J . Phys. Chem. 1992,96, 6598-6610
6598
I nner-Shell Spectroscopy of p -Benzoquinone, Hydroquinone, and Phenol: Distingulshb Quinoid and Benzenoid Structures J. T.Francis and A. P.Hitchcock* Department of Chemistry, McMaster University, Hamilton, Ontario U S 4M1,Canada (Received: April 14, 1992)
Oscillator strengths for C 1s and 0 1s excitation of gas-phasep-benzoquinone,hydroquinone, and phenol have been derived from electron energy loss spectra recorded under electric dipole scattering conditions. Spectral assignments have been aided by extended Huckel (EHMO) calculations camed out within the equivalent core analogy, as well as by comparisons to previously recorded spectra of benzene, acrylic acid, 1,4-cyclohexanedione, and acetone. Two methods of relating EHMO results to experimental core excitation spectra are explored. That derived from the complete charge matrix is found to be preferable. Several features of the C 1s spectrum of p-benzoquinone can be associated with the quinoid structure of this species, namely ~ * ~ ~transition ~ ~ to( 1.6beV~below ~ ) a second A* resonance more intense than the first, and a shift of the C ls(C=C) that of the lowest C 1s r* transition in benzenoid species. This shift is associated with the loss of aromatic stabilization. EHMO calculations of p-benzoquinone in nonequilibrium geometries and of related molecules provide further insight into spectral assignments.
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1. Introduction Core excitation spectroscopy, using either photoabsorption or inelastic electron scattering, is finding increasing application for studies of electronic structure and chemical bonding in isolated molecules, solids, and ~urfaces.~-~ In a recent transmission EELS study, Litzelmann et a1.4 interpreted pH-dependent structure in the C Is and N Is spectra of polyaniline in terms of distortions of the benzenoid ring to a quinoid structure. In particular, an additional low lying peak was observed in both the C Is and N 1s spectra at pH = 7 which was not observed in acidic conditions (see Figure 1). At neutral pH, deprotonation of the nitrogen leads to increased conjugation with the ring and adoption of a quinoidal structure. This structural transformation, and the narrowing of the HOMO-LUMO gap associated with the introduction of the lower energy state, is believed to be related to a semiconductormetal transition and the pH-dependent conductivity of polyaniline-like materiak4v5 Similar prepeaks have been observed in the core excitation spectra of the conducting states of other doped aromatic polymers such as polythiophene and polypyrrole.6 It is possible that structural distortions analogous to the proposed aromaticquinoid transformation are also involved in the conductivity of these aromatic organic conductors. Thus clarification of any possible structural basis for the observed changes in core spectra is of interest. It has been found in many previous cases that studies of model compounds can greatly assist the interpretation of the spectra of complex materials. In order to investigate the spectroscopicbasis for a correlation between the observed spectral changes and transformation from a benzenoid to a quinoid structure, we have examined the C Is and 0 1s spectra of phenol (l), hydroquinone (p-benzenediol) (2), and p-benzoquinone (2,s-cyclohexadienedione) (3). The C 1s near edge X-ray absorption (NEXAFS) spectrum of phenol, as a multilayer solid and monolayer on Ag(llO), has been reported r e ~ e n t l y but , ~ its 0 1s spectrum and the spectra of the other molecules have not been reported previously to our knowledge. In these three compounds there is a clear transformation from a benzenoid structure in 1 and 2, to a quinoid structure in 3. The geometry of the C6-ring changes from one with a very small range of C-C bond lengths of 1.38-1.40 A in 1 and 2,to one with a very clear separation into essentially single (1.481 A) and double (1.334 A) bonds in 3.* Recently we have examined the C 1s spectrum of 2,1,3benzothiadiazole in connection with a study of aromaticlantiaromatic effects in S-N rings? The basic premise of that study was in fact complicated by the strong quinoid character of 2,1,3-benzothiadiazole. The close similarities between its C 1s spectrum and that of 3 support the thesis outlined in this paper that core excitation (and other spectroscopies based on unoccupied orbitals) provides a relatively simple means to distinguish ben-
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zenoid and quinoid ring structures. Extended Hiickel (EHMO) calculations, camed out within the equivalent ionic core virtual orbital model (EICVOM),'O have been used to assist spectral assignments. The EHMO results indicate different assignments for the C 1s spectrum of phenol than those proposed recently.' EHMO results have been used previously to help interpret core excitation spectra of transition metal organometallic c o m p l e x t ~ . ~This ~ - ~is~one of the first applications of EHMO to the interpretations of core excitation spectra of gas-phase organic species. As such we have used this project to explore the optimum procedures for connecting EHMO and experimental results. A combined EHMO and energy loss study of organic molecules related to poly(ethy1ene terephthalate) has also been camed out recently.I5 The quality and relevance of the EHMO calculations to the discrete regions, and to some extent the continuum, are demonstrated by comparison of experimental and calculated spectra. Even though much more sophisticated quantum chemical algorithms, both semiempirical and ab initio, are readily available and may provide more accurate quantitative results for organic molecules, the ease and minimal resources in time or computing power that are required for EHMO calculations make it a powerful tool to assist empirical procedures for spectral assignment.
