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A theoretical study of HN3 reaction with the C(1 0 0)-2×1 surface
3 August 2001
Chemical Physics Letters 343 (2001) 212±218
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A theoretical study of HN3 reaction with the C(1 0 0)-2 1 surface Xin Lu a,*, Gang Fu a, Nanqin Wang a, Qianer Zhang a, M.C. Lin b a
Department of Chemistry, State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, China b Department of Chemistry, Cherry L. Emerson Center for Scienti®c Computation, Emory University, Atlanta, GA 30322, USA Received 9 April 2001; in ®nal form 31 May 2001
Abstract The reaction of HN3 with the C(1 0 0)-2 1 surface has been investigated by means of density functional cluster model calculations. The calculations revealed the following: (i) HN3 undergoes dissociative adsorption on the surface, forming CAN3 and CAH surface species. The predicted reaction energy and the barrier height are )61.0 and 2.5 kcal/ mol, respectively; (ii) N2 elimination from surface azide leads to the formation of HACAN@C< surface species; (iii) The 1,3-dipolar cycloaddition of HN3 onto surface dimer is kinetically less favorable than the dissociative chemisorption. This ®nding, however, suggests the plausibility of functionalizing the C(1 0 0) surface by means of 1,3-dipolar cycloadditions. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Diamond is promising as an electronic-device material [1]. The chemical modi®cation of diamond surfaces might introduce some new physical and chemical properties for speci®c applications [1±7]. For example, it was found that incorporation of nitrogen into the diamond lattice aects its thermal conductivity and optical transparency [1]. More speci®cally, it was theoretically predicted that carbon nitride has some superlative properties, such as a large bandgap and a hardness superior than diamond [8,9]. Consequently, it is meaningful to investigate the chemical modi®ca-
*
Corresponding author. Fax:+86-592-2183047. E-mail address: [email protected] (X. Lu).
tion of diamond, especially by nitrogen-containing precursors [10±12]. So far, little has been known about the chemistry of nitrogen-containing precursors on diamond surfaces. Miller and Brown [11] investigated the functionalization of diamond powers with amine groups by reacting ammonia with the chlorinated surface. Thomas and Russell [12] examined the reaction of HN3 with bare and hydrogenated diamond (1 0 0) surfaces by means of high resolution electron energy loss spectroscopy (HREELS). They found that HN3 reacts with bare C(1 0 0) at 100 K by breaking the NAH bond to form CAN3 and CAH surface species, whereas on the hydrogenated diamond surface it adsorbs molecularly at 100 K and desorbs without reacting at <273 K. Further thermal evolution of the so-formed surface azide led to extrusion of N2 and nitridation of diamond
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X. Lu et al. / Chemical Physics Letters 343 (2001) 212±218
surface [12]. The reaction is by far dierent from that occurring on Si(1 0 0) and Ge(1 0 0) surfaces, where the breaking of the HNAN2 bond is observed at low temperature [13±15]. This implies a dierent reaction mechanism for the reaction on the C(1 0 0) surface from that on Si(1 0 0) and Ge(1 0 0) surfaces. It is well known that the reconstructed C(1 0 0), Si(1 0 0), and Ge(1 0 0) surfaces have a similar bonding motif that pairs of atoms bond to each other via a strong r bond and a weaker p bond, forming ethylene-like dimers. This suggests that the chemistry of C(1 0 0), Si(1 0 0) and Ge(1 0 0) might show some similarity to the chemistry of compounds such as alkenes. Indeed, previous experimental and theoretical studies revealed that on all three surfaces, the surface dimers, working as a dienophile, undergo Diels±Alder reactions with incoming conjugated dienes to form 6-member ring surface species [16±26]. On the other hand, it is noteworthy that in organic chemistry, organic azide can undergo pericyclic 1,3-dipolar addition reaction with an ole®n to form triazole [27,28]. Accordingly, one might expect similar 1,3-dipolar addition of organic azide with the surface dimers on C(1 0 0), Si(1 0 0) and Ge(1 0 0) surfaces. A recent theoretical study [29] demonstrated that on the Si(1 0 0) surface, the decomposition of HN3 is initiated by the 1,3-dipolar cycloaddition of HN3 on the Si@Si dimer (a `dipolarophile') with the formation of a 5-member ring HAN3 Si2 surface species, and subsequent N2 elimination from the surface species gives rise to >NH adspecies. Why the reaction of HN3 with the C(1 0 0) surface at 100 K does not follow a similar mechanism to form a 5-member ring HAN3 C2 surface species, but selectively produces H(a) and surface azide is an interesting question. It should be mentioned that in organic chemistry, HN3 can react with ole®n to form organic azide by direct cleavage of the HAN3 bond [30]. It seems that the reaction of HN3 with the C(1 0 0) surface at a low temperature preferably follows this latter mechanism. To go into the details of the surface reaction, we have performed a theoretical investigation on the reaction of HN3 with the C(1 0 0)-2 1 surface by means of density-functional cluster model calculations and report the results in this Letter.
