The Competitive OâH versus CâH Bond Activation of Ethanol and

Article pubs.acs.org/JPCA

The Competitive O−H versus C−H Bond Activation of Ethanol and Methanol by VO2+ in Gas Phase: A DFT Study Lianming Zhao,*,†,‡ Min Tan,†,‡ Juan Chen,†,‡ Qiuyue Ding,†,‡ Xiaoqing Lu,†,‡ Yuhua Chi,†,‡ Guangwu Yang,†,‡ Wenyue Guo,*,†,‡ and Qingtao Fu§ †

College of Science, China University of Petroleum, Qingdao, Shandong 266580, P.R. China Key Laboratory of New Energy Physics and Materials Science in Universities of Shandong, China University of Petroleum, Qingdao, Shandong 266580, P.R. China § Key Laboratory of Resources and Environmental Analytical Chemistry, Linyi University, Linyi, Shandong 276005, P.R. China ‡

S Supporting Information *

ABSTRACT: The activation of ethanol and methanol by VO2+ in gas phase has been theoretically investigated by using density functional theory (DFT). For the VO2+/ ethanol system, the activation energy (ΔE) is found to follow the order of ΔE(Cβ−H) < ΔE(Cα−H) ≈ ΔE(O−H). Loss of methyl and glycol occurs respectively via O−H and Cβ−H activation, while acetaldehyde elimination proceeds through two comparable O−H and Cα−H activations yielding both VO(H2O)+ and V(OH)2+. Loss of water not only gives rise to VO(CH3CHO)+ via both O−H and Cα−H activation but also forms VO2(C2H4)+ via Cβ−H activation. The major product of ethylene is formed via both O−H and Cβ−H activation for yielding VO(OH)2+ and VO2(H2O)+. In the methanol reaction, both initial O−H and Cα−H activation accounts for formaldehyde and water elimination, but the former pathway is preferred.

1. INTRODUCTION Chemical reactivity involving transition metal oxides is used widely and plays a promising role in heterogeneous catalysis, exhibiting high selectivity and activity.1,2 A quarter of the most important organic chemicals are produced by heterogeneous oxidation catalysis, and more than a third of worldwide catalyst production is based on oxides.3,4 In particular, vanadium oxides are an important class of heterogeneous catalysts used in both industry and laboratory. To identify new modes of reactivity and bonding in the gas phase that might be relevant to condensed-phase chemistry, the reactivity of a range of cationic and anionic vanadium oxide centers toward neutral reagents, such as alkanes, alkenes, and alcohols, has been examined.5−22 Among them the reaction of vanadium oxide with alcohols may be particularly interesting due to the industrial application of heterogeneous vanadium oxide catalysts in the oxidation of alcohols, and the likely intermediacy of vanadium alkoxide species in other oxidation reactions mediated by vanadium oxides.16−19,23−26 A number of ion/molecule experiments in the gas phase have been carried out on reactivity toward alcohols of VO+, VO2+, VO3−, and so on.18,20−22 In combination with quantum chemistry calculation, it is found that the dehydrogenation of methanol by VO + proceeds via initial C−H bond activation.18 However, VO3− reacts with methanol for loss of neutral water, and with ethanol for loss of water and ethylene through initial O−H bond activation. 21 Thus, it is very interesting to unveil the selectivity of the C−H and O−H bond activation of alcohols by VO 2+. Here, we select the gas-phase ion/ molecule reactions of VO 2+ with ethanol and methanol as © XXXX American Chemical Society

the model of our theoretical study, to reveal the mechanism of vanadium dioxide with alcohols.

2. COMPUTATIONAL DETAILS The geometry optimization and frequency calculations were carried out for all the relevant species using the B3LYP27,28 functional in conjunction with the 6-311G(2d,p) basis set.29 Single-point energies of all the optimized species were calculated using B3LYP with the DZVP(opt) basis set30 for V+ and the 6-311++G(2d,2p) basis set29 for nonmetal atoms. All the energies are reported with zero-point energy (ZPE) corrections with a scaling factor of 0.961.31,32 The intrinsic reaction coordinate (IRC) was calculated to check if the correct transition state was located. All these calculations were performed using the Gaussian 09 package.33 The analyses of the electron structure by various methods were performed on some of the key species involved. Natural bond orbital (NBO)34,35 analysis has been performed using the NBO 3.1 program implemented in Gaussian 09.33 Atoms in molecules (AIM) analysis of the wave function was carried out with the AIM2000 software.36 The minimum energy crossing point (MECP) is located via single-point energy calculation at the B3LYP/DZVP(opt):6311++G(2d,2p) level for the relevant IRC points along both the singlet and triplet pathways until they reach an equal Received: March 1, 2013 Revised: May 21, 2013

