PES Study of
The Journal of Physical Chemistry, Vol. 83, No. 23, 1979 2997
Aromatic Hydrocarbon Metabolites
Ultraviolet Photoelectron Studies of Polycyclic Aromatic Hydrocarbons. The Ground-State Electronic Structure of Aryloxiranes and Metabolites of Benzo[ a Ipyrene I. Akiyama, K. C. Li, P. R. LeBreton,* Department of Chemistry, University of Illinois at Chicago Circle, Chicago, Illinois 60680
P. P. Fu, and R. G. Harvey Ben May Laboratory, University of Chicago, Chicago, Illinois 60637 (Received May 3 1, 1979) Publication costs assisted by the University of Illinois at Chicago Circle
UV photoelectron spectroscopy was employed to study the ground-state electronic structure of a series of aryloxiranes, of phenanthrene g,lO-oxide, and of two metabolites of benzo[a]pyrene including the highly The phocarcinogenic diol epoxide, trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene. toelectron spectra of the aryloxiranes and the benzo[a]pyrene metabolites were assigned by comparison with the spectra of the parent polycyclic arenes and ethylene oxide. Spectral assignments were confirmed by comparison of experimental ionization potentials with energy levels predicted by semiempirical molecular orbital calculations based on the CNDO/S 3 method. The photoelectron spectrum of the diol epoxide of benzo[a]pyrene is approximated by the combined spectra of pyrene and ethylene oxide. However, electronic coupling of the saturated six-membered ring with its substituent groups to the A system of the pyrene moiety does occur. In the diol epoxide of benzo[a]pyrene all five of the uppermost A orbitals are destabilized compared to those in pyrene. This destabilization, which in the second highest occupied R orbital has a magnitude greater than 0.4 eV, is expected to aid in the stabilization of intermediates associated with biologically important electrophilic alkylation reactions.
Introduction An understanding of the structural features of polycyclic hydrocarbons which are important to the behavior of these molecules in biochemical systems has long been sought.' Much of the work in this area has focused upon relationships between electronic structure and biochemical activity. Before the identification of hydrocarbon metabolites as primary carcinogens, attempts were made to relate the electronic structure of the parent hydrocarbons directly to their carcinogenic activitya2 More recently, studies have been carried out in attempts to elucidate the role electronic structure plays in the biochemical behavior of important metabolites of polycyclic hydrocarbons. For example, a diol epoxide of benzo[alpyrene is thought to participate in biologically important electrophilic reactions involving alkylation of nucleotide bases such as guaninea3The details of these reactions have not been totally elucidated. Most current speculation has focused upon a two-step s N 1 mechanism which involves formation of a 7,8,9-trihydroxy-7,8,9-trihydrobenzo[a]pyrene carbonium ion. While it is not yet certain whether the reaction does indeed proceed via an s N 1 mechanism or whether a proton-assisted SN2 mechanism plays an important role, either mechanism involves the development of positive charge on the saturated ring as reaction proceed^.^ In one study, perturbational molecular orbital calculations have been carried out in order to gain greater understanding of the reactivity of diol epoxides of polycyclic hydrocarbon^.^ In this work the difference in delocalization energies between diol epoxides and the triol carbonium ions from which they are formed was examined. Results of the study suggest that the ease with which carbonium ion formation takes place is an important index of carcinogenic activity. Much of the work associated with the investigations of electronic structure in polycyclic hydrocarbons and metabolites of these hydrocarbons has been undertaken from a theoretical point of view. There is a current need to obtain experimental verification of the descriptions pro0022-3654/79/2083-2997$01 .OO/O
vided by these theoretical studies. In recent photoelectron studies of polycyclic hydrocarbons6 and of even more complicated heterocyclic aromatic molecules such as the nucleotide base^,^^^ it has been found that photoelectron spectroscopy can provide very detailed descriptions of valence electrons in biologically important molecules. A major goal of the present study has been to provide spectroscopic descriptions of important metabolites of benzo[alpyrene. The present photoelectron study has focused on three series of polycyclic aromatic hydrocarbons. The first series contains oxiranyl aromatic hydrocarbons. This series contains phenyloxirane (I), 2-oxiranylnaphthalene (II),9-oxiranylanthracene (III), and l-oxiranylpyrene (IV). The second series contains phenanthrene (V), biphenyl (VI), and some derivatives of phenanthrene which are partially saturated and which contain epoxide and hydroxyl groups. This set of phenanthrene derivatives includes 9,lO-dihydrophenanthrene (VII), phenanthrene 9,lO-oxide (VIII), and trans-9,10-dihydroxy-9,lO-dihydrophenanthrene (IX). This series demonstrates in a simple manner the spectroscopic effects associated with the disruption of the T system which occurs as an epoxide group is introduced into the ring system of Phenanthrene. The last series contains benzo[a]pyrene (X), pyrene (XI), trans-7,8-dihydroxy7,8-dihydrobenzo[a]pyrene (XII), and trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,lO-tetrahydroben~o[alpyrene (XIII).