213
2. Computational details A C9 H12 cluster model has been employed to represent a dimer site on the C(1 0 0)-2 1 surface. This cluster model was used in previous theoretical studies of cycloadditions of 1,3-butadiene and cyclopentene onto the C(1 0 0)-2 1 surface [25,26]. All calculations were performed with the GA U S S I A N 94 package [31]. Analytical gradients, the standard 6-31+G basis set and the hybrid density functional theory B3LYP method, including Becke's 3-parameter nonlocal-exchange functional [32] with the correlation functional of Lee±Yang± Parr [33] (B3LYP), were used for geometry optimizations with no constrained degrees of freedom. Single-point B3LYP calculations were done at selected critical points using the 6-311+G(2df, p) basis set to investigate the eect on the reaction energetics. Reported energies are obtained at the B3LYP/6-311+G(2df, p) level, but include the unscaled zero-point-energy (ZPE) corrections calculated at the B3LYP/6-31+G level, unless otherwise speci®ed. The optimized geometries of the C9 H12 cluster model and free HN3 are presented in Fig. 1. 3. Results and discussion 3.1. Reaction mechanism As alluded above, there are at least two possible pathways for the reaction of HN3 with the dimer atoms of the C(1 0 0)-2 1 surface. One (channel A) is the direct cleavage of the HAN3 bond upon
Fig. 1. Optimized geometry of C9 H12 cluster model and HN3 molecule.
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adding to a surface dimer; another (channel B) is initiated by 1,3-dipolar cycloaddition of HN3 onto the surface dimer. Both pathways have been carefully investigated in our calculations. Optimized geometries of various stable species and transition states are shown in Fig. 2. The pro®le of energy surfaces for both pathways is depicted in Fig. 3. Channel A. As shown in Figs. 2 and 3, the cleavage of HAN3 bond on the C9 H12 cluster model surface leads to a dissociative chemisorption state, LM1, with the formation of CAH and CAN3 surface species. In LM1 (see Fig. 2a), the so-formed CAH and CAN bond lengths are 1.093 respectively; the azide group (AN3 ) is and 1.481 A, slightly distorted when compared with that of the free HN3 . Owing to the addition of AH(a) and AN3 (a) adspecies, the surface dimer becomes fully saturated. As a result, the dimer bond length is
from 1.364 A in the bare elongated by 0.225 A cluster to 1.589 A in LM1. Furthermore, the dissociative chemisorption is found to be thermodynamically much favorable with the calculated exothermicity of )61.0 kcal/mol at the B3LYP/ 6-311+G(2df, p) level. The high exothermicity implies an early transition state, if it exists, or no transition state for the dissociative chemisorption of HN3 . Indeed, we located an early transition state (TS1), which has a rather low barrier height of 2.5 kcal/mol predicted at the B3LYP/ 6-311+G(2df, p) level. TS1 has a loose 6-member ring structure (see Fig. 2b), in which the forming H C and N C bond distances (1.612 and are quite large. Detailed intrinsic reaction 2.560 A) coordinate (IRC) calculations revealed that the dissociative chemisorption of HN3 on the surface dimer proceeds in such a way that while the H atom is abstracted from one end of the azide group
Fig. 2. Transition states and local minima in the reaction of HN3 with C(1 0 0)-2 1 surface predicted at the B3LYP/6-31+G level with a C9 H12 cluster model.
X. Lu et al. / Chemical Physics Letters 343 (2001) 212±218
215
Fig. 3. Calculated pathways of HN3 reaction with C(1 0 0)-2 1 surface as well as the energetics predicted at the B3LYP/ 6-311+G(2df, p) level of theory. (Data in parentheses are relative energies predicted at the B3LYP/6-31+G level of theory.)
