Photoelectron spectra of some aromatic mono- and di-ketones

Journal of Electron

Spectroscopy

and Related

PHOTOELECTRON D&KETONES

E J McALDUFF?

Department

17 (1979) 81-89 Prmted m The Netherlands

Phenomena,

0 Elsevler Sclentlflc Pubhshmg Company, Amsterdam -

SPECTRA OF SOME AROMATIC

MONO- AND

and D L BUNBURY

of Chemistry,

St Fmnczs Xavaer Unzverslty, Antagonzsh, Nova Scotza (Canada)

(First received 5 July 1978, in final form 6 March 1979)

ABSTRACT The photoelectron spectra of benzophenone, 4-OH benzophenone, benzll, 4-OH benzll, dlbenzoylmethane, 2,2dlbenzoylpropane, 1,2dlbenzoylethane and 2-(p-methoxybenzoyl)-2-benzoylpropane have been determined and ionization potentials lying below 15 eV have been assigned These assignments lead to the result that for compounds contaming substltuents on the phenyl ring, an aromatlc n orbital IS the highest occupied For the enohzable ketone, dl-benzoylmethane, an enol a 18 the HOMO, whereas for all other compounds investigated an n orbital IS the highest occupied

INTRODUCTION

In order to rationalize the photoreactivity of mono- and di-ketones, it ISnecessary to have information about the ground and excited states of these species Wlthm the context of Koopmans’ theorem [I] , photoelectron spectroscopy provides a method of obtammg mformation about the drstnbution of energy levels m the ground state of the molecule The common absorption band m the near-UV for mono-ketones is related to an n + ?r* transition mvolvmg the non-bonding electrons of the carbonyl group In dicarbonyls, strong through-bond couplmg sphts the otherwise degenerate n orbit& whereas the magnitude of the interaction and the extent of sphttrng between the ?r* levels is more ambiguous [ 21. The benzoyl group IS an 8 x-electron system, so that there 1s some murmg of the carbonyl 7r and s* orbitals with the benzene orbit& as long as the two n systems are not perpendicular The highest occupied orbital is mostly benzene-like with a httle carbonyl character, while the lowest a” resides prrmanly on the carbonyl In mono-ketones the n orbital is located on the carbonyl oxygen whereas the non-bonding orb&& of di-ketones can interact by direct overlap ’ Author to whom correspondence should be addressed

82

(“through-space”) or by mteractlons with connecting IS bonds (“throughbond”) [3] Through-space rnteraction of the n orbltals will produce a bondmg combmatron n,, and an anti-bonding combmatlon, n_ In configurations m which the n orbltals overlap very little, the through-space mteractlon 1s small and through-bond mteractions will dominate Electrondonatmg substltuents on the phenyl can rarse the orbital energy of the highest occupied R orbital (lower its IP) to such an extent that INSenergy can approach or exceed that of the non-bonding n orbital In this mvestxgatlon we have deterrnmed the photoelectron spectra of a mono-ketone (benzophenone) a 1,2dl-ketone (benzll), 1,3dl-ketones which are both enohzable (dlbenzoylmethane) and non-enohzable (2,2dlbenzoylpropane) and a 1,4dl-ketone (1,2dlbenzoylethane). In the course of asslgnmg the ionization potentials, rt, was found useful to determme the photoelectron spectra of 4-OH benzophenone, 4-OH benzrl and 2-(p-methoxybenzoyl)-2-benzoylpropane

RESULTS

AND DISCUSSION

The lomzatron potentials and then assignments are hsted m Table 1 The followmg discussion is concerned prnnanly with IP values less than 12 eV, those lymg above 12 eV are tentatively assigned and listed m Table 1

Benzophenone

(1) and P-OH benzophenone

(2)

