Dramatic Acceleration of the Hopf Cyclization on Gold(111): From Enediynes to Peri-Fused Diindenochrysene Graphene Nanoribbons

Hopf et al. reported the high-temperature 6π-electrocyclization of cis-hexa-1,3-diene-5-yne to benzene in 1969. Subsequent studies using this cyclization have been limited by its very high reaction barrier. Here, we show that the reaction barrier for two model systems, (E)-1,3,4,6-tetraphenyl-3-hexene-1,5-diyne (1a) and (E)-3,4-bis(4-iodophenyl)-1,6-diphenyl-3-hexene-1,5-diyne (1b), is decreased by nearly half on a Au(111) surface. We have used scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc-AFM) to monitor the Hopf cyclization of enediynes 1a,b on Au(111). Enediyne 1a undergoes two sequential, quantitative Hopf cyclizations, first to naphthalene derivative 2, and finally to chrysene 3. Density functional theory (DFT) calculations reveal that a gold atom from the Au(111) surface is involved in all steps of this reaction and that it is crucial to lowering the reaction barrier. Our findings have important implications for the synthesis of novel graphene nanoribbons. Ullmann-like coupling of enediyne 1b at 20 °C on Au(111), followed by a series of Hopf cyclizations and aromatization reactions at higher temperatures, produces nanoribbons 12 and 13. These results show for the first time that graphene nanoribbons can be synthesized on a Au(111) surface using the Hopf cyclization mechanism.

On-surface reactions over coinage metals have gained prominence for their extraordinary capacity to generate elaborate designs that are often not attainable via solution syntheses.−38 On-surface coupling reactions of alkyne substrates have included Glaser 30,33,39−48 and Sonogashira reactions, 43,46,49 dehalogenative or desilylative homocoupling of halo-or silylalkynes, 42,50−54 decarboxylative coupling of alkynyl carboxylic acids, 55 as well as various terminal alkyne coupling reactions generating enediynes or enetriynes, 56,57 a [4]radialene via [1 + 1 + 1 + 1] coupling, 58 or benzofulvenes via [2 + 2 + 1] coupling. 47,59o the best of our knowledge, an on-surface Hopf cyclization reaction has not been reported.Known on-surface, concerted cyclization reactions include the Bergman 60−63 and alternate Schreiner-Pascal 64−68 cyclization pathways of cis-enediynes (Figure 1c), as well as Huisgen "click" reactions, 69−71 and an intramolecular hexadehydro-Diels−Alder reaction of a diyneyne macrocycle. 72One should note that these concerted cyclizations take place exclusively via in-plane π-orbitals forming the new C−C bonds (e.g., Figure 1c, middle) and thus do not require partial molecule detachment from the surface of Au(111) at the transition state, which can be energetically costly.In the current study, the Hopf cyclization requires an outof-plane approach between an alkenyl/aryl p-orbital at C 6 and an alkynyl p-orbital at C 1 , Figure 1d), to form a C 1 −C 6 bond, which is facilitated by the nonplanar conformation of the starting material 1a on Au(111) (see below, Figure 2e, left) and the catalytic involvement of a gold atom (see the Calculations section).
In our previous work, the characterization of graphene nanoribbons was accomplished by solid-state CP-MAS 13 C NMR, Raman, and IR spectroscopy on strongly aggregated bulk material, as well as by HR-TEM imaging. 12,13,19To gain better insight into the Hopf cyclization mechanism at the atomic level, it was clear that the structure of [8] A GNR would need to be reduced to one of its simplest constituents, chrysene (cyan subunit in Figure 1b, right).The latter would hence form by double Hopf cyclization of (E)-1,3,4,6-tetraphenyl-3-hexene-1,5-diyne (1a, Figure 1e).
We describe here our initial results on enediyne 1a and its corresponding polymer formed from the precursor, (E)-3,4bis(4-iodophenyl)-1,6-diphenyl-3-hexene-1,5-diyne (1b), to reveal the reaction paths on Au(111) using STM and nc-AFM imaging.We find that the sequential Hopf cyclization of 1a is completed entirely below 160 °C for the first cyclization, affording intermediate naphthalene derivative 2, while the second cyclization, affording chrysene 3, is completed by ∼190 °C (Figures 1e and 2e).The unexpected halving of the Hopf cyclization temperature, and its corresponding reaction barrier, compared to our bulk cyclization reactions 12,13,19 is supported by DFT calculations.We find that a gold-alkyne interaction stabilizes intermediates in the Hopf cyclization on Au(111) (Figure 3), resulting in the lowering of the reaction barrier.

