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Author(s): Cole, WTS; Wei, H; Nguyen, SC; Harris, CB; Miller, DJ; Saykally, RJ | Abstract: Angle-resolved second harmonic scattering is used to probe the adsorption dynamics of aqueous cationic and anionic dye molecules onto polystyrene surfaces. The adsorptions of malachite green to negatively charged polystyrene and naphthol yellow S to positively charged polystyrene are both highly favorable, with I"GAds values of -10.9 ± 0.2 and -10.27 ± 0.09 kcal/mol, respectively. A competitive displacement methodology was employed to obtain values for the adsorption free energies of various smaller neutral organic molecules, including the important micropollutant ascorbic acid, caffeine, and pentoxifylline. For charged adsorbers, electrostatic interactions appear to significantly contribute to adsorption behavior. However, electrostatic repulsion does not necessarily deter the adsorption of molecules with large uncharged moieties (e.g., surfactants). In these cases, the mechanism of adsorption is dominated by van der Waals interactions, with the surface charge playing a relatively minor role.

Ring slip is among the most widely invoked ligand distortions in organometallic chemistry, yet very few ring slipped geometries have been directly characterized. Here we investigate the ultrafast photochemistry of (η1-C5Cl5)Mn(CO)5 to characterize the “reverse ring-slip” processes that result upon ligand dissociation from this complex in polar and nonpolar solvents. (η1- C5Cl5)Mn(CO)5 readily undergoes dicarbonyl-loss upon photoexcitation across a range of UV-excitation wavelengths, and the fac- ile ejection of a second-CO during the reverse ring-slip process is shown to occur due to a mechanism that relies on the enthalpy released during the reverse ring-slip. This mechanistic paradigm is potentially widespread in organometallic reactions involving changes in ligand hapticity, carrying implications for the expansive range of Cp-ligated organometallic complexes. Experiments in CH2Cl2 solution observe formation of a solvent-coordinated product upon coordination of CH2Cl2 to the monocarbonyl-loss spe- cies. An energetic barrier to solvent coordination is present due to the need for rearrangement of the ring geometry from a three- center M-C-Cl coordination to η1 coordination to accommodate the incoming solvent molecule. Density functional theory calculations are used to investigate the structures of the experimentally observed intermediates, as well as to explore the relevance of these experiments to analogous complexes containing the more commonly encountered Cp (C5H5) and Cp* (C5Me5) cyclopentadienyl ligands.

Ultrafast response of the room temperature ionic liquid (RTIL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpyr][NTf2]) to a photoinjected electron is investigated in few-mon...

Dynamic IR peak coalescence and simulations based on the optical Bloch equations have been used previously to predict the rates of intramolecular electron transfer in a group of bridged mixed valence dimers of the type [Ru3(O)(OAc)6(CO)L]-BL-[Ru3(O) (OAc)6(CO)L]. However, limitations of the Bloch equations for the analysis of dynamical coalescence in vibrational spectra have been described. We have used ultrafast 2D-IR spectroscopy to investigate the vibrational dynamics of the CO spectator ligands of several dimers in the group. These experiments reveal that no electron site exchange occurs on the time scale required to explain the observed peak coalescence. The high variability in FTIR peak shapes for these mixed valence systems is suggested to be the result of fluctuations in the charge distributions at each metal cluster within a single-well potential energy surface, rather than the previous model of two-site exchange.

Suppose that is an i.i.d. symmetric α-stable noise, 1 < α < 2, and consider the moving average process given by . Conditions are obtained for the convergence rate of the moving average series, as well as that of the inverted (autoregressive) representation . These conditions are expressed in terms of the associated function and its reciprocal belonging to certain mixed-norm spaces of functions on the open unit disc. Properties of these spaces are explored. Criteria are also derived for the rate of mixing in a certain sense.

Metal carbonyl dimers and clusters constitute a diverse class of organometallic reagents and catalysts. The photochemistry of these complexes is a topic of significant and long-standing interest, as preparative-scale photolyses constitute many of the most synthetically powerful reactions in organometallic chemistry. The metal–metal bonding present in dimers and clusters is varied and significantly influences their overall reactivity. In this review we discuss the primary photochemical dynamics of transition metal carbonyl metal dimers and clusters, with a focus on the changes in metal–metal bonding that occur upon visible and/or ultraviolet photochemical excitation. The bulk of the results discussed here were obtained using ultrafast time-resolved infrared spectroscopy, a technique with high structural sensitivity afforded by the carbonyl reporter ligands of the complexes studied. Picosecond time resolution allows detailed monitoring of the photochemical reaction dynamics, including observation of initially excited complexes and short-lived transient metal–metal bond cleavage intermediates, as well as formation of the longer-lived, yet reactive, intermediates responsible for reactivity occurring on diffusion-limited time scales and beyond.