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dx.doi.org/10.1021/jp4021454 | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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of the π(V+O) bonds is opened, leading to two unpaired electrons on V+(3d) and O(2p) atoms, respectively, which is supported by Mulliken atomic spin densities of 0.88 on V+ and 1.00 on O. Thus, the single V+−O bond increases to 1.742 Å, while the other double V+−O bond slightly shortens to 1.540 Å. Energetically, VO2+(3A′) is located at 30.6 kcal/mol above the singlet (1A1), according to other theoretical results (35.6 kcal/mol at the B3LYP/6-311G(2d,p) level47). The analysis of the frontier molecular orbitals, i. e., the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), suggests that in the singlet state, the HOMO is the highest occupied 2p(O) lone pair (LP) orbital, while the LUMO is the unoccupied 3d4s(V+) orbital. The HOMO and HOMO−1 in the triplet state are the single occupied 3d(V+) and 2p(O) orbitals, respectively, while the LUMO is the π*(V+O) orbital on the other double V+O bond. For the VO2+/CH3OH system, the encounter complex 1a in both the singlet and triplet states bears a V+O2(η1-O-CH3OH) structure (see Figure 1), with the diabatic BDE of 65.4 and 64.5 kcal/mol, respectively. AIM analysis reveals that the existence of one bond critical point (BCP) connects the metal ion with the hydroxyl O atom for both considered states. As shown in Table 2, in both states, the electron density (ρ(r)) at the BCP is small, and the Laplacian of the electron density (∇2ρ(r)) has positive value, indicating a characteristic of closedshell interaction. Moreover, the value of the total energy density (H(r)) for the BCP is negative, indicating the bond involves medium interactions. Also, the criterion 0.5 < −G(r)/V(r) < 1.0 is satisfied for the BCP corresponding to both types of bonds, indicating that, even though the bond is an electrostatic bond, it is also faintly covalent in nature. These results are supported by the NBO analysis, where the O2V+−CH3OH association in both states favors electron donations from the 2s2p(O) LP orbital to the unoccupied 4s*(V+) and π*(V+O) orbitals (Table S2, SI). A similar bonding characteristic has also been found in the interactions between group IIb divalent transition-metal cations (Zn2+, Cd2+, and Hg2+) and 3-mercaptopropionic acid.48 The products of VO2(H2O)+ and VO2(C2H4)+ are featured by the V+O2(η1-O-H2O) and V+O2(η2-C,C-C2H4) structures, respectively. Similar to the O2V+−(CH3OH) association, both O2V+− OH2 and O2V+−C2H4 associations are mainly electrostatic interactions with faintly covalent properties (see Table 2), which result in low-spin singlet ground states, lying 35.4 and 32.9 kcal/mol below those of the corresponding triplet species, respectively (see Table 3). Although we put VO2+ around any position of ethanol, only one adduct was found for the VO2(C2H5OH)+ complex: 1b, with the strong vanadium dioxide−ethanol diabatic BDE of 68.3 (singlet) and 66.4 (triplet) kcal/mol. Different with O2V+−CH3OH, the association of VO2+ with ethanol in the singlet and triplet states bears a chelate structure, in which vanadium dioxide binds to both the hydroxyl and methyl groups. Topological analysis suggests that the BCP of hydroxylO−V+ in both states is characterized by small ρ(r), ∇2ρ(r) > 0, negative H(r), and 0.5 < −G(r)/V(r) < 1.0 (see Table 2), suggesting the hydroxyl-O−V+ bond is an electrostatic bond with some covalent property in nature. On the other hand, for 1 1b, one O atom of dioxide and one H atom of methyl forms a typical OV+O···HCH2CH2OH hydrogen bond with small ρ(r), ∇2ρ(r) > 0, and positive H(r) at the BCP, as proposed by Macchi et al.,49 while 31b presents an agnostic-type interaction of a metal center with one of the C−H bonds of the methyl group (see Table 2).49,50

energy. These calculations were carried out using the Gaussian 09 package.33 The calculation methods of spin−orbit coupling (SOC) and crossing probability at the MECP, which have been described previously,32−35,37−39 are presented in detail in the Supporting Information (SI). In brief, by using the GAMESS package,40 CASSCF calculations were first performed for both singlet and triplet states at the low-spin MECP to get the converged CASSCF wave functions; the SOC matrix elements were then computed using the SOC-CI method.41 Finally, a crude estimation of the crossing probability at the MECP was calculated using the Landau−Zener formula.42−44

3. RESULTS AND DISCUSSION In the following sections, we will first establish the accuracy that is expected from the chosen level of theory for the VO2+/C2H5OH and VO2+/CH3OH systems. Then, we will examine the title reactions in detail, including geometries of all relevant stationary points and potential energy surfaces (PESs) for all possible product channels. Last, we will give the reaction mechanisms by comparing our theoretical results with the experimental findings.20 For simplicity, optimized geometries, selected structural parameters, calculated total energies, zeropoint energies as well as values for all the species involved in the reaction are given in the SI. 3.1. Calibration. To evaluate the reliability of the level of theory chosen, we compare the known experimental binding energies (BDEs)20,45,46 with the results from the B3LYP approach. Table 1 collects the theoretically predicted adiabatic Table 1. Adiabatic Bond Dissociation Energies (in kcal/mol) at 0 K Determined by Calculations and Experiments species

calcda

V+−O V+−OH VO+−O VO+−H2O VO+−OH

146.3 110.4 83.9 33.0 77.1

expt 138.1 107.0 86.3 36.2 80.2

± ± ± ± ±

3.9b 3.0c 3.8d 3.0c 8.5d

a

At the B3LYP/DZVP(opt):6-311++G(2d,2p)//B3LYP/6-311G(2d,p) level. bReference 45. cReference 46. dReference 20.

