Photoelectron spectroscopy of benzophenone, acetophenone and their ortho-alkyl derivatives

Journal of Molecular Structure, 44 (1978) 203-210 OElsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

PHOTOELECTRON SPECTROSCOPY OF BENZOPHENONE, ACETOPHENONE AND THEIR ortho-ALKYL DERIVATIVES

G. CENTINEO, Istituto

I. FRAGALA’,

di Chimica

Genemle,

G. BRUNO and S. SPAMPINATO Uniuersitti

di Catania,

V. le A. Doria,

Catania

(Italy)

(Received 6 October 1977)

ABSTRACT The photoelectron spectra of benzophenone, acetophenone and their alkyl derivatives were measured. The photoelectron bands are discussed in the light of the influence of the molecular geometry. In benzophenone the v conjugation between the two benzene rings is the main factor determining the splitting of the original benzene orbitals. The deviation from the coplanarity between the two benzene rings accounts well for the observed effects. In the ortho-alkyl substituted compounds the strong steric inhibition of resonance due to the substituents allows their spectra to be discussed in terms of overlapping ionization of the two isolated benzene fragments. In the acetophenone derivatives the effects of alkyl substitution split the low energy overlapping band system of the acetophenone. Ionization energy values estimated through Heilbronner’s relationship agree with the experimental findings for the n and R (a,) ionization. INTRODUCTION

Photoelectron spectroscopy (PE) has been used in recent papers to investigate the dependence of higher occupied orbital energies on molecular geometry. The effects of steric inhibition of resonance has been claimed in respect to the PE spectra of the biphenyls [ 11, phenylethylenes [ 23, anilines [ 31, aromatic amines [ 43, etc. In the benzophenone systems the allowed conformations depend on the ring substituents [ 5 ] . In order to verify how steric hindrance can influence the spectral pattern we investigated a series of molecules whose molecular geometries change from a planar situation to a non-planar one. EXPERIMENTAL

Commercial samples were generally used. The alkyl substituted benzophenones and acetophenones were kindly supplied by Prof. G. Montaudo. All the compounds were purified by distillation and vacuum sublimation just prior to their use. He (I) PE spectra were run on a PS 18 (Perkin-Elmer) photoelectron spectrometer using a heated probe. All the spectra were calibrated by reference to the Xe and Ar peaks. Ionization energy (IF) data are collected in Table 1.

a.42

(TUB) 8.14

9.45 9.15 9.66 9.65 9.35 8.77 8.60 9‘42 8.57 8.46

9.05 8.29 9.46 98:;;: 8.50 8.30 8.98 8.32 8.18

LPBC) (TMA) (TIA) (BMC) (TMB) (TIB)

(FN) (API

(BP)

*Kobajashi and Nagakura’s results [ 61.

Phenylbenzylketone 2,4,6trimethylacetophenone 2,4,6-triisopropylacetophenone Benzylmethylketone 2,4,6-trimethylbenzophenone 2,4,6-triisopropylbenzophenone 2,4,6+riisopropyl-2.methylbenzophenone