2. Experimental Section The inner-shell electron energy loss spectrometer and operating procedures have been described previously.' The spectra were obtained by inelastic electron scattering of a high velocity electron beam (incident energy of 2.5 keV plus the energy loss) through a small scattering angle ( < 2 9 with an overall spectral resolution of 0.7 eV fwhm. The samples were obtained commercially (Aldrich, purity >99%, except for pbenzoquhone where the purity was only 98%) and used without further purification, aside from phenol which was distilled prior to use. The C 1s and 0 1s spectra were converted to absolute optical oscillator strengths Cf values) using a procedure described and tested previously.16 A precision of 5% has been estimated from the reproducibility of analyzing independent data sets while the absolute accuracy is better than 20%, according to comparisons with literature optical oscillator strength data.I6 3. Results 3.1. Presentationand Qualitative Aspects of the Speetna. 3.1.1. C 1s Spectra. The C 1s oscillator strength spectra of acrylic acid,'* p-benzoquinone, hydroquinone, phenol, and benzeneIgare shown in Figure 2. The hatched lines indicate the ionization limits as determined directly by gas-phase photoelectron spectroscopy (XPS)*O or inferred with the aid of solid-state XPS results.21*22 In each case the spectra are plotted on a common scale (oscillator
0022-365419212096-6598$03.00/00 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6599
Distinguishing Quinoid and Benzenoid Structures
TABLE I: Energies, Term Values, and Proposed Assignments for Features in the C 1s and 0 1s Spectra of Phenol
c 1s energy (hO.2eV) 285.4 287.4
NEXAFS' assignment C, 1a,
289.1
5bi
this work
76 (ev)
energy
C,
COH
4bl
4bl
5bl
291.0
5bl
295.0 302
(f0.1 eV) 285.2 1' 287.1 288.4 (wk) 289.2 290.2d 290Ad 291.0 292.od 294.5 302 (1) (br)
Cp,C, 5.0
C, 5.4
1.8 1.o
2.2 1.o
-4.3
assignment CCoH
CRFm
CCOH
-3.9 -10
0 1s T (ev) 4.0 1.7
energy (eV) 534.9' 537.2 538.9d 539.4 544 554 (1)
CO
assignment **(4bd, U*O-H **(5bd IP U*C-O 2e u*-(delocal)
-0.5 -5 -15
-
-
'C 1s spectrum of solid phenol, a multilayer on Ag(110)'. T = E - IP. 'Calibration: -5.53 eV relative to Is T* of C 0 2 (290.74 eV). dIPs from XPS." 'Notation for u* resonances is by analogy to the characteristic u* features of benzene. ,Calibration: +4.1(1) eV relative to 1s T* of O2 (530.8 eV). A small signal from O2was subtracted to obtain the spectrum presented in Figure 3.
J w
-
5
Energy Loss (eV)
Figure 1. C 1s and N Is transmission energy loss spectra of polyemeraldine films treated in buffer solutions at pH = 7 (upper) and pH = 0 (lower)! The proposed structures of the polymer units at the extreme pHs are indi~ated.'.~The features indicated by the asterisks are interpreted as evidence of the quinoid structure at neutral pH.