by a surface C atom, the end N atom of the azide group is attracted by another C atom of the same surface dimer. It is known that organic azide can be thermally decomposed with N2 extrusion [34]. Accordingly, we have considered the possibility of thermal decomposition of the surface azide. We located a transition state (TS2) for N2 elimination from surface azide, as shown in Fig. 2c. In TS2, the The acbreaking N N bond length is 1.848 A. tivation energy for N2 elimination is predicted to be 36.9 kcal/mol with respect to LM1. After N2 elimination, the remaining N atom inserts into the surface dimer, giving rise to the HACAN@C< surface species (see Fig. 2d). This surface species might be the precursor state leading to the formation of CNx ®lm. It is noteworthy that N2 elimination from surface azide is exothermic by 5.2 kcal/mol with respect to surface azide, and TS2 is by 24.1 kcal/mol lower in energy than the initial reactants, free HN3 bare cluster model. Hence,
N2 elimination from surface azide is facile and can be thermally activated at moderate temperature. Channel B. The 1,3-dipolar cycloaddition of HN3 on the surface dimer results in another chemisorption state, LM10 , which has a 5-member ring HAN3 C2 surface species (see Fig. 2e). The calculated adsorption energy is )62.5 kcal/mol at the B3LYP/6-311+G(2df, p) level. In accord with its high exothermicity, the 1,3-dipolar cycloaddition reaction has an early transition state TS10 with a loose 5-member ring structure, in which the two forming C N bond lengths are 2.391 (see Fig. 2f). At the B3LYP/ and 2.619 A 6-311+G(2df, p) level of theory, the activation energy of TS10 is predicted to be 7.3 kcal/mol, which is 4.8 kcal/mol higher than that required by the dissociative chemisorption. As such, the dissociative chemisorption is kinetically favorable over the 1,3-dipolar cycloaddition of HN3 on the C(1 0 0)-2 1 surface. This explains why the reaction of HN3 with C(1 0 0) surface selectively
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produces surface azide and H adatom at 100 K [12]. However, the low activation energies of both processes suggest that at ambient or higher temperature, both chemisorptive processes be feasible and products of both processes can thus be detected on the surface. Furthermore, a transition state leading to N2 eliminatiom from the HAN3 C2 surface species, TS20 , was located. At the B3LYP/6-311+G(2df, p) level, the barrier height at TS20 is predicted to be 53.7 kcal/mol with respect to LM10 . TS20 has a loose 5-member ring structure with the breaking C N and N N bond lengths of 2.664 and respectively (see Fig. 2g). After the elim2.712 A, ination of N2 , an NH(a) adspecies is left and di-r bonded onto the surface dimer, forming a triangle HANC2 surface species. Large steric strain is expected to be present in this triangle surface species, as the NACAC and CANAC bond angles are 59.0° and 61.9°, respectively. Nevertheless, the formation of such a triangle HANC2 surface species by N2 elimination from the HAN3 C2 surface species is found to be exothermic, but the predicted heat of reaction is small, only )8.2 kcal/mol at the B3LYP/6-311+G(2df, p) level of theory. 4. Vibrational frequencies of the surface azide The calculated vibrational frequencies of free HN3 and surface azide are given in Table 1, along with the available experimental data. A scaling factor, 0.96, was used in our B3LYP/6-31+G
calculations of the vibrational frequencies. For the free HN3 molecule, our B3LYP/6-31+G calculations faithfully reproduce the experimental results, as shown in Table 1. In their HREELS study of the HN3 /C(1 0 0) system at 100 K, Thoms and Russell [12] observed three intensive peaks at 2950, 2100 and 1220 cm 1 . They assigned the peak at 2100 cm 1 to the asymmetric stretch mode of surface azide, the peak at 2950 cm 1 to the CAH stretch mode, and the peak at 1220 cm 1 to the overlap of the CAH bend mode and the symmetric stretch mode of surface azide [12]. The assignment is con®rmed to be reasonable by our B3LYP/ 6-31+G cluster model calculations. 5. Concluding remarks The gas-surface reaction of HN3 with the C(1 0 0)-2 1 surface has been investigated by means of ®rst-principles density functional cluster model calculations. The following remarks can be drawn from the study: (i) HN3 undergoes dissociative adsorption on the >C@C< dimer of the C(1 0 0)-2 1 surface with a rather low activation barrier (2.5 kcal/mol at the B3LYP/6-311+G(2df, p) level), giving rise to CAN3 and CAH surface species. The reaction is exothermic by )61.0 kcal/mol predicted at the B3LYP/6-311+G(2df, p) level. The calculated vibrational frequencies of the so-formed surface azide are in good agreement with the experimental HREELS spectra. Further thermal elimination of
Table 1 Selected experimental and theoreticala ;b vibrational frequencies of HN3 /C(1 0 0) and free HN3 molecule Mode HAN str. CAH str. N3 asym. str. NAH bend N3 sym. str. CAH bend a
HN3 /C(1 0 0)
Free HN3 c
Theor.
Exp.
