The Hel photoelectron (PE) spectra of anthracene and ten of its
9-substituted derivatives with methyl, t ert-butyl, fluoro; chloro,
bromo, methoxy, aldehyde, cyano, nitro and phenyl groups as substituents
have been recorded. Systems arising from Jt-ionizations
are assigned. The assignment is supported by molecular orbital calculations.
The effect of the substituent on the lowest ionization
energy is well described by Hammett substituent constants based
on the benzene system

B. Kovač

H. Gusten

L. KLasinc

S. Schoof

English

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CROATICA CHEMICA ACTA CCACAA 51 (4) 307-315 (1978) CCA-1126 YU ISSN 0011-1643 547.672 Original Scientific Paper Photoelectron Spectroscopy of 9-Substituted Anthracenes L. KLasinc and B. Kovac »Ruder Boskovic« Institute, 41001 Zagreb, Croatia, Yugoslavia and S. Schoof and H. Giisten Kernforschungszentrum Karlsruhe, Institut fiir Radiochemie, 7500 Karlsruhe, Postfach 3640, Bundesrepublik Deutschland Received July 7, 1978 The Hel photoelectron (PE) spectra of anthracene and ten of its 9-substituted derivatives with methyl, tert-butyl, fluoro; chloro, bromo, methoxy, aldehyde, cyano, nitro and phenyl groups as sub-stituents have been recorded. Systems arising from Jt-ionizations are assigned. The assignment is supported by molecular orbital cal-culations. The effect of the substituent on the lowest ionization energy is well described by Hammett substituent constants based on the benzene system. INTRODUCTION The question of the electronic structure of 9-substituted anthracenes arose in course of our recent study of their electron impact induced fragmentation1 . It was supposed that the electronic structure might be responsible for the different behaviour regarding the loss of substituent. Photoelectron (PE) spe-ctroscopy is effective in getting information concerning electronic structures of molecules within the concept of occupied molecular orbitals (Koopmans' theorem). Regarding the ionization process it is also a complementary method to low energy mass spectrometry. As opposed to anthracene2-s the substituted anthracenes were scarcely studied by PE spectroscopy6- 9. Recently, ionization energies determined mass spectrometrically10 as well as a comparison b etween ionization energies and charge transfer excitation energies of their TCNE and TCNQ complexes11 have been reported for several mesa-substituted anthracenes. EXPERIMENTAL The PE spectra of anthracene and the ten 9-substituted anthracenes were recorded on a Vacuum Generators UV-G3 instrument using the Hel line for excitation. For details of the instrument, see ref 12. Elevated temperature of the sample inlet system (120-180 °C) was employed to produce satisfactory spectra. The energy scale was calibrated and controlled by admitting an Ar/Xe gas mixture to the sample. Besides anthracene, the PE spectra of all compounds were recorded at low resolution (15 meV and 35 meV, respectively). The accuracy of the ionization energies quoted is better than 0.03 eV. The anthracenes are either of commercial origin (anthracene, I 103•1 18 16 14 12 15.00 18 16 14 12 I 10.71. rn 10 9.17 8.53 7_43 ~ ~ 8 E1 /eV ~ ~ 8 Er /eV "88 16 14 12 14.77 12.83 16 14 12 10 8 10 8 ~ ~ Er/eV CCOC(CH3J 3 " /. 