BDEs and the most reliable experimental data for some relevant species. As shown in Table 1, the theoretical values agree well with the experimental findings in most cases except for [V−O]+, where the calculated value is overestimated by 8.2 kcal/mol. However, because the VO+ unit never ruptures in the whole reaction, the selected computational strategy has the ability to correctly reproduce the relative energies of various parts of the PES. Thus, it is satisfactory to describe the features of PES for the titled reactions. 3.2. Reactants, Encounter Complexes, and Product Species. In this section, we will discuss structures and energies of the reactants, encounter complexes, and products involved in the titled reactions. Figure 1 shows the optimized geometries and selected structural parameters for these species. The group 5 element of neutral V atom has five valence electrons and forms dioxide with two strong double bonds, leaving one unpaired electron on the metal center. The corresponding cation arises from ionization of metal core and is thus a closed-shell species. As shown in Figure 1, VO2+(1A1) presents two short V+−O bonds (1.558 Å) with the BDE of 83.9 (calcd) kcal/mol (86.3 (expt)20 kcal/mol). When it forms the triplet (3A′), one B



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Figure 1. Geometries and selected structural parameters optimized at the B3LYP:6-311G(2d,p) level for the encounter complex and some product species involved in the VO2+ + ethanol/methanol reaction. Bond lengths are in angstroms, and bond angles are in degrees.

Table 2. Values of Electron Density (ρ(r)), Laplacian of the Electron Density (∇2ρ(r)), Kinetic Energy Density (G(r)), Potential Energy Density (V(r)), and Electronic Energy Density (H(r)) (in au) and the Ratio of G(r) and V(r) at the Bond Critical Point (BCP) for the Encounter Complexes and Some Products at the B3LYP/6-311G(2d,p) Level species 1

+

1

VO2(CH3OH) ( 1a) VO2(CH3OH)+ (31a) 1 VO2(CH3CH2OH)+ (11b) 3

3

VO2(CH3CH2OH)+ (31b)

1

VO2(H2O)+ VO2(H2O)+ 1 VO2(C2H4)+ 3 VO2(C2H4)+ 1 VO(H2O)+ 3 VO(H2O)+ 1 VO(CH2O)+ 3

3

VO(CH2O)+ VO(CH3CHO)+ 3 VO(CH3CHO)+ 1

bonds

ρ(r)

∇2ρ(r)

G(r)

V(r)

H(r)

−G(r)/V(r)

V −O V+−O1I V+−O1 O2−H1 V+−O1 V+−H1 V+−O1 V+−O1 V+−C1 V+−C1 V+−O1 V+−O1 V+−O1 V+−C V+−O1 V+−O1 V+−O1 O2−H1

0.093 0.088 0.096 0.007 0.084 0.028 0.083 0.078 0.060 0.060 0.076 0.067 0.162 0.095 0.082 0.154 0.084 0.007

0.454 0.458 0.457 0.025 0.461 0.116 0.423 0.431 0.098 0.116 0.359 0.381 0.542 0.161 0.449 0.540 0.463 0.022

0.127 0.124 0.129 0.005 0.122 0.029 0.113 0.112 0.038 0.041 0.097 0.096 0.209 0.071 0.119 0.201 0.124 0.005

−0.139 −0.133 −0.143 −0.004 −0.129 −0.028 −0.121 −0.116 −0.051 −0.054 −0.103 −0.097 −0.283 −0.101 −0.126 −0.266 −0.132 −0.004

−0.013 −0.009 −0.014 0.001 −0.007 0.001 −0.007 −0.004 −0.013 −0.012 −0.007 −0.001 −0.074 −0.030 −0.007 −0.066 −0.008 0.001

0.914 0.932 0.902 1.25 0.946 1.036 0.934 0.966 0.745 0.759 0.942 0.990 0.739 0.703 0.944 0.756 0.939 1.25

+

1

which is much stronger than the OV+−O bond. Because VO+ has a 1σ22σ21π41δ2 electronic configuration, it favors a highspin triplet ground state (3Σ), rather than the low-spin singlet

The monoxide VO+, the glycol loss product in the ethanol/ VO2+ reaction, presents a bond length of 1.527 Å (singlet) and 1.586 Å (triplet), with an adiabatic BDE of 146.3 kcal/mol, C



