Quantum Mechanical Study of N-Heterocyclic Carbene Adsorption on

Mar 14, 2017 - We found that NHCs prefer to bind to the top site with adsorption energies (ΔEs) varying from 1.69 to 2.34 eV, depending on the type o...
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Quantum Mechanical Study of N‑Heterocyclic Carbene Adsorption on Au Surfaces Kuan Chang,† Jingguang G. Chen,†,‡ Qi Lu,*,† and Mu-Jeng Cheng*,§ †

Department of Chemical Engineering, Tsinghua University, Beijing, China Department of Chemical Engineering, Columbia University, New York, New York 10027, United States § Department of Chemistry, National Cheng Kung University, Tainan, Taiwan ‡

S Supporting Information *

ABSTRACT: There is increasing interest in using N-heterocyclic carbenes (NHCs) as surface ligands to stabilize transitionmetal nanoparticles (NPs) and to replace thiols for the preparation of self-assembled monolayers (SAMs) on gold surfaces. This type of surface decoration is advantageous because it leads to improved catalytic activity of NPs and increased stability of SAM, as shown by recent experiments. In this work, we used quantum mechanics combined with periodic surface models to study the adsorption of NHCs on the Au(111) surface. We found that NHCs prefer to bind to the top site with adsorption energies (ΔEs) varying from 1.69 to 2.34 eV, depending on the type of NHC, and the inclusion of solvents in the calculations leads to insignificant variation in the calculated ΔEs. Three types of NHCs were found to bind to Au(111) more tightly and therefore should be better stabilizers than those commonly used. Importantly, by analyzing electronic structures using the Bader charge and energy decomposition analysis, we find that during adsorption NHC acts as an electron donor, transferring its electron density from the lone pair orbital at the carbene center to the empty d orbital of Au with negligible π-back-donation. This binding pattern is very different from that of CO, a ligand commonly used in organometallics, where both interactions are equally important. This leads to the identification of the protonation energies of NHCs as a descriptor for predicting ΔEs, providing a convenient method for computational highthroughput screening for better NHC-type surface ligands. ability to readily fine-tune the properties of organometallic complexes through various structural modification of NHC (through the change in the N-substituent or backbone or through the replacement of N atoms by C, S, and O atoms, Scheme 1), this class of carbenes has broad applications in metal coordination chemistry and homogeneous catalysis.4 The most well-known example of the application of NHC in homogeneous catalysis is in transition-metal-catalyzed olefin metathesis:5,6 the ubiquitous second-7,8 and third-generation9−11 Grubbs catalysts and Hoveyda-Grubbs catalysts12 contain NHC ligands. In addition to being ligands in organometallics, NHCs have recently been used as surface ligands to stabilize transitionmetal nanoparticles (NPs).13 With a high surface-to-volume ratio and a large fraction of highly reactive coordinationunsaturated atoms, NPs are in general more reactive than their bulk counterparts in catalysis.14−23 However, they are prone to aggregation, which would lead to a loss of catalytic activity. Recently, it was found that NHCs can be used to stabilize Ru,24−26 Ir,27 Pd,28−31 Pt,32,33 Au,13,31,34−36 and Ag37 NPs by

1. INTRODUCTION N-Heterocyclic carbene (NHC, Scheme 1) has played an important role in modern chemical science since the first report Scheme 1. Schematic Description of N-Heterocyclic Carbenea

a

The N substituent and the backbone are colored red and blue, respectively.

of the stable NHC isolated by Arduengo et al.1 In contrast to classic carbenes, NHC exhibits a singlet ground state electronic structure with a substantial energy gap to the triplet first excited state.2 Its highest occupied molecular orbital (HOMO) is a formal sp2-hybridized lone pair, whereas the lowest unoccupied molecular orbital (LUMO) is an empty p orbital, both of which are localized primarily at the carbene center.3 NHC behaves as strong σ donor and weak π acceptor and often forms strong bonds to metals (Ms). Because of the enhanced stability in the metal complexes due to those strong M−NHC bonds and the © 2017 American Chemical Society