Time resolved two-photon photoemission (TPPE) is used to probe the unoccupied electronic structure of monolayer films of dicarbonitrile-quaterphenyl (NC-Ph 4 -CN) on Ag(1 1 1) and cobalt phthalocyanine (CoPc) on Ag(1 0 0). For both samples, photoelectron spectra show a well-formed series of electronic states near the vacuum level. These are assigned as the 1 ≤ n ≤ 4 image-potential states (IPS's) based on the scaling of their binding energies and lifetimes. Time domain measurements at energies near the vacuum level exhibit intensity oscillations (quantum beats) which are due to excitation of an electronic wave packet of the n ≥ 4 IPS's. The wave packets remain coherent until population decay renders them unobservable. These measurements clearly demonstrate that the classical image-potential state structure is retained to high order ( n ∼ 6) in the presence of aromatic organic adlayers. This represents the first definitive observation via TPPE of quantum beats of electronic origin at the metal/organic interface.

Among the most popular and widely studied CO-releasing molecules (CO-RMs) for biological research is Ru2Cl4(CO)6 (CORM-2). When dissolved in DMSO (the solubilizing agent used in preclinical trials), the dimer cleaves into monomeric DMSO-ligated Ru subunits; the rate of this reaction and behavior of the products formed are thus highly relevant to the efficacy of CORM-2 in vivo. FT-IR spectroscopy was used to monitor this reaction in DMSO and also to monitor the reactivity of CORM-2 and of the monomeric DMSO-ligated derivatives in mouse serum. While the CORM-2 dimer readily liberates CO in mouse serum under ambient conditions, the monomeric Ru subunits formed in DMSO solution do not. This demonstrates that a substantial fraction of the CO-delivery agent being introduced via DMSO solutions would appear to be, in fact, inactive. Photochemical irradiation readily liberates CO from the inactive species, and thus a combined thermal–photochemical approach can greatly improve the yield of CO delivery. Ultrafast ex...

A study of the scope and limitations of varying the ligand framework around the dinuclear core of FvRu(2) in its function as a molecular solar thermal energy storage framework is presented. It includes DFT calculations probing the effect of substituents, other metals, and CO exchange for other ligands on Delta H-storage. Experimentally, the system is shown to be robust in as much as it tolerates a number of variations, except for the identity of the metal and certain substitution patterns. Failures include 1,1',3,3'-tetra-tert-butyl (4), 1,2,2',3'-tetraphenyl (9), diiron (28), diosmium (24), mixed iron-ruthenium (27), dimolybdenum (29), and di-tungsten (30) derivatives. An extensive screen of potential catalysts for the thermal reversal identified AgNO3-SiO2 as a good candidate, although catalyst decomposition remains a challenge.

Fischer carbenes are commonly used as reagents in the synthesis of new carbon–carbon bonds, a reaction made possible by the unique chemistry of the formal metal–carbon double bond. Nevertheless, the photoinduced reactions of these complexes are relatively poorly understood. For instance, it has been postulated but not confirmed that visible irradiation leads to photocarbonylation, in which a CO ligand inserts into the metal–carbon bond to form a metal ketene intermediate. Here, we report the first direct observation of this intermediate following 400 nm photoexcitation of the model group 6 Fischer carbene Cr(CO)5[CCH3(OCH3)]. Using ultrafast time-resolved infrared spectroscopy (TRIR), we observe the formation of three distinct metal ketene structures, which we assign as a singlet and two isoenergetic triplet excited-state structures. The singlet relaxes to the ground state on a time scale of ∼35 ps, whereas the two triplets are long-lived (>2 ns). TRIR of the tungsten analogue yields no evidence for a met...

We report the excited-state electron dynamics for ultrathin films of NaCl on Ag(100). The first three image potential states (IPSs) were initially observed following excitation. The electrons in the spatially delocalized n = 1 IPS decayed on the ultrafast time scale into multiple spatially localized states lower in energy. The localized electronic states are proposed to correspond to electrons trapped at defects in the NaCl islands. Coverage and temperature dependence of the localized states support the assignment to surface trap states existing at the NaCl/vacuum interface. These results highlight the importance of electron trapping in ultrathin insulating layers.