■ RESULTS AND DISCUSSION
We epitaxially deposited (E)-1,3,4,6-tetraphenyl-3-hexene-1,5diyne (1a) by sublimation onto a Au(111) surface at 20 °C in an ultrahigh vacuum chamber with a base pressure of 1 × 10 −10 mbar.The as-grown sample surface is shown in Figure 2a.Most molecules are isolated, with a preference to reside between the herringbone stripes of Au(111).All the molecules have transconfiguration. 73Enediyne 1a has a nonplanar conformation on the surface of Au(111), with an inversion symmetry between the out-of-plane rotated four phenyl groups (Figure 2e, left).
To track all intermediates of the on-surface Hopf cyclizations, annealing was performed step by step at the temperatures shown in Figure 2e (bottom gray scale bar) with a 15 min stay at each temperature.After each annealing step, the sample was characterized in situ by a combination of STM and nc-AFM at 4.5 K. Surprisingly, the first Hopf cyclization, affording naphthalene derivative 2, can already be observed after annealing at 130 °C (Figures 2b,f and S1).It is characterized by its apparent asymmetry in the topographic image (Figure 2e, middle).The phenyl group connected to the naphthyl ring is highly tilted and located significantly higher than the rest of the molecule, as revealed by its bright protrusion.As a result, monomers of 2 are not as "sticky" as 1a on the gold surface and self-assemble into dimeric, trimeric, and tetrameric clusters away from uncyclized molecules of trans-enediyne 1a (Figures 2b and S2a−e).Chrysene derivative 3 is not present anywhere on the surface, implying that the second Hopf cyclization has a substantially higher energy barrier.After annealing at 160 °C, we observe roughly half of the molecules becoming doubly cyclized with chrysene 3 (Figure 2c,f).As indicated by the topographic STM image (Figure 2e, right), the molecule is now nearly symmetric and exhibits an out-of-plane conformation of the two phenyl substituents (i) and (ii), with an inversion of symmetry between the tilted rings.Thus, chrysene 3 is also fairly mobile on the surface and forms self-assembled clusters, as revealed by the periodic chains and lattices shown in the topographic images (Figures 2c and S2f−h).Finally, highertemperature annealing at 300 °C generates 5-memberedannulated species, named post-Hopf (Figures 2d,f and S3), which involves a well-known dehydrocyclization process at higher temperatures on Au(111). 23Statistical analysis of the molecular distributions at 20, 130, 160, 190, and 280 °C, respectively, reflects the evolution of ratios between structures 1a, 2, and 3 (Figure 2f).Most of the molecules undergo single Hopf cyclization below 160 °C, and double Hopf cyclization between 160 and 200 °C.Calculations.To compare the gas phase and on-surface mechanisms, DFT calculations were carried out on the conversion of enediyne 1a to naphthalene 2 and chrysene 3, both in the gas phase and on a Au(111) slab model (Table 1, Figure 3). 73In the gas phase, the intermediates in the Hopf cyclization calculations were analogous to those previously described by Prall et al. and our own work. 7,12,13More specifically, the Hopf cyclization steps were verified to proceed via an initial 6π-electrocyclization, followed by two consecutive 1,2-H shifts, where the first H-shift was the rate-determining step (Table 1, Figure 3).All gas phase structures were optimized at the B3LYP-D3/6-31G(d) level, with D3 dispersion correction applied to account for steric interactions, and single-point energy calculations were performed at the M06-2X/6-311+G-(d,p) level of theory, with B3LYP-D3/6-31G(d) frequency calculations to obtain the free energy values (see Table 1). 73or the on-surface calculations, all structures were computed at the PBE level of theory, 73,74 with D3 dispersion correction also applied, and with electronic states expanded using a TZV2P contracted Gaussian basis set for carbon and hydrogen and a DZVP basis set for gold.Transition states were assessed by performing, for each reaction, a sequence of constrained geometry optimizations that identified a minimum energy path (MEP).A reaction coordinate was defined and constrained to a value that was changed in small increments at each subsequent optimization.The highest energy structures in each MEP were taken as approximations for the corresponding transition states for each of the reaction steps.The Au(111) surface was modeled with a periodic slab geometry constructed from a four-layer gold supercell with a top surface size of 38.3 × 32.4 Å 2 (768 gold atoms).The on-surface reaction steps using this gold slab proceeded generally in parallel to those in the gas phase (Table 1, Figures 3, S5), namely, through the initial 6πelectrocyclization followed by the two consecutive 1,2-H shifts.However, an essential difference was that most of the steps involved unforeseen bonding to a gold atom.The latter proved to be key to explaining the dramatic acceleration of the Hopf cyclization on Au(111).
