The He I photoelectron (PE) spectra of stilbene (1), 2,2\u27-dichlorostilbene
(2) 2-amino-2\u27-chlorostilbene (3), 2,2\u27-diaminostilbene
(4) ,and 2,2\u27-dinitrostilbene (5), all in their trans configuration are
recorded and discussed. The electronic structure of 1-5 can well
be described on the basis of the composite molecule method and
simple Hiickel molecular orbital (HMO) calculations. Results indicate
the importance of substituent effects causing characteristic
shifts of low energy electronic band systems in the PE spectra

Krešimir Jakopčić

Leo Klasinc

Mladen Mintas

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CROATICA CHEMICA ACTA : CCACAA 56 (2) 263-267 (1983) CCA-1375 YU ISSN 0011-1643 UDC 541/547/535.33 Original Scientific Paper PhotOelectron Spectroscopy of Some trans-2,2'-Disubstituted Stilbenes* Mladen Mintas and Kre8imir Jakopcic Department of Organic Chemistry, Faculty of Technology University of Zagreb and Leo Klasinc# »Ruder Boskovic« Institute, Zagreb, Croatia, Yugoslavia Received August 30, 1982 The He I photoelectron (PE) spectra of stilbene (1), 2,2'-di-chlorostilbene (2) 2-amino-2'-chlorostilbene (3), 2,2'-diaminostilbene (4) ,and 2,2'-dinitrostilbene (5), all in their trans configuration are recorded and discussed. The electronic structure of 1-5 can well be described on the basis of the composite molecule method and simple Hiickel molecular orbital (HMO) calculations. Results indi-cate the importance of substituent effects causing characteristic shifts of low energy electronic band systems in the PE spectra. INTRODUCTION Elucidation of substituent effects on electronic structure of conjugated molecules in a general manner is an intriguing problem which has been the subject of numerous investigations. PE spectroscopy and MO calculations, within the validity of Koopmans' theorem provide a powerful tool for such investigations. In the present study we wish to report such results on some trans-2,2'-disubstituted stilbenes, 2-5, which as compared with the expected splitting of low energy n:-ionizations caused by the substituents (»conjugative effect«) indicate the overwhelming importance of their characteristic shifts (»inductive effect«). Besides numerous results on the PE spectra of trans and cis-stilbene6- 8 so far only one· paper reported 'Spectra of some 4-substituted stilbenes.9 EXPERIMENTAL The He! PE spectra of 1-5 have been recorded on a Vacuum Generators UVG3 spectrometer employing elevated temperatures of the inlet system (100 °c, 110 °c, 140 °c, 200 oc, and 230 °c for 1-5, respectively). Details of the instrument are descri-bed in ref. 1. No decomposition of the samples under these conditions was obserwed. The energy scale was calbirated by admitting small amounts of Xe and/or CH3J t0 the sample flow. * Presented at The IUPAC International Symposium on Theoretical Organic Chemistry, held in Dubrovnik, Croatia, August 30 - September 3, 1982. * Reprints requests to Professor L. Klasinc, »Ruder Boskovic« Institute, 41001 Zagreb, FOB 1016, Yugoslavia. · 264 M. MINTAS ET AL. Besides 1, which was of commercial origin (zone melted) compounds 2-5 were synthesized as described in ref. 2. Standard HMO calculations for 1-5 have been performed using the same para-meter set as in ref. 3, 4 (NH2, N02) and he1 = 2.0, kee1 = 0.4 for chlorine.5 RESULTS The He! PE spectra of 1-5 are reproduced in Figure 1. The numbers above each spectrum correspond to vertical ionization energies of observed band systems which are assigned below according to standard notation. The mk coefficients of the HMO calculations defining the energy Ek of the molecular orbital by Ek = ae + mk (Jee, k = 1, 2, ... n are listed for compounds 1-5 in Table I.Correlation of low energy n-ionizations in 1-5 with the calculated 7.79 ~ -~ 2 18 16 14 12 10 B E1 /eV Figure 1. The Hel photoelectron spectra of 2,2'-dlaminostilbene (4), 2-amino-2'chlorostilbene (3), 2,2'-dlchlorostilbene (2), stilbene (1) and 2,2'-dinitrostilbene (5) PHOTOELECTRON SPECTROSCOPY OF STILBENES 265 n-electron distribution within the corresponding MO's (area of circles are proportional to squared coefficients of atomic orbitals) is given in Figure 2. 