Bonding Structure, Dehydrogenation and Dimerization of 1,3-C6H4

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Bonding Structure, Dehydrogenation, and Dimerization of 1,3-C6H4 from Decomposition of 1,3-C6H4I2 on Cu(100) Yung-Hsuan Liao, Yi-Shiue Lin, Tz-Shiuan Wu, Shu-Kuan Lin, and Jong-Liang Lin* Department of Chemistry, National Cheng Kung University 1, Ta Hsueh Road, Tainan, Taiwan, Repubic of China

Liang-Jen Fan and Yaw-Wen Yang* National Synchrotron Radiation Research Center 101, Hsin-Ann Road, Hsinchu, Taiwan, Repubic of China

Jiing-Chyuan Lin Institute of Atomic and Molecular Sciences Academia, Sinica P.O. Box 23-166, Taipei, Taiwan, Repubic of China ABSTRACT: Temperature-programmed reaction/desorption, Auger electron spectroscopy, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure in combination of calculations based on density functional theory have been employed to investigate adsorption and reaction of 1,3-C6H4I2 on Cu(100). At 100 K, the surface species after 1,3-C6H4I2 adsorption are found to be 1,3-C6H4I2, C6H4I, and 1,3-C6H4. The formation of these adsorbates is dependent on the adsorption sites of 1,3-C6H4I2. 1,3-C6H4I2 adsorbed with the ring at a hollow site and parallel to the surface is predicted to be unstable and preferentially leads to C
I bond dissociation. 1,3-C6H4, the intermediate from 1,3-C6H4I2 decomposition, has a tilted adsorption geometry with a distorted ring. H2 is the only reaction product observed after 550 K in the 1,3-C6H4I2 decomposition on Cu(100), with all of the carbon atoms left on the surface. Dimerization of 1,3-C6H4 molecules on Cu(100) has been described computationally, showing an activated and exothermic process. With the theoretically obtained activation energy of 28.2 kcal/mol and estimated surface coverages, coupling of 1,3-C6H4 can occur by second-order kinetics before H2 evolution. Dimerization of 1,3-C6H4 on Cu(100) shows a different intermolecular interaction behavior from those of 1,2-C6H4 and 1,4-C6H4 on copper single crystal surfaces.

’ INTRODUCTION Benzynes (C6H4), reactive intermediates, have useful applications in organic synthesis and can be found in the combustion of gasoline containing small aromatics, such as benzene and toluene.1,2 Poly(phenylene)s, with C6H4 repeating units, are an important class of conducting polymers.3,4 Materials with phenylene (C6H4) or substituted phenylene may have unique properties, such as ferroelectricity and outstanding toughness.5,6 The ortho-phenylene (o-C6H4 or 1,2-C6H4) has been found to be bonded to the metal centers of HOs3(CO)8(μ-C6H4)(μ-C23H19O3P2).7 On transition metal surfaces, although there have been extensive investigations for six-membered aromatic species, the studies for C6H4 are few, mainly emphasizing on scanning tunneling microscopy (STM).8
13 The para-phenylene (p-C6H4 or 1,4-C6H4) fragments produced on Cu(111) by p-dihalobenzene decomposition have been observed by STM.9,10,13 The para-C6H4 species can align on Cu(111) to form the so-called protopolymer in which the monomers are not chemically bonded to each other as in poly(para-phenylene).10 The STM images of m-C6H4/ Cu(111) and o-C6H4/Cu(100) and the bonding structures of o-, m-, and p-C6H4/Cu(111) and o-C6H4/Cu(100) obtained from density functional calculations have been compared and reported r 2011 American Chemical Society

previously.9,11 However, in these previous C6H4 studies, using halogenated compounds as precursors for generating phenylene groups on the surfaces, carbon
halogen bond dissociation processes were not fully characterized and thermal stability of phenylenes was not explored. In the present study, we investigate the bonding structure and thermal chemistry of m-C6H4 from decomposition of 1,3-C6H4I2 by C
I scission on Cu(100), using temperature-programmed reaction/desorption (TPR/D), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) measurements, with the aid of density functional theory calculations. On single crystal copper surfaces, cyclization of ethyne and propyne to form benzene has been reported.14
16 Therefore, the possibility of forming small hydrocarbons, such as ethyne, ethylene, or propyne, from ring rupture of m-C6H4 on Cu(100) is worth studying. Although p-C6H4 can form protopolymer chains on Cu(111),10 direct interaction between o-C6H4 species on Cu(100) has been claimed to be negligible.11 Therefore, the Received: August 8, 2011 Revised: September 29, 2011 Published: October 08, 2011 23428

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interaction and binding strength between m-C6H4 species become another intriguing research topic.

