyzing the enantioselective hydrogenation of β-ketoesters (e.g., methylacetoacetate).1 The adsorption of (R,R)-tartaric acid onto Ni{111} from solution was investigated with reflection absorption infrared spectroscopy as functions of modification temperature and pH. Under all conditions tested, RAIR spectra are consistent with the formation of several layers of nickel and/or sodium bitartrate. After washing, the majority of the overlayer is dissolved, leaving submonolayer quantities of adsorbate. At 350 K, the optimum modification temperature in terms of catalytic enantioselectivity, the washed surface exhibits a proportionately greater hydrogen tartrate content than at 300 K. When the modified surfaces are exposed to methylacetoacetate, the presence of the protonated tartrate species correlates with an enhancement in the diketone to enol tautomeric ratio of methylacetoacetate. Similarly, when the promoter NaBr is added to the modification solution, although little difference is observed in the RAIR spectra of adsorbed tartrate, there is a further increase in the diketone:enol ratio after exposure to methylacetoacetate. The implications for understanding the behavior of the enantioselective nickel catalyst are discussed.