And 5-Member Ring Aromatic Compounds on the Si(001)-2 6 1 Surface: Correlation Between Binding Energy and Resonance Energy

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Theoretical study of [4 z 2] cycloadditions of some 6- and 5-member ring aromatic compounds on the Si(001)-2 6 1 surface: correlation between binding energy and resonance energy

Xin Lu,*a M. C. Lin,b Xin Xu,a Nanqin Wanga and Qianer Zhanga Paper aState Key Laboratory for Physical Chemistry of Solid Surfaces & Department of Chemistry, Xiamen University, Xiamen, 361005, China. E-mail: [email protected]; Fax: 86 592 2183047; Tel: 86 592 2181600 bCherry L. Emerson Center for Scientiﬁc Computation & Department of Chemistry, Emory University, Atlanta, GA 30322, USA. E-mail: [email protected]; Fax: 1 404 727 6586; Tel: 1 404 727 2825

Received 14th May 2001, Accepted 18th June 2001 Published on the Web 21st June 2001First published as an Advanced Article on the web 25th August 2000

By means of ﬁrst-principles density functional cluster model calculations, we demonstrate that the binding energies of the [4 z 2] cycloaddition products of the 6- and 5-member ring aromatic compounds on the Si(001) surface depend strongly on their resonance energies.

Introduction aromatic compounds and that the binding energies of the corresponding [4 z 2] cycloaddition products on the Si(001)- The interaction of p-conjugated aromatic compounds with 2 6 1 surface would show strong dependence on the resonance silicon surfaces is not only of fundamental interest, but also of energies of the aromatic compounds. practical importance to the development of highly ordered In Scheme 1(a), the [4 z 2] cycloaddition process is divided thin-ﬁlm of conducting polymers.1,2 Growing efforts devoted into two ideal steps. First, the p-conjugated benzene is ideally to this topic include the experimental studies of the adsorption localized into a nonaromatic analog, i.e., 1,3,5-cyclohexatriene. 3 4 5 of benzene (C6H6), pyridine (C5H5N), furan (C4H4O), The energy cost (DE1) in this step should correspond to the 5,6 5b 10 thiophene (C4H4S) and pyrrole (C4H5N) on the Si(111) so-called resonance energy (RE) of benzene. Second, the surfaces and the adsorption of benzene7 and pyrrole8 on the nonaromatic analog undergoes [4 z 2] cycloaddition onto a Si(001)-2 6 1 surface, as well as the theoretical studies of SiLSi dimer. This step is exothermic with a reaction heat (DE2) benzene adsorption9 on the Si(001)-2 6 1 surface. Of parti- close to the heat of the analogous reaction of 1,3-cyclohexa- cular interest is that recent experimental and theoretical studies diene on the same surface site. Therefore, the difference in revealed the occurrence of [4 z 2] cycloaddition (Diels–Alder binding energy between the [4 z 2] cycloaddition products of reaction) of benzene onto the p-bonded, ethylene-like SiLSi benzene and 1,3-cyclohexadiene should have a value close to dimer of the Si(001)-2 6 1 surface7,9 and that the binding the difference in RE between benzene and 1,3-cyclohexadiene. energy of the thus-formed di-s bonded, butterﬂy-like adspecies This is indeed the case, as previous B3LYP/6-31G** cluster is far lower than that of the analogous reaction of simple dienes model calculations9b did predict a difference of 32.6 kcal mol21 7a,9b [e.g., 1,3-cyclohexadiene (C6H8)] on the same surface. in binding energy between benzene and 1,3-cyclohexadiene, This intriguing phenomenon can be ascribed to the aromatic which is comparable to the difference (ca. 28.3 kcal mol21)10,11 stabilization in benzene, and can easily be understood with the in RE between benzene and 1,3-cyclohexadiene. help of a simple energy decomposition scheme that correlates Scheme 1(a) can be generalized to the cases of other the binding energy of the [4 z 2] cycloaddition product 6-member ring, aromatic compounds containing one (or more) with the resonance energy (RE) of benzene, as shown in heteroatom(s) and to those of the 5-member ring, aromatic Scheme 1(a). We will show in this communication that this compounds depicted in Scheme 1(b). For example, for a series simple scheme is applicable to other 6-, and 5-member ring of 6-member ring compounds listed in Scheme 1(a), their nonaromatic analogs contain the same localized dienes as has the 1,3-cyclohexadiene and would have similar values of DE2 when undergoing [4 z 2] addition on the Si(001) surface. Note that among them, benzene has the largest value of RE.10a It is thus deducible that the [4 z 2] cycloaddition reactions of the heterocyclic ones would be more exothermic than that of benzene and would follow such a trend that the compound having higher RE would give lower binding energy. A similar trend could also be expected for the 5-member ring compounds listed in Scheme 1(b).