Benzophenone Fluorenone Acetophenone

Ionization energy data (eV) for the studied molecules

TABLE 1

8.88

9,77 9.58 so20 9.00 9.75 8.99 8.95

9.65 9.28 9.83

9.13

9.18 9.06

9.75 9.47 11.95 11.91

9.47 9.41

10.20

10.84

11,74 11,20 11.00 11.64 10,52 lo,40

11,78 12,12 12,70

11.70

12.05 12.60 11.90 12.16 12.05 10.86

12.10 12.66 14.05

12.00

12.57 13.40 13.60 13.60 12.82 11.90

12.60 14.60 16.20

13.36

14.20 14.30 14.7 14.8 14.05 13.05

14.2 16.35 16.62

14.35

13.90

205 RESULTS AND DISCUSSION

The PE spectrum of benzophenone (Fig. 1) can be interpreted by reference to the spectra of acetophenone (Fig. 1) and fluorenone (Fig. 1) and through CNDO/B calculations. The PE spectrum of fluorenone closely resembles the spectrum of the parent fluorene except for a new sharp band at 9.28 eV. A comparison of the IE of the bands present in the 7-10 eV region indicates that the fluorenone bands are shifted by 0.3 eV with respect to the corresponding ones in the fluorene spectrum (Table 2). In particular the separation between the two extreme bands is preserved in the two compounds. According to Maier and Turner’s results [l] on the fluorene molecule, it can be argued that in fluorenone the interaction between the n systems located on the benzene rings is also a leading factor in determining the ordering of the upper filled molecular orbitals (MO). Consequently, the new 9.28 eV fluorenone band can be attributed to the n carbonyl oxygen lone pair. FN

% Jn

I,

I,

B

10

9

I

’

IO

10

0

eV

Fig. 1. PE spectrum of fluorenone and PE spectra (expanded scale) of benzophenone and acetophenone (low energy regions). TABLE 2 Vertical ionization potentials (eV) and corresponding band assignments of some molecules studied. Some literature data are reported for comparison Fltmrene

Fluorenone Benzylmethylketone PhenyIacetaldehyde 2-Methylacetophenone 2.4.GTrimethylacetophenone 2.4.6-TdisopropyLacetophenone

7.93

P6

8.29 (9.5)a 8.98 n(b *) 9.15 n(b,) 9.15 n
8.83

A~

9.15 <10.3Ja 9.42 n(a,) 9.56 rr(a,) 9.32 n (9.30)b 8.77

I@’

(8.83) 8.63 n(a,)

9.13 ‘IT, 9.28 (10.4)” 9.73 n 10.08 n 9.44 g<“z’ (9.43) 9.18 n (9.10)b 9.14 n

9.41 n, 9.47 tll.lja

Ref. 10.2 (12.6)a Ref. Ref.

potentials computed through CNDO/B calculations (bond angles and distances from ref. 7). bIonization potentials calculated through Heilbronner’s relationship.

aIonization

206

The corresponding ionization gives rise to the 9.72 eV band in acetone and the 9.55 eV band in acetophenone. The trend to lower IE on going from dimethylketone to diphenylketone through methyl-phenylketone can be connected with the electron migration from the benzene ring to the carbonyl group, thus increasingthe negative charge on the oxygen atom. CNDO/B calculations confirm the above experimental assignments. In Table 2 the IE relative to the five upper filled MO’s, according to the Koopmans’ approximation, are reported and compared with the experimental tidings of both fluorenone and fluorene. The fluorene MO’s are labelled according to Maier and Turner’s notation. The PE spectrum of benzophenone shows a considerable change when compared to fluorenone and a close resemblance to the spectrum of acetophenone. The benzophenone spectrum shows, in the 8-10 eV region, an ill-resolved overlapping band where two peaks (9.05 and 9,45 eV) can be distinguished, with two shoulders on the higher energy side. The peak arising from the n ionization, on the basis of the band shape and of the n acetophenone IE, can be attributed to the 9.45 eV band. The CNDO/2 calculation agrees with this interpretation. In Fig. 2 we report a pictorial representation of CNDO/Z MO’s for fluorenone, for a hypothetical planar structure of benzophenone and for the “real” structure of benzophenone where the benzene rings are rotated, out of the plane determined by the carbonyl group, by Qi = # = 33” (C,, molecular symmetry) [5].

-*bl

“Planar” beruophenone

-vb

.

l3enzophenone

Fig. 2. Pictorial representation of localization orbitals of fluorenone, “planar” benwphenone

3

properties of the upper fiied and benwphenone.