Figure 1 shows the photoelectron spectra of benzophenone and 4-OH benzophenone The first band at 9.0eV m benzophenone IS assigned as no. The analogous lomzation occurs at 8 37 eV m acetophenone [4] The second band m benzophenone contams the two phenyl orbltals (Ph, and PhA ) which are not resolved and are centred at 9 4 eV. The phenyl S (b, m c 2v symmetry) orbital LS characterized by high electron density at the positron of substitution of the carbonyl group and para to it and smaller electron density m the ortho and meta positions The phenyl A (a2 m CZv) orbrtal IS characterized by a node at the position of substitution and para to it and high electron den&y m the ortho and meta positions In acetophenone the Phs and PhA are separated and located at 9 55 and 9 77 eV, respectmely. Centmeo et al [5] have obtained bands at 9 45 and 9.65 eV for benzophenone and assigned the IP values to Ph, and PhS, respectively, but their conclusrons are based on an incorrect acetophenone assignment By analogy with phenol [4] the principal effect of substitution of the hydroxy group on benzophenone is expected to be the removal of the accidental degeneracy of the phenyl S and phenyl A levels. In 4-OH benzophenone the first IP at 8.8 eV is aswgned to the phenyl S orbital which has been destabilized from benzophenone by the mductlve effect of the OH group The decrease of 0 6 eV m IP 1s comparable to the decrease of 0 52 eV observed from benzene to phenol [4] The IP at 9 1 m 4-OH benzophenone

142n(C=O)

145n(C=O)

90n_ 9 3 Phs 96PhA,n+ 12 2 Ph

8 45 (C = C) en01 9 4 Phg, PhA , no

118 Ph

2,2-Dlbenzoylpropane (6)

8 6 Phs 8 9 n_ 9 3 PhA,n+ 117Ph (methoxy benzoyl) 143n(C=O) 11 1 no(CH30)

9 2 n, 9 45 Phs, n_ 9 7 PhA 120Ph 143n(C=O)

2-(p-Me0 benzoyl)-2benzoylpropane (8)

1,2-Dlbenzoylethane (‘7)

1106 no (CH3 0)

9 22 PhA 1152 Ph

8 39 Phs

Amsole [ 4 ]

8 73 Ph, 9 40 PhA 1199 Ph 1169 no(OH)

8 9 n,, Phs 96PhA 10 7 n_ 12 1 Ph, no(OH) 1447r(C=O)

8 9 n, 9 5 Phs, PhA 10 8 n12 2Ph 143n(C=O)

8 80 Phs 9 1 no 9 4 PhA 120Ph 142n(C=O) 114 no(O-H)

9 05 no 9 42 Phs, PhA 12 0 14 2

9 37 no 9 55 Phs 9 77 PhA 1191 Ph

I)lbenzoylmethane (5)

Phenol[4]

4-OH Bend (4)

Benz11(3)

4-OH Benzophenone (2)

Benzophenone (1)

._.

Acetophenone [ 4 ]

TABLE 1 VERTICAL IONIZATION POTENTIAL IN eV (+ 0 OS) AND ASSIGNMENTS

8

+-+I

lonlzatlon

Potential

Fig 1 Photoelectron spectra (A) Benzophenone, (B) 4-OH benzophenone

asslslgned to the n0 which IS mrtually unchanged from benzophenone Substltutlon of OH 1s not expected to alter Ph, wgnlficanfly and the band at 9 4 1s assigned to It The prmclpal difference between the energy levels of benzophenone and 4-OH benzophenone IS that the HOMO for the former is assigned as no whereas for the latter it is Ph,

IS

Benzgl(3) and 4-OH benzll(4) Figure 2 shows the photoelectron spectra of benzll and 4-OH benzll The photoelectron spectrum of benzrl has previously been determmed 121. The order of energy levels is identical to that reported here, but the lone pair combmatlons are higher by 0 2 eV and the phenyl levels are higher by 0.4 eV m the previous study In 4-OH benzll the first IP at 8 9 eV gives a peak with a much larger area than the third IP at 10 7 eV, unhke the benzll spectrum where the n+ and n- ionizations resulted m peaks of approximately the same size. This suggests that the fust peak m 4-OH benzll contams ionization from two levels One of these is assigned to the symmetric lone pair combination (n+) By analogy