Experimental Section Spectra were measured with a Perkin-Elmer PS18 photoelectron spectrometer, equipped with a heated probe and a He(1) lamp. Compounds 11, 111, IV, VIII, IX, XII, and XI11 were synthesized according to previously described proced~res.~-l'Compounds I, V, VI, VII, and X I were obtained from Aldrich Chemical Co. while compound X was obtained from Sigma Chemical Co. Since several of the molecules studied are relatively unstable, there was concern that these compounds might decompose under the conditions required to measure their
0 1979 American Chemical Society
2998
The Journal of Physical Chemistry, Vol. 83, No. 23, 1979
Akiyama et al.
I V
L
J
I l l l l l l l / l l l l l / 1 1 1 1 1 1 1 / 1 1 1 8.0 10.0 120 140 16.0 18.0 200 IONIZATION POTENTIAL (eV)
Figure 1. He I photoelectron spectra of phenyloxirane, 2-oxiranylnaphthalene, 9-oxiranylanthracene, and 1-oxiranylpyrene. Assignments and vertical ionization potentials are given for the higher occupied orbitals.
spectra. However, it was found that for all compounds, except XI1 and XIII, several spectra measured from a single sample over a time period of 1 h were identical, indicating that no decomposition occurred. Furthermore, after the measurements were carried out, no discoloration of samples of I-XI was observed. This also suggests that no decomposition occurred. In order to further investigate the possibility of chemical alteration of phenanthrene 9,lO-oxide during the experiments, the spectrum of this compound was compared with that of trans-9,10-dihydroxy-9,lO-dihydrophenanthrene and found to be different. This indicated that no hydrolysis of phenanthrene 9,lO-oxide occurred as the sample was transferred to the spectrometer and the measurements were made. It was found that XI1 and XI11 decomposed during the time required to measure a full spectrum. For these molecules spectra were measured in two steps, using a fresh sample for each step. In the first step measurements were carried out in the energy region 7.0-10.0 eV. In the second
\
i 18 0 1 1 1 100 1 l I I 120 l l l 1 1140 ! l l l160 l 1 l 180 l l l l 200 l l l l l
IONIZATION POTENTIAL (eV)
He I photoelectron spectra of phenanthrene, biphenyl, 9,iO-dihydrophenanthrene, phenanthrene 9,lO-oxide, and trans-9,IO-
dihydroxy-9,lO-dihydrophenanthrene.