3334 ± 2186 1251 1138 ±
3336 ± 2140 1264 1151 ±
(39) (393) (2.6) (218)
(m) (vs) (m) (vs)
Scaled by a factor of 0.96. Values in parentheses are calculated IR intensities, unit in KM/Mol. c Ref. [35]. d Ref. [12]. b
Theor.
Exp.d
± 2990 2142 ± 1274 1222
± 2950 (s) 2100 (s) ± ± 1220 (m)
(27) (640) (141) (13)
X. Lu et al. / Chemical Physics Letters 343 (2001) 212±218
N2 from surface azide requires an activation energy of 36.9 kcal/mol, giving rise to the formation of HACAN@C< surface species, which might be the precursor for further formation of CNx ®lm. (ii) The pericyclic 1,3-dipolar addition of HN3 on the >C@C< dimer of the C(1 0 0) surface is possible, but requires an activation energy of 7.3 kcal/mol predicted at the B3LYP/6-311+G(2df, p) level. Hence, this reaction pathway is kinetically less favorable than the dissociative chemisorption. Yet, the 1,3-diplor cycloaddition process is found to be highly exothermic by )62.5 kcal/mol at the B3LYP/6-311+G(2df, p) level. Elimination of N2 from the thus-formed 5-member ring HAN3 C2 surface species leads to the formation of a 3-member ring HANC2 surface species. This reaction pathway, though insigni®cant at low temperature, might be competitive with the dissociative pathway at ambient or higher temperature due to its low activation barrier. Finally, the ®nding that the pericyclic 1,3-dipolar addition of HN3 on the >C@C< dimer of the C(1 0 0) surface has a low activation energy of 7.3 kcal/mol implies that the reaction of organic azide, such as alkyl azide and aryl azide, with >C@C< dimer of the C(1 0 0)-2 1 surface be feasible by following the 1,3-dipolar cycloaddition mechanism. Accordingly, we propose herewith a plausible way to functionalize the C(1 0 0)-2 1 surface by 1,3-dipolar cycloadditions of organic azide and other 1,3-dipolar molecules onto the solid surface. Further theoretical study on this subject is in progress. Acknowledgements This work was sponsored by the Nature Science Foundation of China, the Ministry of Education (China), the Fok Ying-Tung Educational Foundation, and Emory University through the Robert W. Woodru professorship. References [1] J.E. Field (Ed.), The Properties of Natural and Synthetic Diamond, Academic Press, London, 1992.
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[2] S.A. Grot, G.Sh. Gildenblat, C.W. Hat®eld, A.R. Wronski, A.R. Badzian, T. Badzian, R. Messier, IEEE Elect. Dev. Lett. 11 (1990) 100. [3] H. Nakahata, T. Imai, N. Fujimori, Diamond Materials, Proceedings, vol. 91-8, The Electrochemical Society, Pennington, NJ, 1991, p. 487. [4] D.P. Malta, J.B. Posthill, T.P. Humphreys, R.E. Thomas, G.G. Fountain, R.A. Rudder, G.C. Hudson, M.J. Mantini, R.J. Markunas, Appl. Phys. Lett. 64 (1994) 1929. [5] W.E. Pickett, Phys. Rev. Lett. 73 (1994) 1664. [6] J. Van der Weide, R.J. Nemanich, J. Vac. Sci. Technol. B 12 (1994) 2475. [7] B.L. Mackey, J.N. Russell Jr., J.E. Crowell, J.E. Butler, Phys. Rev. B 52 (1995) 17009. [8] A.Y. Liu, M.L. Cohen, Science 245 (1989) 841. [9] J.L. Corkill, M.L. Cohen, Phys. Rev. B 48 (1993) 17622. [10] C. Niu, Y.Z. Lu, C.M. Lieber, Science 261 (1993) 334. [11] J.B. Miller, D.W. Brown, Langmuir 12 (1996) 5809. [12] B.D. Thoms, J.N. Russell Jr., Surf. Sci. 337 (1995) L807. [13] J.C.S. Chu, Y. Bu, M.C. Lin, in: Proceddings of the 6th International Conference on Electronic Materials and Processing, SAMPE, Baltimore, MD, June, 1992. [14] N.B.H. Jonathon, P.J. Knight, A. Morris, Surf. Sci. 275 (1992) L640. [15] C. Tindall, J.C. Hemminger, Surf. Sci. 330 (1995) 67. [16] R. Konecny, D.J. Doren, J. Am. Chem. Soc. 119 (1997) 