Received: February 5, 2017 Revised: March 12, 2017 Published: March 14, 2017 2674

DOI: 10.1021/acs.jpca.7b01153 J. Phys. Chem. A 2017, 121, 2674−2682

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Figure 1. Two-dimensional schematic description of carbenes examined in this work. In this study, we focus on investigating the electronic effects of NHCs on the Au(111)−NHC bond strength; therefore, hydrogen was used as the substituent on N or C next to the carbene center to minimize steric effects.

investigation of this chemical interaction remains unexplored. In this work, we conducted a quantum mechanical study to provide insights into this chemical interaction. Au(111) was selected as the model surface in the present work because gold is a typical metal that is widely used as a substrate for SAMs. The binding interaction between NHC and the Au(111) surface was studied, and the nature of the NHC−Au bonds was then explored. It should be possible to extrapolate the conclusions from the Au system to other transition metals. The remainder of this article is organized as follows. In Section 2, we detail the computational approaches utilized in this work. In Section 3, we present our results, beginning with the investigation to find the most energetically favorable binding site for the NHC on Au(111) and the analysis of electron transfer during the adsorption of NHC using the Bader charge and energy decomposition analysis based on the absolutely localized molecular orbitals. Then, we calculated the adsorption energies for various NHCs with Au(111) in vacuum and in solvents to screen for better molecular anchors. Finally, through the comparison of the inner properties of NHCs such as ΔES−T’s (the energy gaps between the singlet ground states and the triplet first excited states), PEs (protonation energies), and TEPs (Tolman electronic parameters) with the corresponding adsorption energies on Au(111), we proposed a descriptor that can be used to estimate the adsorption energy for future computational high-throughput screenings.

preventing them from aggregating in organic solvents, aqueous solution, and even biologically relevant media. For example, Tilley and co-workers have prepared NHC-stabilized Au NPs with different NHCs via reduction of the well-defined NHCAu(I) complexes.34 The resulting monocrystalline 5−7 nm NPs were stable for months in organic solvents.34 Experimental results have also shown that transition-metal NPs with NHC adsorbates exhibit improved reactivity and selectivity24,28−32 and sometimes even catalyze the reactions through different mechanisms.35 For instance, Glorius and co-workers have synthesized and applied NHC-modified Ru NPs supported by K−Al2O3 for the catalytic hydrogenation of phenyl acetylene. At low NHC loading, it was found that both the benzene moiety and the alkyne unit were hydrogenated, whereas at high loading only the alkyne unit was hydrogenated.24 Moreover, Cao et al. have used Au NPs for CO2 electrochemical reduction.35 Their experimental results showed that the Faradaic efficiency increases from 53% for Au NPs to 83% for NHC−Au NPs at pH ∼ 7.0 with an applied voltage of U = −0.57 VRHE, and importantly, the Tafel slope changes dramatically from 72 to 138 mV/decade, indicating a change in the reaction mechanism.35 Additionally, there is increasing interest in using NHCs as molecular anchors for the formation of self-assembled monolayers (SAMs) on metallic surfaces, in particular on gold, for molecular electronics, surface patterning, and biosensing.13 Previously, thiols were used for such a purpose; however, thiol-based SAMs suffer from degradation in air, thermal instability, and sensitivity to oxidation, limiting their widespread commercial applications.38,39 Recent experimental investigations by Weidner,40 Johnson,41 Wang,42 and Crudden43,44 have demonstrated the possibility of using NHCs as alternatives to thiols in the preparation of SAMs on gold. With stronger C−Au bonds compared to S−Au bonds in thiol-based SAMs, those novel NHC-based SAMs show improved chemical, pH, oxidative, and electrochemical stability.41,43,44 The aforementioned experimental results clearly show that NHCs are excellent stabilizers for transition-metal NPs and superior molecular anchors for metallic surfaces. In all of these cases, the interaction between NHCs and metal surfaces plays a very important role. However, a detailed systematic theoretical