step measurements were carried out between 9.0 and 20.0 eV. In this manner well-resolved reproducible spectra were obtained for both molecules. The spectra of XI1 and XI11 shown in Figure 3 are composite spectra assembled by combining data obtained by this procedure. Temperatures at which the spectra were measured are given in Figures 1-3. During the measurement of a spectrum the probe temperature was maintained constant to within fl "C. All spectra were calibrated by comparison with photoelectron bands associated with 2P1/2and *P3p ionic states of xenon and argon. Results and Discussion Figure 1 shows He I photoelectron spectra along with assignments and vertical ionization potentials for the up-
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The Journal of Physical Chemistry, Vol. 83, No. 23, 1979
PES Study of Aromatic Hydrocarbon Metabolites
TABLE I: Vertical Ionization Potentials of Upper Occupied $i Orbitalsa T1
benzeneb phen yloxirane naphthaleneC 2-oxiranylnapthalene anthraceneC 9-oxiranylnapthalene pyreneC 1-oxiranylpyrene trans-I ,8-dihydroxy1,8-dihydrobenzo[a1 PYrene trans-l,8-dihydroxyanti-9,lO-epoxy1,8,9,10-tetrahydrobenzo[ a ] pyrene
X
9.24 9.23 8.15 8.2 1 1.41 7.41 1.42 1.43 I.21
*2
*3
1.13 1.85 -9.1
a All ionization potentials given in eV. ref 14. Taken from ref 6.
(223'C)
n
1 60
1
80
1
1 100
1
1
120
1
1 140
1
1
160
1
1
180
1
1
200
IONIZATION POTENTIAL (eV)
Figure 3. He I photoelectron spectra of benzo[a]pyrene, pyrene, trans-7,8-dihydroxy-7,8-dihydrobenzo[ a ] pyrene, and trans-7,8-dihydroxy-anti-9,lO-epoxy-7,8,9,10-tetrahydrobenzo [ a ] pyrene.
permost occupied orbitals in phenyloxirane, 2-oxiranylnaphthalene, 9-oxiranylanthracene, and 1-oxiranylpyrene. The photoelectron spectra arising from the H orbitals of the aryloxiranes are very similar to the spectra of the parent hydrocarbons. This similarity simplified the assignment of the spectra of the aryloxiranes. In the energy region 7.0-11.0 eV the only qualitative difference in the spectra of the aryloxiranes and those of the parent hydrocarbons is that the spectra of the aryloxiranes contain an extra band in the energy region 10.0-10.5 eV. This band is associated with an oxygen atom lone-pair orbital of the epoxide group. In the spectrum of ethylene oxide the oxygen atom lone-pair orbitals appear a t 10.57 and 11.84 eV.12J3 In the aryloxiranes the band arising from the more stable of these lone-pair orbitals appears in a region of poor resolution and has not been assigned. The T symmetry assignments given in Figure 1are based
* 4
* 5
9.24 9.41 8.88 10.08 8.I 4 10.20 8.57 9.23 10.26 8.68 9.20 -10.1 9.36 10.04 8.32 9.13 9.33 -10.0 8.38 9.20 -9.4 8.1 3 8.52 -9.0 -9.2
9.18
* Taken from
on assignments of orbital symmetries associated with the parent hydrocarbons, In benzene the highest occupied molecular orbitals are degenerate and have vertical ionization potentials of 9.24 eV. In phenyloxirane this degeneracy is removed and the uppermost occupied 7~ orbitals have ionization potentials of 9.23 and 9.47 eV. The broad structural feature in the energy region 11.3-13.5 eV in the spectrum of phenyloxirane arises from three overlapping bands, Two of these are a 7~ band and a 0 band associated with the phenyl group,14 and one of them is a u band associated with a delocalized orbital in the epoxide gro~p.~~J~ The assignment of bands associated with the upper occupied T orbitals in 2-oxiranylnaphthalene, 9-oxiranylanthracene, and 1-oxiranylpyrene was carried out by comparison with the previously reported photoelectron spectra of the parent hydrocarbons.6 The similarity of the H systems in the aryloxiranes to those in the parent hydrocarbons is demonstrated in Table I which lists vertical ionization potentials associated with the uppermost occupied H orbitals in several of the molecules studied here. This similarity indicates that in molecules 1.