’ EXPERIMENTAL SECTION All TPR/D and AES experiments were performed in an ultrahighvacuum (UHV) apparatus equipped with an ion gun for sputtering, a differentially pumped mass spectrometer for TPR/D and a cylindrical mirror analyzer for Auger electron spectroscopy. The chamber was evacuated by a turbomolecular pump and an ion pump to a base pressure of approximately 2  10
10 Torr. The quadrupole mass spectrometer used for TPR/D studies was capable of detecting ions in the 1
300 amu range and of being multiplexed to acquire up to 15 different masses simultaneously in a single desorption experiment. In TPR/D experiments, the Cu(100) surface was positioned ∼1 mm from an aperture, 3 mm in diameter, leading to the mass spectrometer and a heating rate of 2 K/s was used. The Cu(100) single crystal (1 cm in diameter) was mounted on a resistive heating element and could be cooled with liquid nitrogen to 110 K and heated to 1100 K. The surface temperature was measured by a chromel
alumel thermocouple inserted into a hole on the edge of the crystal. Cleaning of the surface by cycles of Ar+ ion sputtering and annealing was done prior to each experiment until no impurities were detected by Auger electron spectroscopy. Ar (99.9999%) was obtained from Matheson. 1,3-C6H4I2 (>98%), purchased from Aldrich, was subjected to several cycles of freeze
pump
thaw before introduction of its vapor into the vacuum chamber. Photoemission measurements were carried out at the wide range spherical grating monochromator beamline (WR-SGM) at the National Synchrotron Radiation Research Center of ROC. Total instrumental resolution, including the beamline and energy analyzer, was estimated to be better than 0.3 eV. The photoelectrons were collected at an angle of 50° from the normal surface. All of the XPS spectra presented here were first normalized to the photon flux by dividing the recorded XPS signal by the photocurrent derived from a gold mesh situated in the beamline. The binding energy scale in all of the spectra was referenced to a well-resolved spin
orbit component of the bulk Cu2p3/2 peak at 75.10 eV. The size of X-ray beam used was approximately 2  2 mm2. In the study of 1,3-C6H4I2 decomposition as a function of temperature, the spectra were measured at different positions on Cu(100), which were obtained by moving the crystal. The 620 eV photon energy was used for iodine measurement. The X-ray photoelectron spectra obtained were fitted with Gaussian
Lorentzian functions based on a nonlinear least-squares algorithm after Shirley background subtraction. Polarization-dependent carbon K-edge NEXAFS measurements were performed on the basis of total electron yield (TEY) method with a current amplifier measuring the drain current. The photon energy scale was calibrated against an intense 1s f π* transition of the HOPG graphite located at 285.38 eV. The spectra were first normalized to the photon flux, obtained by measuring the current of freshly evaporated gold mesh situated in front of the Cu(100) surface, to form a so-called I0-normalized spectrum. The X-ray absorption feature of the substrate was eliminated by dividing the I0-normalized TEY spectrum of an adsorbate-covered Cu(100) surface by the I0-normalized TEY spectrum of a clean Cu(100) surface, with the spectra measured separately but under the same photon incident conditions. All of the spectra presented were mathematically treated in the same way. In our theoretical cluster-model calculations for the bonding geometries and reaction paths, two slabs with a total of 41 or 59

Figure 1. Temperature-programmed desorption spectra of 1,3-C6H4I2 on Cu(100) at the indicated exposures. The inset shows the relative desorption yield as a function of exposure.

