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Grand Canonical Quantum Mechanical Study of the Effect of the Electrode Potential on N-Heterocyclic Carbene Adsorption on Au Surfaces Kuan Chang, Jingguang G Chen, Qi Lu, and Mu-Jeng Cheng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07866 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 18, 2017

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The Journal of Physical Chemistry

Grand Canonical Quantum Mechanical Study of the Effect of the Electrode Potential on 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 100084, China Department of Chemical Engineering, Columbia University, New York, NY 10027, USA § Department of Chemistry, National Cheng Kung University, Tainan 701, Taiwan

Submit to JPC C Aug. 8, 2017

*Email: [email protected] and [email protected]

1 ACS Paragon Plus Environment


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Abstract. Increasing interest has been focused on using N-heterocyclic carbenes (NHCs) as surface ligands to replace thiols in the preparation of self-assembled monolayers (SAM) on gold due to their larger adsorption energies. However, one of the drawbacks of these NHC-based SAMs is that they are unstable under electrochemically reducing conditions. In this study, grand canonic quantum mechanics (GC-QM) were used to study the effect of the electrode potential (U) on the adsorption of NHC on Au(111). The NHC adsorption energies were significantly weaker (~0.92 eV) under constant U conditions compared to those under constant charge conditions, demonstrating the importance of using GC-QM for studying electrochemical systems. Consistent with experiments, the results from our calculations indicated that the adsorption energy decreased as U became more negative but increased as U became more positive. These results were rationalized using the frontier orbital theory.

Importantly, based on the same

analysis, when NHCs or their analogs with a smaller gap between the singlet ground and triplet first excited states (∆ES-T < 1.1 eV) were employed as molecular anchors, the adsorption energy was much less affected by U. The same results were obtained for other common SAM substrates (i.e., Ag(111), Cu(111), and Pt(111)). Therefore, based on our GC-QM calculations, we propose that the key to developing a stable NHC-based SAM under electrochemical reducing conditions is to focus on NHCs or their analogs as surface ligands with small ∆ES-Ts.


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Introduction N-heterocyclic carbenes (NHCs) have received increasing attention since the first report of stable and isolable NHCs by Arduengo et al.1 Currently, NHCs are commonly used as ligands for organometallic complexes and have been broadly applied in the field of homogeneous catalysis.2-5 In addition, NHCs have been used as surface ligands to stabilize transition metal nanoparticles.6-7

In some cases, such surface modifications have resulted in catalytic

improvements8 and changes in the reaction mechanisms.9 Recently, NHCs have found new applications as a promising replacement for thiols as molecular anchors to form self-assembled monolayers (SAMs) on Au(111) for use in sensor, drug delivery, microelectronics, and surface protection.7,

10-15

Due to a stronger adsorption

energy (1.64 eV)13 compared to their thiol counterparts (1.31 eV),16 these NHC-based SAMs are typically more resistant to heat and chemicals.

For example, Johnson and co-workers

demonstrated that NHC-based SAMs on gold were stable even under the condition for metalcatalyzed cross-coupling reactions,11 and Crudden and co-workers reported that the SAMs were stable under high temperature and extreme pH conditions as well as in the presence of boiling water and organic solvents.12 However, the stability of these NHC-based SAMs under repeated electrochemical cycling, especially in reducing environments, remains unacceptable, which limits their further application in electrochemistry. Previous experimental studies have demonstrated that the desorption of NHCs occurs under a voltage of only -0.2 VSHE, which is less negative compared to their thiolbased counterpart (-0.9 VSHE).12 In this theoretical study, we used grand canonical quantum mechanics (GC-QM) 17-21 to investigate the effect of the electrode potential (U) on the Au(111)NHC bonding interaction. Our goal was to provide insight into the lack of stability of the NHC-



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based SAMs under reducing conditions and develop NHC or NHC-like molecular anchors that can adsorb stably under reducing environments on gold and other metals that are typically used for SAMs. We describe our computational approach in Section 2. Our results are presented in Section 3 beginning with the study of the adsorption of six selected NHCs on Au(111) under constant U (const-U with U = 0.0 V vs SHE) and under constant charge (const-Q with Q = 0) to illustrate the importance of using GC-QM to study electrochemical systems. Then, we report the effect of U on the NHC-Au(111) adsorption strength and determine the key factor for identifying surface ligands that form stable SAMs under electrochemical conditions. Finally, we expand the scope to Cu(111), Ag(111), and Pt(111), which are also commonly used SAM substrates.

