Adsorption, Dissociation, and Dehydrogenation of Water Monomer

Subscriber access provided by LUNDS UNIV

Article

Adsorption, Dissociation and Dehydrogenation of Water Monomer and Water Dimer on the Smallest 3D Aluminum Particle. The O-H Dissociation Barrier Disappears for the Dimer Jerzy Moc J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b08278 • Publication Date (Web): 07 Oct 2016 Downloaded from http://pubs.acs.org on October 16, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 53

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

The Journal of Physical Chemistry

1

Adsorption, Dissociation and Dehydrogenation of Water Monomer and Water Dimer on the Smallest 3D Aluminum Particle. The O-H Dissociation Barrier Disappears for the Dimer

Jerzy Moc* Faculty of Chemistry, Wroclaw University, F. Joliot-Curie 14, 50-383 Wroclaw, Poland

Abstract. We present a detailed mechanistic study on the interaction and reaction of water monomer and water dimer with the smallest 3D aluminum particle (Al6) by employing density functional and explicitly correlated coupled cluster CCSD(T)-F12 theories. The water adsorption, dissociation and dehydrogenation is considered. For the monomer reaction, where core-valence correlation and an extrapolation to the complete basis set limit is allowed for, our coupled cluster calculations predict the O-H dissociation barrier of about 2 kcal/mol. For the dimer reaction, two distinct reaction paths are identified, initiated by forming separate dimer complexes wherein (H2O)2 adsorbs mainly via the oxygen atom of the donor H2O molecule. The key O-H dissociation transition states of the dimer reaction involve a concerted migration of two H atoms resulting in the dissociation of the donor molecule and formation of the OH-water complex adsorbed on the metal cluster’s surface. The most remarkable feature of both dimer reaction energy profiles is the lack of the overall energy barrier for the (rate-determining) O-H dissociation. The hydrogen bond acceptor molecule is suggested to have an extra catalytic effect on the O-H dissociation barrier of the hydrogen bond donor molecule by removing this barrier. A similar effect on the dehydrogenation step is indicated.

Corresponding Author: Tel. (48)(71) 375-726 *Email address: [email protected] (Jerzy Moc)

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 2 of 53

2 1. Introduction The interaction of water with aluminum plays a role in catalysis as well as in the atmospheric and aqueous corrosion of this metal.1 Early X-ray and UV photoelectron spectroscopic measurements found that the initial H2O interaction with an Al(100) surface at 100 K resulted in molecularly adsorbed species, suggested to be an intermediate in the oxidation of Al.1 A study on H2O + Al(100) based on the electron energy loss spectra (EELS) and thermal desorption revealed a molecular adsorption followed by the water dissociation to the surface bound OH species accompanied by H2 evolution.2 For H2O + Al(111), both the molecular (associative) adsorption with H2O bound through the O atom, and dissociative one leading to the adsorbed OH species was detected at 80 or 130 K by electron stimulated desorption ion angular distributions (ESDIAD)3 and vibrational spectroscopy,4 along with a complete water dehydrogenation at 300 K.4 Experimental studies of the water-metal systems refer usually to the adsorbed water layers, and therefore more than one H2O molecule was suggested to be involved in the observed dissociation at low surface temperatures.2,5 A density functional theory (DFT) study of the interaction of an isolated water molecule with Al clusters simulating the Al(100) adsorption sites pointed to the on-top configuration with the oxygen end down as the most stable.5 However, despite the thermodynamically favourable H2O/Al(100) dissociation found in Ref.5, the process was described therein as "highly activated". The energetically preferred on-top configuration for the molecularly adsorbed water monomer on Al(100) was confirmed by the plane-wave pseudopotential DFT study.6 For the interaction of an isolated H2O molecule with an Al(111) surface modeled by the Al clusters7 and by the slab with periodic boundary conditions,8,9 the top site was found likewise to be most favourable. As in the H2O + Al(100) case,5 a high energy barrier was predicted by DFT for dissociation of a water monomer on this surface, reported to be 59.3 kcal/mol.9 The reaction of Al atoms/clusters with water is of practical relevance to the processes involved in combustion of nanoaluminum10 and combustion enhancement of hydrocarbon fuels.11 Similar to the H2O-Al(100) and H2O-Al(111) systems, an important aspect of the Al cluster-water reactions is the elucidation of the nature of the intermediates formed, especially those that can lead to H2 release.12 The most comprehensively examined reaction of this kind involves Al atom(s),12-23 and the relevant results will be now succinctly reviewed.


Page 3 of 53

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


3 The study of co-condensation of Al atoms with water in noble gas and adamantane matrices at low temperatures resulted in the formation of the HAlOH insertion product, observed using IR14 and EPR12,16-17 spectroscopies. Because reliable theoretical calculations (see below) predict an energy barrier for the Al + H2O reaction to form HAlOH,20 one plausible interpretation of the detection of HAlOH in the matrix isolation experiments was that more than one water molecule took part in the reaction,22 consistent with the suggestion made by the H2O + Al(100) investigation.5 The most accurate potential energy surface (PES) of the reaction of H2O monomer with Al atom was computed by Álvarez-Barcia and Flores (Á-BF)20 using the QCISD/6-311++G(2df,p) structures (QCI stands for Quadratic Configuration Interaction) and coupled cluster singles and doubles (CCSD) energies with the effect of triple excitations as well as corrections for corecorrelation and zero-point energy accounted for. Á-BF found that although the insertion product HAlOH was exothermic by 46.7 kcal/mol, the associated barrier height of 3.8 kcal/mol with respect to the Al(2P) and H2O reactants prevented a spontaneous reaction at low temperatures. These authors also predicted an energy barrier of 7.4 kcal/mol for the HAlOH → AlO + H2 decomposition relative to the separated Al(2P) and H2O reactants. The complementary dynamics study of H2O + Al using TST and RRKM theories confirmed the insignificance of the AlO + H2 decomposition channel.21 The ab initio results of Á-BF20 are in agreement with the experimental study of the reaction of aluminium atoms with H2O in helium droplets (at T=0.37 K) by Krasnokutski and Huisken who observed that, under these conditions, single Al atoms did not react with a single H2O molecule.19 The prediction of energy barriers of chemical reactions with chemical accuracy (±1 kcal/mol) is essential for their accurate modeling. It has been found, however, that affording simultaneously accurate molecular binding energies and barrier heights for gas phase reactions with currently available density functionals may be difficult to attain.24 By the same token, it has been suggested that obtaining chemically accurate results for both molecular adsorption and energy barriers of molecule-metal surface reactions may be hard to accomplish with contemporary DFT.25 For the latter reactions, a viable solution was proposed that involves correlated wave function approach with DFT embedding.25,26 As adsorption energies and activation barriers of metal cluster - molecule reactions are computed nowadays mostly by means of density functionals, they might not always be of chemical accuracy. The two quantities can be



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 4 of 53

4 reliably determined with a correlated wave function based energy calculations on the DFT established potential energy surface of the metal cluster - molecule reaction, especially for smallsize metal clusters (see, e.g., ref.27). The latter approach is adopted in this work for the associative and dissociative adsorption of water monomer and water dimer on the smallest (ground state) 3D aluminum particle, Al6. We show herein that contrary to H2O + Al(100)5 and H2O + Al(111),9 the water monomer dissociation barrier on Al6 is significantly lower, amounting to only about 2 kcal/mol, and that unlike the H2O + Al system,20 the H2 loss barrier lies considerably below separated H2O + Al6. Most importantly, our findings for the water dimer reaction indicate that the overall O-H dissociation barrier is removed.