O t ) . . . . I
strength per carbon atom) in order to emphasize the systematics of the intensities. The energies and proposed assignments of the C 1s spectral features for the three newly studied species (1-3) are presented in Tables 1-111. Qualitatively, one notes that the C 1s spectrum of phenol is rather similar to that of benzene.lg However the oscillator strength lr*(4bl) tranfor the lowest energy feature, the C ls(C-H) sition at 285.2 eV, is about 20% less than that of its counterpart in benzene and there is a sccond T* resonance, the C ls(C-0H) 1#(4bl) transition at 287.1 eV, separated from the first feature by 1.6 eV. This separation is similar to the chemical shift between the C ls(C-OH) (292.0 eV) and the C ls(C-H) (290.2, 290.6 eV) ionization potentials.20 In addition, there is a change in the shape of the C 1s continuum around 294 eV in part because of overlap of three benzenoid C 1s continua separated by the chemical shifts of the C 1s IPS and, in part, because of the additional C T * ~ + resonance which is expected to m u r around 294.5 eV (IP(C0H) = 292.0 eV plus 6 = E - IP = 2.5 eV) based on the bond length c o r r e l a t i ~ n .The ~ ~ phenol C 1s spectrum is very similar to that
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280
. . . .
290
1 . . , . I I .
300 310 Energy (eV)
Figure 2. Oscillator strengths for C 1s excitation of acrylic acid,'* pbenzoquinone, hydroquinone, phenol, and benzene,19 derived from inner shell electron energy loss spectra (ISEELS) recorded with a final electron energy of 2.5 keV, an average scattering angle of 2O, and a fwhm resolution of 0.7 eV. The spectra are plotted on common intensity scales of oscillator strength per carbon atom. The hatched lines indicate the C 1s IPS based on experimental XPS
of the monohaloben~enes,~~ particularly fluorobenzene and chlorobenzene which have similar C 1s chemica! shifts at the substituted carbon (2.0 and 1.2 eV, respectively). The C 1s spectrum of gas-phase phenol (1) is very similar to the recently reported C 1s NEXAFS spectrum of solid phenol (multilayer on Ag(1 lo)).' The energies of all spectral features agree within mutual errors bars (see Table I). Our assignments are generally in agreement, except that we attribute the lowest T*(4bl) transitions instead energy T* resonance to C ls(C-H)
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6600 The Journal of Physical Chemistry, Vol. 96, No. 16, I992 TABLE II: Energies, Term Values, and Proposed Assignments for Features in the C 1s and 0 1s Spectra of Hydroquinone
c 1s T (ev) C-H C-OH
energy (hO.1 eV)
285.24"
5.0
287.25 289.4 290.36 291.8
C-OH
**(3b3J
4.7
a*(2a,) (small)
0.9
2.5
**(3bo)
-1.6
0.1
r*(3b3,)
IP
291.gb 292.4 295.5 303 ( I )
assignment
C-H
**(3b2g)
IP 2e- excitation ~*c-p(I)
-2.1
-5.2
-3.6
-1 1
U*C--O
U*C-C(2)
0 1s
T (ev)
energy (eV) 534.6'
4.3 1.4
537.1 538.g6
~ * G H , r*(3b3J r*(3b2g)
-0.8
539.6 (4) 545 (1) 556 (2)
assignment
o*C-o"(blu)(+l-)d
2e
-6 -17
~*C-oH(ag)(+l+)d
"Calibration: -5.50 (3) eV relative to C02. bLowest(C-H) IP estimated from that of phenol.20 The splitting of the C-H and C-OH IPSis 1.6 ( I ) eV from the solid-state spectrum.21 eCalibration: +3.8 (1) eV relative to O2 A small O2contribution was subtracted to obtain the spectrum presented in Figure 3. dNotation refers to relative signs of the atomic orbitals contributing at the Olef,and Olightatoms in the calculated molecular orbitals which are believed to correspond to these experimental features. TABLE III: Energies, Term Value., and Proposed Assignments for Features in the C 1s and 0 1s Spectra of p-Benzoquinone
c 1s T (eV)
energy ( i O . 1 eV) 283.14O
c=c
286.00
4.3
assignment
c=O
c==c
7.0
**(2b2A **(3bJ
6.6
288.3 290.36
5.2
290.7
-0.4
293.0b 293.6
-2.4
296.2 303
energy (eV)
529.85' 533.2
537.56 538.2 sh 542.7 554 (1)
IP **(3b2g)
-5.9
C=O r*(2bzg) **(3b,u)
IP
-0.6
U*C+
**(3b2,)
-3.2
U*C
3 AOs which are not included in the minimal basis EHMO treatment. The EHMO prediction of the C 1s spectrum of hydroquinone (2) is also in excellent agreement with experiment with respect to the relative energies and intensities of the u* resonances and in less good agreement with regard to the shape of the C 1s continuum. The EHMO results for 2 nicely reproduce the dif-
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Figure 6. Energy diagram for the C 1s 7' electric dipole allowed transitions for p-benzoquinone. The approximate energies and orbital sketches are taken from the EHMO-EICVOM core excitation results, with that for C ls(C=C) on the left and that for C l s ( C 4 ) on the right. The asterisk in the 'birds-eye" sketches of the r* orbitals indicates the excited atom. The spacing of the core orbital energies has been exaggerated for clarity.