-IV the interaction between the T system of the aromatic hydrocarbon moiety and the lone-pair orbitals of the ethylene oxide group is small. Figure 2 shows He I photoelectron spectra along with assignments and vertical ionization potentials for bands arising from the low-energy region of phenanthrene (V), biphenyl (VI), 9,lO-dihydrophenanthrene(VII), phenanthrene 9,lO-oxide (VIII), and truns-9,10-dihydroxy-9,10dihydrophenanthrene (IX). An examination of the data in Figure 2 indicates that the spectra arising from the T systems of molecules VII-IX are more similar to the spectrum of biphenyl than they are to the spectrum of the parent hydrocarbon, phenanthrene. The similarity of spectra obtained from aromatic epoxides to those of hydrocarbons with the same aromatic residue has been observed in previous UV absorption experiments.' However, unlike the photoelectron data the UV absorption data fails to point out the detailed degree to which this similarity occurs. For example, in the photoelectron spectrum of biphenyl, bands arising from five of the highest occupied orbitals can be associated with similar bands observed in the spectrum of phenanthrene 9,lO-oxide. The photoelectron spectra of phenanthrene and biphenyl have been previously studied16and the assignments of the spectra for these molecules given in Figure 2, are the same as those reported earlier. The assignment of the
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The Journal of Physical Chemistry, Vol. 83, No. 23, 1979
Akiyama et al.
spectra obtained from molecules VII-IX is based on a [alpyrene (XIII) is expected to have one more low energy comparison of these spectra with the spectrum of biphenyl. band than the spectrum of pyrene. In this case the addiThe a symmetry assignments given in Figure 2 are based tional band arises from an oxygen atom lone-pair orbital on a orbital symmetries associated with biphenyl in a of the epoxide group. The lone-pair orbital associated with planar conformation. Recent crystallographic and specthe epoxide group has an ionization potential of 10.6 eV troscopic results indicate that the most stable geometry and is overlapped by other bands in the spectrum of XIII. of biphenyl is actually one in which the dihedral angle Like the lone-pair bands associated with the hydroxyl associated with the planes containing the phenyl groups groups of IX and XI1 the hydroxyl lone-pair bands are not is in the range 22-45O.I’ However, symmetry assignments resolved in the spectrum of XIII. of orbitals based on a planar geometry simplify the comThe possible thermal decomposition of XI11 under the parison of the spectra shown in Figure 2. present experimental conditions has also been considered. The spectrum of 9,lO-dihydrophenanthreneis very simiThe fact that the spectrum shown in Figure 3 arises prilar to that of biphenyl in the energy region 8.0-10.0 eV. marily from trans-7,8-dihydroxy-anti-9,lO-epoxyThe only major difference is that the r4 band is better 7,8,9,10-tetrahydrobenzo[a]pyreneis indicated by the obresolved in 9,lO-dihydrophenanthrenethan it is in biservation of the spectral structure at 10.6 eV. No simiphenyl. The spectrum of phenanthrene 9,lO-oxide is larly resolved band is observed in the spectra of other diols nearly identical with that of 9,lO-dihydrophenanthrenein such as IX or XII. Furthermore, spectra measured from the 8.0-10.2-eV energy region except that phenanthrene samples of XI11 which have undergone thermal decompo9,lO-oxide contains an additional band at 10.11 eV. This sition exhibit bands in the energy region 7.0-10.1 eV which are similar to those in the same region of the spectrum of band is due to the less stable of the lone-pair orbitals of the epoxide group. pyrene. However, spectra from the decomposed samples The spectrum of trans-9,10-dihydroxy-9,lO-dihydro- do not exhibit the 10.6-eV band which arises from the fragile epoxide group. phenanthrene (IX) is also very similar to that of 9,lO-dihydrophenanthrene. In IX, bands associated with the Molecular Orbital Calculations. In order to further confirm the assignments of the spectra contained in Figoxygen atom lone-pair orbitals of the hydroxyl groups are ures 1-3 the spectroscopic results have been compared with expected to lie a t higher energies than the 10.11-eV band energy levels predicted by CNDO/S3 molecular orbital of the epoxide group in phenanthrene 9,lO-oxide. A meacalculations.21 The choice of the CNDO/S3 method was surement of the spectrum