2. COMPUTATIONAL DETAILS The PBE functional45 with projector augmented wave pseudopotentials46,47 (400 eV energy cutoff) as implemented in the Vienna ab initio simulation package (VASP)48−51 was employed for all slab and molecule calculations. To account for the van der Waals (VDW) interactions, the DFT-D2 approach implemented in VASP was used with the default settings.52 Because VDW parameters for gold were not provided in the original article,52 we adopted the values of 40.62 J·nm6/mol for the dispersion coefficient (C6) and 1.772 Å for the van der Waals radius (R0) used by Amft et al. in their study of Au adsorption on graphene.53 To accelerate SCF convergence, a Gaussian smearing technique was adopted with a smearing 2675

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previous experimental43 and theoretical studies.43,57 The bond distance between the carbene center (C:) and the closest gold atom (RC:‑‑Au) is 2.06 Å, in good agreement with those from the previous theoretical investigations (2.12 and 2.08 Å).43,57 In the following discussion, attention will be focused on this particular adsorption mode. Comparing the optimized structures of isolated and adsorbed 1 (Figure 3a,b), we find that upon adsorption to Au(111) the

parameter of kBT = 0.1 eV for slabs and 0.01 eV for molecules for the fractional occupation of the one-electron energy levels. All calculated values of energy were then extrapolated to kBT = 0. The Au(111) surface was simulated by a (3 × 3) seven-layer symmetric slab (cell parameter = 4.078 Å for the bulk Au) with two NHCs adsorbed on both sides. During the geometric optimizations, the fourth layer was fixed in its bulk position, whereas the remaining six layers and the adsorbed NHCs were allowed to relax. A Monkhorst−Pack k-point net of 5 × 5 × 1 was chosen to sample the reciprocal space for the slab calculations, whereas only the gamma point was sampled for the molecule calculations. The use of a denser k-point net of 7 × 7 × 1 led to insignificant changes in NHC−Au(111) bond energies (less than 0.01 eV), indicating that the 5 × 5 × 1 k point is sufficient to describe the system. The effect of solvent was simulated using the Poisson−Boltzmann implicit solvation model54 with dielectric constants of 2.4, 33.0, and 80.1 for toluene, methanol, and water, respectively. To prevent interactions between the periodic replicas along the z direction, a vacuum separation of at least 15 Å between adjacent images was used for the slab calculations and a 20 Å × 20 Å × 20 Å box was used for molecular calculations. Although spin-polarized wave functions were used for all calculations, we found that for each calculation the magnetic moment converged to zero after the SCF convergence. For the gold cluster system, all calculations were performed using the QChem package55 with the same PBE-D2 functional. For the basis sets, LanL2DZ was used for Au and Ni,56 and 6-311+ +G** was used for the others.

Figure 3. Geometric parameters for optimized 1 (a) and Au(111)−1 (c) and the corresponding Bader charge analysis (b and d).

structural perturbations in NHC are insignificant, with the variations in bond lengths being less than 0.02 Å. Among the small geometric changes, the most substantial one is the N− C:−N bond angle, θ(N−C:−N), but it increases only slightly from 100.3 to 104.4°. Similar structural variations were observed by Frenking et al. in their DFT study of [(1)AuCl] organometallic complexes,58 in which it was found that when 1 is bound with Au, the variations in the bond lengths are less than 0.01 Å and θ(N−C:−N) widens from 100.2 to 102.8°. Bader charge analysis59−62 was then performed to gain insight into the redistribution of the electron density upon adsorption. On the basis of the summation of the charges for each atom in 1 (Figure 3c,d), we find that 0.35 electron is transferred from 1 to the Au(111) surface. It is mainly from C:, whose charge increases from 1.77 in isolated 1 to 2.23 in adsorbed 1. For the other atoms in 1, the variations in charge are very small, within the range of ±0.11 electron. These results clearly indicate that during adsorption, 1 behaves as an electron donor, transferring its electron density from the carbene center to Au(111), which acts as an electron acceptor. To gain more insight into the charge transfer (CT) between 1 and Au(111), an energy decomposition analysis (EDA) based on absolutely localized molecular orbitals (ALMO-EDA), developed by Head-Gordon et al.,63−66 was performed. Such an EDA scheme decomposes the total molecular binding energy into physically meaningful components such as dispersion, electrostatic, polarization, charge transfer, and geometry relaxation terms. This EDA scheme has been applied in the studies of C−H67,68 and Si−H activation69 by organometallic complexes, the physical adsorption of H2 with metal−organic frameworks,70,71 and the adsorption of H2S in zeolites.72 For example, Goddard et al. employed ALMO-EDA to study methane C−H cleavage by metal complexes and found that on the basis of the CT between methane and metal complex fragments in the transition states, the reactions can be categorized into electrophilic, amphiphilic, and nucleophilic C− H activation.68