2. Computational Methods The B3LYP density functional28,29 and the basis set aug-cc-pVTZ30,31 were employed to characterize stationary points on the potential energy surface (PES) for the reactions of water monomer and water dimer with Al6. Intrinsic reaction coordinate (IRC)32 was found out to assure the connection of transition states with appropriate local minima, sometimes with subsequent geometry optimization. All the search of the PES was performed using the Gaussian 09 package,33 with the stability of the restricted HF and Kohn-Sham (B3LYP) references for the OH bond dissociation and H2 loss transition states of both the monomer and dimer reactions analyzed via the wave function stability routine in this program. The stability tests showed that the RKS (B3LYP) determinants were always stable for these transition state structures; on the other hand, the RHF → UHF instability was detected in these structures. Consequently, we have performed single point coupled cluster calculations (see below) for the O-H bond dissociation and H2 loss transition states (as well as for the relevant reactants) based on the RKS references. For those cases, only the RKS (B3LYP) based correlated ab initio energy barriers are quoted. To show the effect of using the RKS reference on the barrier heights, the TSs’ relative energies obtained with the RHF reference are also included in the relative energy tables. In general, we found that the use of the RKS determinant lowered the correlated barriers by 0.0-0.4 kcal/mol with respect to those calculated using the RHF determinant. The improved relative energies were obtained with the explicitly correlated coupled cluster theory CCSD(T)-F1234-36 (CCSD(T) stands for coupled cluster with single, double, and


Page 5 of 53

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


5 perturbative triple excitations) at the B3LYP/aug-cc-pVTZ optimized structures. Both the CCSD(T)-F12a and CCSD(T)-F12b variants were utilized at various stages of this investigation along with the 3C(FIX)34,35 ansatz and the basis sets cc-pVnZ-F1237 (n=T,Q) designed specifically for use with F12 methods. With the basis set cc-pVTZ-F12, the auxiliary basis sets aug-cc-pVTZ/JKFIT (for the density fitting of the Fock and exchange matrices), aug-ccpVTZ/MP2FIT (for the density fitting of the remaining integrals), and cc-pVTZ-F12/OPTRI (for the resolution of the identity)38-41 were employed (the auxiliary basis sets utilized in conjunction with the basis set cc-pVQZ-F12 are detailed in section 3.5). Comparatively, the relative energies computed using the conventional CCSD(T)42 theory and the basis set aug-cc-pVTZ30,31 are presented. In addition, symmetry-adapted perturbation theory (SAPT)43-45 was applied to the molecularly adsorbed/physisorbed complexes of the monomer reaction to describe their binding nature. In SAPT scheme, the interaction energy of the system is decomposed into the first-order electrostatics (EElst(1)) and exchange-repulsion (EExch-Rep(1)), and the second-order induction (EInd(2)), dispersion (EDisp(2)), exchange-induction (EExch-Ind(2)), and exchange-dispersion (EExchDisp

(2)

) contributions. The DFT based SAPT scheme (DFT-SAPT)44-45 was used in combination

with the LPBE0AC functional46-48 and the aug-cc-pVTZ30,31 basis set (DFT-SAPT_aug-ccpVTZ) (the performance of the latter functional for the DFT-SAPT calculations was tested for the adsorption of the water monomer on the Mg(100) surface represented by the cluster models49). All of the coupled cluster and SAPT calculations were carried out with the Molpro2012.1 package.41

3. Results and Discussion In sections 3.1-3.5, the results pertinent to the reaction of Al6 with a water monomer are described, with the relative energies of different isomers of bare metal cluster reported in Table 1. A neutral aluminum hexamer has been investigated recently by various authors50-52 who concluded that the singlet octahedral-like D3d isomer is of lowest-energy. With the CCSD(T)F12a/cc-pVTZ-F12 calculations, expected to be most accurate in Table 1, the singlet R1 (D3d,1A1g) isomer (Figure S1a) is confirmed to be more stable than the triplet prism R2 (D3h,3A1') (Figure S1b) and singlet prism R3 (C2v,1A1) (Figure S1c) structures by 5.8 and 7.3 kcal/mol, respectively. For the Al6O product oxide, the non-planar P1 (C2v,1A1) and P3 (Cs,1A'),53 and



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 6 of 53

6 planar P2 (Cs,1A') singlet isomers of low-energy are predicted (Figure S1d-f), along with the transition state TSP1-P3 (Cs,1A') (Figure S1g) for interconversion between the P1 and P3 structures. The last column of Table 1 shows that the P1 isomer of Al6O is favored by 4.9 and 2.5 kcal/mol over the P2 and P3 ones, respectively.

3.1 Molecularly Adsorbed Monomer Complex We predict that the reaction of single water molecule with Al6 proceeds by the formation of an initial associative complex H2O-Al6 1 (Figure 1), wherein H2O is bound through the O atom, the binding mode seen for the molecularly adsorbed water on Al(100) and Al(111).1-5 In the complex 1, H2O molecule interacts with Al6 at the O-Al distance of 2.06 Å, which coincides with the DFT LDA interaction distance in the H2O-Al(100) system.5 The CCSD(T)-F12a/ccpVTZ-F12 calculated adsorption energy (Eads) of 9.5 kcal/mol (Table 2) is consistent with the modest increases in the water’s bond length (about 0.01 Å) and bond angle (about 2.5 degrees) accompanying the adsorption. To get insight into the binding nature of the complex 1, the DFT-SAPT_aug-cc-pVTZ interaction energy in the "heterodimer" H2O-Al6 (-8.5 kcal/mol) has been dissected into physically meaningful contributions as shown graphically in Figure 2 (part a), with the individual components given in Table 3 (see also the footnotes under this table). The SAPT results indicate that the electrostatic interaction energy of -56.5 kcal/mol is the largest attractive contribution to the interaction energy, owing to a large permanent electric dipole moment of an isolated water monomer (Table 3). However, because the combined electrostatic and exchange interaction energy is repulsive (79.0 kcal/mol), the induction (-19.7 kcal/mol) and dispersion (-11.4 kcal/mol) energies are also important contributors to the net binding in the complex 1 (the binding energy is the negative of the interaction energy). As for the analogous cluster cation reaction,27 we find the initial complex to be a key species for the Al6 neutral + H2O reaction because starting from 1 three different dissociation/dehydrogenation channels can be followed, denoted herein as pathi (i=1-3). The CCSD(T)-F12a/cc-pVTZ-F12 relative energy profile for the monomer reaction is depicted in Figure 3, and those obtained for this system from the CCSD(T)/aug-cc-pVTZ and B3LYP/augcc-pVTZ calculations (all the profiles using the B3LYP/aug-cc-pVTZ geometries) are included in Figures S2 and S3, respectively, with the associated energies in Table 4.


Page 7 of 53

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


7

3.2 Monomer Path 1 Starting from 1, an H2O molecule can dissociate on Al6 through the four-center transition state TS1-2, thus entering the monomer reaction path 1 (Figure 4). At the TS1-2 structure, the OAl distance is decreased to 1.89 Å (from 2.06 Å in the complex), and the length of the dissociating O-H1 bond is 1.16 Å. This transition state also features the partially formed Al-H1 bond at 2.01 Å along with the broken "active" Al-Al edge. The reaction profile in Figure 3 shows that the O-H dissociation is the rate-limiting step, and for the monomer path 1, the energy difference between the transition state TS1-2 and separated reactants is 11.8 kcal/mol. In the formed intermediate HAl6OH 2, the OH2 and H1 moieties are bound side by side on the bridged and top sites, respectively. The O-H dissociation of the water monomer is found exothermic by 48.8 kcal/mol at the CCSD(T)-F12a/cc-pVTZ-F12 level (Table 4). To reveal the electronic structure features relevant to the dehydrogenation, being the next reaction step, the natural population analysis (NPA)54,55 was performed. The NPA results indicate the interaction between the protic H2 (+0.52 e) and hydridic H1 (-0.41 e) hydrogens in 2. The approaching of H2 towards H1 requires, however, breaking of the O-H2 bond which induces the transition state TS2-3, wherein the H2 unit and Al-O-Al fragment are co-planar (Figure 4). This step demands correspondingly overcoming the sizeable energy barrier of 22.2 kcal/mol relative to 2, but at the same time, TS2-3 lies 26.6 kcal/mol below the reactants. So, similar to the cation system,27 the dehydrogenation step does not require an overall energy barrier (Figure 3). The latter feature of the neutral reaction’s PES remains in clear contrast to the kinetically unfavourable H2 loss for the H2O + Al reaction.20 For the ensuing physisorbed complex Al6O…H2 3 (Figure 4), the decomposition of the DFT-SAPT_aug-cc-pVTZ interaction energy (-0.60 kcal/mol) is presented graphically in Figure 2 (part b) (note the different energy scale compared to Figure 2a), and numerically in Table 3. This analysis points to the dispersion (-0.83 kcal/mol) and electrostatic (-0.77 kcal/mol) interactions as the largest attractive contributors to the interaction energy of 3. The H2 detachment from the latter species leaves the Al6O product oxide P1 (Figure S1d). Because at the CCSD(T)-F12a/cc-pVTZ-F12 level an interconversion of the P1 isomer of Al6O into the P3 one (Figure S1f) via TSP1-3 (Figure S1g) is thermodynamically disfavoured (Table 1), the Al6O (P1) plus H2 products are most exothermic at this level, by 52.0 kcal/mol (Table 4).



