A Study of thethermal C-C and C-H Bond Cleavage in the Aromatic Molecules: Acenaphthene and Acenaphthylene

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1 JJC Jordn Journl of Chemistry Vol. 7 No.4, 2012, pp A Study of thetherml C-C nd C-H Bond Clevge in the Aromtic Molecules: Acenphthene nd Acenphthylene Muthn Shnshl nd Hssn H. Abdullh Deprtment of Chemistry, College of Science, University of Bghdd, Jdirriy, Bghdd, Irq. Received on April 23, 2012 Accepted on Oct. 15, 2012 Abstrct The therml decomposition of the two romtic hydrocrbons Acenphthene nd Acenphthylene is studied pplying the open shell semiempiricl (Hrtree-Fock) PM3 method. In this study ll internl coordintes were vried, except the coordinte chosen to describe the rection. All possible intermedites were considered on describing the rection scheme. The preferred rection pth ws ssigned on the bsis of compring the ctivtion energies of the different pths. The tretment shows tht the preferred rection pths led to the formtion of cetylene s finl product. This result is similr, but in the reverse direction, to former theoreticl nd experimentl results reported in the literture, which showed tht cetylene is bsic building unit within the nturl formtion rections of Polycyclic Aromtic Hydrocrbon molecules. Keywords: Acenphthene; Acenphthylene; Decomposition; PM3. Introduction Due to its importnce for the chemicl nd petrochemicl industry [1] the therml decomposition of polycyclic romtic hydrocrbons (PAH) ws subject of vrious theoreticl nd experimentl studies [1-8]. Ren et l. [3] investigted the rections s initited by insitu H toms, produced from the therml decomposition of methnol t elevted tempertures. The isolted products were formed through recombintion of the formed rdicls with the H toms. Theoreticl, quntum mechnicl studies were crried out for the decomposition rections too [4-7]. On the other hnd, theoreticl s well s experimentl studies were done for the ddition rections of C 2 H 2 to smll romtic hydrocrbons to yield higher PAH molecules nd soot [8-11]. In this pper we report theoreticl study for the therml decomposition rections of two romtic hydrocrbons, Acenphthene nd Acenphthylene. Corresponding uthor: e- mil: mshnshl2003@yhoo.com In prt from the PhD thesis of H. H. A., Bghdd University,

2 Acenphthylene Acenphthene Method of tretment The semiempiricl PM3 [12] method, in the open shell nd unrestricted form, ws pplied s it is progrmmed within the MOPAC progrm system [13]. For the study of bond breking rection, PM3 clcultions were done for the molecule with vrious lengths of the bond. These bond lengths were kept constnt while the other (3N-5) internl coordintes were vried. The energy vlues were plotted then ginst the "frozen" bond length vlues to yield the required rection pth. It ws possible then to ssign grphiclly the trnsition stte nd the products of the rection (Figure 1). This type of clcultion ws repeted for ll the succeeding rection intermedites iming towrds the smllest cceptble rection product. The cceptnce of rection product is judged on the bsis of the ctivtion energy required for its genertion. Accordingly, the described rection pths in this work represent summry of big number of PM3 clcultions (hundres) done for the rupture of ll the bonds in ech "intermedite". Figure 1: PM3 clculted rection pth (dotted line) of the C 1 C 8 bond rupture rection of C 2- cenphthyl rdicl 330

3 As seen in figure 1 the rection energy is clculted s the difference; H rect = H f, product - H f,rectnt nd considering the chnge in entropy being negligible within the rection, S rect. 0.0, H rect E rect. Results nd discussion For the choice of the initil rection step, it ws necessry to evlute the ctivtion nd rection energies ( E ) for the rupture of the different C-C nd C-H bonds s clculted ccording to the working scheme followed in this pper. The choice of E for the discussion of the rection mechnism is cceptble here for two resons; - ll clevge rections re endothermic nd thus follow the Hmmond postulte i.e. their trnsition sttes should be product like in structure; nd b- they follow the Evns-Poliny principle, i.e. their ctivtion energies exhibit similr ordering in mgnitudes s the finl products. Tble 1: PM3 clculted ctivtion energies for the bond rupture rections in cenphthylene. Bond(s) E (kcl/mol) C 2 -H C 3 -H C 4 -H C 5 -H C 1 -C C 2 -C C 2 -C 8b C 2 -C C 3 -C C 4 -C C 5 -C C 2 -C 2 + C 1 -C C 2 -C 3 + C 4 -C C 2 -C 3 + C 5 -C

4 Inspecting the vlues of tble 1, it is seen tht the smllest required ctivtion energy E (75.69kcl/mol) belongs to the C 2 -C 2 bond, followed by those for the bonds C 5 -H 13 nd C 1 -C 2. Scheme 1 shows the clculted 3 rection pths with lest ctivtion energies for bond rupture rections of cenphthylene. Scheme 2 shows two other pths, of lower priority, considered for the sme rection. III I II Scheme 1 332

5 Scheme 2 The two schemes, combined together, show tht, ccording to the E vlues, nd in order for the rection to proceed, the competition is expected to be mong pth I ( E = kcl/mol) nd pth IV ( E = kcl/mol). The initil step for pth I is the rupture of C-H bond,tht for pth IV is the rupture of n essentil C-C single bond.the second step for pth I is the rupture of C-C bond ( E = 53.07kcl/mol). Comprison of the rection pths revels tht pth IV includes the steps with the smllest ctivtion energy vlues followed by the pth I. In both cses the rection proceeds towrds the elimintion of C 2 H 2 (cetylenic) frgment. The role of C 2 H 2 s templte in these rections is similr, but in the reverse direction, to tht of the formtion rections of higher PAH's nd soot s suggested by Frenklch et l. [7,8 ] nd experimentl studies [11]. Inspection of ll 5 rection pths shows tht the formtion of C 2 H 2 frgment is preferred due to the smller E vlues. Other formed cyclic romtic compounds re either benzene or nphthlene derivtives. In the presence of H 2, s ws inspected by Ren's experiments [3] one should expect the following rection products (Tble2 ). 333

