Effect of Alkyl Group on Mx

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Effect of Alkyl Group on MO + ROH (M = Mo, W; R = Me, Et) Reaction Rates Manisha Ray, Sarah E. Waller, and Caroline Chick Jarrold J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b00102 • Publication Date (Web): 15 Feb 2016 Downloaded from http://pubs.acs.org on February 17, 2016

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The Journal of Physical Chemistry

Effect of Alkyl Group on MxOy− + ROH (M = Mo, W; R = Me, Et) Reaction Rates Manisha Ray,a Sarah E. Waller, and Caroline Chick Jarrold*,a a Indiana University, Department of Chemistry, 800 East Kirkwood Avenue, Bloomington, IN 47405 b Department of Chemistry, SUNY Stony Brook, Stony Brook, NY 11794-3400 * EMAIL: [email protected]; Fax: 812-855-8300 Abstract A systematic comparison of MxOy− + ROH (M = Mo versus W; R = Me versus Et) reaction rate coefficients and product distributions combined with results of calculations on weakly-bound MxOy−⋅ROH complexes suggest that the overall reaction mechanism has three distinct steps, consistent with recently reported results on analogous MxOy− + H2O reactivity studies. MxOy− + ROH → MxOy+1− + RH oxidation reactions are observed for the least oxidized clusters, and MxOy− + ROH → MxOyROH− addition reactions are observed for clusters in intermediate oxidation states, as observed previously in MxOy− + H2O reactions. The first step is weakly-bound complex formation, the rate of which is governed by the relative stability of the MxOy−⋅ROH charge-dipole complexes and the Lewis acid-base complexes. Calculations predict that MoxOy− clusters form more stable Lewis acid-base complexes than WxOy−, and the stability of Consistent with this result, MoxOy− + ROH rate

EtOH complexes is enhanced relative to MeOH.

coefficients are higher than analogous WxOy− clusters. Rate coefficients range from 2.7 x 10-13 cm3 s−1 for W3O8− + MeOH to 3.4 x 10−11 cm3 s−1 for Mo2O4− + EtOH. Second, a covalently-bound complex is formed, and anion PE spectra of the several MxOyROH− addition products observed are consistent with hydroxyl-alkoxy structures that are formed readily from the Lewis acid-base complexes. Calculations indicate that addition products are trapped intermediates in the MxOy− + ROH → MxOy+1− + RH reaction, and the third step is rearrangement of the hydroxyl group to a metal hydride group to facilitate RH release. Trapped intermediates are more prevalent in MoxOy− reaction product distributions, indicating that the rate of this step is higher for WxOy+1RH− than for MoxOy+1RH−. This result is consistent with previous computational studies on analogous MxOy− + H2O reactions predicting that barriers along the pathway in the rearrangement step are higher for MoxOy− reactions than for WxOy−.

Keywords: gas-phase reactivity, ion-molecule reaction kinetics, mass spectrometry, transition metal oxides, cluster anions, H2 production, methanol, ethanol.

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1. INTRODUCTION Transition metal oxides (TMO’s) are found in a wide range of technological applications,1−9 including heterogeneous catalysis for processes that are important from economic, energy, and environmental perspectives.10,11 Optimization and design of TMO catalysts can be challenging due to the low abundance of active sites, which tend to be on defect sites such as oxygen vacancies,12 and the dynamic nature of metal oxide surfaces surface-adsorbate interactions.13 The cluster-like properties14 of TMO surfaces have therefore stimulated numerous studies that use cluster models to explore the atomicand molecular-scale interactions of TM centers in various oxidation states with a range of reactants, including alcohols.15-30 Our research group has investigated the reactivity of molybdenum and tungsten oxide anions with several small molecules, including CO,31,32 CO2,33 CH4 and C2H6.34 We recently completed a series of experimental and computational studies on the reactions between Group 6 transition metal oxide cluster anions and water, MxOy− + H2O (M = Mo, W; x = 1 – 4; y ≤ 3x), with the goal of understanding the molecular scale interactions between water and oxygen vacancies on TMO catalysts that can lead to photocatalytic H2 production.35−41 Results of the studies have underscored the importance of the relative Lewis acidity of the suboxide species, ionic radii, metal-oxygen bond energies, and subtle differences in barrier heights that determine whether free H2 production or dissociative addition will occur in watercluster reactions.

Calculated reaction pathways, which were verified by anion photoelectron (PE)

spectroscopy on trapped intermediates, also suggested that proton mobility via hydroxide fluxionality was an important feature in H2 production. To further test this finding, a study on reactions between MoxOy− and ROH [R = Me (CH3) and Et (C2H5)] was completed.42 The hypothesis driving the study was that the heavier R group would slow the reaction kinetics, and new trapped-intermediate addition products might be observed. Due to a simple but large systematic error in the equation used to approximate the number density of ROH in the cluster reactor, we incorrectly reported a significant decrease in MoxOy− + ROH reaction rates relative their analogous MoxOy− + H2O reactions. We have corrected the reaction rate coefficients in 2 ACS Paragon Plus Environment

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the current study, finding that MoxOy− + EtOH reaction rates are higher than, MoxOy− + H2O and MoxOy− + MeOH rates. In addition, we report this corrected analysis in comparison with results of parallel WxOy− + ROH reactivity studies, the anion PE spectra of addition products (MxOy+1RH−) formed in several reactions (which are high-energy trapped intermediates in the MxOy− + ROH → MxOy+1− + RH reaction, vide infra) and calculations on MoxOy−+ ROH and WxOy− + ROH reaction complexes. We compare these results to our more recently reported experimental results43 showing that MoxOy− + H2O and WxOy− + H2O have different rates of reactive complex formation, and that MoxOy− + H2O reactions have modest antiArrhenius temperature dependence, while WxOy− + H2O showed no distinct temperature dependence. These results point to the importance of free energies of weakly-bound complexes leading to reactive complexes. The grand picture that emerges from these combined studies is that the cluster anion reactions with H2O, MeOH, and EtOH have very similar mechanisms, MoxOy− clusters form reactive complexes with ROH with higher rate coefficients than WxOy− clusters, and WxOy− suboxide clusters are less likely to form trapped intermediates (MxOy+1RH−). Disparities in rate coefficients can again be related to the relative stability of weakly bound complexes at the entrance to the reaction channels.