85

P

\

“\ L\. \

a

I 10

12

iontzotton Fig 2 Photoelectron

I 14

-t

16

Potential

spectra (A) 4-OH benzd, (B) benzd

with previous substltuent effects observed m phenol [4] the second lonlzation m thrs band at 8.9 eV IS assigned to the phenyl S lonlzatlon which represents a decrease m its IP by 0 55 eV from benzll. The second band at 9 5 eV 1s shghtly sharper than m benzll and u asslgned to the PhA level The band at 10 7 eV 15 asslgned to the antlsymmetnc lone pan combmatlon n-which would represent a destabtizatlon of 0.15 eV from its posltlon m benzll The sphttmg between the anti-symmetnc and symmetnc lone pm combmatlons 18 18 eV m 4-OH benzll mdlcatmg that there IS a strong through-bond mteraction between the lone parrs as m benzll. Dzbenzoylmethane (5) The photoelectron spectrum of dlbenzoylmethane 1s shown m Fig 3 Schwelg et al. [S] have used vanable-temperature PES to demonstrate the existence and temperature dependence of the concentration of keto and enol tautomers m acetylacetone. The band at 8.45 eV 1s assigned to the w lomzation of the enol tautomer Smce through-bond mteractlon between

14

tonizatlon

PotentmI

Fig 3 Photoelectron spectra (A) Dlbenzoylmethane, methoxybenzoyl)-2-benzoylpropane

(B) 2,2&benzoylpropane,

(C) 2-@-

the lone pans should be weaker than m benzil, owmg to the mtervenmg CH2 group the symmetric lone pan combmation would not be expected to be destabilized by 0.44eV compared to benzll and, therefore, the keto tautomer 1s ruled out as the prmcipal species. The separation between the x enol IP and the n of the enol of 0 9 eV is comparable to that observed by Schweig et al [6] for acetylacetone and 3-methyl-acetylacetone The broad band at 9 4 eV is assigned to Phs and Ph, This band probably also contams the oxygen lone pan of the non-enohzed carbonyl group 2,2-Dabenzoylpropane (6) and 2-(p-methoxybenzoyl)-2-benzoylpropane (8) The spectra of compounds 6 and 8 are also shown m Fig. 3. Substitution of methyl for hydrogen m dlbenzoylmethane produces compound 6 and precludes the possibility of formmg the enol tautomer Compound 8 has been included to ard m the assignment of the spectrum of 6

87

Both expernnent [ 71 and calculatrons [8] suggest that the order of lone pair energy levels is n_ > n+ m p-dicarbonyls. In a study of 24 fl-dicarbonyls, McGlynn and co-workers, [7] obtamed an average separation between the levels of 0.65 eV If the first IP at 9 0 m 6 is assigned to the n- lonlzatlon, then the n+ ionization should occur at 9.65 eV which would be obscured by the broad band at 9 5 eV. Substitution of the methoxy group m 8 is assumed to destabilize the n_ level to a minor extent to 8.9 eV The second IP m 6 at 9.3 eV ls assigned to the phenyl S level which undergoes a destabilization of 0 7 to 8.6 m 8 This IS comparable to the change of 0 86 eV from benzene to arnsole [4] The third IP m both 6 and 8 IS assigned to ionization from phenyl A and occurs at 9 5 eV and 9 3 eV, respectively The tar1 of this band 1s assumed to contam the n+ ionizations m each case The prmclpal difference between the molecular energy levels of 6 and 8 as a result of this assignment 18that m the former the HOMO is the n_ orbital whereas m the latter the HOMO IS the phenyl S owmg to the destablllzmg effect of the methoxy group 1,2-Drbenzoylethane (7) Any two lone pair orbit& separated by an odd number of bonds should exhibit the MO order n+ > n_ if the carbonyl groups are non-coaxial [9] The calculated separation 1173between the levels IS 0.2 eV. The PE spectrum of 1,2-dlbenzoylethane 1s shown m Fig. 4 The first broad band centred at 9 45 eV has shoulders on both sides at 9.15 and 9.7 eV If the former 1s