3. RESULTS AND DISCUSSION 3.1. Nature of the Chemical Bond between NHC and Au(111). As the first step in our investigation, the adsorption of molecule 1 (Figure 1), the reduced model of the first stable crystalline carbene,1 on Au(111) was studied to predict the most energetically favorable adsorption site. All symmetrydistinct sites, including 3-fold fcc, hcp, 2-fold bridges, and onefold top sites, were considered (Figure 2). The adsorption

Figure 2. Schematic description of the fcc, hcp, top, and bridge sites on the Au(111) surface. The gold atoms in the first, second, and third layers are pink, gold, and silver, respectively. The atoms in the fourth layer overlap those in the first layer and therefore cannot be seen.

energy was calculated as ΔE = 0.5 × [(EAu(111) + 2 × ENHC) − EAu(111)−2NHC], where EAu(111), ENHC, and EAu(111)−2NHC are the electronic energies for Au(111), NHC, and Au(111) with two adsorbed NHCs, respectively. The ΔEs were 1.99 eV for the fcc, 1.98 eV for the hcp, and 2.05 eV for the top site. Our attempts to locate the adsorption of 1 at the bridge site failed: the geometry optimizations pushed 1 that was originally bound at the bridge site to either the fcc or hcp site. On the basis of the calculated ΔEs, we concluded that 1 prefers to adsorb on the top site, which is consistent with the 2676

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← Au10. By analyzing the complementary occupied/virtual pairs (COVPs), we find that 96.2% of the 1 → Au10 CT stabilization energy is contributed by the σ-donation of the lone pair electrons at C: of 1 to the empty d orbital of Au (1.82 eV, Figure 5a). In contrast, for the 1 ← Au10 CT stabilization energy, there is no single COVP with a contribution higher than 25%. The largest one is the π-back-donation from the Au dπ orbital to the empty p orbital of 1, mainly localized at the carbene center (24.4%, Figure 5b). However, it stabilizes the system by only 0.15 eV, which is ∼12 times smaller than that of the σ-donation from the NHC lone pair orbital to the Au d orbital. On the basis of this analysis, we concluded that the major CT interaction between Au10 and 1 is the σ-donation from the lone-pair orbital of the carbene center in 1 to the empty d orbital of Au. In the following text, we will show that this dominant CT interaction allows us to predict ΔEs based on the inner electronic properties of NHCs. Because carbonyl (CO) and NHC are both common ligands in organometallics, we compared the charge-transfer pattern in Au10−CO to that in Au10−NHC and found that the two patterns are very different. Our calculations showed that CO → Au10 and CO ← Au10 CT stabilize the system by almost the same amount (1.10 vs 1.19 eV). The dominant CT interaction in CO → Au10 (92.5%, Figure 6a) is the donation of the lone pair from CO to the empty d orbital of Au, similar to that in Au10−1, and the stabilization energy for this interaction is 1.02 eV, which is smaller than that in Au10−1. The main CT interactions in CO ← Au10 are the two sets of π-back-donations from occupied dπ orbitals of Au to the empty π orbitals of CO, which is in good agreement with the previous theoretical study by Hoffmann et al.74 The stabilization energies are 0.48 and 0.51 eV (Figure 6b,c), the sum of which (0.99 eV) is almost the same as the σ-donation from CO to Au. Importantly, it is much larger than the π-back-donation in the Au10−1 system. This clearly indicates that compared to Au10− CO, the π-back-donation in Au10−1 is less significant. 3.2. Adsorption of NHCs on Au(111) in Vacuum and Solvents. Because of the isolation of the first stable crystalline NHC, many carbenes of this type has been synthesized in the