Cu atoms fixed at their lattice positions were used. All of the calculations were performed in the framework of density functional theory using the program package Cerius2-DMol3, in which generalized gradient approximation proposed by Perdew and Wang (GGA-PW91) was employed for the exchange-correlation functional. A double-numeric quality basis set with polarization functional (DNP) was used for the all electron calculations including relativistic effect for the core electrons. The geometry optimization convergence threshold for energy change, maximum force and maximum displacement between optimization cycles were 0.00001 Ha, 0.001 Ha/Å, and 0.0005 Å, respectively. In the reaction path calculations, the preliminary transition states for the surface reactions investigated were obtained using the integrated linear synchronous transit/quadratic synchronous transit method, followed by TS optimization. The confirmation of the pathway connecting the reactant, transition state, and product was achieved using intrinsic reaction path calculation, based on the nudged elastic band algorithm.

’ RESULTS AND DISCUSSION Adsorption and C
I Bond Dissociation of 1,3-C6H4I2 on Cu(100). To understand the adsorption and thermal stability of

1,3-diiodobenzene on Cu(100), the temperature-programmed desorption is studied first. Figure 1 shows the TPD spectra, represented by C6H4I+ (m/z 203), after adsorption of 1,3-C6H4I2 at the indicated exposures. The detected ion is possibly due to C6H4I or C6H4I2 desorption. The parent ion of C6H4I2 exceeds the detection limit (300 amu) of our mass spectrometer. At an exposure less than 3 L, no C6H4I or C6H4I2 molecules desorbing from the surface are measured. From 5 to 20 L, desorption states emerge between 210 and 280 K. In the cases of 5 and 7.5 L, the desorption is peaked around 250 K. At 10 L, this desorption state becomes a shoulder and the main peak appears at 233 K. This peak is shifted to 240 K at 20 L. In the inset showing the relative desorption yield against exposure, it is found that the desorption represented by m/z 203 starts to appear at ∼3.0 L and then 23429

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Figure 3. Relative I4d peak area as a function of temperature. The lines through the data points are drawn only to guide the eyes.

Figure 2. Temperature-dependent I4d X-ray photoelectron spectra of 1 L 1,3-C6H4I2 on Cu(100). All of the spectra were measured at ∼100 K after the 1,3-C6H4I2-covered surface was briefly heated to the temperatures indicated.

the yield increases linearly with exposure, which is typical for molecular desorption from multilayers on a surface. Therefore, the detected C6H4I+ ion is attributed to 1,3-C6H4I2 desorption. This result also suggests that saturation of the first adsorption layer (monolayer) of 1,3-C6H4I2 on Cu(100) occurs at ∼3.0 L. On the basis of the exposure required to form a monolayer, the desorption feature at ∼250 K is attributed to second-layer 1,3C6H4I2 molecules. The 233 K peak observed in the 10 L trace can be ascribed to the desorption mainly from the third-layer molecules and is 17 K lower than the desorption from the second layer. It is also found that the leading edges of the 10 and 20 L desorption traces overlap. This result, together with the peak temperature shift from 233 to 240 K, indicates that the desorption of 1,3-C6H4I2 molecules in the third and higher layers follows a zero-order kinetics.17 Because no C6H4I2 desorption is measured below 3 L, the first-layer molecules are subjected to thermal decomposition. The previous study of iodobenzene on Cu(111) has shown that the C
I bond in this molecule cleaves at ∼175 K to produce phenyl groups (C6H5), which are stable up to ∼300 K. Above 300 K, biphenyl is generated by phenyl coupling and desorbs promptly from the surface.18 The primary reaction path of C6H5I on Cu(111) is C
I bond scission (∼175 K), with the stronger C
H bonds remaining intact. X-ray photoelectron spectroscopy has been employed to monitor the decomposition of 1,3-C6H4I2 on Cu(100) via C
I bond rupture. Figure 2 shows the I4d binding energy changing with temperature after adsorption of 1 L 1,3-C6H4I2 on Cu(100) at 100 K. We start the analysis from the high temperature spectra, because they are well-resolved. In the 370 K spectrum, the peaks of I4d3/2 at 51.3 eV and I4d5/2 at 49.6 eV are ascribed to I adsorbed on Cu(100) in accordance with previously reported assignments.19 This result, together with the absence of other iodine signals,