reversal with 0 1s excitation since none of the u* orbitals experiences significant energy shifts. The agreement in the C 1s continuum region between the EHMO and experimental spectra of 1 is much poorer than for the r* region. EHMO does indicate that there are a number of I
I
I
]\
I
i
J'i-
1
HOGOH
>&?I/-------
l . . . . i . . . . I . . . . I . . .
530
550 560 Energy (eV)
540
IJ-1 . . . . 1 . . . . 1 . . . .
530
540
550
Energy (eV)
I...
560
I . . . . I . . . . I . . . . I . . ,
530
540
550
560
Energy CeV)
Figure 7. Comparison of EHMO and experimental 0 1s spectra for phenol, hydroquinone, and p-benzoquinone. See caption of Figure 5 for further details.
6606 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
ferential core hole relaxation between the (C ls(COH)-’,r*(3b3,,)) and (C ls(C-H)-11,A*(3b3,,)) states. There is 0.3 eV difference in the 3b3, orbital energy when the core hole is on the different sites which, when added to the 1.6 (1) eV difference in C 1s IPS, predicts a separation of 1.9 (1) eV between these two states, in very good agreement with the 2.01 (6) eV separation of the first two A* resonances of 2. As expected from the similarity of molecular structure and the qualitative discussion given earlier, the C 1s spectrum of 2 is very analogous to that of 1, with essentially a doubling of C ls(C0H) component. While the large number of different chemical environments in 1 rmght be expected to lead to a more complicated spectrum than that of 2, the chemical shift between the ortho, meta, and para carbons in 1 is small compared to our spectral resolution (and possibly also smaller than the vibrational band envelope expected3’) so that the first ?r* resonance has a similar shape in both species. According to curve fits, the first peak in the C 1s spectra of benzene, 1 and 2 can be adequately represented by a single Gaussian line of 1.13, 0.97 and 1.06 eV fwhm, respectively. At both the C-OH and C-H carbons the lowest energy C 1s r*(3b3,,)transition is the most intense feature even though there is greater C 2p contributions at the C-H carbons in the other two A* orbitals in the ground state of 2 (compare the orbital plots in Figures 4 and 5 ) . This is a consequence of orbital relaxation in the presence of the core hole. As found in other EHMO studies, the core hole greatly stabilizes the energy and enhances the C 2p component of the lowest energy A* orbital, with much smaller stabilization and “antilocalisation” (Le. a reduction of the C 2p component at the core hole site) in higher energy A* 0rbita1s.I~ Since a very strong low-lying T* resonance is a characteristic feature of the core excitation spectra of unsaturated molecules with multiple r* orbitals, it is clear that EHMO-EICVOM calculations do a good job at reproducing this aspect of the spectroscopy. It also emphasizes the need to take core hole relaxation into account so that ground-state calculations are not adequate by themselves. As discussed above, the 3b3, drops below the la, orbital upon C 1s excitation at either the C-OH or C-H site of 2. In contrast **(la,,) contribution to the to 1, there is some C ls(C-H) EHMO spectrum of 2. However it is not sufficient to result in an identifiable structure in the summed spectrum. This is in agreement with experiment in that the first two peaks can be fit nicely to two Gaussians and there is no improvement in the fit if a third component is introduced between the first two peaks. As in 1, EHMO overemphasizes the first structure in the C 1s continuum of 2. The C-OH component spectrum of 2 has a large intensity at 294 eV corresponding to an orbital of dominant u*C--OHcharacter, and little intensity from other features in the 290-3 IO-eV region, those which have a large C T * character. ~ ~ While the experimental features at 289.4 and 291.8 eV are x*(3bze) and C lsidentified by EHMO as the C ls(C-H) (C-OH) r*(3bzg)transitions, the EHMO spectrum does not predict a structure corresponding to the weak feature at 292.4 eV. This could be associated with the onset of the C ls(C-OH) continuum (estimated IP of 291.9 eV) or it could arise from a two-electron excitation. The agreement between the EHMO and experimental C 1s spectra of p-benzoquinone (3) is less satisfactory than that for 1 or 2. In particular EHMO greatly overestimates the intensity of the second feature and underestimates that of the first feature. There is much better agreement between EHMO and experiment if the C - 0 component is positioned such that its first transition occurs at 283.7 eV (see dot-dash plot in Figure 5 ) . In that case the relative intensities of the first two peaks predicted by EHMO match the experimental spectrum almost perfectly. However, this is not considered acceptable since the r*(2bZg)orbital energy with C ls(C-0) excitation is only 0.4 eV lower than that with C ls(C=C) excitation. A shift of 2.6 eV (the difference in the C 1s IPs2’*22)is needed for exact energy alignment of the C 1s r*(2bzg)transition at both carbon sites. Also, when one aligns both the C - C and C - 0 component spectra on the first experimental feature, the EHMO spectrum (dash-dot) does not