We first confirmed the accuracy of our on-surface calculations by reproducing the adsorptional geometries of experimentally observed structures 1a, 2, and 3 (Figure 4).The optimized geometry of enediyne 1a on Au(111) has its two phenyl groups (i) and (ii) nearly coplanar with the gold surface, while phenyl groups (iii) and (iv) are tilted out-of-plane due to steric interactions with the rest of the molecule (Figure 4a).Thus, the conformation of 1a is the same as that in Figure 2e, left.A simulated AFM image using this ground-state geometry is compared to the experimentally observed nc-AFM image of 1a (Figure 4b, left), showing good consistency between the two.A small difference is that this experimental image does not simultaneously show the flat-lying (E)-1,6-diphenyl-3-hexene-1,5-diyne moiety and its tilted 2,3-diphenyl substituents ((iii) and (iv) in Figure 2e).However, a series of nc-AFM images using a range of tip-molecule distances confirms all the features in the simulated images of the molecule (Figure S4).Similarly, naphthalene 2 and chrysene 3 are also well replicated by our onsurface optimizations, confirmed by the uniformity between simulated and experimental nc-AFM images (Figure 4b, middle and right).
On-Surface Reaction Mechanism and Involvement of a Gold Atom.Calculated geometries, relative free energies, and a comparison of the progression of the Hopf cyclization along the reaction coordinate in the gas phase and on-surface are provided in Table 1 and Figures 3 and S5.Geometries, bond lengths, and angles of selected intermediates obtained in MEP calculations for the first 6π-electrocyclization step are shown in Figure S6A.The calculations reveal an unexpected turn in the reaction mechanism, one involving the participation of a gold atom from the Au(111) surface in the ring-forming and 1,2-H shift reaction steps (Figures 3 and S6A).This participation plays a crucial role in lowering the reaction barrier, switching from the anticipated high-barrier 6π-electrocyclization mechanism to the much lower-barrier Au-catalyzed cyclization.
To understand how the potential catalytic effect of the gold surface could take place, we first calculated how the structure of enediyne 1a changes during the 6π-electrocyclization step, as the    75 The C�C bond lengths in these complexes range from 1.220 to 1.233 Å, 76 and alkyne bending angles range from 155.0 to 168.1°. 77urther constriction of the C 1 •••C 6 distance to 1.9 Å results in the highest energy intermediate in the 6π-electrocyclization step, labeled 4b, which we define as the transitions state (Figures 3, 5a, and S6A).The C 1 •••C 6 distance in 4b is slightly longer, by 0.1 Å, to that found (1.801 Å) for the analogous calculated transition state 5b in the gas phase calculations (Figure 3).Additionally, the Au 138 •••C 2 distance has shortened further to 2.204 Å from that in 4a.The barrier for this first 6πelectrocyclization is 28.6 kcal/mol, which is nearly half (55%) of that calculated for the gas phase (52.3 kcal/mol, Table 1).