Computational details The PBE functional22 with projector augmented wave pseudopotentials23-24 (400 eV energy cutoff) as implemented in the Vienna ab initio Simulation Package (VASP)25-28 was employed for all slab and molecular calculations. To account for the van der Waals (VDW) interactions, the empirical D2 approach29 was used as implemented in VASP. Because the VDW parameters for gold were not provided in the original article, we adopted 40.62 J·nm6/mol for the dispersion coefficient (C6) and 1.772 Å for the van der Waals radius (R0). These values were used by Amft et al. in their study of Au adsorption on graphene.30 To accelerate SCF convergence, a Gaussiansmearing technique was adopted with a smearing 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 energy values were extrapolated to kBT = 0. Although spin-polarized wavefunctions were used for all slab calculations, we found that the magnetic moment converged to zero after the SCF


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convergence in each calculation. The Au(111) surface was simulated using 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, and the remaining six layers and adsorbed NHCs were allowed to relax (Figure 1 (a)). This symmetric surface model leads to zero dipole perpendicular to Au(111) surface, which was essential in our GC-QM calculations. A Monkhorst-Pack k-point net of 3×3×1 was chosen to sample the reciprocal space for the slab calculations.

However, only the gamma point was sampled in the molecular

calculations. In addition, to prevent interactions between the periodic replicas along the zdirection, a vacuum separation of at least 35 Å between adjacent images was used for the slab calculations, and a 20 Å×20 Å×20 Å box was used for the molecular calculations.



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Figure 1. (a) Side view of the 3×3 seven-layer symmetric model for Au(111) that was used in this quantum mechanical study. (b) Top view of the model with possible adsorption sites for NHC and corresponding adsorption energies under const-Q (Q=0) and const-U conditions (0.0 VSHE, numbers in parentheses). The gold atoms in the 1st, 4th, and 7th layers are colored gold, the gold atoms in the 2nd and 6th layers are colored pink, and the gold atoms in the 3rd and 5th layers are colored blue. The energy unit is eV.

The electrochemical model proposed by Head-Gordon et al.,18 Goddard et al.,20-21 and Sautet et al.19 was used to investigate the effect of U. In this model, the Fermi energy is adjusted to a target value by varying the number of electrons in the system during each step in the


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geometry optimization, which allowed us to change the work function of the system. Then, a linearized Poisson-Boltzmann implicit solvation model31 was used to neutralize the non-zero charge in the simulation cell and simulate the electrical double layer of the electrolytes aqueous solution (with a dielectric constants of 78.4 for water and a Debye screening length of 3.0 Å), allowing for a more realistic description of the electrical double layer. The U of the electrode was calculated by relating the work function (Φ) of the system to the experimental work function of the standard hydrogen electrode (SHE) as follows:

U=

Φ − ΦSHE . e

For ΦSHE, we used a value of 4.3 eV, which is consistent with the experimentally determined value (i.e., 4.2 ± 0.4 eV32) and employed by Anderson and co-workers in their studies of various electrochemical interfaces.33-34 This type of model has also been applied to investigate the effects of U on CO + CO coupling on Cu(100),18 pH and U on CO electrochemical reduction mechanisms,20-21 and U on the pyridine adsorption mode on Au(111).19 All these studies have provided satisfactory explanations of the corresponding experimental observations.

Results and Discussion Six NHCs, which form strong dative bonds with Au(111) (with adsorption energy larger than 2.06 eV, Scheme 1) based on our previous study,35 were chosen for use in this study. First, a reduced model of the first isolated crystalline carbene (1) was employed to probe the adsorption energy profile under const-U with U = 0 V vs SHE (noted as VSHE in the following text), and the results were compared to those calculated under const-Q with Q = 0. We noted that under the latter condition, the system is neutral and the corresponding U is not necessary zero. For clean



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Au(111), U is 0.64 VSHE, whereas for Au(111) adsorbed with 1, it is -0.56 VSHE.

Scheme 1. 2D schematic representation of the NHCs selected for this study.