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Francis and Hitchcock predict the third A* resonance observed experimentally at 288.3 eV . While the EHMO spectrum of 3 prepared with the correct chemical shift (solid line in Figure 5 ) does not give very good agreement with experiment with respect to the relative intensities of the first two peaks, it does predict the correct number of A* resonance features and is in accord with the measured C 1s chemical shifts. The origin of the relatively poor agreement between EHMO and experiment with regard to the relative intensities of the first two transitions is not clear. One possibility that was considered was that the EHMO 2p coefficients are not adequate estimates of relative transition probabilities when comparing C 1s excitation from “qualitatively different” sites, since the main intensity of the first peak is from the C=C site and that of the second from the C=O site. However, the intensity of the transition in formaldehyde relative to the C 1s C 1s ?r*c=c transition in ethylene is predicted by EHMO to be 1.9. This is in very good agreement with the value of 1.8 determined oscillator strengths from experifor the ratio of ?r*c-.o:~*c=c mental studies of f ~ r m a l d e h y d eand ~ ~ ethylene.16 The EHMO results indicate the first five features in the experimental spectrum of 3 arise from six C 1s A* transitions, with overlap of the second C ls(C=C) n*(3b3,) and first C l s ( C 4 ) 7r*(2bzg)transitions. This overlap is the origin of the considerably greater breadth and asymmetry of the second peak, which is better fit by two rather than one Gaussian line. As in 1and 2, the “*(la,,) orbital in 3 is “silent” even though there is a dipole allowed C ls(C=C) **(lau) transition (see Table V), since the orbital “rotates” away from the C ls(C=C) core hole. An alternative presentation of the C 1s A* transitions in 3 is given in Figure 6. The A* energy levels are labeled using the virtual orbital symmetries of the ground state. The left-hand side of the diagram exhibits sketches of the A* MOs when there is a C ls(C=C) core hole (at the location indicated by the asterisk), while the right-hand side sketches the corresponding MOs when there is a C ls(C-0) core hole. The diameters of the circles in the orbital plots are proportional to the atomic 2p(7r*) coefficients at each atom. The various (C 1s A * ) transitions are represented in Figure 6 by arrows originating from the different carbon 1s energy levels (separated by 2.6 eV, the difference in the IPS) and ending at a common set of virtual MOs which are generally of mixed and character. In fact there are four C l s ( C 4 ) and two C ls(C=O) orbitals but there are A* transitions, as only seven electric dipole allowed C 1s outlined in Table V. The relative intensity of each transition is proportional to the area of the circle on the core excited atom in a particular virtual molecular orbital. EHMO predicts that the higher energy C ls(C=C) ?r*(3b3,) transition is more intense than the lower energy C ls(C=C) r*(2bzg)transition, in contrast to previous cases where core hole relaxation has been found to make the lowest energy A* resonance the most intense spectral feature. Comparison of the sketches of the 2b2, orbital in the core excited states (Figure 6) with that in the ground state (Figure 4) indicates that there is very little change in the 2bz, orbital when the C ls(C=C) core hole is introduced (if anything, there is a slight reduction in the C 2p component at the core excited carbon). In contrast, the ?r*(2b2,) develops a very pronounced localized character when the core hole is located on the C=O carbon. The r*(3b3,) orbital exhibits the changes expected upon introduction of the core hole at the C ls(C=C) core hole-increase in the C 2p component at the core excited atom and energy stabilization relative to the ground statewhereas it becomes less ?r*--like (relative to the ground state) when the core hole is on the C I O carbon. These core hole relaxation effects, along with overlap of two A* resonances, explain why the second feature is stronger than the first in the C 1s spectrum of 3. While EHMO seems to provide qualitative understanding of the A* resonances, the underestimation of the intensity of the first peak relative to the experiment suggests that EHMO does not fully account for core hole relaxation. The **(2b2& orbital in the C ls(C=C) excited state