Final constriction of the C 1 •••C 6 distance arrives at the strained gold allene η 1 -complex 4c, which is a local minimum (Figures 3, 5a The calculated structure of 4c is intriguing in that it resembles two known gold(I) allene η 1 -complexes (8 and 9, Figure S7), which are very similar in geometry despite the +1 gold oxidation state for these complexes.Stephan and co-workers have reported the synthesis and complexation of tetraphenyl carbodicyclopropenylidene, an all-carbon carbidodicarbene, with an Nheterocyclic carbene (NHC) gold(I) species (8, Figure S7), 79 and Furstner et al. have described the complexation of tetrakis(dimethylamino)allene with Ph 3 PAuCl to give another stable carbidodicarbene complex (9), 80 both characterized by Xray diffraction.The two complexes have identical gold(I)− carbon distances at 2.071 Å, which are comparable to the length calculated for gold(0) complex 4c (2.152 Å).The allene C�C bonds in Stephan's carbidodicarbene complex are 1.372 and 1.362 Å, which compare well with that of the C 1 �C 2 bond in 4c (1.380 Å).In addition, the C 2 −C 3 bond length (1.451 Å) is similar to the allene bond lengths in bis-NHC carbidodicarbene 9 (1.407 and 1.424 Å); these bonds have single bond character due to the π-delocalizing effect of the adjacent dimethylamino groups, as shown in the polar resonance structure of 9 (Figure S7).Intriguingly, the lengths of the C 3 −C 4 and C 4 −C 5 bonds in intermediate 4c are 1.426 and 1.432 Å, respectively (Figure 3).These bonds can therefore be considered a delocalized allylic system, as similar C−C bond lengths in the crystal structures of three different uncomplexed allylic cations range from 1.36 to 1.43 Å (Figure S8), 81 while the neutral allyl radical C 3 H 5 • in the gas phase has a bond length of 1.428 Å. 82 With these considerations, we assign intermediate 4c the delocalized zwitterionic structure shown in Figure 3, whose gold atom Au 138 is partially negatively charged, while the balancing positive charge for this overall neutral species is delocalized between C 2 and C 3 −C 4 −C 5 in the form of an allylic cation.
−86 Upon coordination of a gold atom to one of the triple bonds of a cis-enediyne, the mechanism becomes exergonic, and the positive charge of the zwitterionic form of p-benzyne gets stabilized dramatically to afford an intermediate that is the global minimum on the potential energy surface.Correspondingly, our Hopf cyclization of 1a on Au(111) is likely also electrophilic in nature, and we are currently exploring this exciting prospect for increasing the rate of Hopf cyclizations in solution.
A Hirshfeld charge population analysis, which yields chemically meaningful charges, 87 was performed on gold allene complex 4c to support this assignment (Figure 5b−d).The charge values for the carbon and gold atoms of 4c are plotted as spatially resolved charge deviations from the average value (Figure 5b−d).Carbon atom C 2 has the most charge deviation (positive) from the average value (Figure 5c) and is the one formally bonded to the gold surface.Correspondingly, the gold atom Au 138 , adjacent to C 2 , has an opposite, negative charge (Figure 5d), reinforcing the case for the formation of a polar bond between these two atoms.Furthermore, the projected density of states (PDOSs) of individual atoms, shown in Figure 5e,f, adds further support for the formation of a polar Au−C bond between C 2 and Au 138 .This is revealed by the emergence of new electronic states marked by black arrows.Overall our analysis strongly supports the formation of a Au−C bond, which is essential in lowering the barrier of the on-surface Hopf cyclization.
The next step of the on-surface reaction, which is ratedetermining for the overall enediyne-to-naphthalene transformation (1a−2), is the first of two 1,2-H migrations, affording the gold-complexed carbene intermediate 4e (Figures 3 and  S6B).The barrier (32.3 kcal/mol) for this step is transition state 4d, whose structure parallels that of the gas phase, but for which C 2 remains bound to Au 138 .Again, bonding to gold appears to contribute significantly to halving (54%) the overall reaction barrier from 59.7 kcal/mol in the gas phase down to 32.3 kcal/ mol.The involvement of Au 138 in the transition state for the first 1,2-H shift 4d, its carbene product 4e, and the transition state for the second 1,2-H shift 4f is clearly visible in Figure S6B.Gold stays bound to C 2 in all intermediates 4d−f, with the Au 138 −C 2 bond length varying between 2.07 and 2.17 Å. Hirshfeld charge population analysis of the transitions state structures 4d and 4f again supports the electrophilic nature of the overall rearrangement, with C 2 being most positively charged, and Au 138 negatively charged (Figure S11).
The final steps of the Hopf cyclization on the gold slab model proceed through a second, gold-bound 1,2-H shift, giving initially rotamer rot-2 (−48.9 kcal/mol in Figure 3, value not shown in Table 1).Then, the naphthalene-bound phenyl ring ii rotates to give the final product 2 (−50.7 kcal/mol), which is 1.8 kcal/mol lower in energy.
For the second Hopf cyclization sequence (Table 1, Figure 3), gold-bound intermediates 6a−e have geometries similar to those of the species calculated in the first cyclization sequence.This sequence provides a second rate-determining barrier for this reaction, embodied by transition state 6c.The reaction barrier, at −15.0 kcal/mol (+39.7 kcal/mol relative to 2), is 7.4 kcal/mol higher than the barrier for the first cyclization sequence (+32.3kcal/mol).We attribute this increase in the reaction barrier to the presence of the additional naphthyl ring obtained during the first Hopf cyclization, whose aromaticity needs to be broken during the 6π-electrocyclization and first 1,2-H shift steps.