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aldehyde-O and C atoms, respectively (Tables S2 and S3, SI). On the other hand, AIM analysis shows that for the triplet OV+−(CH2O) and OV+−(CH3CHO), the aldehyde-O−V+ bond is mainly electrostatic interaction with a weakly covalent property, because the corresponding −(r)/V(r) value (0.944 and 0.939) is only slightly less than 1 (see Table 2). For the singlet V+O(CH2O), two BCPs are found to connect the metal center with the aldehyde-O and C atoms, respectively, and the corresponding −G(r)/V(r) values are 0.739 and 0.703, which indicates that, even though these bonds are electrostatic bonds, they are much more covalent in nature than the triplet association. However, for the OV+−(CH3CHO) in the singlet state, only one aldehyde-O−V+ BCP is located, though NBO analysis suggests another new σ(V+Cα) binding orbital is also formed. Similar to VO+ and VO(H2O)+, both VO(CH2O)+ and VO(CH3CHO)+ present a high-spin triplet ground state (with the triplet→singlet excitation energy of 24.2 and 28.6 kcal/mol, respectively, see Table 3). V(OH)2+ is the common product in the reactions of VO2+ with ethanol and methanol. Different from the linear structure in the singlet state, the triplet is featured by a bent structure with the ∠OV+O angle of 112.8°, slightly bigger than 90°, indicating an obvious sd-hybridization of metal center. NBO analysis detects that, although the linear structure is helpful to decrease the repulsive interaction among groups, the singlet metal center just forms a binding orbital with one hydroxyl group (see Table S3, SI). In the triplet state (3A″), however, the 3d4s(V+) hybridized orbital favors formation of binding orbitals with two hydroxyls, leading to a high spin ground state lying at 10 kcal/mol below the singlet state. VO(OH)2+ is the major product of ethylene elimination in the ethanol reaction. One feature of the product is the

Table 3. Summary of all the Possible Products and the Relative Energies (E’s, in kcal/mol) Associated with the Reaction of VO2+ with Methanol/Ethanol at the B3LYP/ DZVP(opt):6-311++G(2d,2p)//B3LYP/6-311G(2d,p) Level E’sa products

singlet

triplet

V(OH)2+ + CH2O (P1a) VO(H2O)+ + CH2O (P2a) VO(CH2O)+ + H2O (P3a) VO(CH3CHO)+ + H2O (P1b) VO(H2O)+ + CH3CHO (P2b) V(OH)2+ + CH3CHO (P3b) VO(OH)(CH2O)+ + CH3 (P4b) VO(OH)2+ + C2H4 (P5b) VO2(H2O)+ + C2H4 (P6b) VO2(C2H4)+ + H2O (P7b) VO+ + (CH2OH)2 (P8b)

−38.1b −31.2b −27.7b −40.6 −35.3 −42.1 −28.7 −55.9 −46.4 −38.7 17.9

−48.1b −52.6b −51.9b −69.2 −56.7 −52.1 38.7 −14.9 −11.0 −5.8 −5.9

a

Energies are relative to the total energy of VO2+(1A1) and ethanol, except as noted. bEnergies are relative to the total energy of VO2+(1A1) and methanol.

state as the dioxide VO2+. Energetically, VO+(3Σ) is located 23.8 kcal/mol below the singlet state. Furthermore, similar to the O2V+−(H2O) association, the OV+−(H2O) association presents mainly electrostatic interaction as well as a faintly covalent nature (see Table 2), which leads to a triplet ground state (with the triplet→singlet excitation energy of 21.4 kcal/ mol). For the adducts of OV + −(CH 2 O) and OV + − (CH3CHO), NBO analysis suggests that the triplet associations favor electron donation from aldehyde-O to V+, whereas in the singlet state, V+ forms two new binding orbitals with the

Figure 2. Energy profile of the reaction of VO2+ with methanol. Numbers refer to the relative stabilities (in kcal/mol) with respect to the reactants of 1 VO2+ + methanol evaluated at the B3LYP/DZVP(opt): 6-311++G(2d,2p)//B3LYP/6-311G(2d,p) level including ZPE corrections. Scaling factor for ZPE is 0.961. D



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Figure 3. Energy profile for the initial O−H and Cα−H activations undergoing VO(OH)(CH3CHOH)+ (3b) involved in the reaction of VO2+ with ethanol. Numbers refer to the relative stabilities (in kcal/mol) with respect to the reactants of 1VO2+ + ethanol evaluated at the B3LYP/DZVP(opt): 6-311++G(2d,2p)//B3LYP/6-311G(2d,p) level including ZPE corrections. Scaling factor for the ZPE is 0.961.