7.13 E1 /eV Figure 1. HeI .Photoelectron spectra of anthracene, 9-methylanthracene, 9-tert-butylanthracene and 9-fluoroanthracene. w 0 Qj !."' p > Ul H z 0 t<J .., > !."' PE S P ECTRA OF ANTHRACENES 309 9-methyl- and 9-phenylanthracene) or were synthetized by procedures well described in the literature. All compounds were purified by column chromatography on Al.,O. in benzene or cyclohexane and final recrystallization. RESULTS AND DISCUSSION The PE spectra of anthracene, 9-methylanthracene, 9-tert-butylanthracene and 9-fluoroanthracene (Figure 1), 9-chloroanthracene and 9-bromoanthracene (Figure 2) and those of 9-anthraldehyde, 9-methoxyanthr acene, 9-cyanoanthra-cene and 9-nitroanthracene (Figure 3) are r eproduced. The characteristic values of the maxima (»vertical ionization energies«) and onsets (»adiabatiG ionization energies«) of the systems observed are listed above and below the spectra, respectively. Uncertain values are given in parentheses. High resolution measurements on ant hracene showed that the first four systems exhibit a vibrational _fine structure with a progression in 1400 cm-1 - - - -(X), 320 and 750 cm-1 (A), 600 and 1400 cm-1 (B) and 1100 cm-1 (C) . The most I 970 18 16 14 12 10 8 E1 / eV 1031 Br 12.78 cCo ..-; 11Ll 7'8 977 927 6.67 16.L.1 ds l 103, -1 I 10 87 18 16 11. 12 10 8 E1/eV Figure 2. H el photoelectron spect ra of 9-chloroan th racene and 9-bromoanthracene. 18 j 3x103,-1 18 14.93 1022 985 9.09 ~ ~ •7.21 12,t.3 950 1~40 10.39 890 16 14 12 10 8 E1 /eV ; 18 16 14 12 .10 16 15.08 ~ ~ 7.67 1033 14 12 10 8 E1 /eV 18 16 14 12 10 Figure 3. Hel photoelectron spectra of 9-methoxyanthracene, 9-anthraldehyde, 9-cyanoanthracene and 9-nitroanthracene. ~ ~ 7.80 8 E1 /eV ~ ~ 8 E1 /eV w ...... 0 !'."' :i'1 t-' > Ul z () M .., > !'."' PE SPECTRA OF ANTHRACENES 311 prominent progression in 1400 cm-1 of the X-system is characteristic of all compounds investigated and can readily be observed at low resolution. This progression is also observed in PE and electronic absorption spectra4,13 of other polycyclic aromatic hydrocarbons, representing a normal vibration that involves alternate stretching of adjacent CC bonds4 • For the assignment of individual band systems in the PE spectra, semiempirical SCF MO calculations within the framework of Pariser, Parr and Pople's (PPP) method were per-formed. In the PPP calculations standard molecular geometries and parameters were employed (Table I). The resulting canonical SCF MO energies of the n:-orbitals for the calculated molecules are given in Table II. TABLE I Parameters Used in PPP Calculations Atom Wµ+/eV Yµµ +/eV ~c---1.JeV rG-µ/pm c 11.16 11.13 -2.39 140 Cl 24.01 11.27 - 0.93 171 Br 21.65 10.03 - 0.70 18,6 CHaO 26.65 14.53 -2.55 136 CHO uo 11.16 YO 11.13 -2.65 122 uo 17.7 YO 15.23 uo 11.16 YO 11.13 -3.5 115 CN UN 14.18 YN 12.52 uo 11.16 YO 11.13 148 C-N N02 UN 19.72 YN 14.80 __:2.37 uo 17.28 YO 15.27 122 N-0 * 1 ev = 96.487 kJ mo1-1 TABLE II Canonical SCF MO Energies in the Ground State of 9-Substituted Anthracenes in Electron Volts 9-X-Anthracene £1 £2 £3 £4 £5 E.6 £7 Es £9 £10 H 14.38 13.26 11.59 11.59 10.36 10.13 8.54 Cl 14.47 13.38 13.13 11.75 11.55 10.57 10.33 8.64 Br 14.46 13.37 12.11 11.74 11.44 10.60 10.32 8.JO CHaO 14.78 13.33 13.22 11.53 11.02 10.33 10.09 8.22 CH3-inductive CHO 14.76 13.41 13.24 11.75 11.51 10.49 10.29 8.76 CN 14.79 13.45 13.39 11.72 11.45 10.49 10.24 8.66 N02 20.90 16.83 15.80 14.14 13.96 12.91 12.61 11.02 9.56 Ph planar 14.59 13.47 13.25 11.88 11.58 10.36 10.35 10.31 10.12 8.29 35° rot. 14.51 13.43 13.25 11.80 11.58 10.36 10.35 10.32 10.12 8.36 312 L. KLASINC ET AL. The assignment of the four lowest energy ionizations known to be of n-type is in agreement with a linear relationship with calculated orbital energies, naturally implying the validity of Koopmans' theorem. Assuming that