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--
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Distinguishing Quinoid and Benzenoid Structures
The Journal of Physical Chemistry. Vol. 96, No. 16, 1992 6607
probably has a larger a*- character than that predicted by the EHMO calculations. Relative to the experimental spectrum of 3, EHMO overestimates the lower energy structure in the C 1s continuum, as was found in 1 and 2. The EHMO C ls(C=C) component spectrum has two dominant u* transitions,which have distinguishable (298 eV) character, in accord with the bond (294 eV) and u*length correlation (cf. b~tadiene’~.~~). In contrast, while a localized picture would predict a well-defined u*c4 resonance, the C l s ( C 4 ) component spectrum predicted by EHMO has four u* features of similar intensity, each of which is very delocalized. This is consistent with observations in other species where delocalization in the A* manifold is accompanied by delocalization in the u* manifold. 3.2.3.2. 0 Is Spectra. Figure 7 compares the experimental and EHMO 0 1s spectra. “Wave” and =charge” presentations and stick plots of the “charge” orbital components are included. Overall the level of agreement is good. In particular EHMO provides a clear distinction between the **-dominated spectrum of 3 and the a*-dominated spectra of 1 and 2. EHMO indicates that the first two features in the 0 1s spectra of 1 and 2 have contributions from 0 1s A* transitions. The 0 2p coefficient in the A* MOs in the 0 1s excited states is less than in the ground state (compare the MO sketches in Figure 7 with those in Figures 4 and 6) and the orbital energies are either unchanged (1) or even slightly destabilized (2) (Table VI). 0 1s A* transitions are only observed because of delocalizationof the preliminary ring-** orbitals onto the oxygen atom. While EHMO indicates only 0 1s A* transitions as the origin of the lowest energy feature in the 0 1s spectra of 1 and 2, previous work suggests that there are also contributions from 0 1s transitions. Features of similar shape, oscillator strength and energy are seen in the gas-phase spectra of all alcohols, including the 0 1s spectra of saturated alcohols,’*species where there cannot be any 0 1s A* transitions. The orbital in alcohols could have a rather large size, and thus appreciable Rydberg character. This would be consistent with the fact that 0 1s transitions are not predicted by the EHMO calculation of 1 and 2 and they are not observed or have a much reduced intensity in the 0 1s NEXAFS of condensed alcohol films.40 The 0 1s continuum of 1 and 2 in the EHMO calculation is dominated by a single transition, which corresponds to an orbital of localized character. There is a second orbital * in 2~which~ gives~a much weaker about 2 eV above the main u contribution. While this latter orbital is delocalized over the whole molecule, it does have a*CaHcharacter at both C-OH sites with a combination of blu(+/-) symmetry whereas the main orbital, while strongly localized at the core excited oxygen atom, is the a,(+/+) combination. In both 1 and 2,EHMO predicts the resonance about 2-3 eV above its experimental location, based on alignment of the first feature in the EHMO spectrum with the first feature in the experimentalspectrum. This is rather different from the C 1s continua where there is rather good alignment between the energy of the main a* features in EHMO and experiment. A possible misalignment of the EHMO relative to the 0 1s experimental spectrum was considered. However the 4.0-4.3 eV shift between the experimentalterm value (4.0-4.3 eV) and the EHMO orbital energy (8.3 eV) is consistent with that found in all of the other EHMO-experiment comparisons. As with the C 1s spectrum, EHMO overestimates the transition. intensity of the 0 1s EHMO predicts two strong 0 1s A* resonances in p benzoquinone (3),corresponding to excitation to the 2b2gand 3b3, A* orbitals. These are the 2ba(+/-) and 3b3,(+/+) combinations orbitals (seeorbital sketches in Figure 7). The of localized A*A*~-,, character of these delocalized orbitals is enhanced in the 0 1s excited state relative to the ground state. The assignment of the first two spectral features in 3 as 0 1s ?r*2b2,(+/-) and 0 1s 7r*3b3,(+/+) transitions agrees with that derived qualitatively by comparing the 0 1s spectrum of 3 with those of acetone and 1,4-~yclohexanedione,as outlined in section 3.1.2. However EHMO overestimates the intensity of the higher energy
-
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-
-
-
-
-
-
-
+
-