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 8 of 53

8

3.3 Monomer Path 2 An alternative path we have identified for the water monomer dehydrogenation after the formation of the complex 1 is called path 2 (Figure 5). The involved O-H dissociation transition state TS1-4 has a geometry with the O-Al distance of 1.87 Å and the breaking O-H1 bond length of 1.28 Å. We note that the TS1-4 structure is apparently stabilized by the Al-H1-Al bridge bonding, with the bridging distances of 1.96 and 2.02 Å, and this stabilization has shifted TS1-4 down to lower energy in comparison to the TS1-2 counterpart (Figure 3). Indeed, according to the CCSD(T)-F12a/cc-pVTZ-F12 calculations, the energy difference between TS1-4 and separated reactants is only 1.7 kcal/mol (Table 4). As the O-H dissociation is rate-determining, the path 2 appears to be kinetically more favored than the path 1. Note that the resultant intermediate 4 is less exothermic compared to the analogue 2 of path 1 due to the energetically less preferred adsorption pattern of the dissociated water in the former HAl6OH isomer: the bridge bonded H1 and terminally bonded OH2 hydroxy group. Past the rotational interconversion 4 → TS4-5 → 5, the H1 movement to the terminal site affords 6 with the corresponding transition state TS5-6 lying 5.1 kcal/mol above 5 (note that 6 in Figure 5 is shown using a different perspective from that employed for TS5-6). The following concerted double-bridging of H1 and OH2 leads to the "octahedral-like" intermediate 7, which involves the transition state TS6-7 located 14.6 kcal/mol above 6. The partial H-H bond forming which can now be achieved is preceded by the O-H2 bond breakage in 7, so the protic hydrogen can come nearer the hydridic one. This gives rise to the transition state TS7-3 lying about 23 kcal/mol above 7, but 20.7 kcal/mol below the separated reactants (Figure 3, Table 4). The final H2 detachment occurs from the physisorbed complex 3. 3.4 Monomer Path 3 It follows from the above considerations that when the molecularly adsorbed water monomer more closely approaches the surface of Al6, the O-H bond starts to break. The complete O-H bond dissociation takes place provided that the reacting system possesses enough energy to overcome the kinetic barrier involved. For the monomer path 3 (Figure 6), the transition state for the O-H dissociation TS1-8 is reached at the O-Al distance of 1.87 Å, with the breaking O-H1 bond length at 1.26 Å. The TS1-8 resembles the TS1-4 analog (path 2) in that both are stabilized


Page 9 of 53

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


9 by the Al-H1-Al bridge bonding. Consequently, the CCSD(T)-F12a/cc-pVTZ-F12 computed TS1-8 energy barrier of 3.0 kcal/mol is only slightly higher than the TS1-4 barrier (Figure 3). The similarity between the two routes also lies in the structure of the formed H2O dissociation intermediate, because 8 like 4 exhibits the bridge bonded H1 and terminally bonded OH2. With the subsequent shift of the bridged H1 atom to the OH2-occupied Al site, via TS8-9, the Al6 core undergoes a profound rearrangement to end up with the species 9 with the OH2bridged "semi-planar" structure (Figure 6). This step has an energy barrier of 14.0 kcal/mol relative to 8. Making the partial H-H bond requires a prior breaking of the Al-Al bond involving the out-of-plane Al atom, followed by a rotation of the Al-OH2 unit and O-H2 bond rupture, bringing about the transition state TS9-10. The latter TS is found to lie 23.5 kcal/mol above 9 and 13.4 kcal/mol below the reactants H2O + Al6 (Figure 3). After the H2 departure from the physisorbed complex Al6O…H2 10, the planar Al6O product oxide P2 (Figure S1e) is left. Note that due to the lower stability of the planar P2 isomer of Al6O relative to the non-planar P1 (Table 1), this water monomer dehydrogenation is less exothermic compared to the paths 1-2. By directly comparing the conventional CCSD(T)/aug-cc-pVTZ and explicitly correlated CCSD(T)-F12a/cc-pVTZ-F12 energy profiles (Table 4) one infers that those obtained at the latter level are generally shifted down in energy with respect to the ones generated by the former scheme (this is a combined coupled cluster method plus basis set effect). This implies that the intermediates and products at the CCSD(T)-F12/cc-pVTZ-F12 level are more exothermic, and the O-H dissociation barriers (TS1-4 and TS1-8) are somewhat lower compared to the CCSD(T)/aug-cc-pVTZ results.

3.5 Additional Relative Energy Calculations for the Monomer Reaction To determine the importance of using larger basis set and including core-valence correlation in the calculations of the relative energies of the water monomer reaction, we have carried out additional explicitly correlated coupled cluster CCSD(T)-F12 computations for the chosen stationary points of the monomer path 2, i.e., one with the lowest energy barrier for the OH dissociation. To that end, cc-pVQZ-F1237 and cc-pCVTZ-F1256 basis sets have been used, respectively, along with the associated auxiliary38-41 basis sets: aug-cc-pVQZ/JKFIT (aug-ccpVTZ/JKFIT), aug-cc-pVQZ/MP2FIT (aug-cc-pwCVTZ-MP), and cc-pVQZ-F12/OPTRI (cc-



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 10 of 53

10 pCVTZ-F12/OPTRI) (the auxiliary basis sets in parentheses were utilized together with the corevalence orbital basis set). Table 5 presents the additional CCSD(T)-F12 energies (relative to separated reactants) for the complex 1, O-H dissociation transition state TS1-4, and products Al6O (P1) + H2, both the CCSD(T)-F12a and CCSD(T)-F12b values are included (for TS1-4, the RKS (B3LYP) reference was used). A comparison of the appropriate CCSD(T)-F12a results in Tables 4 and 5 shows that the corrections due to the use of the quadruple-ξ F12 basis set (valence correlation) are 0.1-0.3 kcal/mol. With the cc-pVQZ-F12 basis set, the F12b results are found to be within 0.1 kcal/mol of the F12a ones. Finally, the explicitly correlated correlation energies (valence correlation) were extrapolated to the complete basis set (CBS) limit using the procedure of Hill et al.,57 with the CCSD-F12b and (T) contributions extrapolated separately with a two-point CBS formula (1) by Schwenke58

ECBS= (Elarge – Esmall) • F + Esmall with the optimized57 CBS coefficients F =

(1)

1.363388 (the CCSD-F12b contribution) and

1.769474 (the (T) contribution) employed in a cc-pVTZ-F12/cc-pVQZ-F12 extrapolation. The CBS extrapolation has been carried out for the "crucial" stationary points of the water monomer path 2 and are displayed in the bottom row of Table 5 (note that these values also include the core-valence corrections). The extrapolation results indicate that our most reliable TS1-4 O-H dissociation barrier amounts to 2.1 kcal/mol.

3.6 Molecularly Adsorbed Dimer Complexes In sections 3.6-3.8, the results pertinent to the reaction of Al6 with water dimer are reported. The water dimer is a well characterized hydrogen bonded species with a Cs equilibrium geometry (Figure 7a) (for the recent review article, see Ref.59). In isolation, the interchange of hydrogen bond donor and acceptor molecules within the water dimer can be achieved via transition state TSdim (Figure 7b). Our CCSD(T)-F12a/cc-pVTZ-F12 computed barrier for this hydrogen bond rearrangement of 0.46 kcal/mol are compared with the previous high-level ab initio evaluations of 0.87 and 0.59 kcal/mol.59,60


Page 11 of 53

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


11 Molecular adsorption of the water dimer on Al6 results in formation of the (H2O)2-Al6 complex D1 (Figure 7c) or D2 (Figure 7d). In either of the two complexes, the water dimer adsorbs predominantly through the oxygen atom of the donor water molecule, with the Al-O distances of 1.98 (D1) and 1.96 Å (D2); the interaction of the (more distant from Al6) water acceptor molecule through the H atom is much weaker, with the respective Al…H distances of 2.91 and 2.87 Å. Notice that the O…O (O…H) distances in D1 and D2 of 2.60 (1.62) Å are 0.32 (0.33) Å shorter than the respective isolated water dimer equilibrium distances. At the CCSD(T)F12a/cc-pVTZ-F12 level, the computed adsorption energy (Eads) corresponding to the complex D2 is 18.9 kcal/mol (Table 6), or 9.5 kcal/mol per H2O molecule. For the complex D1, the respective adsorption energy is about 5 kcal/mol smaller and amounts to 13.8 kcal/mol, or 6.9 kcal/mol per H2O molecule. Both molecularly adsorbed dimer complexes appear to play a similar role in the mechanism of the dimer reaction as the molecularly adsorbed complex 1 does in the mechanism occuring for the monomer reaction.