Computational method and model Scheme 1 Energy decomposition schemes for the adsorption of 6- and 5-member ring, aromatic compounds on a SiLSi dimer site of To conﬁrm the above inference, we have investigated the the Si(001)-2 6 1 surface. [4 z 2] cycloaddition of ﬁve 6-member ring compounds,

DOI: 10.1039/b104187h PhysChemComm, 2001, 13, 1–3 1 This journal is # The Royal Society of Chemistry 2001 Table 1 Key structural and energetic parameters for the products of [4 z 2] cycloaddition of some 6 or 5-member rings, aromatic compounds on the Si(001)-2 6 1 surface (predicted at the B3LYP/6-31G* level). Resonance energies (RE) of the aromatic compounds extracted from ref. 10 are listed

6-Member ring 5-Member ring

C6H6 C5H5PC5H5NC5H6Si C4H4N2 C6H8 C4H5NC4H4SC4H4OC5H6

C–Si/A˚ 1.976 1.979 1.977 1.978 1.965 1.953 2.010 1.965 1.993 1.965 1.976 1.954a 1.973a 1.958a 1.965 1.953 Si–Si-C/u 96.2 97.2 95.6 98.3 94.8 96.6 89.2 92.5 88.5 90.4 96.2 98.3a 95.3a 99.9a 94.8 96.6 21 Eb/kcal mol 221.8 229.0 225.5 226.3 229.6 256.2 225.2 236.0 230.8 252.0 RE/kcal mol21 28.3 26.0 25.6 25.3 22.7 0.0 22.5 16.5 12.1 0.0 aParameters relevant to the carbon atom neighboring the heteroatom.

including benzene, pyridine, silabenzene (C5H6Si), phospha- also found for the 5-member ring compounds, except that the benzene (C5H5P), pyrazine (C4H4N2) and 1,3-cyclohexadiene, more aromatic thiophene gives larger binding energy than does and four 5-member ring compounds, including pyrrole, thio- the less aromatic furan. phene, furan and cyclopentadiene (C5H6), onto a p-bonded SiLSi The calculated binding energy was plotted as a function of 9c dimer site of the Si(001) surface modeled by a Si9H12 cluster. the corresponding resonance energy, as depicted in Fig. 1. 12 The hybrid B3LYP density functional method and the The correlation equations, Eb ~ 1.17 6 RE – 56.3 for the 13 standard 6-31G* basis set were used in our calculations. 6-member ring compounds and Eb ~ 1.12 6 RE – 50.4 for the Adsorption geometries were optimized at the B3LYP/6-31G* 5-member ring compounds, obtained by linear fitting demons- level of theory with no geometric constraint. All calculations trate the nearly linear dependence of the binding energy of the were performed using the Gaussian94 program.14 [4 z 2] cycloaddition adspecies on the resonance energy of those compounds. The appreciable deviations at phosphaben- Results and discussion zene for the 6-member ring compounds, and at furan and thiophene for the 5-member ring compounds, reflect the rather The calculated key structural and energetic parameters for the different ring strains of those compounds from those of other [4 z 2] addition products of those 6- and 5-member ring compounds of the same lines. compounds on the SiLSi dimer sites are listed in Table 1, In summary, we have demonstrated, by means of density together with the theoretical resonance energies reported by functional cluster model calculations, that the binding energies Dewar et al.10a Our B3LYP/6-31G* calculations reveal that: of the [4 z 2] cycloaddition products of some 6- and (i) The [4 z 2] cycloaddition process on the Si(001)-261 5-member-ring, aromatic compounds on the Si(001) surface surface is exothermic for all the 6- and 5-member ring depend strongly on their resonance energies. We should note compounds concerned. The products are di-s bonded adspe- that other bonding configurations of those 6- and 5-member cies with the thus-formed Si–C bond lengths ranging within ring compounds on the Si(001)-2 6 1 surface, e.g., the di-s 1.95–2.01 A˚ . bonding configurations involving direct heteroatom-surface (ii) The nonaromatic ones, i.e., 1,3-cyclohexadiene and bond and the tetra-s bonding configurations, have also been cyclopentadiene, give the largest binding energies, whereas the considered in our calculations. The results will be published most aromatic ones, benzene and pyrrole, give the smallest elsewhere. binding energies in the 6- and 5-member ring compounds, respectively. Acknowledgements (iii) The calculated binding energies for the 6-member ring compounds give the following order of magnitude: C6H8 This work is supported by Natural Science Foundation of 21 21 (256.2 kcal mol ) w C4H4N2 (229.6 kcal mol ) w C5H5P China, the Ministry of Education of China, Xiamen University 21 21 (229.0 kcal mol ) w C5H6Si (226.3 kcal mol ) w C5H5N and Emory University through the Robert. W. Woodruff 21 21 (225.5 kcal mol ) w C6H6 (221.8 kcal mol ). Taking professorship. C5H5P as an exception, we found this is the reverse order of their RE values. That is, the higher RE the compound has, the References lower binding energy does it give in the Diels–Alder addition- like adsorption on the Si(001)-2 6 1 surface. A similar trend is 1 Y. Onganer, M. Saglam, A. Turut, H. Efeoglu and S. Tuzemen, Solid-State, Electron., 1996, 39, 677. 2 M. C. Lonergan, Science, 1997, 278, 2103. 3(a) M. Carbone, M. N. Piancastelli and L. Hellner, Phys. Rev. Sect. B, 2000, 61, 8531; (b) R. A. 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