6 Tble 2.Expected romtic products to be formed from the therml degrdtion of cenphthylene in the presence of H 2. No. Compound H f (kcl / mol) 1 Acenphthylene Nphthlene vinyl-nphthlene methylene-7-vinyl-1H-indene methylene-1H-indene ,2-divinyl-benzene vinyl-benzene Butdiene but-1,3-dienyl-3-vinyl-benzene Benzene Therml decomposition of Acenphthene molecule. In similr tretment, the ctivtion energies of the therml bond breking rections were clculted for the different bonds in cenphthene molecule. Tble 3 shows the clculted E vlues for the different C-H nd C-C bonds of the molecule. Apprent from the vlues of tble 3 re the reltively low vlues of ctivtion energies for the clevge of the bonds C 1 -C 2 (51.14kcl/mol) nd C 2 -H 11 (67.32kcl/mol). The clculted E vlues for the other bonds re exceedingly higher, 92.49kcl/mol (C 5 - H 15 ) t lest. Accordingly, the discussion of the decomposition rection pth hs to be limited to those pths tht strt with the clevge of either of these two bonds; i.e. pth I for the C 2 -H 11 nd pth II for the C 1 -C 2 bond. Scheme 3 shows the rection strting with the clevge of the C 2 -H 11 bond. Tble 3: PM3 clculted ctivtion energies for the bond rupture rections in cenphthene. Bond(s) E (kcl/mol) C 2 -H C 3 -H C 4 -H C 5 -H C 1 -C C 2 -C C 2 -C 8b C 2 -C C 3 -C C 4 -C C 5 -C C 2 -C 2 + C 1 -C C 3 -C 4 + C 5 -C C 2 -C 3 + C 4 -C C 2 -C 3 + C 5 -C

7 Scheme 3 In this scheme one my follow the pth with the lest E vlues (recognized with the bold color). Obviously the route with the successive E vlues; 67.22kcl/mol, 61.29kcl/mol, 76.84kcl/mol nd 46.33kcl/mol is of higher priority. It leds to the formtion of C 2 H 2 nd nphthylbirdicl.the other brnch of the "rection tree" includes higher ctivtion energy vlues, such s kcl/mol, 87.79kcl/mol nd 81.16kcl/mol, nd is to be neglected. Scheme 4 shows the 2 nd probble rection pth strting with clevge of the C 1 - C 2 bond (E =51.14kcl/mol). 335

8 Scheme 4 It includes n E vlue of kcl/mol which is higher thn ny vlue in pth I. For this reson, this pth should be eliminted from the discussion of the mechnism of this decomposition rection. Pth I is left then s the only cceptble description of the rection mechnism. 336

9 Conclusion According to the PM3 (semiempiricl H-F) tretment, the decomposition of the romtic cenphhene nd cenphthylene molecules proceeds towrds the formtion of n cetylenic frgment nd n romtic molecule. In this rection, the cetylenic frgment constitutes the bsic templte, similr to the therml formtion of PAH's nd soot in which cetylene forms the bsic templte too, s found in former works, both theoreticlly nd experimentlly. References [1] Hrvey, R.G. "Polycyclic Aromtic Hydrocrbons; Chemistry nd Cr-cenogenicity", Cmbridge University Press, Cmbridge, [2] Hrvey, R.G. "Polycyclic Aromtic Hydrocrbons", Wiley-VCH, New York, [3] Ren, R.-L.; Itoh, H.; Ouchi, K., Fuel, 1989, 68, [4] Ninomiy, Y.; Dong, Z.; Suzuki, Z. Y., Fuel, 2000, 79, [5] Ling, Y.; Mrtin, J. M. L.; Lifschitz, C., J. Phys. Chem. A, 1997, 101, [6] Mebel, A. M., Lin, S. H., Yng, X. M.; Lee,Y. T., J. Phys. Chem A, 1997, 101, [7] My, K.; Dopperich, S.; Furd, F.; Untereiner, B. V.; Ahlrichs, R., Phys. Chem. Chem. Phys., 2000, 2, [8] Frenklch, M.; Wng, H., Proc. Combust. Inst., 1991, 23, [9] Frenklch, M.; Moritry, N. W.; Brown, N., Proc. Combust. Inst., 1998, 27, [10] Untereiner, B. V.; Sierk, M.; Ahlrichs, R., Phys. Chem. Chem. Phys., 2004, 6, [11] Böhm, H.; Jnder, H., Phys. Chem. Chem. Phys., 1999, 1, [12] Dewr, M. J. S.; Grdy, G. L.; Stewrt, J. J. P., J. Am. Chem. Soc., , ; Dewr, M. J. S; Zoebisch, E. G.; Hely, E. F., J. Am. Chem. Soc., 1985, 107, ; Dewr, M. J. S.; Reynolds, C. H., J. Comp. Chem., 1986, 2, [13] Stewrt, J. J. P., J. Comp. Chem., 1989, 10, ; Stewrt, J. J. P., J. Comp. Chem., 1989, 10,