of 2-cyclohexen-1-01indicated based upon the fact that this computational procedure has that in this molecule the oxygen atom lone-pair orbitals have an ionization potential of 10.38 eV. In benzyl alcohol been parameterized to fit spectroscopic data from polycyclic aromatic hydrocarbons. The original version of and phenol the oxygen atom lone-pair orbitals have ioniCNDO/S3 does not have oxygen atom parameters. Values zation potentials of 10.58 and 11.3 eV, respectively.18 In of these parameters employed in the present calculations IX the lone-pair bands associated with the hydroxyl groups are the same as those suggested by Del Bene and JaffeaZ2 lie in a poorly resolved region of the spectrum and have not been assigned. In the present study the geometries of ~ h e n a n t h r e n e , ~ ~ phenanthrene 9,10-0xide,~~ pyrene? and ben~o[a]pyrene~~ Figure 3 shows spectra along with assignments and veremployed in the calculations were obtained from crystaltical ionization potentials for benzo[a]pyrene (X), pyrene (XI), trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene lographic data. For biphenyl, calculations were carried out (XII), and trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10- by using crystallographic datal7sz7which indicates that the dihedral angle between the planes containing the phenyl tetrahydrobenzo[a]pyrene (XIII). As in the case of the groups is 42O.l’ parent hydrocarbons of molecules I-IV and VII-IX the spectra of benzo[a]pyrenelgand pyrene6 have been preThe geometries of the aryloxiranes were obtained by viously studied and the assignments given in Figure 3 are combining crystallographic and electron diffraction data the same as those reported earlier. The assignment for the for ethylene o ~ i d e with ~ ~ vcrystal ~ ~ data for benzene, naphspectra of molecules XI1 and XI11 is facilitated by a comthalene, anthracene, and ~ y r e n e . ~ ~The J O C-C distance parison with the spectrum of pyrene. All symmetry asassociated with the bond linking the epoxide group to the signments in Figure 3 are based on assignments of orbital hydrocarbons was taken to be 1.54 symmetries associated with pyrene except for benzoThe geometries used in the calculations for 9,lO-di[alpyrene, which is treated by itself. hydrophenanthrene (VII) and trans-9,10-dihydroxy-9,10In the low energy region of 7.0-10.5 eV the spectrum of dihydrophenanthrene (IX) were based on the geometry of phenanthrene 9,lO-oxide which has a planar aromatic systrans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene(XII) is tem. However, in VI1 and IX the C4a-C4bbond was twisted expected to contain one more a band than the spectrum of pyrene. The rlthrough 7r5 orbitals in XI1 have ionizaso that the dihedral angle associated with the two planes containing the phenyl rings was 28’. This modification tion potentials and symmetries similar to those of the a orbitals which occur in pyrene. As indicated from the gave rise to a reasonable Cg-C10 bond length of 1.52 A.32 For IX the 0-H and C-0 bond lengths used in the calcuresults of molecular orbital calculations which are considered later in this discussion, the T’ orbital with an ionizalation were 0.95 and 1.43 A, re~pectively.~~ The C,-Cg-O tion potential of 8.52 eV has large electron density in the and Cla-Cl0-O bond angles used in the calculation were 110.2O and the C-0-H bond angles were 109.5°.31 The Cg-Clo region of XII. However, this orbital mixes with the numbering and lettering system used to describe positions T system of the pyrene moiety and is destabilized compared to the a orbital in cyclohexene which has an ionizaof atoms in this discussion corresponds to IUPAC nomention potential of 8.94 eV.20 As in the case of trans-9,lOclature. dihydroxy-9,10-dihydrophenanthrene, the lone-pair orbitThe geometry of trans-7,8-dihydroxy-7,8-dihydrobenzoals of XI1 which are associated with the hydroxyl groups [alpyrene (XII) used in the calculation was based upon give rise to bands that occur in a poorly resolved energy the geometry of benzo[a]pyrene. However, in XI1 the region of the spectrum. partially saturated ring was puckered so that the hydroxyl groups were in a diaxial conformation. To accomplish this Like the spectrum of XI1 the spectrum of trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,lO-tetrahydrobenzo- the dihedral angle between the plane of the Cg, Cl0, and