The importance of spin state changes in organometallic reactions is a topic of significant interest, as an increasing number of reaction mechanisms involving changes of spin state are consistently being uncovered. The potential influence of spin state changes on reaction rates can be difficult to predict, and thus this class of reactions remains among the least well understood in organometallic chemistry. Ultrafast time-resolved infrared (TRIR) spectroscopy provides a powerful tool for probing the dynamics of spin state changes in organometallic catalysis, as such processes often occur on the picosecond to nanosecond time scale and can readily be monitored in the infrared via the absorptions of carbonyl reporter ligands. In this Account, we summarize recent work from our group directed toward identifying trends in reactivity that can be used to offer predictive insight into the dynamics of coordinatively unsaturated organometallic reaction intermediates. In general, coordinatively unsaturated 16-electron (16e) singlets are able to coordinate to solvent molecules as token ligands to partially stabilize the coordinatively unsaturated metal center, whereas 16e triplets and 17-electron (17e) doublets are not, allowing them to diffuse more rapidly through solution than their singlet counterparts. Triplet complexes typically (but not always) undergo spin crossover prior to solvent coordination, whereas 17e doublets do not coordinate solvent molecules as token ligands and cannot relax to a lower spin state to do so. 16e triplets are typically able to undergo facile spin crossover to yield a 16e singlet where an associative, exothermic reaction pathway exists. The combination of facile spin crossover with faster diffusion through solution for triplets can actually lead to faster catalytic reactivity than for singlets, despite the forbidden nature of these reactions. We summarize studies on odd-electron complexes in which 17e doublets were found to display varying behavior with regard to their tendency to react with 2-electron donor ligands to form 19-electron (19e) adducts. The ability of 19e adducts to serve as reducing agents in disproportionation reactions depends on whether the excess electron density localized at the metal center or at a ligand site. The reactivity of both 16e and 17e complexes toward a widely used organic nitroxyl radical (TEMPO) are reviewed, and both classes of complexes generally react similarly via an associative mechanism with a low barrier to these reactions. We also describe recent work targeted at unraveling the photoisomerization mechanism of a thermal-solar energy storage complex in which spin state changes were found to play a crucial role. Although a key triplet intermediate was found to be required for this photoisomerization mechanism to proceed, the details of why this triplet is formed in some complexes (those based on ruthenium) and not others (those based on iron, molybdenum, or tungsten) remains uncertain, and further exploration in this area may lead to a better understanding of the factors that influence intramolecular and excited state spin state changes.

A phthalocyanine/Ag(111) interface state is observed for the first time using time- and angle-resolved two-photon photoemission. For monolayer films of metal-free (H2Pc) and iron phthalocyanine (FePc) on Ag(111), the state exists 0.23 ± 0.03 and 0.31 ± 0.03 eV above the Fermi level, respectively. Angle-resolved spectra show the state to be highly dispersive with an effective mass of 0.50 ± 0.15 me for H2Pc and 0.67 ± 0.14 me for FePc. Density functional theory calculations on the H2Pc/Ag(111) surface allow us to characterize this state as being a hybrid state resulting from the interaction between the unoccupied molecular states of the phthalocyanine ligand and the Shockley surface state present on the bare Ag(111) surface. This work, when taken together with the extensive literature on the 3,4,9,10-perylene tetracarboxylic dianhydride/Ag interface state, provides compelling evidence that the hybridization of metal surface states with molecular electronic states is a general phenomenon.

Picosecond to microsecond time-resolved infrared spectroscopy (TRIR) has been used to investigate the photochemistry of [CpFe(CO) 2 ] 2 . The early-time TRIR spectra are dominated by vibrationally hot transient species and this leads to spectra with broad featureless bands. The majority (84%) of the well characterized Cp 2 Fe 2 (μ-CO) 3 , with a band at 1824 cm −1 , grows in on a timescale (15–20 ps) similar to the cooling rate of many of the ν (CO) bands, and this arises from presumably the initially formed species, [CpFe(CO)(μ-CO) 2 FeCp] of which we were unable to find clear evidence. However, Cp 2 Fe 2 (μ-CO) 3 is formed by two different processes and under CO the remaining slower formation (16%) occurs at the same rate (46 ± 2 ns) as a transient band at 1908 cm −1 decays. The 1908 cm −1 had been previously observed in an earlier study but its identity remained unclear. We find that this 1908 cm −1 intermediate has no corresponding bridging ν (CO) band and is formed by a 1-photon process. This band is assigned to a dicarbonyl-loss photoproduct, [CpFe(CO)] 2 and is a rare example of double CO loss occurring following a single-photon excitation in the condensed phase.

A common misconception among critics is that a young David Foster Wallace fell under the influence of John Barth and other postmodernist writers, only to wrest himself free of this sinister authority as he matured as a writer, steering his own fiction away from its sway and becoming one of postmodern fiction's strongest detractors in the process. But a close reading of Wallace's early novella “Westward the Course of Empire Takes Its Way” reveals that Wallace's literary relationship with Barth is better understood as agonistic rather than antagonistic, an example of what Harold Bloom iconically describes as “the anxiety of influence.” “Westward” should be read as not only a misprision of “Lost in the Funhouse,” Barth's predecessor text, but as a self-aware misprision, a knowing enactment of the anxiety of influence, as well as a fulfillment of the putatively unrealized possibilities of Barth's fiction and postmodern fiction in general.