Because ALMO-EDA was developed for finite molecules rather than for expended surfaces, we used a cluster model with the same functional for this EDA analysis. The Au(111) surface is simulated by a 2-layer, 10-atom cluster cleaved from the bulk structure (Au10).73 The first layer was formed by seven Au atoms, and the second layer was composed of three. During the geometry optimizations, only the Au atom in the center of the first layer and adsorbed 1 were allowed to relax, whereas the rest of the Au atoms were fixed in their bulk positions. The optimized structure (RC:‑Au = 2.12 Å and θ(N−C:-N) = 104.0°) is similar to that based on the periodic surface model (RC:−Au = 2.06 Å and θ(N−C:−N) = 104.4°), and importantly, the calculated value of ΔE is 1.87 eV, which is also similar to that from our surface calculations (2.05 eV). These results indicate that this cluster model is able to represent the Au(111) surface.

Figure 4. Au10 cluster model used for the ALMO-EDA analysis.

Our ALMO-EDA calculation shows that 1 → Au10 CT stabilizes the system by 1.89 eV, whereas it is only 0.63 eV for 1

Figure 5. Complementary occupied/virtual pairs (COVPs) in the Au10−1 system. Only the COVPs that make a major contribution to the charge transfer (CT) for both 1 → Au10 and 1 ← Au10 are shown. The arrows indicate the direction of electron donation from the occupied orbital to the virtual orbital. 2677

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Table 1. Adsorption Energies (eV) and Bond Lengths (Å) for Au(111)-NHCs in Vacuum, Toluene, Methanol, and Watera vacuum

toluene

methanol

water

NHC

ΔE

RC:‑‑Au

ΔE

RC:‑‑Au

ΔE

RC:‑‑Au

ΔE

RC:‑‑Au

1 2 3 4 5 6 7 8 9 10 11 12 13

2.05 1.65 1.77 2.06 1.69 1.83 2.26 2.29 2.39 1.96 2.13 2.13 2.23

2.06 2.05 2.07 2.07 2.06 2.07 2.05 2.09 2.10 2.10 2.06 2.08 2.09

2.03 1.66 1.79 2.07 1.72 1.86 2.28 2.30 2.39 1.99 2.11 2.13 2.27

2.06 2.05 2.07 2.07 2.06 2.07 2.05 2.09 2.10 2.10 2.06 2.08 2.09

1.93 1.62 1.77 2.02 1.70 1.86 2.24 2.24 2.32 1.98 1.98 2.06 2.23

2.05 2.05 2.07 2.06 2.06 2.07 2.04 2.09 2.10 2.09 2.05 2.07 2.09

1.92 1.62 1.77 2.01 1.70 1.86 2.23 2.22 2.31 1.98 1.97 2.05 2.22

2.05 2.06 2.07 2.06 2.06 2.07 2.05 2.09 2.09 2.09 2.06 2.07 2.09

Despite the large variation in ΔE, the variation of RC:‑Au is small, within 0.05 Å. a