the linear correlation also holds for the corresponding systems in the n-type allows one to predict the positions of the corresponding systems in the PE spectrum within a range of about 0.5 eV. An inspection of the PE spectra at these positions usually yields an assignment supported also by the characte-ristic shape of such peaks. Thus, for anthracene (Figure 1, upper left) the systems at 7.43 (:rt7), 8.53 (n,;), 9.17 (:rt5) , 10.18 (:rt4 and :n;3), 11.82 (1t 2) and 13.5 eV (:rt1 ) were assigned as :rt-ionizations, indicating systems at 11.0, 11.50, 12.66 and 12.94 eV in this region as a-ionizations. The procedure mentioned above also allowed the prediction of :rt-ionization energies of those substituted anthra-cenes which were calculated by the PPP-method. The assignment of the system at 13.5 eV in anthracene as n, is also in agreement with the position of that ionization in phenanthrene (at about 14 eV)14. However, the assignment of :rt-ionizations in 9-methyl-, 9-tert-butyl- and 9-fluoroanthracene (Figure 1) can easily be deduced by comparison with the PE spectrum of anthracene and by taking into account the inductive effects of the alkyl and fluoro substituents15 . Thus, systems at 7.24 and 7:13 (:rt7), 8.41 and 8.35 (:rt6), 9.02 and 8.91 (n5), 9.83 and 9.71 (:rt4), 10.21 an d 10.1 (n3) as well as at 11.73 and 11.73 eV (:rt2) are easily recognized. in the 9-methyl- and 9-tert-butylanthracene spectra, respectively. The position of :rt1 is less certain, but systems at 13.36 and 13.74 could correspond to the anthracene system at about 13.5 eV, which was assigned as :n;1 ionization. Similarly, for 9-fluoro-anthracene, systems at 7.46 (:rt8 ), 8.69 (:rt7) , 9.31 (:rt0 ), 10.16 (:rt5 ) , 10.48 (1t.) , 11.85 (:rt3) and 13.73 eV (n2) were ascribed to :rt-ionizations. The position of the highest energy :n;1 ionization in the PE spectrum of 9-fluoroanthracene is expected to be above 16 ev1s. In all three compounds the degeneracy of the :rt4 and n 3 ionizations of anthracene is lifted by substitution and we observe two systems, of which the one at lower energy h as the greater intensity. For 9-chloro- and 9-bromoanthracene (Figure 2) calculations nicely predict their splitting and the position of the new :rt-ionization as well. Additionally, the following assignment results: 7.45 (:rt8 ) , 8.65 (:rt7), 9.27 (rr6) , 9.92 (:rt5), 10.41 (n,), 10.89 (nc1), 11.5 (1t3), 11.94 (:rt2) and 13.8 eV (rt1 ) for 9-chloroanthracene and 7.48 (:rt8 ) , 8.67 (:rt7), 9.27 (:rt6) , 9.77 (:rt5), 10.31 (nBr), 10.5? (:rt,) , 11.16 (:rt3 ), 12.00 (:rt2) and 13.59 eV (:rt1 ) for 9--bromoanthracene. In the latter, the lower intensity component of the split anthracene n)n3 systems is h idden by the nBr system. In the PE spectra of 9-methoxyanthracene, 9-anthraldehyde, 9-cyanoanthra-cene, and 9-nitroanthracene (Figure 3) ionizations which we can interpret as lone pair as well as n-ionizations can cause some trouble in assignment. Cal-culations assigning them as :n;-electrons will tend to predict their energy too high. Thus, for 9-methoxyanthracene, a comparison with calculation would indicate systems at 7.21 (n. ), 8.47 (:rt7), 9.09 (n6), 9.85 (:rt5) , 10.22 (:rt, ), 11.52 (:rt3 , mainly on oxygen), 11.85 (:rt2) and about 13.5 eV (:rt1 ) as :rt-ionizations. Ho-wever, the shape of the systems at 9.85 eV and 10.22 eV (lower intensity PE SPECTRA OF ANTHRACENES 313 component) and the fact that the compound could be interpreted also as an aryJ alkyl ether suggest that the »oxygen lone pair ionization« should be expected