Rotational correlation times ( τ c ) of two pairs of small solutes, CpM(CO) 3 and M 2 (CO) 10 (M = Mn, Re), are determined in various viscous alkane solutions by narrow-band IR pump broad-band IR probe spectroscopy. By choosing these pairs of molecules, which are significantly different in mass but almost identical in volume, shape and in their expected interactions with solvents, we isolate the effects of mass on τ c . The effect of mass was observed clearly for these pairs of solute tracers, with heavier substitutions leading to larger τ c values. In the case of the CpM(CO) 3 pair, in which the moments of inertia do not change much, the effect of mass was seen in the τ c vs. solvent viscosity plot as result of larger slope for heavier substitution, with no clear change in the intercept. For the M 2 (CO) 10 pair, in which the moments of inertia change significantly, this mass effect can be observed through changes in both the slope and intercept.

This paper explores a family of weak parallelogram laws for Banach spaces. Some basic properties of such spaces are obtained. The main result is that a Banach space satisfies a lower weak parallelogram law if and only if its dual satisfies an upper weak parallelogram law, and vice versa. Connections are established between the weak parallelogram laws and the following: subspaces, quotient spaces, Cartesian products, and the Rademacher type and co-type properties.

Electron solvation is examined at the interface of a room temperature ionic liquid (RTIL) and an Ag(111) electrode. Femtosecond two-photon photoemission spectroscopy is used to inject an electron into an ultrathin film of RTIL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpyr](+)[NTf2](-)). While much of current literature highlights slower nanosecond solvation mechanisms in bulk ionic liquids, we observe only a femtosecond response, supporting morphology dependent and interface specific electron solvation mechanisms. The injected excess electron is found to reside in an electron affinity level residing near the metal surface. Population of this state decays back to the metal with a time constant of 400 ± 150 fs. Electron solvation is measured as a dynamic decrease in the energy with a time constant of 350 ± 150 fs. We observe two distinct temperature regimes, with a critical temperature near 250 K. The low temperature regime is characterized by a higher work function of 4.41 eV, while the high temperature regime is characterized by a lower work function of 4.19 eV. The total reorganizational energy of solvation changes above and below the critical temperature. In the high temperature regime, the electron affinity level solvates by 540 meV at 350 K, and below the critical temperature, solvation decreases to 200 meV at 130 K. This study will provide valuable insight to interface specific solvation of room temperature ionic liquids.

The reactivity of five transition metal dimers toward photochemical, in-solvent-cage disproportionation has been investigated using picosecond time-resolved infrared spectroscopy. Previous ultrafast studies on [CpW(CO)3]2 established the role of an in-cage disproportionation mechanism involving electron transfer between 17- and 19-electron radicals prior to diffusion out of the solvent cage. New results from time-resolved infrared studies reveal that the identity of the transition metal complex dictates whether the in-cage disproportionation mechanism can take place, as well as the more fundamental issue of whether 19-electron intermediates are able to form on the picosecond time scale. Significantly, the in-cage disproportionation mechanism observed previously for the tungsten dimer does not characterize the reactivity of four out of the five transition metal dimers in this study. The differences in the ability to form 19-electron intermediates are interpreted either in terms of differences in the 17/19-electron equilibrium or of differences in an energetic barrier to associative coordination of a Lewis base, whereas the case for the in-cage vs diffusive disproportionation mechanisms depends on whether the 19-electron reducing agent is genuinely characterized by 19-electron configuration at the metal center or if it is better described as an 18 + δ complex. These results help to better understand the factors that dictate mechanisms of radical disproportionation and carry implications for radical chain mechanisms.

The picosecond photochemical dynamics of two group 8 transition metal carbonyl clusters, Fe3(CO)12 and Os3(CO)12, have been studied using ultrafast time-resolved infrared spectroscopy. In both the ...