I

I

14 lonmatlon

Fig

I

I

10

4 Photoelectron

spectra

I

I

16

Potential of 1,2dlbenzoylethane

88 attnbuted to n, ionization then the n_ ionization should he m the broad band at 9.45 eV This band also contams Phs lomzation and the shoulder at 9.7 eV is assigned to PhA There is no evidence of a a enol band m the regon where it is found m dlbenzoylmethane Those compounds which contam substltuents on the phenyl group (2, 4 and 8 m Table 1) are assigned an aromatic 7~orbrtal as the highest occupied. This level is accidentally degenerate with an n+ m 4-OH benzll. The enol A of dlbenzoylmethane is the HOMO For all the other compounds mveslzgated an n level is assigned as the HOMO. The change m the nature of the HOMO level for these ketones could have some bearmg on their photochemical reactivity and UV absorption characteristics Wagner et al [lo] have pomted out the effect of electrondonatmg substltuents on tnplet state reactlvlty of substituted valerophenones Those ketones with ?r, n* lowest triplets show considerably reduced tiplet state reactlvlty relative to those with n, r* lowest tnplet Electron-donatmg groups would be most effective at rmsmg the r level up to or surpassing that of the n level The close proximity of the n and R levels m all these cases would make for efficient vrbromc murmg of the 7~,R* and n, T* states, thus making the effect of the substituent on UV spectra or reactivity a difficult matter to Interpret. Correlations between changes m ionization potentials from one compound to another and changes m UV absorption energies are often nsky smce such correlations contam ambiguities concernmg the T* orbital energies and rnterelectronic repulsion terms m the transition energies [ 111. In addition, the UV data contam contmbutxons from solvent effects which are difficult to quantify

EXPERIMENTAL Photoelectron spectra were recorded on a Perkm-Elmer PS-18 Photoelectron Spectrometer using argon and xenon as mternal cahbrants Resolution as determmed from the argon peak at 15.76 eV was 25-30 meV and recorded peak positions represent an average of five determinations Benzophenone, 4-hydroxybenzophenone, benzll, dlbenzoylmethane and dlbenzoylethane were commercial samples and were recrystalhzed before use Hydroxybenzll was prepared as described by Bunbury and Chan [12] 2,2-Dibenzoylpropane was prepared from dibenzoylmethane Monomethylation was done accordmg to Bickel [13] 2-(p-Methoxybenzoyl)-l-benzoylpropane was synthesized by first condensmg ethyl 4-methoxybenzoate with acetophenone m anhydrous ether usmg sodium hydride as the base Dimethylation was accomplished usmg the procedure of Bloomfield [ 143 The final compound was recrystallized from hexane-benzene m p 107109°C uncorr Analysis calculated, 76 10% C, 6 01% H, found, 75 97% C, 6 15% H NMR 6 165 (s, 6H, CHs), 3 76 (s, 3H, OCHJ ), 6 7-7 9 (m, 9H, aromatic)

89 ACKNOWLEDGEMENTS

The authors wmh to acknowledge the financml support of the NatIonal Research Council of Canada and the St Francis Xamer Umvemty Council for Research and the techmcal asmtance of Mr. J V. Burke The cooperation and amstance of Professor K N Houk of Loumana State Unlverslty m whose laboratory the photoelectron spectra were determmed 1salso gratefully acknowledged

REFERENCES 1 2 3 4 6 6

9 10 11 12 13 14

T Koopmans, PhysIca, l(l934) 104 J F Amett, G Newkome, W L Mattlce and S P McGlynn, J Am Chem Sot , 96 (1974) 4386 R Hoffmann, Accounts Chem Res ,4 (1971) 1 T Kobayashl and S Nagakura, Bull Chem Sot Jpn ,47 (1974) 2563 G Centmeo, I Fragala’, G Bruno and S Spanpmato, J Mol Struct ,44 (1978) 203 A Schwelg, H Vermeer and U Weldner, Chem Phys Lett ,26 (1974) 229 D Dougherty, P Brmt and 8 P McGlynn, J Am Chem Sot ,100 (1978) 6697 K N Houk, L P Dams, G R Newkome, R E Duke and R V Nauman, J Am Chem Sot ,95 (1973) 8364 R Hoffmann, A Iwamura and W J Hehre, J Am Chem Sot ,90 (1968) 1499 P J Wagner, A E Kemppamen and H N Schott, J Am Chem Sot , 95 (1973) 5604 H Bock, K Wlttel, M Vleth and N Wlberg, J Am Chem Sot ,98 (1976) 109 D L Bunbury and T M Chan, Can J Chem ,50 (1972) 2499 C L Blckel, J Am Chem Sot ,67 (1945) 2045 J J Bioomfield, J Org Chem ,26 (1961) 4112