7 8 9 10 l _s: ~ ~ _./ 119 ~ 1)--4 5 .,/ ~ •• / 1.;t •• 7.59 ••• A. ~.····'if, 7.79 .-0 ..!'"\ . ...- 19 ~ •• 7 ~ .·---~ 0-0 • .H,"···j>-.. / 7.90 . ...... / 117 - --- ------ --a:: ·--~ 0-o' _840/;;,_···~····T,~ ... --- · · 11, ~/- 8.58 8.68 • • ·18 ~--- • .--, . -· "' .- · .• / e 11',, .... ,,.. . / 9.06~---'!lot_ .~---- · -- -8:1 /~ 'lr,lf~--· .... ---· ... , 7 _.-;.-g:;;-- 9.35 / 1~ 970, ~~--·v;·-~· -- 9::-~---·rr.· ~ 1199.88.~~--~ V4 l>4 ... 9: 11 11108 " ,- "s ,_. .. .. 10.48 - ~ __ ,.,. ~-~---~-- lo.66~ .. -10.61.-0 - ~ --~·- 'iis ·. ~ ...... 'ifs Figure 2. Correlation diagram for 1'-ionizatlons of o,o'-disubstituted stilbenes 1-5 TABLE I HMO Results for Compounds 1-5 Compound mi m2 ms m4 ms ms m1 ms m9 mio mu 1 2.21 2.06 1.50 1.16 1.00 1.00 0.50 2 2.30 2.23 1.98 1.91 1.50 1.15 0.96 0.96 0.49 3 2.34 2.24 1.95 1.69 1.50 1.15 0.96 0.82 0.44 4 2.36 2.27 1.72 1.66 1.50 1.14 0.84 0.79 .0.40 5 4.19 4.19 2.23 2.09 1.51 1.50 1.50 1.17 1.09 1.07 0.56 trans-Stilbene (1) The PE spectrum of 1, as well as of its isomer cis-stilbene has been reported several times6- 8 and the assignment of the five lowest energy systems to n-fonizations can be taken as established. The location of Jt2 at 12.11 eV and n1 at 12.73 eV is less certain, and will be proven in near future either through measurement of the Penning ionization spectrum of stilbene10 or by the >> perfluoro effect« in the PE spectrum of perfluoro stilbene. The correlation 266 M. MINTAS ET AL. of the re-ionizations with HMO coefficients yields a fairly good straight line which practically coincides with those obtained for indene analogs3 and Schiff's bases4• As shown by electron diffraction measurements11 1 is not planar in the gas phase, its phenyl rings being rotated for 30° around the C-Ph bond. This nonplanarity is indicated also in the PE spectrum of 1 by the structureless X and D systems. The vibrational fine structure in these systems can e. g. be observed for diindene12 which is a model compound for planar 1. For the same reason, i.e. structureless systems X and D, we anti-cipate that 2-5 are not" planar in the gas phase too. trans-2,2'-Dichlorostilbene (2) The presence of two chloro substituents is indicated in the PE spectrum of 2 by the two doubly -degenerated band systems at 11.19 eV and 11.53 eV, which correspond to ionization of the 3py (o) and 3px (:n:) »lone pair« electrons of the chlorine atoms, respectively. The observation of such systems ·of close energy (doublets), known as Pxf Py splitting13 is based on symmetry reasons: the 3px orbitals which are perpendicular to the plane of phenyl rings have the right symmetry to interact with the :n:-system what results in their stabilization i.e. higher ionization energy; on the other hand the interaction of 3py (in plane) orbitals with the a-system is small because of the difference in energy. The correlation of the five low energy :n:-orbitals ·of 1 and 2 {Figure 2) shows a stabilization of systems X, C and D (the same probably holds for the two highest energy :n:-ionizations) and a splitting and shift of the degenerate A and B systems (in 1) to lower energy (in 2) . The correlation with HMO indices of the five low energy :n:-ionizations yields practically the same line as for trans stilbene. trans-2-Amino-2'-chlorostilbene (3) In the PE spectrum of 3 we see the separation of the A, B and C systems which was indicated also in 2 and will become complete in 4. This is well predicted by the respective HMO results. In correlation with 2 the low energy n-ionizations are shifted to lower values (Figure 2) in accordance with the expected effect of an amino group. Again, correlation with the corresponding HMO indices yields the same regression line as for 1 and .2. The system at 11.03 eV in 3 corresponds to the Py (in plane) ionization of chlorine and is followed by the system at 11.52 eV