The photochemistry of [CpRu(CO)2]2 in P(OMe)3/CH2Cl2 solution has been studied using picosecond time-resolved infrared (TRIR) spectroscopy. Photolysis at 400 nm leads to the formation of 17-electron CpRu(CO)2(•) radicals, which react on the picosecond time scale to form 19-electron CpRu(CO)2P(OMe)3(•) adducts. The TRIR spectra of this adduct display an unusually low CO stretching frequency for the antisymmetric CO stretching mode, suggesting that one carbonyl ligand adopts a bent configuration to avoid a 19-electron count at the metal center. This spectral assignment is supported by analogous experiments on [CpFe(CO)2]2 in the same solvent, combined with DFT studies on the structures of the 19-electron adducts. The DFT results predict a bent CO ligand in CpRu(CO)2P(OMe)3(•), whereas approximately linear Fe-C-O bond angles are predicted for CpFe(CO)2P(OMe)3(•). The observation of a bent CO ligand in the 19-electron ruthenium adduct is a surprising result, and it provides new insight into the solution-phase behavior of 19-electron complexes. TRIR spectra were also collected for [CpRu(CO)2]2 in neat CH2Cl2, and it is interesting to note that no singly bridged [CpRu(CO)]2(μ-CO) photoproduct was observed to form following 400- or 267-nm excitation, despite previous observations of this species on longer time scales.

© 2016, National Academy of Sciences. All rights reserved. The rise of inorganic-biological hybrid organisms for solar-to-chemical production has spurred mechanistic investigations into the dynamics of the biotic-abiotic interface to drive the development of next-generation systems. The model system, Moorella thermoacetica-cadmium sulfide (CdS), combines an inorganic semiconductor nanoparticle light harvester with an acetogenic bacterium to drive the photosynthetic reduction of CO2to acetic acid with high efficiency. In this work, we report insights into this unique electrotrophic behavior and propose a charge-transfer mechanism from CdS to M. thermoacetica. Transient absorption (TA) spectroscopy revealed that photoexcited electron transfer rates increase with increasing hydrogenase (H2ase) enzyme activity. On the same time scale as the TA spectroscopy, time-resolved infrared (TRIR) spectroscopy showed spectral changes in the 1,700-1,900-cm-1spectral region. The quantum efficiency of this system for photosynthetic acetic acid generation also increased with increasing H2ase activity and shorter carrier lifetimes when averaged over the first 24 h of photosynthesis. However, within the initial 3 h of photosynthesis, the rate followed an opposite trend: The bacteria with the lowest H2ase activity photosynthesized acetic acid the fastest. These results suggest a two-pathway mechanism: a high quantum efficiency charge-transfer pathway to H2ase generating H2as a molecular intermediate that dominates at long time scales (24 h), and a direct energy-transducing enzymatic pathway responsible for acetic acid production at short time scales (3 h). This work represents a promising platform to utilize conventional spectroscopic methodology to extract insights from more complex biotic-abiotic hybrid systems.

Caught in the light: The fulvalene diruthenium complex shown on the left side of the picture captures sun light, causing initial Ru-Ru bond rupture to furnish a long-lived triplet biradical of syn configuration. This species requires thermal activation to reach a crossing point (middle) into the singlet manifold on route to its thermal storage isomer on the right through the anti biradical.

Time-resolved IR spectroscopy and density functional theory calculations indicate that the 14-electron, triplet species, 3Fe(CO)3, generated from photolysis of Fe(CO)5, appears to exist uncoordinated to alkyl groups in alkane solvents. In alcohols of varying lengths, triplet 3Fe(CO)3 forms a hydroxyl-coordinated complex on the time scale of tens of picoseconds, implying that its solvation kinetics are diffusion-limited. Surprisingly, the hydroxyl-coordinated complex remains in a triplet state, in contrast to the activity of triplet 3Fe(CO)4, which must convert to a singlet state to coordinate to a solvent molecule. To our knowledge, this study represents the first investigation into the detailed metal–solvent interactions and rearrangement kinetics of a 14-electron complex on the ultrafast time scale.

The photochemical rearrangement dynamics of Co4(CO)12 were studied using picosecond time-resolved infrared spectroscopy. In cyclohexane and CH2Cl2 solvents, monitoring the kinetics of absorptions i...

The activation of Sn–H bonds in tributylstannane by three triplet organometallic photoproducts (Fe(CO)4, CpCo(CO), and CpV(CO)3) has been studied using picosecond time-resolved infrared spectroscop...

The picosecond dynamics of CpCo(CO), a catalytic intermediate in the cyclotrimerization of alkynes, have been studied using time-resolved infrared spectroscopy and density functional theory calculations. In neat 1-hexyne or 1-hexene solution, the first intermediate to form is a triplet η2-coordinated species, which then converts to a singlet η2 species in ca. 30–40 ps. The η2 triplet is the only solvent-coordinated species observed at early times, suggesting that this is the dominant mechanistic pathway for coordination of alkynes and alkenes to the CpCo(CO) catalyst. These new results complement the recent discovery that CpCo(CO) can coordinate to certain solvent molecules in both singlet and triplet spin states and further support previous studies implicating triplet intermediates in cobalt-catalyzed cyclotrimerization and cyclo-oligomerization reaction mechanisms.