membered ring 4 to 2.29 eV for six-membered ring 8 and then to 2.39 eV for seven-membered ring 9. (3) NHCs without unsaturated CC bonds have only slightly larger ΔEs than their counterparts with CC bonds. For example, ΔEs for 4 (2.06 eV) and for 5 (1.69 eV) are slightly larger than those for 1 (2.05 eV) and 2 (1.65 eV). This suggests that the unsaturated CC bond does not have a large influence on ΔEs. Because NHC-stabilized NPs or surfaces are used in heterogeneous catalysis,24−26,28−31 electrochemical catalysis,35 and biomedical applications,87 all of which are systems in solvents, we then investigated the effects of solvents on ΔE. Three different solvents (toluene, methanol, and water) were considered, and the results are summarized in Table 1. Upon analyzing the data, we find that the influence of solvents on ΔEs is small. The most notable variation occurs for 1 and 10; however, their ΔEs decrease by only ∼0.14 eV when the solvent is methanol or water. Therefore, by using the implicit solvation model, which does not include any explicit solvent molecules in simulations, we find that solvent has an insignificant effect on ΔE and ΔE calculated in vacuum is sufficient to predict accurate adsorption energies in solution. 3.3. Descriptors for Predicting the Adsorption Energy of NHC on Au(111). Next, we correlated the ΔEs to the inner properties of NHCs to identify a descriptor for ΔEs. This would allow one to estimate the adsorption energies through simple and computationally less expensive DFT molecular calculations. From the literature, it is known that the reactivity of NHC is mainly determined by its ΔES−T = Etriplet − Esinglet (the energy gap between the triplet first excited state and the singlet ground state).2,88−90 This is because an unreactive singlet carbene must pay an energy cost to be promoted to the highly reactive triplet state, which is then able to cleave C−H91 and H−H bonds92 or form CC bonds with another triplet NHC without kinetic barriers.93 We calculated the ΔES−T’s for 13 NHCs (Table 2); however, no strong correlation between ΔEs and ΔES−T’s (R2 = 0.18, Figure 7) was observed, suggesting that ΔES−T is not a suitable descriptor. Previous studies have suggested that the electron-donation ability of an NHC is related to its Tolman electronic parameter

Figure 6. Complementary occupied/virtual pairs (COVPs) in the Au10−CO system. Only the COVPs that make a major contribution to the charge transfer (CT) for both 1 → Au10 and 1 ← Au10 are shown. The arrows indicate the direction of electron donation from the occupied orbital to the virtual orbital.

past 25 years.2,4,75−77 However, to date, NHCs used as surface ligands to form SAMs on gold surfaces or to stabilize MNPs are limited to those similar to 1,26,28,30,32−34,43,78−84 4,29,41 or 11.40,43,78,85,86 Aiming to find improved surface stabilizers, aside from 1, 4, and 11, nine additional NHCs were evaluated for their adsorption energies with Au(111) (Figure 1). The goals were (a) to find better NHC-type surface ligands, (b) to understand how the geometry of NHC influences the corresponding ΔE, and (c) to propose a descriptor that allows one to estimate ΔE through simple molecular electronic calculations rather than computationally more demanding periodic DFT calculations. It should be noted that similar to 1 the other 12 NHCs are energetically favorable to binding on the top site of Au(111). The adsorption energies (Table 1) for 1, 4, and 11 are 2.05, 2.06, and 2.13 eV, respectively, all of which are in the range of 2.00−2.20 eV. Using this range as the criterion, we find that 7− 9 and 13 have larger adsorption energies and therefore are better stabilizers, whereas 2, 3, 5, 6, and 10 have smaller adsorption energies. Comparing the ΔEs with the backbones of those NHCs, several interesting trends can be concluded: (1) When one of the two N heteroatoms is replaced with C, ΔE increases, whereas when it is replaced with O or S, ΔE decreases. For example, when N in 4 is substituted by C, ΔE increases from 2.06 to 2.26 eV for 7, whereas when it is replaced by O (or S), ΔE decreases to 1.69 eV for 5 (or 1.83 eV for 6). (2) Given a similar type of NHCs, ΔE increases as the ring size increases: ΔE increases from 2.06 eV for five2678

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HOMO lone-pair electron of the carbene center to empty Au d orbitals. Because the surface was fixed to Au(111) in this study, the energy for the empty Au d orbital is the same in all cases. Therefore, the CT interaction and ΔE should be correlated to the energy of the lone-pair orbital (εLP). NHC with a higher εLP allows easier electron donation and therefore possesses a larger ΔE. Because εLP is correlated to the protonation energy (PE) of NHC,95 a correlation between ΔEs and PEs is expected. Indeed, we find an excellent linear correlation between ΔEs and PEs with an R2 of 0.93 (Figure 9), which is even better than