11098. [17] R. Konecny, D.J. Doren, Surf. Sci. 417 (1998) 169. [18] A.V. Teplyakov, M.J. Kong, S.F. Bent, J. Am. Chem. Soc. 119 (1997) 11100. [19] A.V. Teplyakov, M.J. Kong, S.F. Bent, J. Chem. Phys. 108 (1998) 4599. [20] J.S. Hovis, H. Liu, R.J. Hamers, J. Phys. Chem. 102 (1998) 6873. [21] C.H. Choi, M.S. Gordon, J. Am. Chem. Soc. 121 (1999) 11311. [22] S.W. Lee, L.N. Nelen, H. Ihm, T. Scoggins, C.M. Greenlief, Surf. Sci. 410 (1998) L773. [23] G.T. Wang, S.F. Bent, J.N. Russell Jr., J.E. Butler, M.P. D'Evelyn, J. Am. Chem. Soc. 122 (2000) 744. [24] M.Z. Hossain, T. Aruga, N. Takagi, T. Tsuno, N. Fujimori, T. Ando, M. Nishijima, Jpn. J. Appl. Phys. 38 (1999) L1496. [25] D.R. Fitzgerald, D.J. Doren, J. Am. Chem. Soc. 122 (2000) 12334. [26] Y. Okamoto, J. Phys. Chem. B 105 (2001) 1813. [27] W. Lwowski, in: A. Padwa (Ed.), 1,3-Dipolar Cycloaddition Chemistry, vol 1, Wiley, New York, 1984, p. 560. [28] M.P. Repasky, W.L. Jorgensen, Faraday Disscuss. 110 (1998) 379. [29] F.T. Bacalzo, M.C. Lin, unpublished results. [30] E.A. Kyba, in: E.F.V. Scrien (Ed.), Azides and Nitrenes: Reactivity and Utility, Academic Press, Orlando, FL, 1984, (chapter 1). [31] M.J. Frisch et al., GA U S S I A N 94, Revision B.3, Gaussian, Inc., Pittsburgh, PA, 1995. [32] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
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[33] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1989) 785. [34] J.A. Lerrmakers, J. Chem. Phys. 55 (1933) 2719. [35] T. Shimanouchi, Molecular Vibrational Frequencies, in: W.G. Mallard, P.J. Linstrom (Eds.), NIST Chemistry
WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, Gaithersburg, MD 20899, February 2000 (http://webbook.nist.gov).
Chemical Physics Letters 343 (2001) 212±218
www.elsevier.com/locate/cplett
A theoretical study of HN3 reaction with the C(1 0 0)-2 1 surface Xin Lu a,*, Gang Fu a, Nanqin Wang a, Qianer Zhang a, M.C. Lin b a
Department of Chemistry, State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, China b Department of Chemistry, Cherry L. Emerson Center for Scienti®c Computation, Emory University, Atlanta, GA 30322, USA Received 9 April 2001; in ®nal form 31 May 2001
Abstract The reaction of HN3 with the C(1 0 0)-2 1 surface has been investigated by means of density functional cluster model calculations. The calculations revealed the following: (i) HN3 undergoes dissociative adsorption on the surface, forming CAN3 and CAH surface species. The predicted reaction energy and the barrier height are )61.0 and 2.5 kcal/ mol, respectively; (ii) N2 elimination from surface azide leads to the formation of HACAN@C< surface species; (iii) The 1,3-dipolar cycloaddition of HN3 onto surface dimer is kinetically less favorable than the dissociative chemisorption. This ®nding, however, suggests the plausibility of functionalizing the C(1 0 0) surface by means of 1,3-dipolar cycloadditions. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Diamond is promising as an electronic-device material [1]. The chemical modi®cation of diamond surfaces might introduce some new physical and chemical properties for speci®c applications [1±7]. For example, it was found that incorporation of nitrogen into the diamond lattice aects its thermal conductivity and optical transparency [1]. More speci®cally, it was theoretically predicted that carbon nitride has some superlative properties, such as a large bandgap and a hardness superior than diamond [8,9]. Consequently, it is meaningful to investigate the chemical modi®ca-
*
Corresponding author. Fax:+86-592-2183047. E-mail address: [email protected] (X. Lu).