also indicates that both C
I bonds in 1,3-C6H4I2 have dissociated on Cu(100) at 370 K, leaving atomic iodine on the surface. The 230, 290, and 320 K spectra can be nicely fitted with four peaks. Two of them are due to adsorbed iodine atoms. The others are at 50.5 and 52.2 eV, signifying the existence of another iodine-containing species in this temperature range. Because no desorption products evolve prior to 500 K (shown later in the TPR/D study), this species is likely to be C6H4I from C
I bond scission of 1,3-C6H4I2 or the parent molecule itself. The major difference among the spectra of 230, 290, and 320 K is the relative intensity of 49.6 to 50.5 eV (or 51.3 to 52.2 eV). This ratio increases from 230 to 320 K. In the curve-fitting procedure for the 100 and 170 K spectra, six deconvoluted peaks are used, which are at 49.7, 51.4, 50.6, 52.3, 50.9, and 52.6 eV. The first four positions have been observed in the temperature range of 230
290 K. The appearance of the latter two peaks reveals the presence of a third iodine-containing compound on the surface. Figure 3 shows the iodine photoelectron integrated intensities of the three different iodine-containing species as a function of surface temperature. Surface atomic iodine (49.7 and 51.4 eV), which is from C
I bond scission of 1,3-C6H4I2, grows continuously. The species responsible for the peaks of 50.9 and 52.6 eV, as a contrast, decreases with increasing temperature and is no longer observed at 230 K. In the curve for the peaks at 50.6 and 52.3 eV, the intensity reaches the maximum at ∼230 K and then declines to zero at ∼330 K. Considering a stepwise dissociation for the two C
I bonds of 1,3-C6H4I2, the iodine peaks of 50.9 and 52.6 eV can be attributed to 1,3-C6H4I2. Its decomposition by one C
I bond breakage generates C6H4I, which has the I4d binding energy peaks of 50.6 and 52.3 eV. C6H4I seems relatively more stable than 1,3-C6H4I2 on Cu(100), with the evidence that it accumulates and reaches the maximum surface concentration at 230 K and is still barely observable at 320 K. C6H4I also dissociates by breaking C
I bond, leading to the growth of surface atomic iodine. However, the sequential reaction steps, C6H4I2 f C6H4I + I f C6H4 + 2I, are not the only C
I dissociation pathway of 1,3-C6H4I2. If the reaction proceeds step-by-step, with the intermediate C6H4I remaining intact at 100 K, then the I4d signals of C6H4I and atomic I in the 100 K spectrum would have a similar intensity. But this is not the case, the atomic iodine has much larger peaks, strongly suggesting that 1,3-C6H4I2 can dissociate by breaking both the C
I bonds simultaneously. 23430

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Figure 4. Changes in structure of 1,3-C6H4I2 on Cu(100) after energy-minimizing calculations for the initial adsorption states. Top- and side-view structures are provided.

This reaction process is further supported by DFT calculations. Theoretically in search of the optimized structure of 1,3-C6H4I2 on Cu(100), it is found that this molecule on Cu(100) may exist molecularly and in the dissociative forms of C6H4I and C6H4, which are related to the adsorption sites of 1,3-C6H4I2 on the surface. Effect of Adsorption Site on the C
I Bond Dissociation of 1,3-C6H4I2 on Cu(100). Figure 4 shows the changes in the 1,3C6H4I2 structure from the preset initial adsorption states after performing the energy-minimizing procedure computationally. In this process for obtaining the optimized structures, all of the coordinates, including the bond lengths and angles and the heights and positions of atoms relative to the surface were allowed to be varied. The structure of a single 1,3-C6H4I2 molecule was calculated first separately and then was spaced over the surface, with the ring parallel to the surface and the ring center around the hollow (Figure 4a,b), bridge (Figure 4c), or atop (Figure 4d) site as the initial states. The distances between the iodine atoms and the copper surface were initially set to be 3.4 Å, a sum of van der Waals radii of iodine and copper atoms. As shown in Figure 4a,