The adsorption energy was defined as ∆E = (EAu(111) + ENHC) – EAu(111)-NHC, where ENHC, EAu(111), and EAu(111)-NHC are the electronic energies for the isolated NHC in the gas phase, and clean Au(111) and NHC-adsorbed Au(111) in the aqueous phase, respectively. Our calculated results indicate that under const-U, the ∆E values are 1.12, 1.05, and 1.03 eV for the top, fcc, and hcp sites, respectively (Figure 1 (b)). The average of these values is ~0.92 eV smaller than that calculated under const-Q (2.06, 1.96, and 1.95 eV for the top, fcc, and hcp sites, respectively). This dramatic difference in the adsorption energy between the two cases is related to the lack of


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Φ changes during adsorption under const-U. However, under const-Q, Φ changed from 4.94 to 3.74, 3.82, and 3.79 eV for the top, fcc, and hcp adsorption, respectively. We note that the calculated Φ for 1-adsorbed Au(111) is consistent with a recent theoretical study (3.8 eV)36 and experimental investigation (3.73 ~ 3.98 eV, measured using Kelvin probe force microscopy).37 These results demonstrate that the energetics related to const-U and const-Q are very different, and therefore, GC-QM should be used to study electrochemical systems, especially systems involving significant variations in Φ. Despite the significant difference in the ∆Es calculated under const-U and const-Q, the energetically most favorable adsorption site is the top site under both conditions. In fact, for all the NHCs and their analogs considered in this study, the top site is the most favorable adsorption position for all studied Us, and the following discussion is based on this adsorption model. The calculated ∆Es for the other five NHCs at U = 0.0 VSHE were 1.22, 1.23, 1.44, 1.53, and 1.44 eV for 2, 3, 4, 5, and 6, respectively (Table 1). Similar to 1, the average of these ∆Es is ~0.92 eV smaller than that calculated under const-Q. The order of the binding interaction is 1 < 3 ~ 2 < 4 ~ 6 < 5, with 5 binding with Au(111) most strongly and 1 binding with Au(111) most weakly. This trend is similar to that calculated under const-Q. Apparently, due to similar electronic structures (e.g., similar HOMO and LUMO energies), the interactions of all six NHCs with Au(111) are similar, and therefore, their adsorption energy variations moving from const-Q to const-U are also similar.

Table 1. Calculated NHC adsorption energy on Au(111) for six selected NHCs under cons-Q (Q = 0) and under const-U with U = 0.5, 0.0, and -0.5 VSHE, and the corresponding slopes for the



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response of ∆E under different U. The singlet and triplet energy gaps (∆ES-Ts) for the NHCs are also listed.

NHC 1 2 3 4 5 6

∆E const-Q 2.06 2.15 2.12 2.36 2.45 2.38

U = +0.5 1.44 1.54 1.51 1.75 1.84 1.77

U = 0.0 1.13 1.22 1.23 1.44 1.53 1.44

U = -0.5 0.81 0.89 0.95 1.11 1.20 1.10

Slope

∆ES-T

0.62 0.65 0.56 0.65 0.64 0.67

3.61 2.95 3.32 2.47 2.18 2.08

Using ∆E at U = 0.0 VSHE as the reference, we studied the variation of ∆E under the influence of U. To make the environment more oxidizing (or reducing), an electron was removed from (or injected into) the surface to increase (or decrease) Φ, which increases (or decreases) U. At U = +0.5 VSHE, the Au(111)-NHC bond was strengthened by ~0.31 eV (i.e., ∆Es increase to 1.44, 1.54, 1.51, 1.75, 1.84, and 1.77 for 1, 2, 3, 4, 5, and 6, respectively). In contrast, at U = -0.5 VSHE, the Au(111)-NHC bond was weakened by ~0.32 eV (i.e., ∆Es decrease to 0.81, 0.89, 0.95, 1.11, 1.20 and 1.10 for 1, 2, 3, 4, 5, and 6, respectively). A linear relationship was observed between ∆E and U with R2 ~ 1.0, and similar slopes (ranging from 0.56 to 0.67) were obtained for all six NHCs. Therefore, our study predicts that the chemical bond between Au(111) and NHC is strengthened in positive U but weakened in negative U. Indeed, the experiments by Crudden et al. have shown that a NHC-based SAM on gold is stable under oxidizing conditions up to U = 0.8 VSHE but exhibits desorption at U = -0.2 VSHE.12 We noted that replacing the hydrogen atoms bound to N in 1 and 2 with methyl (1-Me and 2-Me) lead to insignificant change in ∆Es: at U = +0.5, 0.0, and -0.5 VSHE, ∆Es are 1.61, 1.25, and 0.86 eV, respectively, for 1-Me, and 1.62, 1.23, and 0.84 eV, respectively, for 2-Me, all of which are similar to their