-
The Journal of Physlcal Chemistry, Vol. 83, No. 23, 1979 3001
PES Study of Aromatic Hydrocarbon Metabolites
PHENYLOXIRANE
2- OXIRANYL NAPHTHALENE
IIiI
9-OXIRANYL ANTHRACENE
1-OYIRANYL WRENS
PhENAhTHRENE
B PHEUYL
9.80-DIHYDRO. PHENAVTHRENE
PPEhANThRENE9 0 OXIDE
TRANS-9 IS-DIHYDROXY9lQ-DlHYDROPHENANTHRENE
(111)
-
.J
I
a
12.0 I
-t w
z W
9.0
-1 n
11.0 n10)
Flgure 4. Energy level diagram showing the higher occupied molecular orbitals in molecules I-IV. Panel A shows experimental results obtained from vertical ionization potentials. The hatched areas show regions in which there is an overlap of photoelectron bands and for which the precise ordering of the bands is uncertain. The vertical lines appearing at energies above 10.5 eV denote the onset of largely unresohed regions in the He I spectra of molecules 11, 111, and I V where no assignments were made. Panel B shows energy levels obtained from CNDO/S3 molecular orbital calculations.
Cloaatoms and the plane of the aromatic system was 24.0'. The dihedral angle between the plane of the C7, and C8 atoms and the plane of the aromatic system was 41.5'. In this conformation all C-C bond lengths in the partially saturated ring except Cg=Clo were 1.52 A. The Cg=Clo bond length was 1.34 A.28,29 The geometry employed in the calculation for the diol epoxide of benzo[a]pyrene (XIII) was obtained in a manner analogous to that of XII. The geometry of the epoxide ring structure of XI11 was taken to be the same as that in phenanthrene 9,lO-oxide. For XI11 the dihedral angle between the plane containing the Cg, Cl0, and CIOaatoms and that of the aromatic system was taken to be 2 2 . 1 O . The dihedral angle between the plane of the Csa, C7,and C8 atoms and the plane of the aromatic system was 37.9O. This yielded C-C bond lengths of 1.52 A for all carbon atoms in the saturated ring. The C-0 and 0-H bond lengths and the C-0-H bond angles of the hydroxyl groups used in the calculations for XI1 and XI11 were the same
Figure 5. Energy level diagrams showing the higher occupied molecular orbitals in molecules V-IX. Panel A shows experimental results obtained from vertical ionization potentials. Panel B shows energy levels obtained from CNDO/S3 molecular orbital calculations.
as those used for IX. The Cg-C8-0 and CSa-C7-O bond angles were 110.2O. Figure 4 contains a comparison of experimental and theoretical results obtained for the aryloxiranes studied here, Panel A shows vertical ionization potentials obtained from the photoelectron measurements. Panel B shows energy levels predicted by the CNDO/S3 calculations. An examination of Figure 4 indicates that the pattern of ionization potentials associated with the less stable occupied P orbitals of the aryloxiranes studied here is very well represented by the computational results. The calculations also accurately predict the position of the upper occupied oxygen atom lone-pair orbital of the epoxide group in the manifold of occupied x orbitals. The ordering and spacing of the upper occupied u bonding orbitals may be less reliably predicted by the calculations. For example, in previous photoelectron studied3 a comparison of spectra obtained from phenyloxirane, ethylene oxide, and vinyloxirane led to the conclusion that the fourth, fifth, and sixth ionization potentials of phenyloxirane arise from a u orbital of the epoxide group, the x 3 (ak,) orbital, and the u1 (etg) orbital, respectively. In the present calculations, however, the ordering of the u orbital of the epoxide group and the u1 (ez,) orbital is switched. In other semiempirical calculations similar to CNDO/S3 it is found that the relative energies of the P and u systems obtained from the calculations are often inaccurate. Because the spectra of the molecules studied here are poorly resolved in the energy region where the intermixing of the P and u systems occurs, it is not possible to reliably test the detailed ordering of P and u orbitals predicted by the present calculations.