VO2+ to form VO(OH)(CH2OH)+ (2a), which features the η2O,C−OHCH2 ligand in the singlet state and the η1-O−OHCH2 ligand in the triplet state (via transition state TS1−2a), and then a hydroxyl H-transfer to the other oxo ligand of VO(OH)+ to form V(OH)2(CH2O)+ (3a) with an η2-O,C-OCH2 (singlet)/ η1-O-OCH2 (triplet) ligand (via TS2−3a). Alternatively, species 3a could also be formed by initial hydroxyl H transfer followed by methyl H transfer to two oxo ligands (1a → 5a → 3a). Energetically, the initial C−H activation pathway is located above that of initial O−H activation on the triplet PES (Erel = −13.1(3TS1−5a) vs −24.1(3TS1−2a) kcal/mol), whereas on the singlet ground PES, the former is favored over the latter (Erel = −33.8(1TS1−5a) vs −31.1(1TS1−2a) kcal/mol). Along both the initial O−H and C−H activation pathways, a singlet-to-triplet crossing is expected to occur after the second H transfer transition state TS5−3a and TS2−3a, respectively. The SOC constants (SOCC) of the relevant 1MECP5−3a and 1 MECP2−3a are calculated to be 180.0 and 110.5 cm−1, respectively, and the crossing probabilities are found to be 33.3% and 18.9%, indicating that the initial O−H activation has a relatively large crossing probability, compared with the initial C−H activation. Once species 3a is formed, it could generate the major product of formaldehyde (P1a) via the rupture of (OH)2V+−(OCH2), exothermic by 38.1 (48.1) kcal/mol in the singlet (triplet) state. Alternatively, a subsequent H shift between two hydroxyl groups carries 3a into VO(H2O)(CH2O)+ (4a) with an η2-O,C-OCH2 (singlet)/η1-O-OCH2 (triplet) ligand (via TS3−4a lying at −44.3 (singlet) and −61.2 (triplet) kcal/mol). Different bond cleavages of (H2O)−OV+−(OCH2) account for products of formaldehyde + VO(H2O)+ (P2a) and water + VO(CH2O)+ (P3a), exothermic

tricoordination of the metal center with O-atom and two OH ligands, but the difference of the (OH)2V+−O bond lengths in the singlet and triplet states is apparent (1.539 Å vs 1.733 Å). NBO analysis detects that in the V+(OH)2 group, the association of metal center with two hydroxyls has a similar binding situation in both spin cases (see Table S3, SI). However, in the V+O moiety, one of π(V+O) binding orbitals is ruptured when it forms the triplet, which is supported by Mulliken atomic spin densities of 0.93 on V+ and 0.95 on O in the triplet state. These explain the larger stretch of the (OH)2V+−O bond in the triplet state and less stabilization of the triplet than of the singlet by 40.1 kcal/mol. The methyl-loss product is V+O(OH)(η1-O-CH2O). Different with a closed-shell VO2+ (1A1) complex, the doublet hydroxide VO(OH)(CH2O)+ (2A) presents a radical character, and the unpaired electron is located on the metal center, with the Mulliken atomic spin density of 1.1 on V+. However, Mulliken atomic spin densities in the quartet are populated extensively on V+ (1.13) and the formaldehyde ligand (O (0.78) and C (0.87)), suggesting the rupture of π(CαO) orbital and the elongation of C−O bond (1.303 Å). Accordingly, the quartet is located at 67.4 kcal/mol above the doublet. 3.3. VO2+ /Methanol Reaction PESs. Two neutral products, H2O and CH2O, in the VO2+/methanol reaction are suggested by the FT-ICR experiment.20 Both the initial O−H and C−H activation mechanisms have been found to account for the methanol oxidation to formaldehyde by FeO+.51 In the VO2+/methanol system, a similar bond activation mechanism is also found by us. As shown in Figure 2, once encounter complex 1a is formed, there would be two futures to expect. One is transfer of a methyl H atom to an oxo ligand of E



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Figure 4. Energy profile for the reaction of VO(OH)(CH3CH2O)+ (2b) via the direct C−C dissociation, Cα-to-O H transfer, and Cβ-to-O H transfer mechanisms involved in the reaction of VO2+ with ethanol. Parameters follow the same notations as in Figure 3.

(water)V+O(acetaldehyde) structure, resulting in a high-spin triplet ground state, similar to the monoxide VO+. This is in accordance with the stability of 4b (Erel = −105.4 (triplet) and −84.2 (singlet) kcal/mol). Thus, a singlet-to-triplet crossing is expected to occur immediately before species 4b. For the relevant MECP (1MECP3−4b), the activated O1−H bond is stretched to 1.491 Å, and the newly formed O3−H bond length is reduced to 1.028 Å (see Figure S3, SI). The SOCC of MECP3−4b is calculated to be 25.4 cm−1, and the crossing probability is estimated to be only 2.1% at room temperature. Different dissociation of (H2O)−OV+−(OCHCH3) would account for water + VO(CH3CHO)+ (P1b) and acetaldehyde + VO(H2O)+ (P2b), with the overall exothermicities of 40.6 (69.2) and 35.5 (56.7) kcal/mol on the singlet (triplet) PES, respectively. The other exit of species 3b is the hydroxyl-H shift to another terminal oxo ligand to yield V+(OH)2(η1-O-OCHCH3) (5b), which favors a high-spin (triplet) ground state as V(OH)2+. The relevant triplet transition state 3TS3−5b (Erel = −50.1 kcal/mol) is located slightly below the singlet state (Erel = 49.1 kcal/mol), indicating that a crossing occurs immediately before TS3−5b. The relevant MECP (MECP3−5b) presents a geometrical structure much similar to TS3−5b (see Figure S3, SI), suggesting that it is indeed a “late” crossing point. The SOCC is calculated to be 2007.4 cm−1, and the crossing probability is close to 1 at room temperature. Furthermore, V+(OH)2(CH3CHO) (5b) largely stabilizes the system (Erel = −91.4 (singlet) and −114.9 (triplet) kcal/mol). Indeed, the triplet species (35b) forms the global minimum in the whole VO2+/ethanol reaction. Direct dissociation of 5b would account for acetaldehyde loss product V(OH)2+ (P3b), exothermic by 42.1 (52.1) kcal/mol in the singlet (triplet) state. Species 2b could also directly evolve into species 5b (V+(OH)2(η1-OCHCH3)) via direct methylene-H transfer to another terminal oxo ligand of V+O(OH) via transition state TS2−5b (Erel = −46.4 (singlet) and −43.3 (triplet) kcal/mol, see Figure 4), which is more favorable than the stepwise H-shift pathway (2b → 3b → 5b) as described above. In addition, a singlet-to-triplet crossing is expected to occur after transition state TS2−5b. The SOCC of the relevant 1MECP2−5b is