2. METHODS 2.A. Experimental Details. elsewhere.44

The experimental apparatus used has been described in detail

Briefly, tungsten and molybdenum oxide cluster anions were produced in a pulsed

molecular beam, laser ablation source.45 A compressed sample [98/2 mol% International Corp., 99+ enrichment) for WxOy− clusters; pure

98

186

W/98Mo (Trace Sciences

Mo for MoxOy− clusters] was ablated

using the 2nd harmonic output of a Nd:YAG laser (532 nm, 10 mJ/pulse) operating at a 30 Hz repetition rate. The resulting plasma was entrained in a pulse of ultra-high purity (UHP) He carrier gas (40 psig, backing pressure) issued from a pulsed molecular beam valve (BV1) to facilitate cluster formation and cooling. A second beam valve (BV2) was used to introduce a reactant mixture of MeOH (Macron, 3 ACS Paragon Plus Environment


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anhydrous, ACS reagent grade) or EtOH (Pharmco-Aaper, absolute, ACS/USP grade) seeded in UHP He into the reaction channel. The number density of MeOH or EtOH was varied both by changing the concentration of the alcohol in the reactant gas mixture and by varying the duration of the gas pulse issued from BV2. Reactant gas mixtures were prepared at room temperature by introducing alcohol at ambient vapor pressure (13.02 kPa and 5.95 kPa for MeOH and EtOH, respectively) into an evacuated mix tank, then adding 400 psig of UHP He. Lower concentrations were obtained through serial dilutions. The gas mixture expanded from the reactor into a vacuum chamber and was collimated by a 3mm skimmer. Negative anions were then accelerated on-axis into a 1.2-m beam modulated time-of-flight mass spectrometer. After passing through a 3-mm mass defining slit, the ions drifted an additional 80 cm before colliding with a dual microchannel plate assembly ion detector. Mass spectra were recorded using

a digitizing oscilloscope. The mass resolution, ∆, of the mass spectrometer in the range of the ion masses in this study is 300. Mass spectra were analyzed with OriginPro 9.0, a data analysis and graphics program. Overlapping peaks were fit with Gaussian functions to determine the contribution of each mass to the total peak intensity. It was assumed MxOy−–Mx′Oy− and cluster fragmentation reactions do not contribute significantly to the product distributions. To determine whether analogous MxOy− + ROH addition complex ions have similar molecular and electronic structures for different R groups, anion PE spectra of several series of long-lived MxOy+1RH− (R = H, Me, Et) intermediates were recorded.

Mass-selected complexes were photodetached prior to

colliding with the ion detector using the third harmonic output (3.49 eV) of a second Nd:YAG laser. A small fraction of the resulting photoelectrons travelled the length of a 1-m field-free drift tube situated perpendicular to the ion drift path, and collided with an electron detector. Drift times of the electrons were recorded using a digitizing oscilloscope, and converted to electron binding energy, calibrated with well-known PE spectra of Mo− and MoO−.46 The spectra shown below were collected for ca. 1 million laser shots. 4 ACS Paragon Plus Environment

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2.B. Computational Details. Structures and energies of charge-dipole complexes and complexes formed by the most stable Lewis acid-base complex in which a metal center on the cluster acts as an acid toward the lone pairs on the O-atom in water and local dipole-dipole interactions align an H−OR bond with an M−O bond for M2Oy− (M = Mo and W; y = 4, 5) were investigated with density functional theory (DFT) using the GAUSSIAN 0947 suite of electronic structure calculation software. Charge–dipole complexes were obtained using a potential energy surface scan, where the cluster-ROH intermolecular distance varies incrementally (0.2 Å) starting from the most stable Lewis acid-base complex towards a large charge-dipole separation of 10 Å. From the resulting potential energy scan, further tight optimizations were carried out to identify the charge-dipole complex and the transition state connecting the two complexes. Vibrational frequency calculations were performed for all stationary points to confirm that minima had all positive frequencies while transition states had a single imaginary frequency. Based on previous studies, the popular B3LYP functional can underestimate the energy of weakly bound complexes as well as barriers; therefore, all calculations were performed using the unrestricted M06 hybrid density functional method 48 which has demonstrated better accuracy for this application.49,50 The Stuttgart-Dresden (SDD) relativistic pseudopotential was used to replace the 28 core electrons of Mo and 60 core electrons of W, and the 14 remaining valence electrons were treated by an augmented version of the associated double-ζ basis set. 51 Diffuse s-, p-, and d- functions were also included in the basis set to properly describe the increased radial extent of the anion wavefunction. Use of this diffuse basis set results in negligible basis set superposition error (BSSE) when treating weakly-bound cluster−reactant complexes, as others have found,52 so BSSE corrections were not included in the reported energies. Single point calculations were performed on the resulting double-ζ optimized structures of the minima and the transition states in the potential energy surface using augmented triple-ζ basis set as proposed by Martin and Sundermann. 53,54 Free energies and enthalpies of the charge-dipole complex, the Lewis acid-base complex, and the relevant transition states were obtained from a thermodynamic analysis at the augmented double-ζ level



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(using a standard vibrational analysis within the harmonic-oscillator, rigid-rotor approximations), and extrapolated using augmented triple-ζ single point energies. Changes in Gibbs free energies and enthalpies were determined by taking the difference in the free energy and enthalpy of the reaction complexes from the ROH and the cluster at infinite separation. 3. RESULTS AND ANALYSIS 3.A. Reaction product distributions. Figure 1 shows characteristic initial cluster distributions generated in the laser ablation source for

98

Mo2Oy− [black dotted trace, Fig. 1(a)] and

186

W2Oy− [black

dotted traces, Fig. 1(b)] along with their respective typical M2Oy− + MeOH product distributions (solid green traces).