within the system at 10.22 eV. In this case the ionization at 11.52 eV is a a-ioni-zation which is also in agreement with the shape of the anthracene and 9-methylanthracene spectra in this region (compare Figure 3 and Figure 1). In the PE spectrum of 9-anthraldehyde the assignment of the system at 7.67 (rt8 ) is clear, but besides the rt7 and rt6 ionizations also the in-plane oxygen lone pair ionization must occur in the regron between 8.6 and 9.5 eV. Systems at 10.33 and 10.7 (?) eV were assigned as rt5 and rt4 , respectively. The additional rt-ionization arising from the C=O bond in the molecule is expected in agreement with calculation (£ 3) at high energy together with rt2 at about 11.9 eV. The system at 13.88 eV we tentatively assign as rt1-ionization. The electron withdrawing cyano and nitro substituents enhance strongly the energies of rt-ionizations. The additional, nearly degenerate, two rt-ioni-zations of the cyano group are well observed at 12.43 eV in the PE spectrum of 9-cyanoanthracene. Besides the completely uncertain position of rt1 (at 13.54 or 14.2 eV), and assuming that rt2 is at about 12.5 eV, the assignment 7.80 (rt8 ), 8.9 (rt7), 9.50 (rt6), 10.39 (rt5), 10.69 (rt4 ) , 12.43 (7t3 and the additional rtCN ionization) can be taken for granted for 9-cyanoanthracene. Also reasonable, and supported by calculation, is the assignment 7.88 (rt9) , 8.99 (rt8 ), 9.57 (rt7), 10.29 (rt6), 10.77 (rt5), about 11.4 (rt4 ) and 12.5 (7t 3) , 13.63 (rt2) and at about 16 eV (rt1 ) for 9-nitroanthracene. Addit ionally, by comparison with PE spectra of nitrobenzene and other nitro aromatics17, an (in-plane) oxygen lone pair ionization is assigned to be within the system at 10.77 eV (low intensity component). In the PE spectrum of 9-phenylanthracene similarly to Schafer et al.8 we assign systems at 7.18 (rt10), 8.37 (1t9), 8.89 (rt8 and Jt7), 9.23 (n6), 9.94 (7t 5), 10.21 (rt4 ), about 11.8 (n3 and rt2?) and 13.6 (rt1 ) as rt-ionizations. The effect of twisting the phenyl ring is negligible within the calculation, but it is very likely that this compound is non-planar. A steric effect, in addition to the electronic effect, is clearly indicated in Figure 4 where a correlation of the lowest energy 7.8 76 do /' ~v8 / Cl 7.4 72v; ~h 0 - 0.2 00 02 0.4 06 0.8 G para Figure 4. Correlation of the lowest ionization energy of 9-substituted anthracenes with Hammett O"p constants (from ref 19 and ref 20 (for the aldehyde group)). Full points for tert butyl and phenyl are not included in the correlation line. 314 L. KLASINC ET AL. n-ionization of the 9-substituted anthracenes with Hammett Opara values for benzenes19•20 clearly exists for all substituents besides tert-butyl and phenyl, which cause sterical hindrance. The eleven 9-substituted anthracenes in Figure 4 reveal an ionization energy of Ei = ((0.70 ± 0.07) eV) O'p + ((7.34 ± 0.03) eV) with a correlation coeffi-cient of r = 0.956. Omitting the two sterically hindered 9-phenyl- and 9-tert--butylanthracenes from Figure 4 results in Ei = ((0.64 ± 0.04) eV) O'p + ((7.38 ± ± 0.02) eV) with a correlation coefficient of r = 0.984. Since we are dealing with ground state cr-values obtained from kinetic data on mono-substituted benzenes, the correlation is surprisingly good and appears to be no worse than those for other spectroscopic correlations with Hammett o-values21 • Furthermore, the correlation of the lowest energy n-ionization of the 9--substituted anthracenes with Hammett cr-values is of the same quality as the one between the lowest ionization