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The Journal of Physical Chemistry, Vol. 83, No. 23, 1979
/A
TRANSTRANS-7,8-DIHYDROXYANTI-9.10 EPOXY,~.UIHYDRO7 8.9 IO-TETRAhYDROB E N 2 0 ial PYRENE B E N 2 0 la1 FYRENE 7 &@IcYDROXY.
w
BEN20 PYRENE
PYRENE
/ -
”’
g 100 11 0
2 12 0
1
3
Flgure 6. Energy level diagrams showing the higher occupied molecular orbitals in molecules X-XIII. Panel A shows experimental results obtained from vertical ionization potentials. I n panel A the hatched area appearing in the diagram for XI11 in the energy region around 10.6 eV shows the approximate position of the oxygen atom lonspair band of the epoxide group and of other unassigned overlapping bands. Panel 6 shows energy levels obtained from CNDO/S3 molecular orbital calculations.
Figure 5 shows a comparison of experimental and computational results obtained for V-IX. Panel A contains vertical ionization potentials. Panel B contains energy levels obtained from the CNDO/S3 calculations. Here, as in the case of the aryloxiranes, the calculations give a very satisfactory picture of the pattern of experimentally measured ionization potentials. In phenanthrene and biphenyl the calculations give a good prediction of the spacing of the upper occupied a orbitals. As in the case of the aryloxiranes the detailed ordering of a and (T orbitals predicted by the calculation cannot be experimentally verified. However, for phenanthrene the close grouping of the ( T ~ ,o2, and a5energy levels predicted by the calculation, and the prediction that these three orbitals have binding energies 0.6-1.3 eV greater than that of the a4 orbital are consistent with the observation that the spectrum of phenanthrene becomes poorly resolved at energies above 10.5 eV. The calculations do not give an accurate prediction concerning the differences in ionization potentials of the a4 orbitals which are observed in VI-IX. However, they do give a good description of the shifts in ionization potentials associated with the highest occupied a orbital in these molecules. Furthermore, the calculations accurately predict the ordering of the less stable oxygen lone-pair orbital in phenanthrene 9,lO-oxide. Figure 6 shows a comparison of experimental and computational results for X-XIII. These are the most com-
Akiyama et al.
plicated of the molecules considered in this study and there are restrictions as to the accuracy which may be expected from the calculations. However, as in the case of I-IX, the calculations give a good description of the experimental order and energy spacing observed for the five highest occupied a orbitals of benzo[a]pyrene and for the four highest occupied a orbitals of pyrene. For these molecules, as for I-IX, the interspacing of the lower occupied T orbitals and the upper occupied u orbitals predicted by the calculations is expected to be less accurate. For trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (XII) and trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10tetrahydrobenzo[a]pyrene (XIII) the calculations give good descriptions of the less stable occupied a orbitals. The calculations accurately predict that, compared to pyrene, XI1 has an additional occupied T orbital with low ionizaticn potential. They also give a generally good description of the energy spacing of the five highest occupied orbitals in this molecule. For XI11 the calculations accurately locate the energy of the less stable oxygen atom lone-pair orbital of the epoxide group relative to the energies of the uppermost occupied a orbitals. Finally, an overall examination of the results for trans7,8-dihydroxy-anti-9,lO-epoxy-7,8,9,lO-tetrahydrobenzo[alpyrene indicates that, while the spectrum of this molecule is approximately that obtained by simply superimposing the spectrum of ethylene oxide upon that of pyrene, the coupling of the pyrene electronic system to that of the saturated ring and its substituent groups is spectroscopically observable. This is demonstrated in Table I which indicates that all five of the highest occupied a orbitals in XI11 are destabilized compared to those in pyrene and that the magnitude of this destabilization in individual a orbitals is sometimes large. For example, the ionization potential of the second highest occupied a orbital in pyrene is more than 0.4 eV greater than the ionization potential of the same orbital in the diol epoxide. In previous photoelectron studies of nucleotide bases it was found that valence orbital energy shifts of the magnitude which have been observed here sometimes reflect biochemically significant changes in binding7B3and reactive proper tie^.