End of the Road, the 1970 film adaptation of John Barth's 1958 novel The End of the Road, offers an object lesson in the perils of transferring a subjective verbal medium, literary fiction, into an objective visual medium, film. End of the Road is finally unsuccessful because of its often puzzling departures from Barth's novel, its inability to seamlessly graft the filmmakers's explicit political agenda onto a novel whose politics are, at best, implicit, and its failure to find an equivalent in its mise-en-scene for Jake Horner's essential narrational voice.

We study stability of a numerical method in which the backward Euler method is combined with order one convolution quadrature for approximating the integral term of the linear Volterra integrodifferential equation $\mathbf{u}'(t)+\int_{0}^{t}\beta(t-s)\mathbf{Au}(s)\,ds=0$, $t\geq0$, $\mathbf{u}(0)=\mathbf{u}_{0}$, which arises in the theory of linear viscoelasticity. Here $\mathbf{A}$ is a positive self-adjoint densely defined linear operator in a real Hilbert space, and $\beta(t)$ is locally integrable, nonnegative, nonincreasing, convex, and $-\beta'(t)$ is convex. We establish stability of the method under these hypotheses on $\beta(t)$. Thus, the method is stable for a wider class of kernel functions $\beta(t)$ than was previously known. We also extend the class of operators $\mathbf{A}$ for which the method is stable.

Two of the primary hurdles facing organic electronics and photovoltaics are their low charge mobility and the inability to disentangle morphological and molecular effects on charge transport. Specific chemical groups such as alkyl side chains are often added to enable spin-casting and to improve overall power efficiency and morphologies, but their exact influence on mobility is poorly understood. Here, we use two-photon photoemission spectroscopy to study the charge transport properties of two organic semiconductors, one with and one without alkyl substituents (sexithiophene and dihexyl-sexithiophene). We show that the hydrocarbon side chains are responsible for charge localization within 230 fs. This implies that other chemical groups should be used instead of alkyl ligands to achieve the highest performance in organic photovoltaics and electronics.

The photochemical disproportionation mechanism of [CpW(CO){sub 3}]{sub 2} in the presence of Lewis bases PR{sub 3} was investigated on the nano- and microsecond time-scales with Step-Scan FTIR time-resolved infrared spectroscopy. 532 nm laser excitation was used to homolytically cleave the W-W bond, forming the 17-electron radicals CpW(CO){sub 3} and initiating the reaction. With the Lewis base PPh{sub 3}, disproportionation to form the ionic products CpW(CO){sub 3}PPh{sub 3}{sup +} and CpW(CO){sub 3}{sup -} was directly monitored on the microsecond time-scale. Detailed examination of the kinetics and concentration dependence of this reaction indicates that disproportionation proceeds by electron transfer from the 19-electron species CpW(CO){sub 3}PPh{sub 3} to the 17-electron species CpW(CO){sub 3}. This result is contrary to the currently accepted disproportionation mechanism which predicts electron transfer from the 19-electron species to the dimer [CpW(CO){sub 3}]{sub 2}. With the Lewis base P(OMe){sub 3} on the other hand, ligand substitution to form the product [CpW(CO){sub 2}P(OMe){sub 3}]{sub 2} is the primary reaction on the microsecond time-scale. Density Functional Theory (DFT) calculations support the experimental results and suggest that the differences in the reactivity between P(OMe){sub 3} and PPh{sub 3} are due to steric effects. The results indicate that radical-to-radical electron transfer is a previouslymore » unknown but important process for the formation of ionic products with the organometallic dimer [CpW(CO){sub 3}]{sub 2} and may also be applicable to the entire class of organometallic dimers containing a single metal-metal bond.« less

Ground state structures with side-on nitrosyl ({eta}{sup 2}-NO) and isonitrosyl (ON) ligands have been observed in a variety of transition-metal complexes. In contrast, excited state structures with bent-NO ligands have been proposed for years but never directly observed. Here we use picosecond time-resolved infrared spectroscopy and density functional theory (DFT) modeling to study the photochemistry of Co(CO){sub 3}(NO), a model transition-metal-NO compound. Surprisingly, we have observed no evidence for ON and {eta}{sup 2}-NO structural isomers, but have observed two bent-NO complexes. DFT modeling of the ground and excited state potentials indicates that the bent-NO complexes correspond to triplet excited states. Photolysis of Co(CO){sub 3}(NO) with a 400-nm pump pulse leads to population of a manifold of excited states which decay to form an excited state triplet bent-NO complex within 1 ps. This structure relaxes to the ground triplet state in ca. 350 ps to form a second bent-NO structure.