Fig. 3. Correlation diagram of the upper filled molecular benwphenone and benwphenone.

molecular

orbitals of fluorenone,

“planar”

207

Simple correspondences between the upper filled MO’s of the three aforesaid structures can be drawn from the correlation diagram reported in Fig. 3. No meaningful differences arise on going from the coplanar benzophenone structure to the non-planar one. Notable differences, on the contrary, appear when comparing fluorenone with both the benzophenone structures. In particular the 7r4(b ,) benzophenone MQ is strongly stabilized in the fluorenone molecule due to the bonding interaction between the 2,2’centers. In addition, due to the introduction of an antibonding x-orbital in benzophenone, its R~(cz~)level is destabilized becoming the upper filled one in fluorenone. On the whole the above theoretical approach suggests the more important interaction, lifting the degeneracy of the original benzene orbitals, to be the conjugation between the two phenyl moieties. This reduced interaction in the benzophenone system can account for the grouping of the corresponding PE bands in its spectrum with respect to the fluorenone one (Fig. 1). The PE spectrum of phenylbenzylketone (PBC) also indicates 1~conjugation as the main factor in determinin g the splitting of the original benzene levels. The close resemblance (Fig. 4) between its PE spectrum and that of benzophenone leads one to conclude that the deviation from coplanarity in benzophenone gives the same effect as the conjugation rupture in PBC. The PE spectra of alkyl derivatives of benzophenone (Fig. 5) show two band systems in the 8-9 eV region. It seems useful to discuss these spectra with reference to the corresponding ones of acetophenone alkyl derivatives on the basis of Kobayashi and Nagakura’s treatment [6]. Ih the acetophenone spectrum the non-bonding carbonyl-oxygen n orbital is accidentally neardegenerate with the outermost 7~orbitals. The effects of alhyl substitution PBC

Fig. 4. PE spectra (low energy regions)

of phenylbenzylketone

Fig. 5. PE spectra (low energy regions)

of benzophenone

and of benzylmethylketone.

alkyl substituted

derivatives.

split the overlapping band system. In fact the elg benzene orbitals suffer a more marked influence than the rzoxygen orbital because of the alkyl substitution. Consequently, the shift to lower ionization energies will be more pronounced for the n-systems than for the n one. Both 2,4,6-trimethylacetophenone (TMA) and 2,4,6&iisopropylacetophenone (TIA) PE spectra (Fig. 6) show in the 8-9 eV region a first asymmetric band where two distinct peaks can be detected. At higher IE (9.0-9.5 eV) a sharp symmetric band, characteristic for the non-bonding ionization, is detected. In effect this approach to the spectral pattern of the ring substituted acetophenones does not account for the effect of steric inhibition of resonance due to the ortho substituents. By comparing the PE spectrum of benzylmethylketone (BMC) (Fig. 4) with those of TMA and TIA, some correlations between the effects of the conjugation rupture, due to introduction of the CH, group, and the steric inhibition of resonance can be made. The BMC spectrum can be immediately assigned by reference to the PE spectrum of phenylacetaldehyde [S] (Table 2). The reported IE data refer to orbitals conventionally classified in the C 2v symmetry. Noteworthy the 9.73 eV band corresponds to the acetone n ionization (g-72 eV) and the 10.08 eV one to the acetaldehyde n ionization (10.22 eV). On the whole the BMC spectrum can be considered as coming from the simple overlap of the n acetone band and the toluene PE spectrum. In Table 2 some IE values computed through Heilbronner’s method [lo] are compared with the experimental findings. In this Table the orbital notation still refers to a “working” symmetry C&_ This allows a straight comparison with the “original” benzene orbitals. A correlation diagram is also reported in Fig. 7. A comparison between the mesitylene PE data [ 91 and the corresponding ones for TMA (Fig. 7) confirms unambiguously the attribution of the TMA spectrum. In fact in this spectrum the 8.42 eV mesitylene band splits giving two components due to the lifting of degeneracy of err(DSh) orbitals. IE values estimated through

Fig. 6. PE spectra (low energy regions) of acetophenone

aikyl substituted

derivatives.

209 IE

El?”