energy of benzenes and Opara values22 , indi-cating that, similar to the para-position in benzene, the 9-position in anthracenes is higly sensitive to electronic effects and that the response in both classes of molecules is the same. Acknowledgment. - This work was performed on the basis of a German-Yugo-slav scientific cooperation program. The financial support by the Internationales Buro, Kernforschungsanlage Jiilich, and by the Republic Council for Science of Croatia (SIZ II) is gratefully acknowledged. We thank Dr. J. Michl· from the University of Utah, Salt Lake City, for a sample of 9-fluoroanthracene. REFERENCES 1. H. G ii st en, L. K 1 a s inc, S. Koma din o vi c, and D. S t e fan o vi c, Croat. Chem. Acta 48 (1976) 1J9. 2. J. H. D. E 1 and, Int. J. Mass Spectrom. 9 (1972) 214. 3. M. J. S. Dewar and D. W. Go o d man, J. Chem. Soc. Faraday Trans. II 68 (1972) 1784. 4. R. B o s chi, J. N. Murr e 11, and W. S chm i d t, Discuss. Faraday Soc. 54 (1972) 116. 5. P. A. C 1 ark, F. Brog 1 i, and E. He i 1 bro n n er, Helv. Chim. Acta 55 (1972) 1415. 6. D. G. Streets and T. A. W i 11 i ams, J. Electron Spectrosc. 3 (1974) 71. 7. N. S. Hush, A. S. Cheung, and P. R. Hi 1 ton, J. Electron Spectrosc. 7 (1975) 385. 8. W. S ch a fer, A. S ch we i g, F. B i ck e 1 h au pt, and H. Vermeer, Angew. Chem. 84 (1972) 993. 9. S. Hin o and H. In o k u chi, Chem. Lett. (1974) 363. 10. K.-H. Grupe, M. Ruh ten berg, and K.-H. Heck n er, Z. Chem. 16 (1976) 20. 11. J. Sauer, J. Bendig, and M. Siegmund, Z. Chem. 17 (1977) 308. 12. L. K 1 as inc, B. Kovac, and B. Ru sci c, Kem. Ind. (Zagreb) 23 (1974) 569. 13. J. N. Murr e 11, The Theory of the Electronic Spectra . of Organic Molecules, · Methuen, London, 1963. 14. B. Ru sci c, B. Kovac, L. K 1 as inc, and H. G ii st en, Z. Naturforsch. 33a (1978) 1006. 15. H. G ii st en, L. K 1 as inc, and B. Ru sci c, Z. Naturforsch. 3la (1976) 1051. 16. W. v on Niess en, G. H. F. D i er ck s e n, and L. S. C e de r b au m, Chem. Phys. Lett. 45 (1977) 295. 17. T. Kobayashi and S. Nag a k u r a, J. Electron Spectrosc. 6 (1975) 421. 18. L. K 1 as inc, B. Ru sci c, G. Heinrich, and H. G ii st en, Z. Naturforsch. 32b (1977) 1291. 19. H. H. J a ff e, Chem. Rev. 53 (1953) 191. PE SPECTRA OF ANTHRACENES 315 20. A. A. Humffray, J. J. Ryan, J.P. Warren, and Y. H. Yung, Chem. Commun. (1965) 610. 21. A. R. Kat r it z k y and R. D. Tops om in: N. B. Ch a pm an and J . Shor-te r, (Eds.), Advances in Linear Free Energy ReLationships, Chap. 3, Plenum Press, London and New York, 1972. 22. M. Thompson, P.A. Hewitt, and D.S. Wooliscroft in : D. Briggs, (Ed.), Handbook of X-Ray and Ultraviolet Photoelectron Spectroscopy, Chap. 10, Heyden, London-Philadelphia-Rheine, 1977. SAZETAK Fotoelektronska spektroskopija 9-supstituiranih antracena L. Klasinc, B. Kovac, S. Schoof* i H. Gilsten* Snimljeni su HeI fotoelektronski spektri antracena i deset njegovih derivata sa slijedecim supstituentima u polofaju 9: metil, terc.-butil, fluor, klor, brom, metoksi, aldehid, cijano, nitro i fenil. Na 'osnovi molekularno-orbitalnih raeuna u spektrima su oznaceni sistemi koji su posljedica ionizacije elektrona. Hammettove konstante za benzenski sistem dobro opisuju efekt supstituenata na najni:le energije ionizacije. INSTITUT »RUDER BOSKOVIC« 41001 ZAGREB I •KERNFORSCHUNGZENTRUM KARLSRUHE, INSTITUT F-0-R RADIOCHEMIE, 7500 KARLSRUHE, POSTFACH 3640, BUNDESREPUBLIK DEUTSCHLAND Prispjelo 7. srpnja 1978. 

Photoelectron Spectroscopy of 9-Substituted Anthracenes

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