The mass scales are set to facilitate a direct comparison between Mo2Oy− and W2Oy−

clusters. Figs. 1(c) and 1(d) show analogous results for M2Oy− + EtOH reactions (red traces). In all four panels, it is evident that the more reduced clusters in the M2Oy− series are partially depleted in the reactions, and products are observed both at higher mass and between several of the bare oxide masses. The supporting information shows typical series of mass spectra obtained with incremental increases in ROH number density for MxOy− + MeOH and MxOy− + EtOH (x = 2, 3). Oxidation versus ROH addition- While depletion of the most reduced clusters and subsequent build-up of more highly oxidized clusters suggests a direct oxidation via O + OH → O + H

(1)

there are several prominant ROH addition products observed, with the 32 amu mass addition for MeOH indicated with green arrows, and the 46 amu mass addition for EtOH indicated by red arrows, via O + OH → O H

(2)

Additionally, there are numerous less abundant MxOyH− and MxOyCmHn− species (vide infra). To a large extent, the predominant reactions observed in this study are analogous to MxOy− + H2O reactions, in which 6 ACS Paragon Plus Environment

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the most reduced clusters are sequentially oxidized, and the more oxidized clusters undergo addition reactions, though both oxidation and addition reactions are observed for several of the clusters. These results are evident from the evolution of relative peak intensities for both the suboxide clusters and the addition products as the number of cluster-ROH collisions is increased. Figure 2 shows representative plots of the relative intensities of M2O4− clusters (cluster in an intermediate oxidation state), the summed intensities of the suboxide M2Oy− (y ≤ 5) clusters, the stoichiometric M2O6− clusters, and the M2O5− + ROH addition products, which are the most abundant addition products as shown in Figs 1(a)-(d) for R = Me [(a) M = Mo and (b) M = W] and Et [(c) M = Mo and (d) M = W]. Figs. 2(a) and (c) also show a common double-ROH addition product observed for Mo2O4−, Mo2O6R2− (loss of two H-atoms) which involves the dehydrogenation product Mo2O5CH2− in MeOH reactions and Mo2O5C2H4− in EtOH reactions, as shown in Fig. 1(a) and (c). Because the cluster oxide distribution can fluctuate over the course of recording the series of mass spectra with increasing ROH number density, numerous series of mass spectra were collected with different reactant gas concentrations and different cluster oxide distributions, and the final results on reaction rates (presented below) are determined from an average of numerous series of spectra. The plots shown in Fig. 2 are from single series of mass spectra obtained for the four different combinations under the most similar conditions. What emerges is that for both M = Mo, W and R = Me, Et, the intensity of the suboxide species decays with the number of collisions with ROH. The M2O6− peak intensity increases for all four cases, though to widely varying degrees. While the differences in the Mo2Oy− + ROH and W2Oy− + ROH reaction pattern are small and subtle, patterns of Mo3Oy− and W3Oy− + ROH reactions exhibit striking differences. Figure 3 shows mass spectra of typical initial and post-reaction ion distributions measured for the (a) Mo3Oy− + MeOH, (b) W3Oy− + MeOH, (c) Mo3Oy− + EtOH, and (d) W3Oy− + EtOH reactions. As seen in previous studies on M3Oy− + H2O/D2O reactions,38,39,43 Mo3O6− is non-reactive, while W3O6− is highly reactive, though their 7 ACS Paragon Plus Environment


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electronic and molecular structures are nearly identical.55 Additionally, Mo3O5− undergoes ROH addition (forming Mo3O6RH−) along with oxidation (forming Mo3O6− + RH) reactions, whereas W3O5− only undergoes oxidation.

Finally, the nearly stoichiometric Mo3O8− cluster primarily undergoes ROH

addition reactions to form Mo3O9RH−, while its W3O8− congener primarily undergoes oxidation to form W3O9− + RH. Dehydrogenation and abstraction- In addition to direct oxidation and ROH addition, evidence of MxOy− + ROH → MxOy+1R′− + nH2 dehydrogenation reactions is observed, and the extent of dehydrogenation can be correlated with the oxidation state of the cluster. In Fig. 1(a), the small combs situated to the lower-mass side of Mo2O4− and Mo2O5− indicate Mo2O3CHz− and Mo2O4CHz− (z = 0, 1, 2; z ≠ 3), possible because of the electron-rich Mo centers in both Mo2O2− and Mo2O3−. On the other hand, the only dehydrogenation product inferred for Mo2O4− is Mo2O5CH2−, which is the most abundant dehydrogenation product observed in Fig. 1(a), and this ion evidently undergoes an additional MeOH addition reaction, as indicated by the green arrow and pointed out above. Another fairly prominent double addition/dehydrogenation product, Mo2O5C2H4−, is also indicated in Fig. 1(a).

Analogous

dehydrogenation and addition products are observed for MoxOy− + EtOH reactions, and are similarly indicated in Fig. 1(c). Addition/dehydrogentation products are not as cleanly resolved in the W2Oy− mass spectra shown in Figs 1(b) and (d), but ions that have undergone reactions with two ROH molecules are less abundant. Finally, there are additional low intensity peaks in all the mass spectra shown that suggest abstraction of OR or OH by the clusters, e.g.: O + OH → O + ∙ H

(3)

O + OH → O H + ∙

(4)

or

While these products are minor, they are more prevalent than what was observed in previous MxOy− + 8 ACS Paragon Plus Environment

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H2O studies, which is likely a reflection of the larger bond dissociation energy of HO−H (5.18 eV) compared to MeO−H (4.59 eV), EtO−H (4.59 eV), Me−OH (3.64 eV) and Et−OH (4.08 eV).56 A diagram showing all single bond dissociation energies is included in the supporting information, for convenience. 3.B. Comparison of relative rate coefficients. Using data on cluster intensity versus approximate cluster-ROH collisions, we can approximate the relative reaction rate coefficients graphically using procedures described previously.38,39 A detailed description is given in the supporting information. To summarize, we assume the reactions follow pseudo-1o kinetics since the number density of MeOH or EtOH is several orders of magnitude higher than the number density of any given cluster. The experiment follows the initial rate method: The ROH number density, from which we can approximate the number of cluster-ROH collisions over the ca. 15 µs reactor residence time, is varied, rather than the reaction time. We therefore analyze the ROH-cluster reaction kinetics by converting the standard rate expression to the change in cluster ion intensity with respect to the number of collisions, c:

∙

=

(5)

where represents the number densities of the cluster, which we assume to be proportional to the integrated peak intensity, recorded in the mass spectra, and frequency. The value for

,

the inverse of the collision

= kcollisionNROH can be derived with various levels of rigor: The hard-sphere

collision rate coefficient in this case turns out to be close in value to the Langevin collision rate,57,58 & /)

!" = 2$% '

(6)

Using NIST values,59 kL is 7.5 x 10-10 cm3 s−1 for MeOH and 8.2 x 10−10 cm3 s−1 for EtOH. These are the values used in this analysis and to approximate the number of collisions in the plots shown in Fig.2. Using the more sophisticated variation treatment described by Bowers and coworkers,58 which also takes into account the dipole moment of in a charge-dipole collision, kvar = 2.3 x 10−9 cm3 s−1 for MxOy− + 9 ACS Paragon Plus Environment


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MeOH, and 1.7 x 10−9 cm3 s−1 for MxOy− + EtOH. However, as will be seen below, the 2o rate constants recovered from the measurements, use the same rate coefficient to convert from reaction efficiency. The analysis assumes the most reduced clusters in each MxOy− series (e.g. W2O3− is the most reduced cluster in the W2Oy− cluster series) can only be consumed in ROH reactions, and follow simple rate law: ln ,

,- .

- .,0102034

= −! ∙ c

(7)

where k1 in this case is a reaction efficiency. Intermediate clusters are both generated and consumed by sequential oxidation, e.g.: ,- 7

= ! ∙ - . − !) ∙ - 7

(8)

From which a value of k2 can be determined using the previously determined value of k1 by plotting ,- 7

− ! ∙ - . as a function of -7 . Reaction efficiencies are then converted to 2o rate constants

by multiplying through by the collision rate coefficient (kL, in this case). Figure 4 shows the relative reaction efficiencies for reactions with ROH and our most recently measured room-temperature H2O reaction efficiencies. For cases in which there are competing addition and oxidation reactions, we have included solid shapes for the oxidiation and open plus (+) symbols for the addition reactions. Table 1 summarizes the 2o rate constants for all dominant reactions inferred from the study. We point out here that there are a number of approximations made in the analysis and emphasize that the true rate coefficients may be an order of magnitude higher or lower than that the values listed here. However, the relative magnitude of the rate constants is accurate to ca. 20%. We can therefore draw several general conclusions: (1) The rate coefficients of MxOy− + EtOH are twice as high has analogous MeOH reactions. (2) Analogous MoxOy− + MeOH and MoxOy− + H2O reactions have very similar rate constants, while WxOy− + MeOH rate coeffiencts are generally twice the analogous H2O rate 10 ACS Paragon Plus Environment

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coefficients. (3) In cases of competing addition and oxidation reactions, oxidation is significantly more prevalent for WxOy− + ROH reactions than for MoxOy− + ROH reactions, and oxidation by EtOH is more prevalent than oxidation by MeOH. The most striking disparity between the Mo3Oy− + ROH and W3Oy− + ROH reactions lies with M3O6−. Mo3O6− unreactive toward both ROH, as was seen with H2O,39,43 while W3O6− has the highest rate coefficient of all the WxOy− clusters for which this analysis was completed. 3.C. Anion PE spectra of ROH-addition complexes. From comparing Figs. 4(c) and 4(d), it appears that MxOy− + ROH reactions likely follow the same reaction pathways as those explored for reactions with H2O, which is supported by the anion PE spectra of addition complexes. Calculations on the Mo3O5− + H2O → Mo3O6H2− and W3O5− + H2O → W3O6− + H2 reaction free energies40,41 have shown that the barriers associated with the rearrangement of a M3O6H2− dihydroxide complex to a hydride-hydroxide complex are modestly submerged for M = W and modestly non-submerged for M = Mo. This result motivated the current study with the hypothesis that ROH addition complexes would take longer to undergo the necessary rearrangement leading to RH production, increasing the possibility of collisional cooling and formation of trapped intermediates.

While we have not observed any new trapped

intermediates (though we have observed cluster dehydrogenation complexes undergoing reactions with a second ROH molecule), anion PE spectra of addition complexes are nearly identical to H2O addition complexes.

The PE spectra below exhibit broad electronic transitions, and the lack of vibrational

structures prevents definitive structural determination. However, we have found in previous studies that transition energies and band profiles of MxOy−

+ H2O addition products can be reconciled with

calculations on feasible structures of intermediates along reaction pathways.35,37,41 The spectra presented below, while broad, support the possibility that MxOy− + H2O and ROH reactions follow similar pathways, forming similar trapped intermediates. Figure 5 shows the PE spectra of (a) Mo3O6H2− formed from Mo3O5− + H2O, and (b) Mo3O6CH4− 11 ACS Paragon Plus Environment


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and (c) Mo3O6C2H6− formed from Mo3O5− + ROH. The PE spectrum of Mo3O7−, which has the same mass as Mo3O6CH4−, is included in panel (b) to confirm that it is not contributing to the PE spectrum of Mo3O6CH4−, which also indicates that Mo3O7− is depleted in reactions with MeOH, forming Mo3O8CH4−. The Mo3O6H2− structure determined by Ramabhadran et al. is included in Fig. 5(a).41 All three spectra show a prominent electronic transition originating at nearly the same energy (1.85 eV for Mo3O6RH−, 1.90 eV for Mo3O6H2−).