by 31.2 (52.6) and 27.7 (51.9) kcal/mol on the singlet (triplet) PES, respectively. 3.4. VO2+/Ethanol Reaction PESs. The FT-ICR experiment suggests five neutral products of CH3, H2O, C2H4, C2H4O, and C2H6O2 in the gas-phase reaction of VO2+ with ethanol.20 In the following, we shall present a systematic survey of the [V, C2, H6, O3]+ PES to reveal the reaction mechanism associated with all of these products. 3.4.1. Initial O−H and Cα−H Activation. Initial O−H activation could account for the CH3, C2H4, H2O, and CH3CHO eliminations. Figures 3 and 4 show the PESs together with the schematic structures involved. Once 1b is formed, the direct H shift from the hydroxyl of ethanol to one oxygen atom of VO2+ could generate an ethoxo center VO(OH)(CH3CH2O) + (2b), lying at −78.8 (−50.9) kcal/mol on the singlet (triplet) PES. In this process, the C−O bond in 2b is strengthened to 1.466 (1.378) Å in the singlet (triplet) state, while the C−C bond is weakened to 1.513 (1.524) Å. On the singlet PES, this is a facile process, because the transition state (1TS1−2b) for this possibility lies at 31.5 kcal/mol below the singlet entrance channel. Although 3TS1−2b lies at 23.5 kcal/mol above the singlet entrance channel, this process is also accessible on the triple PES, because the expected triplet entrance channel (Erel = 30.6 kcal/mol) is located above in energy. Species 2b is a branching point, where three possible channels can be immediately followed by (i) direct C−C dissociation, (ii) Cα−H activation, and/or (iii) Cβ−H activation. As shown in Figure 3, species 2b undergoes a H shift from Cα to adjacent ethoxyl O, forming VO(OH)(OHCHCH3)+ (3b) via transition state TS2−3b lying at −29.6 (−28.7) kcal/ mol on the singlet (triplet) PES. Alternatively, species 3b could also be formed directly from encounter complex 1b through the initial Cα−H activation, that is, a direct methylene-H shift to a terminal oxo ligand of VO2+ (via TS1−3b). Transition state 1 TS1−3b (3TS1−3b) lying at −31.5 (−26.1) kcal/mol suggests the initial Cα−H activation is comparable with the initial O−H activation. One exit of species 3b is the direct H shift between two hydroxyl groups, yielding (H2O)V+O(OCHCH3) (4b) via transition state TS3−4b lying at −57.9 (−56.3) kcal/mol on the singlet (triplet) PES. The new species 4b is featured by a F



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Figure 5. Energy profile for the initial Cβ−H acitvaton involved in the reaction of VO2+ with ethanol. Parameters follow the same notations as in Figure 3.

calculated to be 53.8 cm−1, and the crossing probability is found to be 26.4%. Direct dissociation of 5b yielding acetaldehyde and V(OH)2+ has been discussed above. Along the Cβ−H activation pathway of species 2b (Figure 4), a direct H transfer from methyl to the terminal oxo ligand of VO(OH)+ in conjunction with the breakage of ethyoxyl O−C bond could carry the system into the VO(OH)2(C2H4)+ adduct (6b). In 16b, the V+O of the OV+(OH)2 group locates approximately parallel above the C−C bond of ethylene, forming a four-membered ring structure, and the complex is largely stabilized by strong donor−acceptor interaction between OV+(OH)2 and ethylene groups. However, 36b is featured by association of the OV+(OH)2 group (via O ligand) with one C atom of ethylene, leading to a population of Mulliken atomic spin densities mostly on V+ (1.13) and terminal C (0.99). Thus, the unfavorable structure of 36b results in much less stability than that of 16b (Erel = −33.7 vs −97.2 kcal/mol). The transition state for this possibility (TS2−6b) lies at −57.0 (singlet) and −20.2 (triplet) kcal/mol. Finally, direct loss of ethylene from 6b would account for VO(OH)2+ (P5b), exothermic by 55.9 (singlet) and 14.9 (triplet) kcal/mol. As discussed above, the strength of the C−C bond in ethyoxyl intermediate (2b) is further weakened in the hydroxylH shift process; thus, the C−C stretching vibration in 2b could lead to direct decomposition of (OH)OV+OCH2−CH3, yielding methyl and OV+(OH)(η1-OCH2) (P4b). Although it should be endothermic by 38.7 kcal/mol on the triplet PES, the ground singlet process is favorable due to the exothermicity of 28.7 kcal/mol. This is in accordance with the methyl-loss mechanism in the VO2+/ethanol reaction proposed by Engester, et al.24 Noted that a similar CH3-elimination process had been observed in the reaction of FeO+ with ethanol but did not