3.7 Dimer Path 1dim The CCSD(T)-F12a/cc-pVTZ-F12 energy profile (using the B3LYP/aug-cc-pVTZ geometries) for the dimer reaction involving complex D1 (path 1dim) is shown in Figure 8, with the relevant energies in Table 7. Forming complex D1 opens up a pathway for the O-H dissociation through a concerted migration of two H atoms that involves transition state TS1d (path 1dim, Figure 9). In this mechanism, the hydrogen atom H2 of the donor water molecule is transferred to the acceptor water molecule, which in turn transfers one of its original hydrogens (H1) to the Al4 atom to form eventually the H1 bridge bond with the Al4-Al3 edge. Overall, in the resulting cluster 1d, one water molecule is dissociated (the H2H3O2 donor), and the other one is non-dissociated (the "recovered" H4H2O1) and hydrogen bonded to the (Al3 bound) O2H3 hydroxy group. On passing from D1 to TS1d to 1d, the Al3-O2 bond distance decreases, acquiring 1.73 Å in the latter intermediate. Producing 1d which holds the Al4-Al3 edge bridged H atom and adsorbed OH-water complex is exothermic by 44.3 kcal/mol at the CCSD(T)F12a/cc-pVTZ-F12 level (Table 7). The most remarkable feature of the dimer reaction is the absence of the overall O-H dissociation barrier, because TS1d lies 9.6 kcal/mol below the reactants (H2O)2 + Al6 (Figure 8). This is in clear contrast to the water monomer reaction where such a barrier exists for all the



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 12 of 53

12 paths examined. Apparently, for the dimer reaction, the hydrogen bond acceptor water molecule has a catalytic effect on the O-H dissociation barrier of the hydrogen bond donor water molecule by removing this barrier. The HAl6OH(H2O) cluster 2d encountered next along the path 1dim differs from 1d mainly by having the H4H2O1 water molecule and O2H3 hydroxy group roles (i.e., H-bond donor or acceptor) interchanged (Figure 9). Correspondingly, 1d and 2d are of very similar stability. Although we could not locate a transition state for the interchange of the H-bond donor and acceptor connecting the two species minima, we believe that the corresponding energy barrier should be low. Our reasoning is based on experimental evidence suggesting that donoracceptor interchange can occur easily within the water dimer bound on a surface as well. For instance, such interchange was reported to be directly observed using a scanning tunneling microscope in water dimers bound on a Cu(110) surface at 6 K.61 For the interchange associated with the interconversion 1d → 2d, only the O2H3 moiety is directly bound to the metal cluster’s surface (Figure 9). The subsequent two steps bring the hydridic H1 and protic H2 hydrogens closer to one another to enable H2 release. The first is 2d → TS2d-3d → 3d that entails breaking of the H1 bridge bond and rearranging the O2-H3…O1 hydrogen bond (i.e., with flipping and internal rotation of the acceptor H4H2O1 water molecule about this H-bond). It requires an energy of 1 kcal/mol relative to 2d (Figure 8). The second is 3d → TS3d-4d → 4d; the transition vector of TS3d-4d (Figure S7) indicates the motion of Al1 and O2. This step also involves some rearrangement of the metal cluster, to end up with 4d comprising the Al1-Al3 edge bridged O2H3 hydroxy group (Figure 9). After traversing the TS3d-4d energy barrier of 11 kcal/mol with respect to 3d, H1 and H2 become 1.75 Å apart in 4d. Finally, the dehydrogenation can occur via transition state TS4d-5d wherein H3 and H2 protons and H1 hydride move concertedly to form the partial H1-H2 bond (Figure 9). Compared to the water monomer reaction with the energy barriers for H2 loss ranging from 22.2 to 23.5 kcal/mol (with respect to the directly preceding species), that associated with TS4d-5d of 15.4 kcal/mol (with respect to 4d) is 6.8-8.1 kcal/mol lower. The barrier lowering was made possible by the presence of the hydrogen bond acceptor (H4H2O1) water molecule within the water-OH complex in 4d. This suggests the catalytic effect of the hydrogen bonded H2O on the dehydrogenation step similar to that on the O-H bond dissociation of the water dimer reaction.


Page 13 of 53

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


13 From the ensuing physisorbed complex 5d, the H2O-Al6O product P1d (Figure S4) is obtained while the hydrogen molecule departed, with the overall dehydrogenation reaction exothermicity of 66.3 kcal/mol (Figure 8, Table 7).

3.8 Dimer Path 2dim The CCSD(T)-F12a/cc-pVTZ-F12 energy profile (using the B3LYP/aug-cc-pVTZ geometries) for the dimer reaction involving complex D2 (path 2dim) is depicted in Figure 10, with the relevant energies in Table 7. The dimer complex D2 is a starting point for the O-H dissociation occurring through the transition state TS6d (path 2dim, Figure 11) by the mechanism described for the path 1 dim. That is to say, it consists of transfer of the atom H3 from the donor molecule to the acceptor molecule and migration of H1 from the latter H2O to bridge the Al1-Al3 edge. This step results in formation of cluster 6d which holds one dissociated water molecule (the H2H3O2 donor) whose O2H2 moiety is bound to the Al3 site, and nondissociated one (H3H4O1) which donates hydrogen bond to the O2H2 group. The binding energy of 6d is 39.2 kcal/mol relative to (H2O)2 and Al6 (Table 7). Similar to path 1dim, the most intriguing feature of the energy profile of path 2dim (Figure 10) is the lack of the O-H dissociation barrier with respect to the reactants (H2O)2 + Al6, implying that the hydrogen bond acceptor (H4H1O1) water molecule has a catalytic effect on the O-H dissociation barrier of the hydrogen bond donor (H2H3O2) water molecule to remove this rate-determining barrier to yield 6d. Related to the species 6d is 7d in which the non-dissociated water molecule binds to the Al3 site through the O1 atom, with the associated exothermicity of about 10 kcal/mol relative to 6d (Figure 10). The H2O coordination proceeds by the low-barrier transition state TS6d-7d wherein the water molecule is weakly bound to the metal cluster. Because the O1-Al3 bond formation in 7d is accompanied with the breakage of the O1-H3…O2 H-bond implies that the hydroxy-water complex occurs within the HAl6OH(H2O) cluster only at an early stage of the reaction path 2dim. As we will see below, the lack of this hydrogen bond significantly affects the barrier height for H2 loss compared to that of path 1dim. As can be seen from Figures 10 and 11, the next two rearrangements correspond to (i) the formation of the Al3-Al6 edge bridged O2H2 site to yield 8d via TS7d-8d lying 11 kcal/mol above 7d, and (ii) subsequent destroying of the Al1-Al3 edge bridged H1 site in 8d through the



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 14 of 53

14 low-barrier transition state TS8d-9d which affords 9d having this edge broken. In fact, 9d is predicted to be the lowest energy species for path 2dim - the corresponding binding energy amounts to 52.6 kcal/mol relative to (H2O)2 and Al6 (Table 7). Both steps (i) and (ii) can be viewed as preparatory ones for the dehydrogenation, that involves the protic (H2) and hydridic (H1) hydrogens within the transition state TS9d-10d. The magnitude of the associated energy barrier of 35.4 kcal/mol relative to 9d is found noticeably larger compared to the corresponding TS4d-5d barrier of path 1dim (cf. Figures 8 and 10). An explanation is that the TS9d-10d does not benefit from the catalytic effect of the hydrogen bonded water molecule as it was the case for TS4d-5d (Figure 9). Secondly, the formation of TS9d-10d from 9d involves "inversion" of the H2O molecule with respect to the Al3-O2-H2-H1 plane (Figure 11). Nevertheless, as Figure 10 clearly shows, TS9d-10d is lying safely below the reactants (H2O)2 + Al6 by 17.2 kcal/mol. Upon the H2 detachment, the P2d H2O-Al6O product (Figure S4) is obtained from the physisorbed complex 10d. The overall dehydrogenation reaction exothermicity for path 2dim of 48.5 kcal/mol (Table 7) is smaller than that of path 1dim by 17.8 kcal/mol, reflecting the stability difference between the non-planar P1d and planar P2d species at the CCSD(T)-F12a/cc-pVTZ-F12 level (Table 8).