arising from the Px-ionizati-on. trans-2,2'-Diaminostilbene ( 4) and trans-2,2'-Dinitrostilbene (5) The effect of electron donating and electron withdrawing substituents on · 1 is illustrated by the spectra of compounds 4 and 5, respectively. What one readiiy observes is the strong shift of :n;-ionizations into ·Opposite direction and a different behaviour respective to conjugative effects. Namely, systems A, B and C come appart in 4, but nearly coincide in the PE spectrum .of 5. Both is qualitatively predicted by simple HMO calculations (Table I). However, PHOTOELECTRON SPECTROSCOPY OF STILBENES 267 correlations with HMO indices for 4 and 5 although yielding straight lines with the same slope as for 1-3 have clearly different intercepts caused by corresponding shifts of about 0.5 eV to lower and higher ionization energies, respectively. DISCUSSION The characteristic influence which certain substituents or heteroatoms exhibit on the PE spectrum of a »parent« compound has been widely reco-gnized and usually intepreted in terms of »perfluoro«, »aza«, »nitro group«, »methyl-group« and other effects. Hereby the observation was meant that characteristic and structurally defined parts of the »parent« PE spectrum remained practically unchanged in shape, but were shifted to higher or lower ionization energies in the related spectrum. In the present case we have a similar situation, namely that besides the splitting of systems which even within the simple HMO method is well described by conjugative effects, in compounds 4 and 5 with strong electron donating and electron withdrawing groups as substituents, respectively, we observe, in correlation to the parent compound, the overwhelming effects of energy shifting: all systems in 4 are shifted to lower and all in 5 to higher ionization energies. One can define such shift as a characteristic of the substituent. The similar has been shown14 in the case of monosubstituted benzenes, naphthalenes and anthracenes. However, in the present compounds enlarged twisting caused by the substi· tuents additionally influences the spectra yielding structureless and rather broad systems X and D. REFERENCES I. L. Klas inc, B. Kovac, and B. Ru sci c, Kem. Ind. (Zagreb), 23 (1974) 569. 2. M. Mint as, K. J a k op c i c, and L. Klas inc, Z. Naturforsch. 32b (1977) 181. 3. H. G i.i st en, L. Klas inc, and B. Ru sci c, Z. Naturforsch. 3la (1976) 1051. 4. L. K 1 as inc, B. Ru sci c, G. Heinrich, and H. G i.i st en, Z. Naturforsch. 32b (1977) 1291. 5. A. Streit w i es er, Jr., Molecular Orbital Theory for Organic Chemists, Wiley, New York 1961. 6. J. P. Maier and D. W. Turner, JCS Faraday II, 69 (1973) 196. 7. T. Kobayashi, K. Yokota, and S. Nagakura, Bun. Chem. Soc. Jpn. 48 (1975) 412. 8. L. J. Krick a, M. Lambert, and A. Ledwith, J.C.S. Perkin I (1974) 52. 9. E. J. Mc Al duff and T. Ch an, Can. J. Chem. 56 (1978) 2714. 10. T. Mun a k at a, K. 0 h no, Y. Hor ad a, and K. Kuch its u, Chem. Phys. Lett. 83 (1981) 243. 11. M. Tr a et t e berg and E. B. Frants en, J. MoL Struct. 26 (1975) 69. 12. L. Klas inc and H. G i.i st en, unpublished results. 13. A. D. Baker, D. P. May, and D. W. Turner, J. Chem. Soc. (B) (1968) 22. 14. L. Klas inc, B. Kovac, and H. Gi.isten, Pure Appl. Chem., in press. SAZETAK Fotoelektronski spektri nekih trans-2,2'-disupstituiranih stilbena M. Mintas, K. Jakopcic i L. Klasinc Snimljeni su i diskutirailli He I fotoelektronski spektri stilbena (1), 2,2'-diklor-stilbena (2), 2-amino-2'-klorstilbena (3), 2,2'-diaminostilbena (4) i 2,2'-dinitrostilbena (5), svih u trans konfiguraciji. Elektronska struktura spojeva 1-5 moze se dobro opisati na osnovu metode sastavljene molekule i jednostavnim racunima Hi.ickelovih molekularnih orbitala. Rezultati ukazuju na znacaj efekata supstituenata, koji iza-zivaju karakteristiene pomake nisko energetskih elektronskih sistema vrpci u foto-elektronskim spektrima. 

Photoelectron Spectroscopy of Some trans-2,2-Disubstituted Stilbenes

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