Interestingly, during our heating experiments with enediyne 1a on Au(111), we observed an inconsequential number of pentafulvene molecules 2′ by STM, out of hundreds of molecules of naphthalene derivative 2 (Figure S9).This observation shows that the alternate Schreiner-Pascal cyclization pathway between C 2 and C 6 can also occur (Figure 1c), 66,67 but with a much lower probability, as underscored by the relative calculated reaction barriers for both pathways pictured in Figure S9.
Synthesis of Novel Graphene Nanoribbons.With the promising results obtained on enediyne 1a, we wanted to demonstrate that the Hopf cyclization can be advantageously exploited to synthesize isolated graphene nanoribbons on a Au(111) surface at moderate temperatures, allowing characterization of the products by tip imaging techniques, as well as providing validation for our bulk GNR synthesis method which exploits the Hopf cyclization. 12,13,19A straightforward way to investigate this path was to bis-halogenate structure 1a with either bromine or iodine substituents and then polymerize the resulting monomers into 1D chains through Ullmann-like coupling on Au(111).We wanted a system that would undergo 1D-polymerization prior to the Hopf cyclization sequence.Thus, iodination, exemplified by diiodo enediyne 1b, was adopted owing to the facile Ullmann-like coupling propensity of C−I bonds on Au(111), which can occur as low as ambient temperature. 88The set of reactions starting from diiodo enediyne 1b is shown in Figure 6a.We deposited diiodo enediyne 1b on a Au(111) surface under the same conditions as those for enediyne 1a.However, unlike the isolated molecules of enediyne 1a, we did not see any free molecules of diiodo enediyne 1b; instead, they were already coupled into the 1D-polymeric chains 10 at 20 °C (Figure 6b,c).Isolated iodine atoms can be seen moving freely on the Au(111) surface, with a strong preference for locating between aggregates of the polymeric enediyne chains of 10.The formation of cisoid and transoid connections between individual enediyne units is also clearly visible in Figure 6c and over a larger area STM image in Figure S12a.Following the same heating protocol carried out on enediyne 1a, we obtained chains of poly(chrysene) 11, clearly showing the chrysene units in a periodically repeating pattern, albeit with many kinks that we attribute to ring fusions deriving from the penultimate enediyne units of 10 in a cisoid relationship (Figure 6d).After annealing at 370 °C, most of the chains are converted into 5-membered-annulated species (12) and further annealed aromatic ribbons with structure 13.While the structure of 13 is quite complex due to its cisoid/ transoid distribution of monomeric units (Figures 6e and S12d), many sections have longer runs of transoid regioisomers that can be clearly seen, for example on the right side of Figure 6e. Figure 6f gives an example of an isolated oligomeric nanoribbon obtained at 370 °C, whose nc-AFM image (Figure 6g) is in good agreement with the structural features of 13a displayed in Figure 6a.Because the all-transoid form of graphene nanoribbon 13 has unusual 5-membered ring fusions, we have calculated its band structure as well as the projected density of states (Figure S10).The band structure of 13 indicates that it has a bandgap of ∼3 eV (left panel), making it a wide-bandgap semiconductor.

■ CONCLUSION
In summary, we have shown that the reaction barrier of the Hopf cyclization can be significantly lowered on a Au(111) surface for two model systems, (E)-1,3,4,6-tetraphenyl-3-hexene-1,5-diyne (1a) and (E)-3,4-bis(4-iodophenyl)-1,6-diphenyl-3-hexene-1,5diyne (1b).We have used scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc-AFM) to monitor the two sequential Hopf cyclizations of enediynes 1a,b on Au(111).The experimental results and DFT calculations show that the reaction barrier is effectively halved through the involvement of a gold atom on the Au(111) surface.These results show that, in principle, a large diversity of graphene nanoribbons may be obtainable through a combination of Ullmann-like and Hopf cyclization reactions on Au(111).The involvement of gold catalysis in the reaction mechanism opens a new avenue for creating novel nanographene structures, both in solution and on Au(111), which we are currently pursuing.

■ ASSOCIATED CONTENT Data Availability Statement
Data related to the simulations will be available on the Materials Cloud repository at DOI: 10.24435/materialscloud:62-ew.