^^ With regard to the diol epoxide of benzo[alpyrene it is interesting to speculate whether the labilization of the a system relative to that in pyrene and l-oxiranylpyrene alters the chemical behavior of the pyrene system and of the reactive epoxide group in biologically important ways. One possibility is suggested by the enhanced ability of a molecular fragment to support positive charge as the ionization potentials associated with neighboring labile orbitals decreases. This is often reflected in the relative stabilities of structurally related carbonium ions.4 For example, the stabilities of aliphatic carbonium ions increase in the order methyl cation < ethyl cation C tert-butyl cation4s3;while the ionization potentials of related neutral molecules decrease in the order methane > ethane > isobutane.I4 This relationship also holds for unsaturated carbonium ions. Allyl cations are less stable than benzyl cations which in turn are less stable than naphthylmethyl ~ a t i o n s . ~The B ~ ionization potentials of related molecules decrease in the order ethylene > benzene > naphthalene. For the diol epoxide of benzo[a]pyrene the labilization of the upper manifold of occupied a orbitals associated with the pyrene ring system is expected to enhance the ability of the metabolite to form intermediates which possess either a full or a partial positive charge on the saturated benzo ring. This would include intermediates involved in either SN2or SN1reaction mechanisms.
The Journal of Physical Chemistry, Vol. 83, No. 23, 7979
Electrochemical Transfer Coefficient
Acknowledgment. Support of this work by the American Cancer Society and the Computer Center of the University of Illinois a t Chicago Circle is gratefully acknowledged. The authors also thank Dr. Charles Duke for providing the CNDO/S3 program and Mr. Terrence 0'Donne11 for computational assistance.
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Potential Dependence of the Electrochemical Transfer Coefficient. An Impedance Study of the Reduction of Aromatic Compounds D. Garreau, J. M. Saveant,* and
D. Tessier
Laboratoire d'Electrochimie de i'Universit6 de Paris VII-2, place Jussieu-75 22 1 Paris, Cedex 05, France (Received March 5, 1979) Publication costs assisted by Laboratoire d' Electrochimie de I' Universit6 de Pari. VII
The electrochemical electron transfer rate of five aromatic compounds (nitromesitylene, nitrodurene, terephthalonitrile, phthalonitrile, p-diacetylbenzene) have been determined in DMF as a function of the dc electrode potential by using an ac impedance technique with frequencies ranging from 1000 to 20 000 Hz. The transfer coefficient was observed to vary with potential beyond experimental uncertainty in all cases. The magnitude of the variation is on the same order as that predicted by the Marcus theory of outer-sphere electron transfers. This behavior observed for various solvents and functional groups appears as a general phenomenon in the reduction of organic molecules in aprotic solvents, i.e., in the case where charge transfer is fast and mainly governed by solvent reorganization.
Introduction The present theories of electron transfer a t electrodes (ref 1and references therein), based on harmonic approximation, predict a linear dependence of the transfer coefficient, a , upon the electrode potential. During the last 0022-3654/79/2083-3003$01 .OO/O
15 years there have been several attempts to detect such a dependence experimentally (ref 2-4 and references therein). In most cases, however, the results were not actually conclusive owing mainly to the large magnitude of the double layer correction and the uncertainties about 0 1979 American
Chemical Society