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hydrogen counterparts (1.44, 1.13, and 0.81 eV for 1, and 1.54, 1.22, and 0.89 eV for 2). Also, it should be noted that the adsorption of NHCs on Au surface will not be infinitely stable when the potential becomes more positive, because the oxidation of Au or surface electrochemical reactions (e.g., oxygen reduction) may take place to destabilize Au(111)-NHC interaction. We rationalized these results using the frontier molecular orbital theory.12,38 As shown in our previous study, two sets of important molecular orbital interactions contribute to the Au(111)NHC dative bond (Figure 2 (b)).35 The first set is the donation of an occupied lone pair orbital at the NHC carbene center (the highest occupied molecular orbital, HOMO) to the unoccupied Au(111) d-band (σNHC→Au(111)), and the other set (πNHC←Au(111)) is the π-back donation from the occupied Au(111) d-band to the empty π orbital of NHC (the lowest unoccupied molecular orbital, LUMO). For the six NHCs, the LUMO is much higher in energy than the HOMO, as indicated by their ∆ES-Ts (> 2.0 eV, Table 1). Therefore, πNHC←Au(111) is less important, and σNHC→Au(111) dominates the Au(111)-NHC interaction. By comparing the density of states (DOS) of clean Au(111) to that of isolated 1 (Figure 3 (a) and (b)), the LUMO of isolated 1 was distant from the Fermi level of Au(111), indicating a weak πNHC←Au(111) interaction.



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Figure 2. Two frontier orbital interactions between Au(111) and NHCs with large (a, b, c) and small ∆ES-Ts (d, e, f) under different U. The red line represents the interaction between the occupied Au(111) d-band and the LUMO of NHC, and the blue line represents the interaction between the HOMO of NHC and the unoccupied Au(111) d-band. The gray, blue, orange, and white balls represent carbon, nitrogen, phosphorus, and hydrogen, respectively. The top of the shaded area represents the Fermi level of clean Au(111).


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Figure 3. Total (uncolored) and projected density of states (DOS, colored) for (a) clean Au(111), and isolated (b) 1 and (c) 7. The electrostatic potentials in the middle of the vacuum for the three systems were aligned to the same value for comparison. In addition, the Fermi level of clean Au(111) is shifted to 0 eV. The gold color represents projected DOS for the Au 3d-orbital, the red color represents projected DOS for the px- and py-orbitals of 1 and 7, and blue color



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represents projected DOS for the pz- and s-orbitals of 1 and 7. Therefore, the projected DOS indicate that the HOMOs of 1 and 7 have σ-orbital character and their LUMOs have π-orbital character.

When an electron is removed from the metal surface to increase U to 0.5 VSHE, the Fermi level of Au(111) decreases by 0.5 eV, which reduces its energy difference with the HOMO of NHC and results in a better σNHC→Au(111) interaction that leads to an increase in ∆E (Figure 2 (c)). However, when an electron is injected into the surface to lower U to -0.5 VSHE, the Fermi level increases by 0.5 eV, resulting in a larger energy difference between the unoccupied d-band and the HOMO of NHC, and therefore, a weaker interaction and a small ∆E were obtained (Figure 2 (a)). To identify a NHC-based surface ligand that adsorbs stably under negative potentials (U < 1.0 V), NHC or its analogs must have a small ∆ES-T. For these carbenes, due to their small ∆ESTs,

both σNHC→Au(111) and πNHC←Au(111) are expected to contribute equally to the strength of the

Au(111)-NHC dative bond (Figure 2 (e)). When U becomes negative, the Fermi level increases, and σNHC→Au(111) weakens. At the same time πNHC←Au(111) strengthens, compensating for the decrease in the σNHC→Au(111) interaction and maintaining the bond strength (Figure 2 (d)). Similarly, when U becomes positive and the Fermi level decreases, σNHC→Au(111) strengthens but πNHC←Au(111) weakens, leading to an unchanged bond strength (Figure 2 (f)). Overall, we expect that ∆Es for these molecules would be less susceptible to U due to the competing effects of the two sets of frontier molecular interactions. To confirm this hypothesis, rather than using NHC-type carbenes, which typically have large