present thus far in other ion/molecule reactions of vanadium oxides with alcohols or hydrocarbons.20,52 3.4.2. Initial Cβ−H Activation. Calculated PES together with schematic structures involved in the product channels is given in Figure 5. Starting with 1b, a direct H shift from methyl to an oxo ligand of VO2+ would carry the system into complex 7b (VO(OH)(CH2CH2OH)+), lying at −79.0 (−62.9) kcal/mol in the singlet (triplet) state. During this process, a new V+−Cβ bond is formed, and the Cα−OH bond is concomitantly enlarged to 1.576 (1.463) Å in the singlet (triplet) state. Compared with the initial O−H and Cα−H activation pathways, this pathway occurs more facilely because the transition sate TS1−7b involved (Erel = −39.5 (−31.3) kcal/mol in the singlet (triplet)) is much lower in energy than that in the two former pathways. Then, the intramolecular rearrangement of species 7b forms dihydroxo species 6b (VO(OH)2(C2H4)+), which could dissociate into ethylene and VO(OH)2+ as discussed above. The energy of ground singlet transition state involved (Erel(1TS7−6b) = −78.8 kcal/mol) is significantly lower than the entrance channel (1VO2+ + C2H5OH). Another pathway from species 7b also accounts for loss of ethylene; that is, direct H transfer between two hydroxyl groups in 7b carries the system conversion into (water)V+O2(η2-C2H4) (8b) via TS7−8b lying at −46.0 (singlet) and −34.2 (triplet) kcal/mol. Different decompositions of 8b could get two neutral products. Loss of C2H4 (for V(O)2(H2O)+ (P6b)) is attributed to the rupture of (H2O)(O)2V+−(C2H4), exothermic by 46.4 (11.0) kcal/mol on the singlet (triplet) PES, while the loss of H2O (for V(O)2(C2H4)+ (P7b)) arises from the rupture of (C2H4)(O)2V+−(H2O), giving out heat of 38.7 (5.8) kcal/mol. The last exit of species 7b is a subsequent coupling of the hydroxyl and terminal methylene ligands, resulting in G



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activation for yielding VO(OH)2+, initial Cβ−H activation for VO2(H2O)+ and VO(OH)2+, where the last product channel is the most favorable in the whole reaction from a viewpoint of both kinetics and thermodynamics. This explains the dominant neutral product of ethylene in the VO2+/ethanol reaction (72%) suggested by the FT-ICR experiments.20 Loss of glycol could occur via initial Cβ−H activation with a large endothermicity (17.9 kcal/mol) on the adiabatic singlet PES, or cross to the triplet PES with slight exothermicity of 5.9 kcal/mol. CH3 elimination is carried out via the simple initial O−H activation with exothermicity of 28.7 kcal/mol. Therefore, the lower exothermicity results in less competitiveness of loss of glycol and methyl than the formation of other neutral products (C2H4, C2H4O, and H2O). This is in accord with the experimental results, where small amounts of CH3 (3%) and C2H6O2 (4%) are formed in the VO2+/ethanol reaction.20 Dehydrogenation of methanol by VO+ has been studied at the B3LYP/TZVP level.18 It is suggested that the initial O−H activation pathway is prevented by the insurmountable second H-shift barrier, whereas the pathway of initial C−H activation is possible at the thermal condition with the highest barrier (the second H-shift barrier) lying at 4 kcal/mol above the entrance channel, explaining a low reaction efficiency of the VO+/methanol reaction (6%).18 Compared with the monoxide of VO+, the dioxide of VO2+, which could afford two oxo ligands, favors consecutive H-shifts from methanol directly to two vanadyl oxygen atoms via both initial O−H and C−H activations, showing a relatively higher methanol reaction efficiency of 10%.20 However, the much more facile Cβ−H activation channel makes the VO2+/ethanol system present the highest reaction efficiency (50%) in all three systems.20