4. Summary We have investigated a reaction mechanism by which water monomer and water dimer are dissociated and dehydrogenated on the smallest 3D aluminum particle, Al6. The associated energetics has been calculated using the explicitly correlated coupled cluster CCSD(T)-F12 theory with the cc-pVnZ-F12 (n=T,Q) basis sets on the B3LYP/aug-cc-pVTZ determined potential energy surfaces of the two reactions. For the "crucial" stationary points of the water monomer reaction, core-valence correlation and an extrapolation to the complete basis set limit have been considered in the coupled cluster calculations. The results show that the water monomer reaction proceeds by the formation of the molecularly adsorbed complex, followed by the O-H dissociation and H2 release, with the formation of the physisorbed H2-complex on the exit channel, the scenario similar to the analogous cluster cation reaction.27 However, by contrast to the cation case, the overall energy barrier for the O-H dissociation of about 2 kcal/mol is predicted for the Al6 + H2O system.


Page 15 of 53

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


15 For the water dimer reaction, two distinct paths have been identified, initiated by forming separate dimer complexes wherein (H2O)2 adsorbs mainly via the oxygen atom of the donor H2O molecule. The key O-H dissociation transition states of the dimer reaction involve a concerted migration of two H atoms resulting in the dissociation of the donor water molecule and formation of the OH-water complex adsorbed on the metal cluster’s surface. The notable feature of both dimer reaction energy profiles is the absence of the overall energy barrier for the O-H dissociation step. Our calculations suggest that the hydrogen bond acceptor water molecule has an extra catalytic effect on the O-H dissociation barrier of the hydrogen bond donor water molecule to remove this rate-determining barrier. A similar effect on the dehydrogenation step is indicated.

5. Acknowledgement The author gratefully acknowledges computational resources provided by the Wroclaw Centre for Networking and Supercomputing, WCSS.

Supporting Information: Full citation for references 33 and 41. R1-R3 isomers of Al6 and P1P3 isomers of Al6O (Figure S1). The CCSD(T)/aug-cc-pVTZ and B3LYP/aug-cc-pVTZ energy profiles for the reaction of H2O with Al6 (Figures S2-S3). P1d and P2d isomers of H2O-Al6O (Figure S4). The CCSD(T)/aug-cc-pVTZ and B3LYP/aug-cc-pVTZ energy profiles for the reaction of (H2O)2 with Al6 (Figures S5-S6). Imaginary mode of the transition state TS3d-4d of the path 1d (Figure S7).

Figure 1. Molecularly adsorbed water monomer complex 1. Selected bond distances are reported in Å.

Figure 2. Graphical representation of the DFT-SAPT_aug-cc-pVTZ interaction energy for the physisorbed complexes (a) H2O-Al6 1 and (b) Al6O…H2 3 (notice different energy scales in the figures a) and b)).



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 16 of 53

16 Figure 3. CCSD(T)-F12a/cc-pVTZ-F12 energy profile for the H2O + Al6 → Al6O + H2 reaction calculated at the B3LYP/aug-cc-pVTZ geometries, with (zero-point energy) ZPE (B3LYP) corrections.

Figure 4. Stationary points of the path 1 of the monomer reaction (imaginary frequencies of transition states are shown). Selected bond distances are reported in Å.



Figure 7. (a) Water dimer. (b) Transition state for the interchange of hydrogen bond donor and acceptor molecules within the isolated water dimer (with the imaginary frequency shown). (c) Molecularly adsorbed water dimer complex D1. (d) Molecularly adsorbed water dimer complex D2.

Figure 8. CCSD(T)-F12a/cc-pVTZ-F12 energy profile for the (H2O)2 + Al6 → H2O-Al6O + H2 reaction initiated with the formation of the dimer complex D1, as calculated at the B3LYP/augcc-pVTZ geometries, with (zero-point energy) ZPE (B3LYP) corrections.

Figure 9. Stationary points of the path 1dim of the dimer reaction (imaginary frequencies of transition states are shown). Selected bond distances are reported in Å.

Figure 10. CCSD(T)-F12a/cc-pVTZ-F12 energy profile for the (H2O)2 + Al6 → H2O-Al6O + H2 reaction initiated with the formation of the dimer complex D2, as calculated at the B3LYP/augcc-pVTZ geometries, with (zero-point energy) ZPE (B3LYP) corrections.

Figure 11. Stationary points of the path 2dim of the dimer reaction (imaginary frequencies of transition states are shown). Selected bond distances are reported in Å.


Page 17 of 53

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


17

References (1) Szalkowski, F. J. Evidence for a Molecular Intermediate During the Oxidation of Aluminum by Water. J. Chem. Phys. 1982, 77, 5224-5227. (2) Paul, J.; Hoffmann, F. M. Decomposition of H2O on Clean and Oxidized Al (100). J. Phys. Chem. 1986, 90, 5321-5324. (3) Netzer, F. P.; Madey, T. E. Adsorption of H2O on Al(111). Surf. Sci. 1983, 127, L102-L109. (4) Crowell, J. E.; Chen, J. G.; Hercules, D. M.; Yates, J. T. Jr. The Adsorption and Thermal Decomposition of Water on Clean and Oxygen-predosed Al(111). J. Chem. Phys. 1987, 86, 5804-5815. (5) Müller, J. E.; Harris, J. Cluster Study of the Interaction of a Water Molecule with an Aluminum Surface. Phys. Rev. Lett. 1984, 53, 2493-2496. (6) Michaelides, A.; Ranea, V. A.; de Andres, P. L.; King, D. A. First-principles Study of H2O Diffusion on a Metal Surface: H2O on Al{100}. Phys. Rev. B 2004, 69, 075409. (7) Jin, S.; Head, J. D. Theoretical Investigation of Molecular Water Adsorption on the Al(111) Surface. Surf. Sci. 1994, 318, 204-216. (8) Ranea, V. A. Potential Energy Surface of H2O on Al{111} and Rh{111} from Theoretical Methods. J. Chem. Phys. 2012, 137, 20472. (9) Guo, F. Y.; Long, C. G.; Zhang, J.; Zhang, Z.; Liu, C. H.; Yu. K. Adsorption and Dissociation of H2O on Al(111) Surface by Density Functional Theory Calculation. Appl. Surf. Sci. 2015, 324, 584-589. (10) Tappan, B. C.; Dirmyer, M. R.; Risha. G. A., Evidence of a Kinetic Isotope Effect in Nanoaluminum and Water Combustion. Angew. Chem. Int. Ed. 2014, 53, 9218-9221. (11) Guerieri, P. M.; DeCarlo, S.; Eichhorn, B.; Connell, T.; Yetter, R. A.; Tang, X.; Hicks, Z.; Bowen, K. H.; Zachariah, M. R. Molecular Aluminum Additive for Burn Enhacement of Hydrocarbon Fuels. J. Phys. Chem. A 2015, 119, 11084-11093. (12) Brunet, F. D.; Joly, H. A. Electron Paramagnetic Resonance Spectroscopic Evidence for the Interaction of HAlOH with Water Molecules. J. Phys. Chem. A 2012, 116, 4267-4273. (13) Oblath, S. B.; Gole, J. L. Aluminum Hydration in the Vapor Phase. J. Chem. Phys. 1979, 70, 581-582.