0 1s 7r*(3b3,) transition, perhaps because it does not fully account for relaxation of the 3b3, orbital in the presence of a localized 0 1s core hole. In general core hole relaxation increases the intensity of lower energy and decreases the intensity of higher energy A* resonances. EHMO also predicts there will be a small 0 1s r*(3b2J contribution, which should occur around 536.5 eV based on alignment of the first peak. While there is not a distinct feature at this energy, there is a poorly resolved shoulder which could correspond to the 0 1s **(3bzr) transition. In the 0 1s continuum of 3 EHMO predicts two a* resonances with a separation of about 10 eV which have u*o-oblu(+/-) and a*&,(+/+) character at the carbonyl sites in addition to some u*character. This agrees with our earlier suggestion that the 543- and 554-eV peaks observed in the experimental spectrum are states based on 0 1s excitation to the two possible combinations of two localized orbitals. While these aspects of the 0 1s EHMO prediction for 3 are in satisfactory agreement with experiment, the positions of the main continuum resonances and their relative intensities are not well reproduced. In particular, the features are each about 5-6 eV above the correEHMO u*sponding experimental features (relative to the lowest energy A* feature) and the second EHMO us- feature of 2 and 3 is more intense than that observed in the experimental spectrum. Overall, the gaxl agreement between the spectra predicted from the EHMO-EICVOM calculations and the experimental results gives confidence that the EHMO results are meaningful and can be used as a guide for spectral interpretation. An essential aspect needed to obtain the level of agreement found in Figures 5 and 7 is a careful selection of the energy alignment of the EHMO components with experiment. However, if this alignment is done incorrectly, as for example it would be if it was guided by erroneous assignments of the experimental spectra, then there is much worse agreement with EHMO and experimental spectra. In the future it would be desirable to combine EHMO predicted spectra (shifted by 4.3 eV) with reliable procedures for estimating chemical shifts of core IPS in order to have a procedure which would be independent of experiment and thus would have predictive value. In retrospect, it is perhaps not surprising that there is good agreement between EHMO and experiment. Core excitation spectra emphasize small-R parts of the virtual orbital wavefunctions and these are well represented by MOs constructed using only a minimal basis set. However the level of agreement is perhaps better than expected given the primitive character of the extended HUckel method. The reasonable agreement in the near continuum is particularly encouraging. This suggests these features are primarily oneelectron excitations and that contributions from two-electron excitations are relatively minor. Recently other semiempirical quantum chemical calculations, including HAM/3”v4’ and CND0,4* have also been applied to studies of core excitation spectra. In the HAM/3 work in particular there was better agreement between orbital eigenvalues and experimental term values, suggesting that HAM/3 might be a better choice than extended HUckel for development as a general tool for assisting core spectroscopic studies.43
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4. Discussion 4.1. Can Quinoid Shwtum Be Identified from Core Excitation Spectra? Four distinct models for the electronic structure of
p-benzoquinone (3)were investigated in our initial consideration of the C 1s and 0 1s spectra. As a means to illustrate the limitations of the “building block” mode13qMfor core spectra of species with highly delocalized electronic structure, and to show in further detail how the C 1s spectrum of 3 reflects its quinoidal structure, these four models are outlined and their deficiencies and/or merits for understanding the observed C 1s spectrum of 3 are discussed. (1) Localized, Independent r*and r*Orbitals. In this very naive picture pbenzoquinone is assumed to consist of isolated C==C and C = O units so that the discrete portion of the C 1s spectrum of 3 would be expected to be similar to the sum of the C 1s spectra of ethyleneI6and f~rmaldehyde.~**~~ This is not the case either experimentally or theoretically. The ground state
6608 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