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∆ES-Ts,2,

39-40

the phosphorus analog of NHC (i.e., P-heterocyclic carbenes (PHCs)) was

employed.41 In contrast to the NHCs, PHCs have theoretically smaller ∆ES-Ts.42 The synthetic route to stable PHCs (7’, Scheme 2) has been reported by Bertrand and co-workers.43 Although the stability of the carbene relies on the bulkiness of the substituents on P to prevent dimerization of two carbenes to form alkene,39 we replaced the bulky 2,4,6-tris(tert-butyl)phenyl and methyl with hydrogens (7) to investigate the electronic structure effect and reduce the computational cost.

Scheme 2. 2D schematic representation of two P-heterocyclic carbenes. Ar corresponds to the 2,4,6-tris-(tert-butyl)phenyl substituent.

The ∆ES-T of 7 was calculated to be 1.06 eV, which is similar to that reported by Melaimi et al. (i.e., 0.97 eV).41 By comparing the projected density of states of clean Au(111) to those of isolated 7 (Figure 3 (c)), the LUMO (s, pz) of isolated 7 is much lower in energy than that for 1 and is close to the Fermi level of Au(111). However, the HOMO (s, pz) of isolated 7 is in the same energetic position as that of isolated 1. The results from our GC-QM calculations indicate that as U decreased from 0.0 to -0.5, 0.75, and -1.00 VSHE, the ∆E of 7 changed slightly from 1.51 to 1.43, 1.51, and 1.55 eV,



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respectively. When U increased from 0.0 to 0.5 VSHE, ∆E of 7 only slightly increased from 1.51 to 1.68 eV (for 1, ∆E increased from 1.13 to 1.44 eV). Clearly, the Au(111)-7 bond strength is much less affected by U and even becomes slightly more stable under negative U. In addition, these ∆Es are larger than those for 1 in the range of -0.5 to +0.5 VSHE, indicating a stronger binding interaction. By analyzing the projected DOS for 7 and 1 for the adsorption, the DOS formed by πcarbene←Au(111) in the Au(111)-7 system is populated by an electron as U becomes more negative (Figure 5 (b)) but remains empty in the Au(111)-1 system (a). These results confirm our assumption that for adsorption of NHCs with small ∆ES-Ts on gold, the adsorption energy is less susceptible to U, and the binding remains stable under negative Us.


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Figure 4. Projected DOS for (a) Au(111)-1 and (b) Au(111)-7. The black, red, and blue lines represent the total DOS and projected DOS for the s- and pz-orbitals for 1 and 7, as well as the projected DOS for the px- and py-orbitals for 1 and 7, respectively.

To further confirm our hypothesis, we studied the adsorption of two NHC carbenes with



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diboron backbones (8a and 8b, Scheme 3) on Au(111). A NHC similar to 8b but with bulky substituents on the two nitrogen atoms has been synthesized by Krahulic et al.44 The calculated ∆ES-T was only 0.74 eV for 8a but much larger for 8b (1.92 eV).

Scheme 3. 2D schematic representation of two NHC carbenes with a diboron backbone.

As shown in Table 2, for 8a, as U increased from 0.0 to 0.5 VSHE, ∆E slightly decreased. However, when U decreased to -0.5 VSHE, ∆E increased from 1.48 to 1.82 eV rather than decreasing. In contrast, for 8b, ∆E decreased with decreasing U from 1.42 eV at U = 0.5 VSHE to 0.97 eV at U = -0.5 VSHE. Therefore, the response of ∆E to U for 8a was similar to that for 7. However, when the hydrogen substituents on the two boron atoms were replaced by amines to form 8b, which leads to an increase in ∆ES-T, the response of ∆E to U is similar to that for the six NHCs.

By analyzing the electronic structures, we determined that the projected DOS of

Au(111)-7 and Au(111)-8a (Figure 6 (a)) were similar, and the projected DOS of Au(111)-1 and Au(111)-8b (Figure 6 (b)) were also similar. This result suggests that the choice of NHC or an analog with a small ∆ES-T is key to identifying molecular anchors that form stable SAMs on Au(111) under a negative U.