The spectra confirm that the addition products have the same electronic

structures, and that the stability of the anions relative to their respective neutrals is insensitive to Ridentity. Given the previous analysis of the Mo3O6D2− PE spectrum,41 the HOMO of the anion is localized in 4d-like orbitals centered on the two Mo centers to which terminal −OD (or OH) groups are bound. The change in electron density upon photodetachment, therefore, is coupled to the −OR and −OH groups, so R-group interactions with the Mo3O6 skeleton must be similar on both the neutral and anionic clusters. Figures 6 and 7 show PE spectra of Mo2O5− + ROH and W2O5− + ROH addition products, respectively. The spectra are broader than the Mo3O6− + ROH addition product spectra, so are less informative. Nonetheless, from Figs. 6(a-c), it is evident that the single broad electronic transitions in the (b) Mo2O6CH4− and (c) Mo2O6C2H6− spectra are nearly identical in profile to the (a) Mo2O6D2− spectrum (the Mo2O6H2− spectrum is also shown, but has a different profile at higher e−BE due to overlap with Mo2O6− and Mo2O6H−). The binding energies of Mo2Ο6RH− complexes are ca. 0.11 eV lower than the Mo2O6D2− complex, again indicating that the relative stability of Mo2O6RH− and the associated neutral is not significantly affected by the R-group. The structure of Mo2O6H2− determined previously37 is included; the ROH addition complexes, given preliminary calculations on the Mo2O5− + ROH reaction path, would also have a bridging hydroxyl group, with a terminal alkoxy group. The W2O6RH− spectra shown in Figure 7 are broad, noisy, and difficult to obtain. However,


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based on the structures determined for W2O6H2− and W2Ο6Η2, the poor spectral quality is the result of a significant structural change upon photodetachment, as shown Fig. 7(a). A partially-resolved 670(15) cm−1 vibrational spacing is observed in all three spectra, indicated with the dashed lines. This spacing is typical of a M-OH stretch frequency.41 To summarize, the PE spectra of these addition products, and similarities in product patterns found when comparing MxOy− + H2O, MxOy− + MeOH, and MxOy− + EtOH reactions indicates that the oxidation and addition reactions follow analogous pathways, though with different rate coefficients. 3.D. Computational results. Results of previous computational studies on MxOy− + H2O reactions indicated that for all suboxide clusters (y < 3x), MxOy+1− + H2 is thermodynamically favorable, with ∆Grxno values on the order of −160 to −250 kJ mol−1, and is typically more negative for WxOy− clusters than for MoxOy− clusters because of the larger W-O bond energy. Table 2 summarizes these calculated reaction free energies, alongside the newly calculated M2Oy− + MeOH → M2Oy+1− + CH4 free energies. The more negative ∆Grxno values for reactions with MeOH are in very good agreement with the difference in free energies of formation, ∆Gfo of [∆Gfo (H2) − ∆Gfo (H2O)] and [∆Gfo (CH4) − ∆Gfo (MeOH)], which is 117 kJ mol−1 lower for MeOH.60 ∆Grxno values for M2Oy− + EtOH → M2Oy+1− + C2H6 reaction are 93 kJ mol−1 more negative than analogous water reactions. The large negative free energies of the oxidation reactions supports the assertion that MxOy+1RH− addition products are trapped intermediates. Results on the temperature-dependence of MxOy− + H2O reactions revealed that weakly-bound complexes play a pivotal role in the rate coefficients measured in this experiment.43 The rate-limiting step for reactions observed to go to completion is forming an association complex, which initiates with the formation of a charge-dipole complex. The dipole must undergo a pivot in the Coulombic field in order for the O-atom on H2O or ROH to come into reactive distances from the M-center, forming a Lewis acidbase complex that aligned by O-H dipole- M-O dipole interactions.


From previous combined


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computational and experimental studies,35,37,41 the Lewis acid-base complexes lead directly to dihydroxide addition complexes (in the case of M2O5− and Mo3O5− + H2O reactions, the PE spectra of addition complexes suggest that covalently-bound addition complexes are formed, and the similarity of the anion PE spectra of ROH addition complexes in the previous section suggests that the addition complexes observed in the current study are also covalently bound). Figure 8 shows schematics of the free energies of charge-dipole complexes (labeled CD) relative to separated M2Oy− + ROH (M = Mo, W; y = 4, 5; R = Me, Et) reactants, along with the Lewis acid-base complexes (AB) that are further stabilized by local dipole-dipole interactions and the polarizability of the −R group. Free energies for analogous cluster-water complexes are included for comparison.43 We note here that deficiencies have been found when treating weakly-bound systems using DFT methods. However, qualitatively, the results are consistent with known physical properties of the ROH reactants. For example, the dipole moment of water (1.855 Debye) is larger than the dipole moments of MeOH (1.700 Debye) and EtOH (1.441 Debye),59 and the most stable charge-dipole complexes are the M2Oy−⋅H2O complexes. The M2Oy−⋅EtOH charge-dipole complexes are more stable than the M2Oy−⋅MeOH analogs because of the extra polarization from the Et-group (average polarizabilities of H2O, MeOH and EtOH are 1.501 Å3, 3.081 Å3 and 5.112 Å3, respectively).

59

Fig. 8(b) includes the structures of the

W2O4−⋅MeOH and W2O4−⋅EtOH charge-dipole complexes to illustrate how the Et-group interacts with the cluster. In all charge-dipole complexes, the O-atom in ROH (R = H, Me, Et) is pointing away from the cluster, and is not in reactive distance from any M-center, and the ROH dipole must pivot in the Coulombic field in order to form the Lewis acid-base complex, which results in a small barrier separating the reactive arrangement from the non-reactive arrangement. Structures calculated for the covalentlybound addition complexes, W2O5RH− (R = Me, Et) formed directly from the Lewis acid-base complexes are also included for comparison. The hydroxy-alkoxy addition complexes are calculated for this series of cluster + ROH reactions to be 80 – 120 kJ mol−1 lower in energy than the separated reactants, and can undergo rearrangement to structures leading to RH production. 14 ACS Paragon Plus Environment

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M2Oy−⋅EtOH form the most stable Lewis acid-base complexes. For Mo2O4− and W2O4−, the calculated free energy of the transition state between the AB complexes and the covalently-bound hydroxide-alkoxide complexes are lower in energy than the AB complexes, indicating a barrierless process. For both Mo2O5− and W2O5−, the barrier is calculated to be greater than zero, and for W2O5−, this barrier is higher than the free energy of the separated reactants. We now discuss how these computational results align with the experimental results. IV. DISCUSSION The results of this study reinforce the conclusions from our previously reported study on the temperature dependence of MxOy− + H2O reactions, which is that the reaction can be broken down into three parts. (1) Weakly-bound complex formation, MxOy− + ROH → MxOy−⋅ROH