Here we report nano- through microsecond time-resolved IR experiments of iron-catalyzed alkene isomerization in room-temperature solution. We have monitored the photochemistry of a model system, Fe(CO){sub 4}({eta}{sup 2}-1-hexene), in neat 1-hexene solution. UV-photolysis of the starting material leads to the dissociation of a single CO to form Fe(CO){sub 3}({eta}{sup 2}-1-hexene), in a singlet spin state. This CO loss complex shows a dramatic selectivity to form an allyl hydride, HFe(CO){sub 3}({eta}{sup 3}-C{sub 6}H{sub 11}), via an internal C-H bond-cleavage reaction in 5-25 ns. We find no evidence for the coordination of an alkene molecule from the bath to the CO loss complex, but do observe coordination to the allyl hydride, indicating that it is the key intermediate in the isomerization mechanism. Coordination of the alkene ligand to the allyl hydride leads to the formation of the bis-alkene isomers, Fe(CO){sub 3}({eta}{sup 2}-1-hexene)({eta}{sup 2}-2-hexene) and Fe(CO){sub 3}({eta}{sup 2}-1-hexene){sub 2}. Because of the thermodynamic stability of Fe(CO){sub 3}({eta}{sup 2}-1-hexene)({eta}{sup 2}-2-hexene) over Fe(CO){sub 3}({eta}{sup 2}-1-hexene){sub 2} (ca. 12 kcal/mol), nearly 100% of the alkene population will be 2-alkene. The results presented herein provide the first direct evidence for this mechanism in solution and suggest modifications to the currently accepted mechanism.

Many properties of chemical reactions are determined by the transition state connecting reactant and product, yet it is difficult to directly obtain any information about these short-lived structures in liquids. We show that two-dimensional infrared (2D-IR) spectroscopy can provide direct information about transition states by tracking the transformation of vibrational modes as a molecule crossed a transition state. We successfully monitored a simple chemical reaction, the fluxional rearrangement of Fe(CO) 5 , in which the exchange of axial and equatorial CO ligands causes an exchange of vibrational energy between the normal modes of the molecule. This energy transfer provides direct evidence regarding the time scale, transition state, and mechanism of the reaction.

Morphology- and layer-dependent electronic structure and dynamics at the PTCDA/Ag(111) interface have been studied with angle-resolved two-photon photoemission. In Stranski-Krastanov growth modes, the exposed wetting layer inhibited the evolution of the vacuum level and valence band to bulk values. For layer-by-layer growth, we observed the transition of electron structure from monolayer to bulk values within eight monolayers. Effective masses and lifetimes of the conduction band and the n = 1 image potential state were measured to be larger for disordered layers. The effective mass was interpreted in the context of charge mobility measurements.

The process of iron- and ruthenium-assisted alkene isomerization is investigated with ultrafast time-resolved infrared spectroscopy under ambient conditions. M(CO)4(η2-1,4-PD) (M = Ru, Fe; PD = pentadiene) was irradiated with a 100 femtosecond pulse of 266 nm light in a solution of cyclohexane, and a single carbonyl was photodissociated. When M = Ru, experimental results indicate that both Ru(CO)3(η4-1,4-PD) and HRu(CO)3(η3-allyl) form within a few picoseconds, and their populations do not change significantly out to 800 ps. When M = Fe, the coordinatively unsaturated triplet metal complex, 3Fe(CO)3(η2-1,4PD), was observed to form rapidly and rearranged to HFe(CO)3(η3-allyl) at a rate of (4.8 ± 0.2) × 108 s-1. Surprisingly, the triplet iron complex did not coordinate the free alkene on the pentadiene ligand on the ultrafast time scale. The difference in reactivity between the iron- and the ruthenium-assisted alkene isomerization is due to differences in spin state of the coordinatively unsaturated metal c...

The initial steps of β-hydrogen elimination from the complex CpW(CO)3(CH2CH3) were studied using ultrafast infrared and step-scan FTIR spectroscopy combined with density functional theory calculations. The reaction was initiated by a pulse of UV light at 266 nm and followed with infrared light on the femto- to microsecond timescale, allowing for the identification and kinetic characterization of transients. In the process of β-H elimination, the primary photoproduct was found to be an α-H stabilized complex and the dynamics of the elusive species cis-CpW(CO)3(H)(CH2CH2) were resolved, allowing for a full kinetic and thermodynamic analysis of the reaction. In a side reaction, CpW(CO)3• is formed via ethyl ligand dissociation; the measured formation rate indicates that the complex forms from a vibrationally excited parent complex. Density functional theory calculations of all relevant transient intermediates and transition states involved in β-H elimination of CpW(CO)3(CH2CH3) are presented. These calculati...