Table 2. Singlet−Triplet Gaps (ΔES−T), Protonation Energies (PE), and Tolman Electronic Parameter (TEP) for NHCsa NHC

ΔE

ΔES−T

PE

TEP

1 2 3 4 5 6 7 8 9 10 11 12 13

2.05 1.65 1.77 2.06 1.69 1.83 2.26 2.29 2.39 1.96 2.13 2.13 2.23

3.61 3.35 2.88 2.95 2.93 2.36 2.08 2.47 2.18 2.26 3.32 2.20 1.96

11.22 10.66 10.87 11.34 10.86 10.95 11.39 11.75 11.83 11.18 11.28 11.45 11.67

2044.16 2056.81 2053.86 2045.63 2053.15 2051.52 2043.89 2039.96 2039.64 2044.55 2049.54 2044.82 2042.15

a The unit is eV for ΔE, ΔES−T, and PE, and it is cm−1 for TEP (Tolman electronic parameter).

Figure 9. Correlation of PEs and ΔEs for 13 NHCs.

that between ΔEs and TEPs. This suggests that PE is a good descriptor for ΔE and can be used to estimate ΔE without performing time-consuming periodic DFT calculations.

4. CONCLUSIONS We have used density functional theory combined with a periodic slab model to study the adsorption of NHCs on the Au(111) surface. We found that NHC prefers to adsorb on the top site with adsorption energies (ΔE) ranging from 1.65 to 2.39 eV, depending on the type of NHC. Our results also showed that ΔE does not change significantly with the inclusion of solvents in the simulations. This suggests that ΔE calculated in vacuum is sufficient to represent ΔE calculated in solvents. Compared to the ΔEs of three commonly used NHC-type surface ligands (1, 4, and 11), we found that 7−9 and 13 bind with Au(111) more strongly and therefore are better stabilizers. By analyzing the electronic structures using the Bader charge and ALMO-EDA, we found that during the adsorption, NHC acts as an electron donor, transferring its electron density (0.35 electron) from the carbene center to Au(111). Importantly, we find that the dominant charge-transfer interaction is the donation of the HOMO lone-pair electrons in the carbene center to the empty d orbital of Au and the π-back-donation from Au(111) to the LUMO of NHC is negligible. This leads to the exclusion of ΔES−T, which is correlated to the HOMO− LUMO gap, as a descriptor for predicting ΔE. Because the energy of this HOMO is correlated to the PE of NHC, it is expected that PE is strongly correlated to ΔE. This is confirmed by the excellent correlation between ΔEs and PEs (R2 = 0.93). Therefore, we propose that PE is a good descriptor for ΔE, providing a convenient way to screen for better NHC-type surface stabilizers.

Figure 7. Correlation of ΔES−T’s and ΔEs for 13 NHCs.

(TEP = νCO (A1) of Ni(CO)3(NHC)),94 which can be measured experimentally and is well documented for numerous NHCs. Therefore, we calculated the TEPs for the 13 NHCs and compared them to the ΔEs. A good correlation between TEPs and ΔEs with an R2 of 0.83 was found, as illustrated in Figure 8. Our ALMO-EDA analysis showed that during the adsorption, NHCs donate their electron density from the

Figure 8. Correlation of TEPs and ΔEs for 13 NHCs. 2679

DOI: 10.1021/acs.jpca.7b01153 J. Phys. Chem. A 2017, 121, 2674−2682

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b01153. Optimized coordinates for NHCs, Au(111), and NHCAu(111) and the corresponding electronic energies (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Mu-Jeng Cheng: 0000-0002-8121-0485 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.C. and Q.L. acknowledge financial support from the National Natural Science Foundation of China (grant number 21606142). M.-J.C. acknowledges financial support from the Ministry of Science and Technology of the Republic of China under grant no. MOST 105-2113-M-006-017-MY2. We thank the supercomputer center at Tsinghua National Laboratory for Information Science and Technology for providing computational resources.

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REFERENCES

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