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 18 of 53

18 (14) Hauge, R. H.; Kauffman. J. W.; Margrave, J. L. Infrared Matrix-isolation Studies of the Interactions and Reactions of Group 3A Metal Atoms with Water. J. Am. Chem. Soc. 1980, 102, 6005-6011. (15) Douglas, M. A.; Hauge, R. H.; Margrave, J. L. Matrix Isolation Studies by Electronic Spectroscopy of Group IIIA Metal Water Photochemistry. J. Chem. Soc. Faraday Trans 1 1983, 79, 1533-1553. (16) Joly, H. A.; Howard, J. A.; Tomietto, M.; Tse, J. S. Characterization of the Intermediates Formed in the Reaction of Al Atoms with H2O, H2S and H2Se by EPR Spectroscopy. J. Chem. Soc. Faraday Trans. 1994, 90, 3145-3151. (17) Knight, L. B., Jr.; Gregory, B.; Cleveland, J.; Arrington, C. A. Laser Vaporization Generation of HAlOH, DAlOD and HAl17 OH for Neon and Argon Matrix Isolation Electron Spin Resonance Investigations. Comparison with Theoretical Calculations. Chem. Phys. Lett. 1993, 204, 168-174. (18) McLean, R. E.; Nelson, H. H.; Cambell, M. L. Kinetics of the Reaction Aluminum(2P0) + Water Over an Extended Temperature Range. J. Phys. Chem. 1993, 97, 9673-9676. (19) Kransnokutski, S. A.; Huisken, F. Low-temperature Chemistry in Helium Droplets: Reactions of Aluminum Atoms with O2 and H2O. J. Phys. Chem. A 2011, 115, 7120-7126. (20) Álvarez-Barcia, S.; Flores, J. R. A High-accuracy Theoretical Study of the AlOH2 System. Chem. Phys. Lett. 2009, 470, 196-202. This paper also includes references to the prior low-level calculations of the Al-OH2 system. (21) Álvarez-Barcia, S.; Flores, J. R. A Theoretical Study of the Dynamics of the Al + H2O Reaction in the Gas Phase. Chem. Phys. 2011, 382, 92-97. (22) Álvarez-Barcia, S.; Flores, J. R. How H2 Can Be Formed by the Interaction of Al Atoms with a Few Water Molecules: A Theoretical Study. Chem. Phys. 2010, 374, 131-137. (23) Álvarez-Barcia, S.; Flores, J. R. The Interaction of Al Atoms with Water Molecules: A Theoretical Study. J. Chem. Phys. 2009, 131, 174307. (24) Zheng, J. J.; Zhao, Y.; Truhlar, D. G. The DBH24/08 Database and Its Use to Assess Electronic Structure Model Chemistries for Chemical Reaction Barrier Heights. J. Chem. Theory Comput. 2009, 5, 808-821. (25) Kroes, G.-J. Toward a Database of Chemically Accurate Barrier Heights for Reactions of Molecules with Metal Surfaces, J. Phys. Chem. Lett. 2015, 6, 4106-4114.


Page 19 of 53

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


19 (26) Libisch, F.; Huang, C.; Liao, P.; Pavone, M.; Carter, E. A. Origin of the Energy Barrier to Chemical Reactions of O2 on Al(111): Evidence for Charge Transfer, Not Spin Selection. Phys. Rev. Lett. 2012, 109, 198303. (27) Moc, J. Theoretical Investigation of the Reaction Paths of the Aluminum Cluster Cation with Water Molecule in the Gas Phase: A Facile Route for Dihydrogen Release, J. Phys. Chem. A 2015, 119, 8683-8691. (28) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648-5652. (29) Lee, C. T.; Yang, W. T.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785-789. (30) Dunning, T. H. Gaussian Basis Sets for Use in Correlated Molecular Calculations 0.1. The Atoms Boron through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007-1023. (31) Woon, D. E.; Dunning, T. H. Gaussian Basis Sets for Use in Correlated Molecular Calculations. 3. The Atoms Aluminum through Argon. J. Chem. Phys. 1993, 98, 1358-1371. (32) Gonzalez, C.; Schlegel, H. B. An Improved Algorithm for Reaction Path Following. J. Chem. Phys. 1989, 90, 2154-2161. (33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, Revision B.01; Gaussian, Inc., Wallingford, CT, 2009. (34) Adler, T. B.; Knizia, G.; Werner, H.-J. A Simple and Efficient CCSD(T)-F12 Approximation. J. Chem. Phys. 2007, 127, 221106. (35) Knizia, G.; Adler, T. B.; Werner, H.-J. Simplified CCSD(T)-F12 Methods: Theory and Benchmarks. J. Chem. Phys. 2009, 130, 054104. (36) Werner, H.-J.; Adler, T. B.; Knizia, G.; Manby, F. R. Recent Progress in Coupled Cluster Methods: Theory and Applications; Čársky, P., Paldus, J., Pittner, J., Eds.; Springer: Berlin, Germany, 2010; pp 573-619.



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 20 of 53

20 (37) Peterson, K. A.; Adler, T. B.; Werner, H. -J. Systematically Convergent Basis sets for Explicitly Correlated Wavefunctions: the Atoms H, He, B-Ne, and Al-Ar. J. Chem. Phys. 2008, 128, 084102. (38) Yousaf, K. E.; Peterson, K. A. Optimized Complementary Auxiliary Basis Sets for Explicitly Correlated Methods: aug-cc-pVnZ Orbital Basis Sets. Chem. Phys. Lett. 2009, 476, 303−307. (39) Weigend, F. A Fully Direct RI-HF Algorithm: Implementation, Optimised Auxiliary Basis Sets, Demonstration of Accuracy and Efficiency. Phys. Chem. Chem. Phys. 2002, 4, 4285−4291. (40) Hättig, C. Optimization of Auxiliary Basis Sets for RI-MP2 and RI-CC2 Calculations: CoreValence and Quintuple-ζ Basis Sets for H to Ar and QZVPP Basis Sets for Li to Kr. Phys. Chem. Chem. Phys. 2005, 7, 59−66. (41) Werner, H.-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz, M.; Celani, P.; Korona, T.; Lindh, R.; Mitrushenkov, A.; Rauhut, G. et al. MOLPRO, version 2012.1; Cardiff University: Cardiff, U.K., 2012. (42) Raghavachari, K.; Trucks, G. W.; Pople, J. A.; Head-Gordon, M. A Fifth-Order Perturbation Comparison of Electron Correlation Theories. Chem. Phys. Lett. 1989, 157, 479-483. (43) Jeziorski, B.; Moszynski, R.; Szalewicz, K. Perturbation Theory Approach to Intermolecular Potential Energy Surfaces of van der Waals Complexes. Chem. Rev. 1994, 94, 1887-1930. (44) Heβelmann, A.; Jansen, G. The Helium Dimer Potential from a Combined Density Functional Theory and Symmetry-adapted Perturbation Theory Approach Using an Exact Exchange–correlation Potential. Phys. Chem. Chem. Phys. 2003, 5, 5010-5014. (45) Heβelmann, A.; Jansen, G.; Schütz, M. Density-functional Theory Symmetry-adapted Intermolecular Perturbation Theory with Density Fitting: a New Efficient Method to Study Intermolecular Interaction Energies. J. Chem. Phys. 2005, 122, 014103. (46) Adamo, C.; Barone V. Toward Reliable Density Functional Methods without Adjustable Parameters: The PBE0 Model. J. Chem. Phys. 1999, 110, 6158-6170. (47) Sala, F. D.; Görling, A. Efficient Localized Hartree–Fock Methods as Effective Exactexchange Kohn–Sham Methods for Molecules. J. Chem. Phys. 2001, 115, 5718-5732. (48) Grüning, M.; Gritsenko, O. V.; van Gisbergen, S. J. A.; Baerends, E. J. Shape Corrections to Exchange-correlation Potentials by Gradient-regulated Seamless Connection of Model Potentials for Inner and Outer Region. J. Chem. Phys. 2001, 114, 652-660.


Page 21 of 53

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


21 (49) Karalti, O.; Alfe, D.; Gillan, M. J.; Jordan, K. D. Adsorption of a Water Molecule on the MgO(100) Surface as Described by Cluster and Slab Models, Phys. Chem. Chem. Phys. 2012, 14, 7846-7853. (50) Moc, J. Hydrogenation of Aluminium Hexamer: Ab Initio Molecular Orbital Theory and Density Functional Theory Study. Chem. Phys. Lett. 2008, 466, 116-121. (51) Pino, I.; Kroes, G. J.; van Hemert, M. C. Hydrogen Dissociation on Small Aluminum Clusters. J. Chem. Phys. 2010, 133, 184304. (52) Ahlrichs, R.; Elliott, S. D. Clusters of Aluminium, A Density Functional Study. Phys. Chem. Chem. Phys. 1999, 1, 13-21. (53) Neukermans, S.; Veldeman, N.; Janssens, E.; Lievens, P.; Chen, Z.; Schleyer, P. v. R. Combined Experimental and Theoretical Study of Small Aluminum Oxygen Clusters. Eur. Phys J. D 2007, 45, 301-308. (54) Weinhold, F.; Landis, C. R. Valency and Bonding: A Natural Bond Orbital Donor-Acceptor Perspective; Cambridge University Press: Cambridge, UK, 2005. (55) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO v 3.1; Theoretical Chemistry Institute and Department of Chemistry, University of Wisconsin, Madison, Wisconsin, 1998. (56) Hill, J. G.; Mazumder, S.; Peterson, K. A. Correlation Consistent Basis Sets for Molecular Core-Valence Effects with Explicitly Correlated Wave Functions: The Atoms B−Ne and Al−Ar. J. Chem. Phys. 2010, 132, 054108. (57) Hill, J. G.; Peterson, K. A.; Knizia, G.; Werner, H.-J. Extrapolating MP2 and CCSD Explicitly Correlated Correlation Energies to the Complete Basis Set Limit with First and Second Row Correlation Consistent Basis Sets. J. Chem. Phys. 2009, 131, 194105. (58) Schwenke, D. W. The Extrapolation of One-Electron Basis Sets in Electronic Structure Calculations: How It Should Work and How It Can Be Made to Work. J. Chem. Phys. 2005, 122, 014107. (59) Mukhopadhyay, A.; Cole, W. T. S.; Saykally, R. J. The Water Dimer I: Experimental Characterization. Chem. Phys. Lett. 2015, 633, 13-26. (60) Smith, B. J.; Swanton, D. J.; Pople, J. A.; Schaefer III, H. F.; Radom, L. Transition Structures for the Interchange of Hydrogen Atoms within the Water Dimer. J. Chem. Phys. 1990, 92, 1240-1247.