(9)

(2) covalently-bound addition product formation, MxOy−⋅ROH → MxOy−1(OH)(OR)−

(10)

And (3) RH production, MxOy−1(OH)(OR)− → MxOy+1− + RH

(11)

The first step is reversible and is complicated by the possibility of pre-equilibrium being established in cases where the free energies of the Lewis acid-base complexes, the structure of which facilitates the –OH and −OR bond formation with the cluster, are close to or higher than the free energies of the charge-dipole complexes, the structure of which does not facilitate −OH and −OR bond formation, and the separated reactants. The Lewis acid-base complex free energies for the W2Oy− clusters are less stable relative to the charge-dipole complexes when compared to the Mo2Oy− complexes (see Fig. 8). We therefore would anticipate that the W2Oy− addition and oxidation reactions would have lower rate coefficients than analogous Mo2Oy− reactions, which indeed is the case for all M2Oy− and M3Oy− + EtOH reactions, with the exception of M3O5− + EtOH (Mo, W have the same rate coefficient) and M3O6− (Mo3O6− has a rate



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coefficient of zero). While tungsten oxide is a stronger Lewis acid, we have found in previous studies that the slightly smaller ionic radius of the Mo-atoms in these clusters facilitates closer dipole-dipole alignment, resulting in lower complex energy and lower barriers to the covalently-bound addition products. The enhanced stability of the M2Oy−⋅EtOH charge-dipole and Lewis acid-base complexes relative to analogous M2Oy−⋅MeOH complexes is in line with the significantly higher rate coefficients measured for MxOy− + EtOH reactions. Surface studies have generally found that the sticking probability for alkanes on metal surfaces increases with alkane length.61 In studies on MgO surfaces, it has further been shown that the activation energy of desorption of alkanes also increases with chain length,62 and a study comparing EtOH and MeOH desorption from graphene showed a similar relationship between the energy of desorption and chain length.63 All studies point to stronger adsorbate-surface interactions proffered by longer alkyl chains. In our study, both the non-reactive charge-dipole complex structures and the reactive Lewis acid-base complex structures formed with EtOH are more stable than the analogous MeOH complexes. The relative MxOy− + MeOH and MxOy− + H2O rate coefficients are similar (Fig. 4), which suggests and interesting interplay between the charge-dipole (most stable for water, least stable for MeOH) and Lewis acid-base (least stable for water, more stable for MeOH) complexes. Step 2 of the reaction is calculated to be barrierless or with a submerged barrier for dissociative adsorption for most of the cluster-ROH reactions presented here. However, for all the addition complexes observed and spectroscopically probed in this experiment, the MxOy−1(OH)(OR)− hydroxy- alkoxy structure is formed. Step 3 requires structural rearrangement of the MxOy−1(OH)(OR)− complex to a MxHOy (OR)− a hydride-alkoxy complex to facilitate RH production. For all cases in which direct oxidiation with RH production is observed, the rate coefficients of step 3 need to be larger than the collision frequency if the energy wells along the rearrangement path are comparable to those calculated for MxOy− + H2O reactions, which is ca. 1010 to 1011 s−1 with He under our experimental conditions. An 16 ACS Paragon Plus Environment

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upper limit on the half-lives of these MxOy−1(OH)(OR)− complexes is therefore ca. 10 ps. Finally, a survey of Fig. 4 shows that in two cases clusters undergoing both oxidation and addition reactions (Mo2O5− + ROH, W2O5− + ROH and H2O), EtOH reactions are more likely to result in oxidation relative to MeOH, and MeOH is more likely to result in oxidation than H2O.

The

thermodynamic drive in terms of the most negative ∆Grxn is greatest for MeOH. Calculations on the free energy paths are underway to determine the extent to which the Et-group lowers the barriers along the rearrangement pathway (step 3).

V. CONCLUSIONS A systematic comparison of MxOy− + ROH (M = Mo versus W; R = Me versus Et) reaction rate coefficients and product distributions along with calculations on weakly-bound MxOy−⋅ROH were completed to further explore recent experimental and computational studies on analogous MxOy− + H2O reactions.

MxOy− + ROH → MxOy+1− + RH oxidation reactions are observed for the least oxidized

clusters, and MxOy− + ROH → MxOyROH− addition reactions are observed for clusters in intermediate oxidation states, generally following the same patterns observed in MxOy− + H2O studies. A distillation of the results suggests that the overall reaction mechanism has three important steps. The reactions initiate with weakly-bound complex formation, the rate of which is governed by the relative stability of the MxOy−⋅ROH charge-dipole complexes and the Lewis acid-base complexes. MoxOy− form more stable MxOy−⋅ROH Lewis acid-base complexes than WxOy−, with the stability of EtOH complexes being further enhanced because of higher polaraizability. As a consequence, bare MoxOy− clusters are consumed by ROH reactions at a higher rate than analogous WxOy− clusters, and reactions with EtOH have higher rates than reactions with MeOH by a factor of two. Experimentally, the rate coefficients were typically in the range of 10−13 to 10−11 cm3 s−1, with Mo2O4− + EtOH having the highest (3.4 x 10−11 cm3 s−1) and W3O8− + MeOH having the lowest (2.7 x 10-13 cm3 s−1). The second step involves ROH dissociation on the cluster; anion PE spectra of the addition products are consistent with hydroxyl17 ACS Paragon Plus Environment


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alkoxy structures. However, addition products are in fact trapped intermediates in the MxOy− + ROH → MxOy+1− + RH reaction, the third step of which is rearrangement of the hydroxyl group to form a hydride group to facilitate RH release. Trapped intermediates are more prevalent in MoxOy− reaction product distributions; this particular elementary step has a much higher rate coefficient for WxOy+1RH− addition complexes. Previous computational studies on analogous MxOy− + H2O reactions predicted that barriers along the pathway in the rearrangement step are higher for MoxOy− reactions than for WxOy−, which is consistent with our experimental observations.