VO(η2-O,O-glycol)+ (9b) via TS7−9b. Both 9b and TS7−9b present a high spin triplet ground state (Erel = −64.4 (−81.9) and −35.0 (−46.5) kcal/mol in the singlet (triplet) state, respectively). The decomposition of 9b could get glycol and VO+ (P8b). Although the singlet process is endothermic by 17.9 kcal/mol, it is exothermic by 5.9 kcal/mol in the triplet state. 3.5. Comparison with Experimental Results. Gas-phase FT-ICR experiments have inferred that the reaction of VO2+ with ethanol mainly produces C2H4, C2H4O, H2O, C2H6O2, and CH3 with a branching ratio of 76:10:5:4:3, while the methanol reaction yields two neutral products, that is, CH2O (72%) and H2O (28%).20 As is well-known, the BDEs of O−H, Cα−H, and Cβ−H bonds in ethanol are 105.5, 95.9, and 100.8 kcal/mol, respectively, while the O−H and Cα−H BDEs in methanol are respectively found to be 105.3 and 96.1 kcal/mol.53 However, our calculation suggests that the initial O−H bond activation is comparable to the Cα−H bond activation in ethanol, and even preferred in methanol, while the initial Cβ−H activation is the most facile channel in the whole ethanol reaction. This behavior is counterintuitive. Different structures of transition states involved may provide a hint for this situation. In both the methanol and ethanol reactions, the transition state of Cα−H bond activation (1TS1−2a and 1TS1−3b) forms a new V+−Cα bond accompanied with rupture of the activated Cα−H bond, leading the shifted H atom to be alone without formation of a binding orbital with other atoms and be stabilized by electron donation from σ(V+−O) and σ(V+−Cα) orbitals to the 1s* (H) orbital. This unfavorable binding situation leads to a relatively high activation energy of the ground singlet state. However, in the moderate O−H activation process, it has a concerted fourcenter H-shift transition state (1TS1−5a and 1TS1−2b), where the LP orbital of the hydroxyl-O atom favors formation of a new V+−O bond without the complete rupture of the activated O−H bond. In addition, the transition state is stabilized by the strong electron donation from the adjacent oxo ligand of VO2+ to the σ*(O−H) orbital. In the Cβ−H activation process of ethanol, the transition state (1TS1−7b) involved forms a much more stable chelate structure in which the metal center binds to both the hydroxyl and the activated methyl of the ethanol group, resulting in the lowest energy barrier in all bond activation processes. For the VO2+/methanol system, both initial O−H and C−H activation could account for formaldehyde and water elimination, but the former product is more dynamically and thermodynamically favorable than the latter on both the singlet and triplet PESs. This explains the primary product of CH2O (72%) with a small quantity of H2O (28%) observed by the FT-ICR experiment.20 For the ethanol/VO2+ system, loss of H2O occurs via both initial O−H and Cα−H activation for yielding VO(CH3CHO)+ and initial Cβ−H activation for VO2(C2H4)+. Although formation of VO(CH3CHO)+ is the most thermodynamically favorable in the whole reaction, it must experience a spin inversion occurring immediately after TS3−4b with the crossing probability of 2.1%. Furthermore, it is well-known that SOC is a slow process.54 Thus, the major ion product in the H2O-loss process is the kinetically favorable product of VO2(C2H4)+. Loss of acetaldehyde could occur via both initial O−H and Cα−H activation for V(OH)2+ and VO(H2O)+, and the former ion product is preferred, considering the spin inversion. Loss of C2H4 in the ethanol/VO2+ reaction facilely occurs adiabatically through three channels, that is, initial O−H

4. CONCLUSION The present theoretical studies shed new light on the experimental observations and provide a basis for understanding the gas-phase reaction mechanism of VO2+ with ethanol and methanol. For the VO2+/C2H5OH system, the initial O−H activation is comparable to the Cα−H activation but less favorable than the Cβ−H activation. Loss of methyl occurs via initial O−H activation followed by direct decomposition, while the glycol loss product (VO+) is produced via the initial Cβ−H activation followed by a C−O coupling process. Acetaldehyde elimination occurs via two comparable pathways of initial O−H and Cα−H activation, yielding both VO(H2O)+ and V(OH)2+. All three pathways of initial O−H, Cα−H, and Cβ−H activation account for water elimination, giving rise to VO(CH3CHO)+ and VO2(C2H4)+. Loss of ethylene occurs via both initial O−H and Cβ−H activation for yielding VO(OH)2+ and VO2(H2O)+. Formation of VO(OH)2+ via the initial Cβ−H activation adiabatically on the singlet PES is the most kinetically and thermodynamically favorable in all channels of the ethanol reaction. Both initial O−H and Cα−H activation accounts for loss of formaldehyde and water in the methanol/VO2+ reaction, but the former pathway is preferred.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed description of spin−orbit coupling calculations, geometries and selected structural parameters of MECPs, results of NBO analysis for some products and encounter complexes, and optimized geometries, selected structural parameters, calculated energies, zero-point energies, and for all species involved in the reaction of ethanol and methanol H



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with VO2+. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.Z.); [email protected] (W.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Program for Changjiang Scholars and Innovative Research Team in University (IRT0759) of MOE, PRC, NSFC (21003158 and 10979077), Shandong Province Natural Science Foundation (ZR2011EMZ002), Promotive Research Fund for Excellent Young and Middle-aged Scientists of Shandong Province (BS2012NJ015), and the Fundamental Research Funds for the Central Universities (12CX02014A, 14CX02004A, 13CX02001A, and 14CX06001A).

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