85-

Fig. 7. Correlation diagram of the upper filled molecular acetophenones and of some related molecules.

orbitals of trialkyl substituted

Heilbronner’s relationship* (Table 2) agree well with the experimental values only for the n and n(a,) ionizations. This fact can be connected with the localization properties of TI(Q*)level in acetophenone-like molecules. This orbital can be described as completely localized on the benzene ring so that it does not suffer any perturbation by the steric inhibition of resonance due to the absence of coplanarity in the ortho alkyl substituted acetophenones. The same is true for the n orbital localized on the carbonyl oxygen. Consequently, the differences between the experimental and computed values for the n(b,) level can be considered as a measure of the deviation from the coplanarity of the carbonyl group with respect to the benzene ring plane. In fact the trend in these differences parallels the increase of the torsion angles; methylacetophenone (MA) 4 = 28”) TMA 4 = 51”, TIA 6 = 53” [l&12]. Looking now at the substituted benzophenone systems, the considerations for the corresponding acetophenones allow a “qualitative” picture of their spectral pattern to be made. The observed effect of the ortho alkyl sub-

stitution shifts the ‘ITionizations towards lower energies, while the n ionizations remain practically insensitive to this effect. Consequently, the bands in the 8.0-8.5 eV region in the PE spectra of all the substituted *In the Heilbronner relationship: --al”(J) = 0.0528 + 1.0110 Zr(3C;r + Cjr + C&)/3, where c denotes the positio.1 of methyl substitution, and r and 7’ the position of the nearest neighbours to r_ Ca is the coefficient of the k-th A0 in the J-th TTMO obtained by CNDO/Z method for planar acetophenone.

210

benzophenones can be attributed to the 7rionization of the more substituted ring. The ill-resolved bands in the 8.9-9.5 eV region belong to both the n ionization and to the ‘ITionization of the unsubstituted or less substituted benzene rings. On the other hand, the corresponding ionizations of benzene and toluene are at 9.25 eV and 8.88-9.30 eV respectively. This proposed spectral pattern agrees well with the intensity ratios between the two low energy PE bands (2 : 3) observed in all the substituted benzophenones. Conformational computations indicate the torsion angle, defined according to Buchanan et al. [ 131, to be cf, = 90”, @ = 0” for the 2,4,6-substituted benzophenone derivatives and Cp= 90”, $ = 30” for the 2,4,6,2’-ones. This data accounts for the strong steric inhibition of resonance increasing with the number of ortho substituents. Consequently, the spectra of the ortho substituted benzophenones can be discussed in terms of overlapping of the two isolated fragments. ACiC.

IWLEDGEMENTS

We thank Prof. G. Montaudo and Prof. G. Condorelli, Catania, for helpful suggestions and discussions.

of Universita di

REFERENCES 1 2 3 4 5 6 7 8 9 10

J. P. Maier and D. W. Turner, Discuss. Faraday Sot., 54 (1972) 149. J. P. Maier and D. W. Turner, J. Chem. Sot. Faraday Trans. 2.69 (1973) 196. J. P. Maier and D. W. Turner, J. Chem. Sot. Faraday Trans. 2,69 (1973) 521. H. J. Haink and J. E. Adams, Ber. Bunsenges. Phys. Chem., 78 (1974) 437. R. Hoffman and J. Swenson, J. Chem. Phys., 74 (1970) 415. T. Kobayashi and S. Nagakura, Bull. Chem. Sot. Jpn., 47 (1974) 2563. H. Kuroda and T. L. Kunii, Theor. Chem. Acta, 9 (1967) 51. J. W. Rabalais and R.. J. Colton, J. Electron Spectrosc. Relat. Phenom., 1 (1972) 83. T. Koenig and M. Tuttle, J. Org. Chem., 39 (1974) 1308. E. Heilbronner, V. Homung, F. H. Pinkerton and S. F. Thames, Helv. Chim. Acta, 5 (1972) 289. 11 K. S. Dhami and J. B. Stothers, Tetrahedron Lett., (1964) 631. 12 K. S. Dhami and J. B. Stothers, Can. J. Chem., 43 (1965) 479. 13 G. W. Buchanan, G. Montaudo and P. Finocchiaro, Can. J. Chem., 51 (1973) 1053.