ASSOCIATED CONTENT Supporting Information Available: Supporting information includes a detailed description of the analysis applied toward determining rate coefficients, representative series of MxOy− + ROH (M = Mo, W; x = 2, 3; R = Me, Et) mass spectra taken with incremental increases in ROH number density in the cluster reactor, and single bond dissociation energies for H2O, MeOH and EtOH. This material is available free of charge at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: (812)855-8300. Notes The authors declare no competing financial interest. ACKOWLEDGEMENTS The authors gratefully acknowledge support for this research from the U.S. Department of Energy (Grant No. DE-FG02-07ER15889).


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List of Figures Figure 1. Typical initial cluster distributions (black dotted traces) and product distributions measured for (a) 98Mo2Oy− + MeOH (green trace), (b) 186W2Oy− + MeOH(green trace), (c) 98Mo2Oy− + EtOH (red trace), and (d)

186

W2Oy− + EtOH (red trace) reactions. Dashed arrows indicate addition of MeOH (green) and

EtOH (red). Product distributions were measured under conditions giving approximately 150 clusterROH collisions. Figure 2. Relative ion intensities plotted as a function of approximate cluster-ROH reactions for (a) Mo2Oy− + MeOH, (b) W2Oy− + MeOH, (c) Mo2Oy− + EtOH, and (d) W2Oy− + EtOH. For simplicity, sums of all suboxides are shown (black traces) along with a single representative suboxide cluster (black open circles), the stoichiometric clusters (gray traces) and ROH addition products (green for MeOH, red for EtOH). Figure 3. Typical initial cluster distributions (black dotted traces) and product distributions measured for (a) 98Mo3Oy− + MeOH (green trace), (b) 186W3Oy− + MeOH(green trace), (c) 98Mo3Oy− + EtOH (red trace), and (d)

186

W3Oy− + EtOH (red trace) reactions. Dashed arrows indicate addition of MeOH (green) and

EtOH (red). Product distributions were measured under conditions giving approximately 150 cluster-ROH collisions. Figure 4. Relative efficiencies of MxOy− + H2O (blue), MxOy− + MeOH (green) and MxOy− + EtOH reactions (red) measured for (a) Mo2Oy−, (b) W2Oy−, (c) Mo3Oy− and (d) W3Oy−. In cases in which both oxidation and addition reactions are observed, addition is indicated with an open “+” symbol, oxidation is a solid symbol. Figure 5. PE spectra of Mo3O5− + ROH addition products for (a) R = H, (b) R = Me, and (c) R = Et, measured using 3.49 eV photon energy. The PE spectrum of Mo3O7− (mass coincident with Mo3O6CH4−) is included as black dotted trace on panel (b). The structure of Mo3O6H2− is from Ref. [41]. Figure 6. PE spectra of Mo2O5− + ROH addition products for (a) R = H, (b) R = Me, and (c) R = Et, 25 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 37

measured using 3.49 eV photon energy. Structure of Mo3O6H2− is from Ref. [37]. Figure 7. PE spectra of W2O5− + ROH addition products for (a) R = H, (b) R = Me, and (c) R = Et, measured using 3.49 eV photon energy. The structure of Mo3O6H2− is from Ref. [36]. Figure 8. Comparison of relative free energies of charge-dipole complexes (CD), the Lewis acid-base complexes (AB), and the transition state between the AB complexes and the covalently-bound addition complexes (ǂ) for MxOy− + H2O (blue), MxOy− + MeOH (green) and MxOy− + EtOH reactions (red) calculated for (a) Mo2O4−, (b) W2O4−, (c) Mo2O5− and (d) W2O5−. Examples of structures of AB, CD and covalently-bound complexes are shown for W2O4−. The covalently-bound complexes initially have hydroxy-alkoxy structures, and range from 80 – 120 kJ mol−1 lower in energy than the separated reactants, and can rearrange to structures leading to RH production.


Page 27 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Rate coefficients (cm3 s−1) for most prominent MxOy− + ROH reactions. Values for MxOy− + H2O reactions are from Ref. [43]. Uncertainty in values is typically 20%. Values for several MxOy− + MeOH could not be unambiguously determined because of mass coincidences. Reaction

MeOH

EtOH

H 2O

Mo2O2− + ROH → Mo2O3− + RH

8.3 × 10−12

2.2 × 10−11

1.3 × 10−11

W2O2− + ROH → WO3− + RH

9.0 × 10−12

1.6 × 10−11

-


1.1 × 10−11

3.0 × 10−11

1.5 × 10−11

W2O3− + ROH → W2O4− + RH

9.8 × 10−12

2.2 × 10−11

6.5 × 10−12


1.1 × 10−11

3.4 × 10−11

1.0 × 10−11

W2O4− + ROH → W2O5− + RH

1.2 × 10−11

1.9 × 10−11

4.1 × 10−12

Mo2O5− + ROH → Mo2O6RH−

9.8 × 10−12

2.5 × 10−11

1.1 × 10−11


3.0 × 10−12

1.7 × 10−11

0

W2O5− + ROH → W2O6RH−

3.0 × 10−12

5.7 × 10−12

1.2 × 10−12

W2O5− + ROH → W2O6− + RH

6.8 × 10−12

1.6 × 10−11

7 × 10−13


7.0 × 10−12

9.0 × 10−12

8.5 × 10−12


1.3 × 10−11

2.0 × 10−11

1.1 × 10−11

W3O4− + ROH → W3O5− + RH

4.9 × 10−12

9.0 × 10−12

-

1.6 × 10−11

9 × 10−12

x=2

x=3

Mo3O5− + ROH → Mo3O6RH− Mo3O5− + ROH → Mo3O6− + RH

1.1× 10−11(a)

2.5 × 10−12

2.1 × 10−12

W3O5− + ROH → W3O6− + RH

6.8 × 10−12

1.7 × 10−11

3.1 × 10−12

0

0

0

1.1× 10−11

2.6 × 10−11

4.2 × 10−12

---

1.8 × 10−11

3.5 × 10−12

3.2 × 10−12

5.9× 10−12

1.2 × 10−12

Mo3O8− + ROH → Mo3O0RH−

---

8.2 × 10−12

9.3 × 10−13

W3O8− + ROH → W3O9− + RH

2.7 × 10−13

~0

1.8 × 10−13

Mo3O6− + ROH → W3O6− + ROH → W3O7RH− Mo3O7− + ROH → Mo3O8RH− W3O7− + ROH → W3O8RH−

(a)

Addition and oxidation reactions cannot be unambiguously separated because of mass coincidence; the total reaction rate coefficient is included.