both C
I bonds of the initial 1,3-C6H4I2 dissociate after structure optimization. The C
I bonds are elongated from 2.15 Å to 4.478 and 4.537 Å in the final structure, with a tilted and distorted C6H4 ring and iodine atoms being adsorbed at hollows sites. In the initial state of Figure 4b, the ring is also adsorbed at a hollow site but is rotated by ∼120° with respect to the initial structure of 1,3-C6H4I2 in Figure 4a. Interestingly, the energy-minimizing procedure leads to the formation of C6H4I; i.e., only one C
I bond breaks, instead of both. In the final state, the C
I bond length of C6H4I, also with a tilted and distorted ring structure, slightly increases to 2.30 Å, about 7% increment as compared to that of 1,3-C6H4I2. However, as the 1,3-C6H4I2 molecule is adsorbed at bridge site (Figure 4c), no C
I bond dissociation is observed. In the final, optimized adsorption structure of the 1,3C6H4I2, the two C
I bonds are only slightly elongated to 2.28 and 2.30 Å and the whole molecule is tilted over the surface, with both iodine atoms attaching to the surface. In Figure 4d, with the ring of 1,3-C6H4I2 at atop site as the initial state, the energyminimizing calculation also results in structurally distorted 23431

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Figure 5. Temperature-programmed reaction/desorption spectra of 1,3-C6H4I2 on Cu(100), represented by m/z 2 for H2 formation, at the indicated exposures. The inset shows the relative hydrogen desorption yield vs exposure.

1,3-C6H4I2 in the final state, with both C
I bond lengths being increased to 2.34 Å. The optimized 1,3-C6H4I2 molecule, similar to the case of Figure 4c, has a tilted adsorption geometry, with both iodine atoms directly in contact to the surface. These theoretical results indicate that the adsorption site of 1,3-C6H4I2 can affect the C
I bond dissociation and that the dissociation of both C
I bonds of 1,3-C6H4I2, upon its adsorption on Cu(100), is possible. Thermal Decomposition and Bonding Structure of 1,3C6H4 on Cu(100). In the TPR/D experiments of 1,3-C6H4I2/ Cu(100), a mass survey from 1 to 254 amu in search of gaseous, carbon-containing products has been performed. The mass of biphenyl ion is (C12H10+) 254 amu. However, none of hydrocarbons was detected, but H2. In Figure 5 showing the TPR/D spectra of hydrogen, it is found that H2 starts to desorb at approximately 550 K and extends up to ∼900 K. This result suggests that no H atoms are present on the surface before 550 K, i.e., the C
H bonds remaining intact before 550 K, because it has been known that recombination of H atoms on Cu(100), forming H2, occurs around 300 K.19 After considering that the two C
I bonds of 1,3-C6H4I2 have dissociated at 370 K, as shown in Figure 2, C6H4 is suggested to be the species present on the surface at this temperature. In the inset of Figure 5, the yield of H2 desorption increases linearly with exposure of 1,3C6H4I2 and levels off as the exposure is approximately above 3.0 L. Figure 6 shows the Auger electron spectra obtained after adsorption of 3 L 1,3-C6H4I2 on Cu(100) at 180 K, followed by briefly heating the surface to 450 and 980 K. Both the carbon (∼276 eV) and iodine (∼512 and 521 eV) peaks are detected at 450 K. The carbon is ascribed to the presence of C6H4 from C
I bond breakage of 1,3-C6H4I2. Increasing the surface temperature to 980 K results in the disappearance of iodine, but with the

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Figure 6. Carbon and iodine Auger electron spectra obtained after adsorption of 3 L 1,3-C6H4I2 on Cu(100) at 180 K, followed by briefly heating the surface to 450 and 980 K.