tion of diamond, especially by nitrogen-containing precursors [10±12]. So far, little has been known about the chemistry of nitrogen-containing precursors on diamond surfaces. Miller and Brown [11] investigated the functionalization of diamond powers with amine groups by reacting ammonia with the chlorinated surface. Thomas and Russell [12] examined the reaction of HN3 with bare and hydrogenated diamond (1 0 0) surfaces by means of high resolution electron energy loss spectroscopy (HREELS). They found that HN3 reacts with bare C(1 0 0) at 100 K by breaking the NAH bond to form CAN3 and CAH surface species, whereas on the hydrogenated diamond surface it adsorbs molecularly at 100 K and desorbs without reacting at <273 K. Further thermal evolution of the so-formed surface azide led to extrusion of N2 and nitridation of diamond
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 7 0 8 - 4
X. Lu et al. / Chemical Physics Letters 343 (2001) 212±218
surface [12]. The reaction is by far dierent from that occurring on Si(1 0 0) and Ge(1 0 0) surfaces, where the breaking of the HNAN2 bond is observed at low temperature [13±15]. This implies a dierent reaction mechanism for the reaction on the C(1 0 0) surface from that on Si(1 0 0) and Ge(1 0 0) surfaces. It is well known that the reconstructed C(1 0 0), Si(1 0 0), and Ge(1 0 0) surfaces have a similar bonding motif that pairs of atoms bond to each other via a strong r bond and a weaker p bond, forming ethylene-like dimers. This suggests that the chemistry of C(1 0 0), Si(1 0 0) and Ge(1 0 0) might show some similarity to the chemistry of compounds such as alkenes. Indeed, previous experimental and theoretical studies revealed that on all three surfaces, the surface dimers, working as a dienophile, undergo Diels±Alder reactions with incoming conjugated dienes to form 6-member ring surface species [16±26]. On the other hand, it is noteworthy that in organic chemistry, organic azide can undergo pericyclic 1,3-dipolar addition reaction with an ole®n to form triazole [27,28]. Accordingly, one might expect similar 1,3-dipolar addition of organic azide with the surface dimers on C(1 0 0), Si(1 0 0) and Ge(1 0 0) surfaces. A recent theoretical study [29] demonstrated that on the Si(1 0 0) surface, the decomposition of HN3 is initiated by the 1,3-dipolar cycloaddition of HN3 on the Si@Si dimer (a `dipolarophile') with the formation of a 5-member ring HAN3 Si2 surface species, and subsequent N2 elimination from the surface species gives rise to >NH adspecies. Why the reaction of HN3 with the C(1 0 0) surface at 100 K does not follow a similar mechanism to form a 5-member ring HAN3 C2 surface species, but selectively produces H(a) and surface azide is an interesting question. It should be mentioned that in organic chemistry, HN3 can react with ole®n to form organic azide by direct cleavage of the HAN3 bond [30]. It seems that the reaction of HN3 with the C(1 0 0) surface at a low temperature preferably follows this latter mechanism. To go into the details of the surface reaction, we have performed a theoretical investigation on the reaction of HN3 with the C(1 0 0)-2 1 surface by means of density-functional cluster model calculations and report the results in this Letter.
213
2. Computational details A C9 H12 cluster model has been employed to represent a dimer site on the C(1 0 0)-2 1 surface. This cluster model was used in previous theoretical studies of cycloadditions of 1,3-butadiene and cyclopentene onto the C(1 0 0)-2 1 surface [25,26]. All calculations were performed with the GA U S S I A N 94 package [31]. Analytical gradients, the standard 6-31+G basis set and the hybrid density functional theory B3LYP method, including Becke's 3-parameter nonlocal-exchange functional [32] with the correlation functional of Lee±Yang± Parr [33] (B3LYP), were used for geometry optimizations with no constrained degrees of freedom. Single-point B3LYP calculations were done at selected critical points using the 6-311+G(2df, p) basis set to investigate the eect on the reaction energetics. Reported energies are obtained at the B3LYP/6-311+G(2df, p) level, but include the unscaled zero-point-energy (ZPE) corrections calculated at the B3LYP/6-31+G level, unless otherwise speci®ed. The optimized geometries of the C9 H12 cluster model and free HN3 are presented in Fig. 1. 3. Results and discussion 3.1. Reaction mechanism As alluded above, there are at least two possible pathways for the reaction of HN3 with the dimer atoms of the C(1 0 0)-2 1 surface. One (channel A) is the direct cleavage of the HAN3 bond upon
Fig. 1. Optimized geometry of C9 H12 cluster model and HN3 molecule.