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Page 28 of 37

Table 2. Reaction free energies calculated for M2Oy− + MeOH and M2Oy− + H2O oxidation reactions for M = Mo, W; y = 2 − 5. ∆Grxno / kJ mol−1 M = Mo M=W

Reaction y=2 M2O2− + MeOH → M2O3− + CH4

−276

−334

M2O2− + H2O → M2O3− + H2

−160

−222

M2O3 + MeOH → M2O4− + CH4

−305

−368

M2O3− + H2O → M2O4− + H2

−192

−251

M2O4− + MeOH → M2O5− + CH4

−297

−355

M2O4− + H2O → M2O5− + H2

−184

−238

M2O5 + MeOH → M2O6− + CH4

−300

−318

M2O5− + H2O → M2O6− + H2

−184

−206

y=3 −

y=4

y=5 −


Page 29 of 37

Relative Ion Intensity

(a) Mo2Oy + MeOH Mo2O5CH2

4

6

3 2

250

300

350

6


5

4 3 400

450 Mass/charge (amu/e)

(c) Mo2Oy + EtOH Relative Ion Intensity

Mo2O5C2H4

5

(b) W2Oy + MeOH

4

6 5

500

Mo2O5C2H4

3 2 250

(d) W2Oy + EtOH 4 Relative Ion Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

300

350

5

3

2 400

450 Mass/charge (amu/e)

500

Figure 1. Ray et al.

ACS Paragon Plus Environment


(c) Mo2Oy + EtOH

(a) Mo2Oy + MeOH

Sum of suboxides


Sum of suboxides

Mo2O6

Mo2O6 Mo2O6CH4/Mo2O7

Mo2O6C2H6





Mo2O4

Mo2O4

Mo2O6C2H6

0

25

50

75

100

125

150

Mo2O6C4H10

0

25

50

75

100

125

150

Approximate cluster-ROH collisions

(b) W2Oy + MeOH

(d) W2Oy + EtOH

Sum of suboxides


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 37

Sum of suboxides W2O6 W2O6

W2O4

W2O4

W2O6C2H6

W2O6CH4/W2O7

0

25

50

75

100

125

150

0

25

50

Approximate cluster-ROH collisions



75

100

125

150

Page 31 of 37


(a) Mo3Oy + MeOH 6 9

7

5

8

4 3 340

360

380

400

420

440

460

480

500




(b) W3Oy + MeOH

8 9

7 6

4 600

620

5 640

660

680

700

720

740

760

Mass/charge (amu/e)


(c) Mo3Oy + EtOH 6 7

5

8

9

4 3 340

360

380

400

420

440

460

480

500

(d) W3Oy + EtOH Relative Ion Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8 6

9

7

5 4 3

600

620

640

660

680

700

Mass/charge (amu/e)

720

740



760


MeOH

(a) Mo2Oy



oxidation + addition

0.06

(b) W2Oy

EtOH

0.06

H2O

oxidation 0.05 Reaction Efficiency Reaction Efficiency

Reaction efficiency Reaction Efficiency

0.05

0.04

0.03

0.02

0

0.04

oxidation + addition

oxidation

0.03

0.02

0.01

0.01

1

2

3

4

5

0

6

1

2

3

y

4

6

(d) W3Oy 0.04

0.04

addition Reaction Efficiency Reaction Efficiency

oxidation + addition oxidation

0.03

addition

0.02

0.03

oxidation

0.02

oxidation 0.01

0.01

0

5

y

(c) Mo3Oy

Reaction Efficiency Reaction Efficiency

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

2

3

4

5

6

7

8

0

2

3

4

5

y

y



6

7

8

(a) Mo3O6H2

Relative e counts

1.00

0.05 eV

1.50

2.00

2.50

3.00

(b) Mo3O6CH4 Mo3O7

1.00


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 33 of 37

1.50

2.00

2.50

3.00

2.00

2.50

3.00

(c) Mo3O6C2H6

1.00

1.50

Electron Binding Energy (eV)





~ 0.11 eV


Mo2O6, Mo2O6H

(a) Mo2O6H2 Mo2O6D2

1.5

2

2.5

3

2.5

3

2.5

3

(b) Mo2O6CH4

1.5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2

(c) Mo2O6C2H6

1.5

2




Page 34 of 37

(a) W2O6D2


1.5

2

2.5

3

2.5

3

2.5

3

(b) W2O6CH4

1.5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 35 of 37

2

(c) W2O6C2H6

1.5

2





H2O

(a) Mo2O4

(b) W2O4

MeOH

Gcomplex- Greactants (T = 298 K)/ kJ mol-1

EtOH 10

10

0

0

-10

-10

CD

ǂ

-20

ǂ

CD

AB

-20

AB -30

-30

Covalentlybound complexes


(c) Mo2O5 Gcomplex- Greactants (T = 298 K)/ kJ mol-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 37

(d) W2O5

30

30

20

20

10

10

ǂ

0

ǂ

0

AB -10

-10

CD

CD -20

-30

AB

-20

Covalentlybound -30 complexes




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[Gcomplex- Greactants (T = 298 K)] / kJ mol-1

Page 37 of 37

10

0

W2O4

-10

H2O MeOH

-20

EtOH -30

Charge-dipole complex

Lewis acid- Covalently-bound base complex complexes

TOC graphic


Effect of Alkyl Group on Mx

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