Figure 7. Theoretically predicted 1,3-C6H4 structure on Cu(100).

carbon unchanged. Note that raising the surface temperature to 980 K also causes hydrogen desorption. Basically, the unreduced carbon Auger intensity observed after H2 desorption suggests that no hydrocarbons are formed and desorbed in the decomposition of C6H4, which is consistent with the TPR/D observation. The optimized adsorption structure of a single 1,3-C6H4 molecule on Cu(100) predicted by DFT calculations is shown in Figure 7 with three different viewing angles. The phenylene group is not in a form of perfect geometric plane. The six C
C bond lengths of the 1,3-C6H4 are different, ranging from 1.404 to 1.449 Å. Although the calculated 1,3-C6H4/Cu(100) shows a distorted ring, no relative shrinkage of the C1
C3 distance is found. That means there is no evidence for the formation of the so-called bicyclic structure.1,2 Note that for a benzene molecule, the C
C bond length is 1.397 Å.20 The distance between C2 and C3 (1.449 Å), the largest among the C
C bonds of the 1,3-C6H4, 23432

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Figure 9. Theoretically predicated H4C6
C6H4 structure on Cu(100). Figure 8. Angle-dependent NEXAFS spectra for 5 L 1,3-C6H4I2 on Cu(100) at 320 K. The inset shows the relative peak height vs incidence angle and the fitting curves.

is elongated by 3.7% in terms of the benzene bond length. The unsaturated C1 of the 1,3-C6H4 is bonded at atop site, whereas another unsaturated carbon (C3) is located near a bridge site. The distances of the C1 and C3 atoms to the copper surface are 2.117 and 1.673 Å, respectively. The C2
H bond is slightly longer than the other three by ∼0.024 Å (approximately 2%). As shown in Figure 7, the tilted angle of the distorted phenylene plane with respect to the surface is estimated to be approximately 28°. This inclination angle is only calculated for a single, isolated molecule, which can be affected by molecular interaction in real adsorption systems. The tilting of phenylene plane of 1,3-C6H4 on Cu(100) has been roughly measured by angle-dependent near-edge X-ray absorption fine structure method. Figure 8 shows the NEXAFS spectra taken at the indicated incidence angles for a 1,3-C6H4 covered Cu(100) surface. The surface was prepared by adsorption of 5 L 1,3-C6H4I2 at 100 K, followed by heating the surface to 320 K. This method has been employed to determine the average inclination angle of C6 rings from surfaces for adsorbed benzene, halobenzenes, and phenyl in terms of the angle-dependent excitation of the carbon 1s electrons to the π* orbitals which corresponds approximately to a photon energy of ∼285 eV.21,22 All of the spectra in Figure 8 show intense C1s-π* NEXAFS transition peaks for 1,3-C6H4 on Cu(100), and the inset shows the relative peak intensity as a function of light incidence angle and three fitting curves obtained by using the reported equation: Iπ(θ,α) ≈ P(sin2 α sin2 θ + 2 cos2 α cos2 θ) + (1
P) sin2 α, where I is the normalized peak intensity, θ is light incidence angle, α is angle of the C6 ring relative to the surface, and P is the degree of linear polarization of the synchrotron radiation.21 On the basis of the fitting result, it can be roughly estimated that the tilted angle of 1,3-C6H4 on Cu(100) is at 40 ( 5°. However, this experimental angle is an average result in the sense that all of the adsorbed 1,3-C6H4 molecules may not have the same angle and the molecule itself is not planar.21 Theoretical Study of Dimerization of 1,3-C6H4 on Cu(100). In the theoretical coupling study of 1,3-C6H4 on Cu(100), the optimized structure of the dimer (C12H8) is obtained first and used as the final state in the calculation of the reaction path. Figure 9 shows the adsorption geometry of the dimer molecule on Cu(100). The C
C bond connecting the two C6H4 groups has a length of 1.479 Å, typical for a carbon
carbon σ bond.

Figure 10. Calculated potential energy vs reaction coordinate in the path of dimerization of 1,3-C6H4 on Cu(100) (a) and the structure of the transition state (b).