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 22 of 53

22 (61) Kumagai, T.; Kaizu, M.; Hatta, S.; Okuyama, H.; Aruga, T.; Hamada, I.; Morikawa. Y. Direct Observation of Hydrogen-Bond Exchange within a Single Water Dimer. Phys. Rev. Lett. 2008, 100, 166101. Table 1. Relative energy (kcal/mol) of the isomers of Al6 and Al6O clusters at various levels of theory.a B3LYP b CCSD(T)b

Species

CCSD(T)-F12ab

Al6 R1 (D3d, 1A1g)

0.0

0.0

0.0

Al6 R2 (D3h, 3A1')

-0.5

4.9

5.8

Al6 R3 (C2v, A1)

2.0

6.6

7.3

Al6O P1 (C2v, 1A1)

0.0

0.0

0.0

Al6O P2 (Cs, A')

-1.6

3.9

4.9

Al6O P3 (Cs, 1A')

-4.1

1.0

2.5

1

1

1

3.8 6.5 7.2 TSP1-P3 (Cs, A') Calculated with the basis set aug-cc-pVTZ (B3LYP, CCSD(T)) and with the basis set cc-pVTZ-F12 (CCSD(T)-F12a), and including ZPE correction calculated at the B3LYP level. For the optimized structures of these clusters, see Figure S1. b At the B3LYP geometries. The CCSD T1 diagnostic (RHF-CCSD) for the R1, R2, R3, P1, P2, P3, and TSP1-P3 structures is 0.021, 0.020, 0.029, 0.024, 0.035, 0.022 and 0.024, respectively (for the R2 structure, this is the ROHF-UCCSD T1 value). a

Table 2. Adsorption energy (Eads, kcal/mol)a of the associative complex H2O-Al6 1 at various levels of theory. B3LYPb CCSD(T)b

CCSD(T)-F12ab

Eads 7.9c 8.8d 9.5d a Eads= E(H2O) + E(Al6) – E(H2O-Al6). Calculated with the basis set aug-cc-pVTZ (B3LYP, CCSD(T)) and with the basis set cc-pVTZ-F12 (CCSD(T)-F12a), and including ZPE correction calculated at the B3LYP level. b At the B3LYP geometry. The CCSD T1 diagnostic (RHF-CCSD) for the complex 1 is 0.023. c With respect to the R2 (D3h, 3A1') structure of Al6 plus H2O (see Table 1). d With respect to the R1 (D3d, 1A1g) structure of Al6 plus H2O (see Table 1).


Page 23 of 53

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


23 Table 3. DFT-SAPT_aug-cc-pVTZ interaction energya (in kcal/mol) for the physisorbed complexes 1 and 3. Elst Exch-Rep Indb Dispc Total H2O-Al6 1 -56.5 79.0 -19.7 -11.4 -8.5 Al6O…H2 3 -0.77 1.10 -0.10 -0.83 -0.60 a Note that the SAPT interaction energy is calculated with respect to the subsystems at their geometries assumed in the complex, whereas (the positively defined) Eads (Table 2) is calculated with respect to the optimized species and includes ZPE correction. bThe induction and exchangeinduction terms were combined. cThe dispersion and exchange-dispersion terms were combined. The dipole moment of an isolated water monomer is 1.855 D.

Table 4. B3LYP, CCSD(T) and CCSD(T)-F12a energy profiles (relative to separated H2O + Al6, kcal/mol) of the H2O + Al6 → Al6O + H2 reaction calculated at the B3LYP geometries with ZPE (B3LYP) corrections (monomer paths 1-3).a path 1 Method B3LYPc CCSD(T)

d

CCSD(T)F12ad Method

TS1-2 (C1)b

2 (Cs)

-7.9

6.2

-48.9

TS2-3 (Cs)b

3 (Cs)

P1 (C2v) + H2

-52.4

-53.0

-49.2

-27.2

TSP1-P3 (Cs) + H2

-8.8

11.6 (12.0)

-46.7

-23.5 (-23.5)

-48.2

-48.1

-41.7

-9.5

11.8 (12.2)

-48.8

-26.6 (-26.3)

-51.9

-52.0

-44.8

P3 (Cs) + H2

B3LYPc CCSD(T)

1 (Cs)

-57.2 d

CCSD(T)F12ad

-47.2 -49.5

path 2 Method B3LYPc CCSD(T)

d

CCSD(T)F12ad

1 (Cs)

TS1-4 (C1)b

4 (C1)

TS4-5 (C1)

-7.9

1.2

-40.0

-39.9

-40.2

-35.6

-44.1

-31.3

-41.6

-8.8

3.3 (3.3)

-37.2

-37.0

-37.3

-32.1

-42.1

-28.5

-40.9

-9.5

1.7 (1.9)

-40.0

-40.0

-40.1

-35.0

-45.9

-31.3

-43.4

5 (C1)

TS5-6 (C1)

6 (C1)

TS6-7 (C1)

path 2 (continued) Method B3LYPc CCSD(T)

d

TS7-3 (Cs)b

3 (Cs)

-19.9

-52.4

-53.0

-49.2

-57.2

-48.2

-48.1

-41.7

-47.2

-17.4 (-17.4)

P1 (C2v) + H2

TSP1-P3 (Cs) + H2


P3 (Cs) + H2

7 (C1)


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 24 of 53

24 CCSD(T)F12ad

-20.7 (-20.4)

-51.9

-52.0

-44.8

-49.5

path 3 Method B3LYPc

1 (Cs)

TS1-8 (C1)b

8 (C1)

TS8-9 (C1)

9 (C1)

-7.9

3.5

-41.4

-30.6

-41.7

TS9-10 (C1)b -20.8

10 (C1)

P2 (Cs) + H2

-54.5

-54.6

d

-8.8

4.0 (4.0)

-39.7

-25.6

-34.7

-11.0 (-11.0)

-44.2

-44.2

CCSD(T)F1ad

-9.5

3.0 (3.3)

-41.7

-27.7

-36.9

-13.4 (-13.2)

-46.9

-47.0

CCSD(T)

a

With the basis set aug-cc-pVTZ (B3LYP, CCSD(T)) and with the basis set cc-pVTZ-F12 (CCSD(T)-F12a). b The coupled cluster values are based on the RKS (B3LYP) reference; the values in parentheses are based on the RHF reference. c Relative to the R2 (D3h, 3A1') + H2O reactants (see Table 1). d Relative to the R1 (D3d, 1A1g) + H2O reactants (see Table 1). Table 5. Relative energy of the "crucial" stationary points of the monomer path 2 (compared to separated H2O +Al6, kcal/mol) calculated with the CCSD(T)-F12a and CCSD(T)-F12b methods and cc-pVQZ-F12 basis set at the B3LYP geometries with ZPE (B3LYP) corrections. TS1-4 (C1)a

Method

1 (Cs)

CCSD(T)-F12a/cc-pVQZ-F12

-9.3

1.8

-51.7

CCSD(T)-F12b/cc-pVQZ-F12

-9.2

1.8

-51.7

-8.9

2.1

-51.6

b

CCSD(T)-F12b/CBS

P1 (C2v) + H2

a

The values were calculated using Kohn-Sham (B3LYP) orbitals. CBS [cc-pVTZ-F12/cc-pVQZ-F12] extrapolation (see the text for the details), with the core-valence corrections. b

Table 6. Adsorption energy (Eads, kcal/mol) of the associative complexes (H2O)2-Al6 D1 and D2 at various levels of theory.a B3LYP

CCSD(T)

CCSD(T)-F12a

D1b

12.1c

12.9d

13.8d

D2b

16.1c

18.2d

18.9d

a

At the B3LYP geometries. Calculated with the basis set aug-cc-pVTZ (B3LYP, CCSD(T)) and with the basis set cc-pVTZ-F12 (CCSD(T)-F12a), and including ZPE correction calculated at the B3LYP level. b Eads= E[(H2O)2] + E(Al6) – E[(H2O)2-Al6)]. The CCSD T1 diagnostic (RHF-CCSD) for the D1 and D2 structures is 0.024 and 0.022, respectively.