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X. Lu et al. / Chemical Physics Letters 343 (2001) 212±218
adding to a surface dimer; another (channel B) is initiated by 1,3-dipolar cycloaddition of HN3 onto the surface dimer. Both pathways have been carefully investigated in our calculations. Optimized geometries of various stable species and transition states are shown in Fig. 2. The pro®le of energy surfaces for both pathways is depicted in Fig. 3. Channel A. As shown in Figs. 2 and 3, the cleavage of HAN3 bond on the C9 H12 cluster model surface leads to a dissociative chemisorption state, LM1, with the formation of CAH and CAN3 surface species. In LM1 (see Fig. 2a), the so-formed CAH and CAN bond lengths are 1.093 respectively; the azide group (AN3 ) is and 1.481 A, slightly distorted when compared with that of the free HN3 . Owing to the addition of AH(a) and AN3 (a) adspecies, the surface dimer becomes fully saturated. As a result, the dimer bond length is
from 1.364 A in the bare elongated by 0.225 A cluster to 1.589 A in LM1. Furthermore, the dissociative chemisorption is found to be thermodynamically much favorable with the calculated exothermicity of )61.0 kcal/mol at the B3LYP/ 6-311+G(2df, p) level. The high exothermicity implies an early transition state, if it exists, or no transition state for the dissociative chemisorption of HN3 . Indeed, we located an early transition state (TS1), which has a rather low barrier height of 2.5 kcal/mol predicted at the B3LYP/ 6-311+G(2df, p) level. TS1 has a loose 6-member ring structure (see Fig. 2b), in which the forming H C and N C bond distances (1.612 and are quite large. Detailed intrinsic reaction 2.560 A) coordinate (IRC) calculations revealed that the dissociative chemisorption of HN3 on the surface dimer proceeds in such a way that while the H atom is abstracted from one end of the azide group
Fig. 2. Transition states and local minima in the reaction of HN3 with C(1 0 0)-2 1 surface predicted at the B3LYP/6-31+G level with a C9 H12 cluster model.
X. Lu et al. / Chemical Physics Letters 343 (2001) 212±218
215
Fig. 3. Calculated pathways of HN3 reaction with C(1 0 0)-2 1 surface as well as the energetics predicted at the B3LYP/ 6-311+G(2df, p) level of theory. (Data in parentheses are relative energies predicted at the B3LYP/6-31+G level of theory.)
by a surface C atom, the end N atom of the azide group is attracted by another C atom of the same surface dimer. It is known that organic azide can be thermally decomposed with N2 extrusion [34]. Accordingly, we have considered the possibility of thermal decomposition of the surface azide. We located a transition state (TS2) for N2 elimination from surface azide, as shown in Fig. 2c. In TS2, the The acbreaking N N bond length is 1.848 A. tivation energy for N2 elimination is predicted to be 36.9 kcal/mol with respect to LM1. After N2 elimination, the remaining N atom inserts into the surface dimer, giving rise to the HACAN@C< surface species (see Fig. 2d). This surface species might be the precursor state leading to the formation of CNx ®lm. It is noteworthy that N2 elimination from surface azide is exothermic by 5.2 kcal/mol with respect to surface azide, and TS2 is by 24.1 kcal/mol lower in energy than the initial reactants, free HN3 bare cluster model. Hence,
N2 elimination from surface azide is facile and can be thermally activated at moderate temperature. Channel B. The 1,3-dipolar cycloaddition of HN3 on the surface dimer results in another chemisorption state, LM10 , which has a 5-member ring HAN3 C2 surface species (see Fig. 2e). The calculated adsorption energy is )62.5 kcal/mol at the B3LYP/6-311+G(2df, p) level. In accord with its high exothermicity, the 1,3-dipolar cycloaddition reaction has an early transition state TS10 with a loose 5-member ring structure, in which the two forming C N bond lengths are 2.391 (see Fig. 2f). At the B3LYP/ and 2.619 A 6-311+G(2df, p) level of theory, the activation energy of TS10 is predicted to be 7.3 kcal/mol, which is 4.8 kcal/mol higher than that required by the dissociative chemisorption. As such, the dissociative chemisorption is kinetically favorable over the 1,3-dipolar cycloaddition of HN3 on the C(1 0 0)-2 1 surface. This explains why the reaction of HN3 with C(1 0 0) surface selectively