The two C6 rings in the dimer are similar in structure in terms of bonding site, ring geometry, and tilting over the surface. The two unsaturated carbons in the dimer are bonded near atop sites of Cu(100) and their heights to the surface are close (1.809 and 1.836 Å). The two distorted C6 rings have a similar inclination angle of ∼50°. In addition, the C
C bond lengths of the two0 0 rings seem symmetric and the lengths of C4
C5, C5
C6, C4
C5 , 0 0 and C5
C6 are relatively smaller. Figure 10 shows the potential energy vs reaction coordinate of the coupling path of 1,3-C6H4 on Cu(100), with the structure of the transition state. The initial state is assumed to be two optimized 23433

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atop or at the bridge site. No hydrocarbon product is observed in the decomposition of 1,3-C6H4I2 on Cu(100). Only H2 desorbs after 550 K, with the carbon atoms left on the surface. Coupling of two 1,3-C6H4 groups on Cu(100) is an activated and exothermic process, and cannot be ruled out in the reaction of 1,3-C6H4I2 on Cu(100).

’ AUTHOR INFORMATION Corresponding Author

1,3-C6H4 molecules, separated by 5 Å. The coupling process is exothermic, with a reaction energy of 73.5 kcal/mol; i.e., the dimer is more stable than two 1,3-C6H4 molecules. But, the reaction is predicted to be activated. The change from the initial state to transition state requires 28.2 kcal/mol. On the basis of the activation energy (Ea), the coupling temperature of 1,3-C6H4 species on Cu(100) in temperature-programmed reaction/desorption can be estimated, using Ea/RTp2 = (2 Npν/β)e
Ea/RTp, where R: gas constant, ν: preexponential factor; β: heating rate (2 K/s); Tp: the temperature of maximum reaction rate; Np: surface concentration at Tp.23ν for second-order surface reactions is typically assumed to be 10
2
10
3 cm2/s.24,25 The maximum surface concentration of 1,3-C6H4 from decomposition of 1,3-C6H4I2 on Cu(100) can be roughly obtained by using estimated molecular and atomic cross-section areas, with consideration of van der Walls interaction. According to the structure of Figure 7, the surface area occupied by a 1,3-C6H4 group is approximately 3.94  10
15 cm2 and two iodine atoms have a total area of 2.51  10
15 cm2 (van der Waals radius of iodine = ∼2.0 Å). Therefore, there is approximately 1.6  1014 1,3-C6H4 groups/cm2 in a closely packed monolayer containing 1,3-C6H4 and I. As Np is assumed to be half of this number, the Tp is calculated to be 471 and 507 K at ν = 10
2 cm2/s and 10
3 cm2/s, respectively. Note that these temperatures are lower than the desorption temperature of H2 as shown in Figure 5. The second-order preexponential factor (ν) of 10
2
10
3 cm2/s is reported for recombination of atoms or small surface groups.24,25 The ν for coupling 1,3-C6H4 may be smaller due to steric effect that would increase the recombination temperature of 1,3-C6H4; however, the possibility for dimerization of 1,3-C6H4 on Cu(100) cannot be ruled out. Because in our TPR/D study there is no hydrocarbon evolution, except H2, the C12H8 dimers, if they are formed, must undergo complete dehydrogenation on Cu(100). The coupling of 1,3-C6H4 groups on Cu(100) to form C12H8 dimers is different from the interaction of 1,4-C6H4 on Cu(111), generating C6H4 chains. Besides, 1,2-C6H4 groups on Cu(100) have been reported to have a negligible interaction.10,11 The thermal reaction sequence of 1,3-C6H4I2 on Cu(100) is summarized in Scheme 1. At 100 K, there are three species (1,3C6H4I2, C6H4I, and 1,3-C6H4) present on the surface after 1,3C6H4I2 adsorption on Cu(100). 1,3-C6H4 is generated when both the C
I bonds of 1,3-C6H4I2 dissociate, with the C
H bonds remaining intact. The adsorbed 1,3-C6H4 may recombine as a C12H8 dimer, before it decomposes to form H2.

’ CONCLUSIONS Our experimental and theoretical results indicate that 1,3C6H4I2 molecules are adsorbed molecularly and dissociatively on Cu(100) at 100 K. It dissociates primarily by C
I bond rupture, which depends on adsorption site. As a 1,3-C6H4I2 molecule is adsorbed with the ring parallel to the surface and at a hollow site, the C
I bond is easier to break, as compared to the molecule

*J.-L.L.: phone, 886-6-2757575 ext. 65326; fax: 886-6-2740552; e-mail: [email protected]. Y.-W.Y.: e-mail, [email protected].

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