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Table 2. Calculated NHC adsorption energy on Au(111) for 8a and 8b under const-U with U = 0.5, 0.0, and -0.5 VSHE. The singlet and triplet energy gaps (∆ES-Ts) are also listed.

NHC 8a 8b

U = +0.5 1.43 1.42

∆E U = 0.0 1.48 1.26


U = -0.5 1.82 0.97

∆ES-T 0.74 1.92


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Figure 5. Projected DOS for (a) Au(111)-8a and (b) Au(111)-8b. The black, red, and blue lines represent the total DOS and projected DOS of s- and pz-orbitals for 8a and 8b, as well as the projected DOS of px- and py-orbital for 8a and 8b, respectively.

During the reviewing process, one of the reviewers raised a question about whether N,N’diamidocarbenes (DACs), which is known to have triplet-like reactivity 45 and have been used to functionalize silicon surfaces,46 are able to bind with Au(111) strongly under negative Us, similar to 7 and 8a. We therefore studied the adsorption of 9 (Scheme 4), a five-membered ring DAC with a calculated ∆ES-T of 1.16 eV, and indeed found this is the case. ∆Es were predicted to be 1.65, 1.68, and 1.83 eV at U = +0.5, 0.0, and -0.5 VSHE, respectively.

Scheme 4. 2D schematic representation of diamidocarbene 9.

In addition to gold, silver, copper, and platinum have been used as metallic supports for SAMs.47 To expand the scope of our concept, we calculated the adsorption energies of 7 on the (111) index surfaces of these face-centered cubic (fcc) metals, which are the most stable surfaces in vacuum based on their surface energies.48 For comparison, the adsorption energies of 1 on the


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(111) index surfaces of these metals under const-U with U = 0.5, 0.0, and -0.5 VSHE were also calculated and the results are summarized in Table 3. Based on these results, the dative bonds of 7 to Ag(111), Cu(111), and Pt(111) are also less affected by the electric potential of the surface and stronger compared to those of 1. Therefore, 7 is a more robust surface ligand, especially under electrochemically reducing conditions.

Table 3. Calculated NHC adsorption energy on different metal surface for 1 and 7 under const-U with U = +0.5, 0.0, and -0.5 VSHE.

Metal Au Cu Ag Pt

1 U = 0.0 1.13 1.60 1.25 2.18

U = +0.5 1.44 1.89 1.50 2.47

U = -0.5 0.81 1.27 0.95 1.91

7 U = 0.0 1.51 2.33 1.76 2.85

U = +0.5 1.68 2.41 1.84 3.00

U = -0.5 1.43 2.24 1.72 2.76

3.5 3.0

Binding Energy (eV)

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2.5 2.0 1.5 1.0 0.5 0.0 -0.6

1 on Au 7 on Au

-0.4

1 on Cu 7 on Cu

-0.2

0.0

1 on Ag 7 on Ag

0.2

1 on Pt 7 on Pt

0.4

0.6

Applied Voltage (V) Figure 6. Binding energies of 1 and 7 on Cu(111), Ag(111) and Pt(111) under different applied



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voltages.

Conclusions In summary, we employed grand canonic quantum mechanics to study the influence of the electrode potential (U) on the adsorption energies of NHC and its analogs on Au(111). Consistent with experiments, we found that for NHCs with larger ∆ES-Ts, the adsorption energies increased as U becomes more positive and decreased as U becomes more negative. These observations were rationalized using the frontier orbital theory. Importantly, based on the same analysis, we proposed and confirmed that the adsorption energy of NHC or its analogs with smaller ∆ES-Ts would be less affected by U. We further expanded the scope of our investigation to Ag(111), Cu(111), and Pt(111), which are also commonly used as substrates for self-assembled monolayers. We found that similar to Au(111), the adsorption of a NHC analog with smaller ∆ES-Ts was less affected by U. Therefore, our grand canonic quantum mechanical results suggest that the key to identifying NHC-based surface ligands to form a stable SAM under electrochemical conditions is to focus on NHC-like carbenes with small ∆ES-Ts.

Supporting Information Available Optimized coordinates for Au(111), and NHC-Au(111) and the corresponding electronic energies are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements 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


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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 the Tsinghua National

Laboratory for Information Science and Technology for providing computational resources.

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