Experimental and computational methods, supporting STM and nc-AFM data, calculations, detailed synthesis description of chemical compounds reported in this study, and crystallographic data for enediynes 1a and 1b 89 (PDF) Accession Codes CCDC 2295204−2295205 contain the supplementary crystallographic data for this paper.These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cam- bridge Crystallographic Data Centre, 12 Union Road, Cam-

Figure 1 .
Figure 1.(a) Mechanism for the Hopf cyclization of a hexa-1,3-diene-5yne (dienyne) to a new arene.(b) Illustration of the light-promoted topochemical polymerization of a diphenyl diacetylene into a polydiacetylene (PDA), and subsequent Hopf cyclization cascade and dehydrogenation to afford [8] A GNR.A chrysene subunit in [8] A GNR is shown in cyan color.(c) Two on-surface cyclization modes for cis-enediynes.(d) Illustration of the nonplanar transition state geometry in the Hopf 6π electrocyclization step.(e) Two consecutive Hopf cyclizations of enediyne 1a to naphthalene 2 and chrysene 3.

Figure 3 .
Figure 3. Mechanism of the Hopf cyclization of 1a to afford naphthalene 2 and chrysene 3, illustrated for a gold slab model and in the gas phase.
distance between carbon atoms C 1 and C6 is progressively restricted to shorter values (Figure5and S6A).The energy for each stepwise C 1•••C 6 distance constriction to intermediate 4c is shown in purple circles in Figure 5a.Gold-catalysis begins upon constriction of the C 1 •••C 6 distance in the MEP calculations, upon which carbon C 2 of one of the C�C bonds in enediyne 1a starts interacting with the gold atom Au 138 .Upon further constriction of the C 1 •••C 6 distance from 2.2 to 2.1 Å, a sudden contraction of the Au 138 •••C 2 distance occurs, changing from 3.145 to 2.311 Å.At this stage, a formal gold−carbon bond has essentially formed, represented by intermediate structure 4a (Figures 3, 5a, and S6A).However, this intermediate structure is not yet a transition state but can be viewed as a transient goldalkyne complex whose C 1 �C 2 alkyne moiety has reached substantial bonding through bending (∠C 1 −C 2 −C 3 = 133.0°,∠C 3 −C 2 −C 20 = 142.7°).For reference, several crystal structures of gold(I) η 2 -complexes with open-chain or cyclic alkynes have been reported.

Figure 4 .
Figure 4. (a) DFT optimized ground-state geometries for 1a, 2, and 3 on a Au(111) slab model.(b) In the same order, experimentally observed nc-AFM images (left) and corresponding simulated AFM images (right).White scale bars are 0.5 nm.

Figure 5 .
Figure 5. MEP calculation of the first 6π-electrocyclization steps and Hirshfeld charge analysis.(a) Changes in the bond length (Å) between C 2 −Au 138 (dark gray squares), C 1 −C 2 (golden brown squares), C 2 −C 3 (mauve triangles), and free energies (eV) at each step of the MEP calculation starting from enediyne 1a, forming gold-allene complex 4c via the transition state 4b.Bond lengths for C 1 −C 2 and C 2 −C 3 are plotted twice their value for clarity.(b) Top view of the geometry of cyclization intermediate 4c calculated at the PBE level of theory on a gold slab (top); side view with Au 138 detaching from the surface of the gold slab (bottom).The color-coded spatially resolved charge values for carbons C 1 −C 30 and Au atoms obtained from a Hirshfeld charge population analysis are also shown.(c, d) Charge deviation from the average population (avg) for carbon and first-layer Au atoms.The avg is 4.01773 electrons for carbon and 11.03194 for first-layer gold.(e) Calculated partial density of states (PDOS) for carbon atom C 2 (gray) and all others (maroon); arrows indicate new states for carbon atom C 2 .(f) Calculated PDOS for gold surface atom Au 138 (gray) and bulk gold atoms (mauve); arrows imply new states for the gold surface atom Au 138 that has formed a new bond to C 2 .

Table 1 .
Calculated Relative Free Energies of the Intermediates in the Hopf Cyclization of Enediyne 1a to Naphthalene 2 and Chrysene 3 on a Au(111) Slab Model and in the Gas Phase a Free energy values are in kcal/mol.b Values shown in bold italic are the transition states.c Rate-determining barrier.