Conventional ultrafast spectroscopic studies on the dynamics of chemical reactions in solution directly probe the solute undergoing the reaction. We provide an alternative method for probing reaction dynamics via monitoring of the surrounding solvent. When the reaction exchanges the energy (in form of heat) with the solvent, the absorption cross sections of the solvent's infrared bands are sensitive to the heat transfer, allowing spectral tracking of the reaction dynamics. This spectroscopic technique was demonstrated to be able to distinguish the differing photoisomerization dynamics of the trans and cis isomers of stilbene in acetonitrile solution. We highlight the potential of this spectroscopic approach for studying the dynamics of chemical reactions or other heat transfer processes when probing the solvent is more experimentally feasible than probing the solute directly.

The room temperature ionic liquid [Bmpyr]+[NT f2]− is studied on Ag(111). Monolayer and bilayer films both show a single state solvating, the timescale of solvation and magnitude of energy relaxation is significantly greater for the bilayer.

The photochemistry of the trimetal cluster,Ru(3)(CO)12, was investigated on the ultrafast timescale using UV-vis pump, infrared probe spectroscopy in order to study the transient intermediates formed upon photoexcitation.

The ultrafast solvation and rearrangement dynamics following the photolysis of M(CO)6 (M = Cr, Mo, W) in terminal alkyne solutions have been investigated. Upon photoinitiated loss of a ligand from the parent metal hexacarbonyl, coordination to an alkyne solvent molecule is followed by rearrangement to a complex that is bound to the terminal alkyne site in a π-bonded fashion. This rearrangement occurs on a time scale that is anticorrelated with the metal−ligand binding energy. A dissociative rearrangement is ruled out for this process in favor of an associative or interchange mechanism. Previous theoretical and experimental studies are examined, and we conclude that the ultimate rearrangement to a vinylidene species is energetically inaccessible at ambient temperatures on the femtosecond to millisecond time scale.

The mechanism of intermolecular ligand exchange has been studied using transition path sampling (TPS) based molecular dynamics (MD) simulations. Specifically, the exchange of solvent molecules bound to unsaturated Cr(CO)5 in methanol solution has been investigated. The results of the TPS simulations have shown that there are multiple steps in the reaction mechanism. The first involves partial dissociation of the coordinated solvent from the Cr metal center followed by association with a new methanol molecule between the normally void first and second solvent layers. After diffusive motion of the exchanging ligands, the last step involves the originally bound methanol molecule moving into the bath continuum followed by solvation of the Cr metal fragment by the exchanging ligand. It has been found that the reaction center (defined as the organometallic fragment and two exchanging ligands only) and the solvent bath have favorable interactions. This is likely due to the adiabatic nature of the ligand exchange transition. The ability to understand the microscopic molecular dynamics of a chemical process based on a free energy analysis is also discussed.

We review recent work in the study of interfacial electronic states and electron–adsorbate interactions with time- and angle-resolved two-photon photoemission (2PPE) spectroscopy. Results for interfaces between noble-metal surfaces and organic as well as inorganic dielectric overlayers are presented. Layer structure and thickness have pronounced effects on the spatial extents, binding energies, and dynamics of image-potential states (IPS) and molecular orbitals. The transition from delocalized to localized states in the plane of the interface can also occur dynamically through electron-induced nuclear motion in the overlayer. An important example of this class of phenomenon is the formation of polaronic states from initially delocalized IPS, which occurs on sub-picosecond timescales. Dynamic energy relaxation through electron solvation by molecular dipoles, the analogue of the dynamic Stokes shift in bulk solvents, may also accompany localization on ultrafast timescales, and has become the focus of much recent experimental and theoretical interest. The interplay of a static layer structure and an evolving potential from the adsorbate creates a rich environment for interfacial electron dynamics, as evidenced by the alcohol/Ag(1 1 1), nitrile/Ag(1 1 1), and D 2 O/Cu(1 1 1) systems. We conclude with a discussion of recent attempts to determine the spatial extent of localization parallel to the interface for electrons following localization and solvation using experimental measurements of the photoelectron distribution in momentum space.

The low-temperature equilibrium and nonequilibrium solvation dynamics of an excess electron in bulk and at a surface interface have been characterized using mixed quantum/classical simulation methods. A methanol bath was modeled using classical molecular dynamics, whereas the properties of an excess electron were calculated from a wave function based approach. The temperature dependence of the bulk response has been found to be minimal, which may indicate that large scale hydrogen bond breaking and diffusive motion may not play important roles in the solvation dynamics. The equilibrium dynamics have also been compared to nonequilibrium simulations of charge injection into the neat bath. An excess electron at a methanol/Pt(100) surface interface has been shown to become solvated by the bath yet is less bound by a factor of ∼2 compared to the bulk. The solvent response function also displays interesting differences when compared to the low temperature bulk glass. These results also reveal many of the micros...