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produces surface azide and H adatom at 100 K [12]. However, the low activation energies of both processes suggest that at ambient or higher temperature, both chemisorptive processes be feasible and products of both processes can thus be detected on the surface. Furthermore, a transition state leading to N2 eliminatiom from the HAN3 C2 surface species, TS20 , was located. At the B3LYP/6-311+G(2df, p) level, the barrier height at TS20 is predicted to be 53.7 kcal/mol with respect to LM10 . TS20 has a loose 5-member ring structure with the breaking C N and N N bond lengths of 2.664 and respectively (see Fig. 2g). After the elim2.712 A, ination of N2 , an NH(a) adspecies is left and di-r bonded onto the surface dimer, forming a triangle HANC2 surface species. Large steric strain is expected to be present in this triangle surface species, as the NACAC and CANAC bond angles are 59.0° and 61.9°, respectively. Nevertheless, the formation of such a triangle HANC2 surface species by N2 elimination from the HAN3 C2 surface species is found to be exothermic, but the predicted heat of reaction is small, only )8.2 kcal/mol at the B3LYP/6-311+G(2df, p) level of theory. 4. Vibrational frequencies of the surface azide The calculated vibrational frequencies of free HN3 and surface azide are given in Table 1, along with the available experimental data. A scaling factor, 0.96, was used in our B3LYP/6-31+G
calculations of the vibrational frequencies. For the free HN3 molecule, our B3LYP/6-31+G calculations faithfully reproduce the experimental results, as shown in Table 1. In their HREELS study of the HN3 /C(1 0 0) system at 100 K, Thoms and Russell [12] observed three intensive peaks at 2950, 2100 and 1220 cm 1 . They assigned the peak at 2100 cm 1 to the asymmetric stretch mode of surface azide, the peak at 2950 cm 1 to the CAH stretch mode, and the peak at 1220 cm 1 to the overlap of the CAH bend mode and the symmetric stretch mode of surface azide [12]. The assignment is con®rmed to be reasonable by our B3LYP/ 6-31+G cluster model calculations. 5. Concluding remarks The gas-surface reaction of HN3 with the C(1 0 0)-2 1 surface has been investigated by means of ®rst-principles density functional cluster model calculations. The following remarks can be drawn from the study: (i) HN3 undergoes dissociative adsorption on the >C@C< dimer of the C(1 0 0)-2 1 surface with a rather low activation barrier (2.5 kcal/mol at the B3LYP/6-311+G(2df, p) level), giving rise to CAN3 and CAH surface species. The reaction is exothermic by )61.0 kcal/mol predicted at the B3LYP/6-311+G(2df, p) level. The calculated vibrational frequencies of the so-formed surface azide are in good agreement with the experimental HREELS spectra. Further thermal elimination of
Table 1 Selected experimental and theoreticala ;b vibrational frequencies of HN3 /C(1 0 0) and free HN3 molecule Mode HAN str. CAH str. N3 asym. str. NAH bend N3 sym. str. CAH bend a
HN3 /C(1 0 0)
Free HN3 c
Theor.
Exp.
3334 ± 2186 1251 1138 ±
3336 ± 2140 1264 1151 ±
(39) (393) (2.6) (218)
(m) (vs) (m) (vs)
Scaled by a factor of 0.96. Values in parentheses are calculated IR intensities, unit in KM/Mol. c Ref. [35]. d Ref. [12]. b
Theor.
Exp.d
± 2990 2142 ± 1274 1222
± 2950 (s) 2100 (s) ± ± 1220 (m)
(27) (640) (141) (13)
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N2 from surface azide requires an activation energy of 36.9 kcal/mol, giving rise to the formation of HACAN@C< surface species, which might be the precursor for further formation of CNx ®lm. (ii) The pericyclic 1,3-dipolar addition of HN3 on the >C@C< dimer of the C(1 0 0) surface is possible, but requires an activation energy of 7.3 kcal/mol predicted at the B3LYP/6-311+G(2df, p) level. Hence, this reaction pathway is kinetically less favorable than the dissociative chemisorption. Yet, the 1,3-diplor cycloaddition process is found to be highly exothermic by )62.5 kcal/mol at the B3LYP/6-311+G(2df, p) level. Elimination of N2 from the thus-formed 5-member ring HAN3 C2 surface species leads to the formation of a 3-member ring HANC2 surface species. This reaction pathway, though insigni®cant at low temperature, might be competitive with the dissociative pathway at ambient or higher temperature due to its low activation barrier. Finally, the ®nding that the pericyclic 1,3-dipolar addition of HN3 on the >C@C< dimer of the C(1 0 0) surface has a low activation energy of 7.3 kcal/mol implies that the reaction of organic azide, such as alkyl azide and aryl azide, with >C@C< dimer of the C(1 0 0)-2 1 surface be feasible by following the 1,3-dipolar cycloaddition mechanism. Accordingly, we propose herewith a plausible way to functionalize the C(1 0 0)-2 1 surface by 1,3-dipolar cycloadditions of organic azide and other 1,3-dipolar molecules onto the solid surface. Further theoretical study on this subject is in progress. Acknowledgements This work was sponsored by the Nature Science Foundation of China, the Ministry of Education (China), the Fok Ying-Tung Educational Foundation, and Emory University through the Robert W. Woodru professorship. References [1] J.E. Field (Ed.), The Properties of Natural and Synthetic Diamond, Academic Press, London, 1992.
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