Page 25 of 53

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


25 c

With respect to the R2 (D3h, 3A1') structure of Al6 plus (H2O)2 (see Table 1). With respect to the R1 (D3d, 1A1g) structure of Al6 plus (H2O)2 (see Table 1).

d

Table 7. B3LYP, CCSD(T) and CCSD(T)-F12a energy profiles (relative to separated (H2O)2 + Al6, kcal/mol) of the (H2O)2 + Al6 → H2O-Al6O + H2 reaction calculated at the B3LYP geometries with ZPE (B3LYP) corrections (dimer paths 1dim-2dim).a path 1dim Method B3LYPc CCSD(T)

TS1d (C1)b

D1 (C1) -12.1 d

CCSD(T)F12ad

1d (C1)

2d (C1)

-44.6

-44.7

-43.6

-44.0

-8.3

TS2d-3d (C1)

3d (C1)

-12.9

-7.7 (-7.7)

-42.0

-42.1

-41.4

-42.6

-13.8

-9.6 (-9.3)

-44.3

-44.7

-43.8

-45.5

path 1dim (continued) Method

TS3d-4d (C1)

B3LYPc

TS4d-5d (C1)b

4d (C1)

5d (C1)

P1d (C1) + H2

-33.9

-41.1

-28.7

-62.9

-63.2

d

-32.0

-41.3

-25.5 (-25.5)

-62.1

-61.9

CCSD(T)F12ad

-34.5

-44.2

-28.8 (-28.4)

-66.4

-66.3

CCSD(T)

path 2dim Method B3LYPc CCSD(T)

TS6d (C1)b

D2 (C1) d

CCSD(T)F12ad

6d (C1)

TS6d-7d (C1)

7d (C1)

-16.1

-9.4

-40.9

-39.8

-47.4

-18.2

-8.6 (-8.7)

-37.1

-36.8

-47.6

-18.9

-10.2 (-9.9)

-39.2

-38.8

-49.9

path 2dim (continued) Method B3LYPc CCSD(T)

8d (C1)

TS8d-9d (C1)

9d (C1)

TS9d-10db

-39.8

-46.0

-44.7

-52.3

-19.8

-36.6

-45.4

-44.1

-49.0

-13.1 (-12.9)

-38.7

-48.5

-47.4

-52.6

-17.2 (-16.8)

TS7d-8d (C1) d

CCSD(T)F12ad

path 2dim (continued) Method B3LYPc CCSD(T)

d

10d (C1)

P2d (C1) + H2

-53.9

-54.4

-45.4

-45.2



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 53

26 CCSD(T)F12ad

-48.5

-48.5

a

With the basis set aug-cc-pVTZ (B3LYP, CCSD(T)) and with the basis set cc-pVTZ-F12 (CCSD(T)-F12a). For the graphical representation of the B3LYP and CCSD(T) energy profiles of the water dimer reactions, see Figures S5 and S6, respectively. b The coupled cluster values are based on the RKS (B3LYP) reference; the values in parentheses are based on the RHF reference. c Relative to the R2 (D3h, 3A1') + (H2O)2 reactants (see Table 1). d Relative to the R1 (D3d, 1A1g) + (H2O)2 reactants (see Table 1). Table 8. Relative energy (kcal/mol) of the isomers of H2O-Al6O clusters at various levels of theory.a B3LYP b

Species H2O-Al6O P1d (C1, 1A) 1

H2O-Al6O P2d (C1, A)

CCSD(T)b

CCSD(T)-F12ab

0.0

0.0

0.0

8.8

16.6

17.8

a

Calculated with the basis set aug-cc-pVTZ (B3LYP, CCSD(T)) and with the basis set cc-pVTZ-F12 (CCSD(T)-F12a), and including ZPE correction calculated at the B3LYP level. For the optimized structures of the P1d and P2d clusters, see Figure S4. b At the B3LYP geometries. The CCSD T1 diagnostic (RHF-CCSD) for the P1d and P2d structures is 0.022 and 0.031, respectively.


Page 27 of 53


27

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 ACS Paragon Plus Environment


Page 28 of 53

28


Page 29 of 53


29



Page 30 of 53

30


Page 31 of 53


31



Page 32 of 53

32


Page 33 of 53


33



Page 34 of 53

34


Page 35 of 53


35



Page 36 of 53

36


Page 37 of 53


37



Page 38 of 53

38


Page 39 of 53


39



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

Figure 1. Molecularly adsorbed water monomer complex 1. Selected bond distances are reported in Å. 36x66mm (300 x 300 DPI)


Page 40 of 53

Page 41 of 53

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


Figure 2. Graphical representation of the DFT-SAPT_aug-cc-pVTZ interaction energy for the physisorbed complexes (a) H2O-Al6 1 and (b) Al6O…H2 3 (notice different energy scales in the figures a) and b)). 165x199mm (300 x 300 DPI)



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

Figure 3. CCSD(T)-F12a/cc-pVTZ-F12 energy profile for the H2O + Al6 → Al6O + H2 reaction calculated at the B3LYP/aug-cc-pVTZ geometries, with (zero-point energy) ZPE (B3LYP) corrections. 189x173mm (300 x 300 DPI)


Page 42 of 53

Page 43 of 53

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


Figure 4. Stationary points of the path 1 of the monomer reaction (imaginary 166x141mm (300 x 300 DPI)



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

Figure 5. Stationary points of the path 2 of the monomer reaction (imaginary frequencies of transition states are shown). Selected bond distances are reported in Å. 196x135mm (300 x 300 DPI)


Page 44 of 53

Page 45 of 53

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


Figure 5 continued. Stationary points of the path 2 of the monomer reaction (imaginary frequencies of transition states are shown). Selected bond distances are reported in Å. 171x122mm (300 x 300 DPI)



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

Figure 6. Stationary points of the path 3 of the monomer reaction (imaginary frequencies of transition states are shown). Selected bond distances are reported in Å. 201x164mm (300 x 300 DPI)


Page 46 of 53

Page 47 of 53

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


Figure 7. (a) Water dimer. (b) Transition state for the interchange of hydrogen bond donor and acceptor molecules within the isolated water dimer (with the imaginary frequency shown). (c) Molecularly adsorbed water dimer complex D1. (d) Molecularly adsorbed water dimer complex D2. 171x88mm (300 x 300 DPI)



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

Figure 8. CCSD(T)-F12a/cc-pVTZ-F12 energy profile for the (H2O)2 + Al6 → H2O-Al6O + H2 reaction initiated with the formation of the dimer complex D1, as calculated at the B3LYP/aug-cc-pVTZ geometries, with (zero-point energy) ZPE (B3LYP) corrections.

195x179mm (300 x 300 DPI)


Page 48 of 53

Page 49 of 53

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


Figure 9. Stationary points of the path 1dim of the dimer reaction (imaginary frequencies of transition states are shown). Selected bond distances are reported in Å. Figure 9 171x178mm (300 x 300 DPI)



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

Figure 9 continued. Stationary points of the path 1dim of the dimer reaction (imaginary frequencies of transition states are shown). Selected bond distances are reported in Å. 248x98mm (300 x 300 DPI)


Page 50 of 53

Page 51 of 53

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


Figure 10. CCSD(T)-F12a/cc-pVTZ-F12 energy profile for the (H2O)2 + Al6 → H2O-Al6O + H2 reaction initiated with the formation of the dimer complex D2, as calculated at the B3LYP/aug-cc-pVTZ geometries, with (zero-point energy) ZPE (B3LYP) corrections. 227x182mm (300 x 300 DPI)



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

Figure 11. Stationary points of the path 2dim of the dimer reaction (imaginary frequencies of transition states are shown). Selected bond distances are reported in Å. 246x177mm (300 x 300 DPI)


Page 52 of 53

Page 53 of 53

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


TOC file 86x75mm (300 x 300 DPI)


Adsorption, Dissociation, and Dehydrogenation of Water Monomer

Recommend Documents