unoccupied molecular orbitals (Figure 4b) cannot be interpreted or u*- but rather have density over the whole as simply molecule, even though the three higher energy orbitals are similar to those of 1 and 2 in the region of the ring. As a test of when such a model might be relevant, EHMO calculations were performed on a series of distorted p-benzoquinone geometries. The two ( ( 2 4 ) moieties were left at their location in the equilibrium geometry while the two (C-C) moieties (with their H atoms) were translated laterally in opposite directions up to a distance of 1.5 A from the equilibrium geometry. EHMO C 1s excitation spectra expected from these hypothetical structures were calculated via the procedures used for the equilibrium molecular geometries. The results are presented in Figure 8 with the C ls(C=C) and C ls(C=O) excitation copponents presented separately. Note that the energy scales are based on exactly the same procedure used to generate the EHMO spectra in Figure 5 . The C ls(C=C) spectrum of a structure with a lateral distortion from the equilibrium geometry of 1.O or 1.5 A (bottom plot of Figure 8a) resembles that of an isolated (C-C) moiety, for example, ethylene.16 However, as the structure approaches the equilibrium geometry of 3, new features appear in the calculated C l s ( C 4 ) spectrum, which eventually converges to the C l s ( C 4 ) component spectrum of 3 (top plot of Figure 8a). The additional low-lying transitions are associated with a* orbitals generated by the interaction of orbitals of the ethylenelike ( C 4 ) and carbonyl (C-0) moieties. Similarly, the C ls(c-0) spectrum of the structure where the c--O and C=C units are well-separated resembles that of an isolated ( C d ) moiety, as in formaldehyde.4s As the structure approaches the equilibrium geometry of 3, the C=O and C = C moieties overlap and new features appear in the C ls(C=O) spectrum, as found in the C l s ( C 4 ) spectrum, eventually converging to the C ls(C=O) component spectrum of 3 (top plot of Figure 8b). These results show that a model in which the r*- and a*orbitals are considered to be independent and localized is not correct. In particular the higher energy a* resonances observed in 3 at 288.3 and 290.7 eV would not be predicted, nor would the red-shift of the lowest u* resonance. While superficiallyone might consider the lower energy C 1s a* transition in 3 to be a a*and the second feature to be a T * ~ + , resonance, the EHMO results and other considerations indicate that excitations from C Is(=) and C l s ( C I 0 ) to the same a*LuMo(bzg) orbital form the major part of both features. Thus the correct interpretation of the C 1s spectrum of 3 must take into account the existence of considerable interaction between the C=O and C=C moieties in the quinoid structure. (2) Benzenoid Structure. One can write resonance structures for 3 which have a normal benzene ring. These involve charge separation and thus are not likely to be strong contributors in a valence bond description. If these structures had any relevance to the C 1s core excitation, one would expect to see a benzene-like C 1s spectrum, which is clearly not the case. However, the 0 1s u* transition in 3 is somewhat weaker than its counterpart in acetone or 1,4-cyclohexanedione (Figure 3) which could be interpreted as evidence for a partial C - 0 bond character as im lied in this model. In fact the C=O bond length in 3 (1.225 ) is only slightly longer than a regular C I 0 double bond such as that in formaldehyde (1.209 A) or acetone (1.214 A), so benzene-like resonance contributions, while perhaps relevant, are not very significant. (3) Two Noninteracting C=C-C=O Units. Acrylic acid is structurally similar to 3 and there are strong similarities between its C 1s spectrum and that of 3. Propenal (CH2==CHC--V) would be the best model for this interpretation of the electronic structure of 3 but its C 1s and 0 1s spectra have not yet been recorded. EHMO predictions of the C 1s and 0 1s spectra of propenal are presented in Figure 9, in comparison to the experimental spectra of 3. There is relatively good agreement for both the C 1s and 0 1s spectra, indicating that propenal already contains many of the orbital interactions important in determining the core spectra of 3. However, the EHMO C 1s spectrum of propenal does not predict the higher energy u* resonanas between
Francis and Hitchcock I
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Figure 8. (a) C ls(C=C) and (b) C l s ( C 4 ) EHMO spectra for p-benzoquinone at geometries distorted such that the C2Hz units are pulled away from the axis of the carbonyl groups by the indicated distance (in A). The top curve in each panel corresponds to the EHMO component spectrum of pbenzoquinone while the bottom panel is the EHMO spectrum for ethylene (left) and formaldehyde (right), the isolated C=C and C 4 moieties. Sketches of selected MOs are included to indicate how the 7' orbital accessed by core excitation localizes on either the C==C or C 4 unit as the structure expands. The energy scale is set by the alignment used for the EHMO spectra calculated at the correct geometry of 3, as shown in Figure 5.
288 and 292 eV, which are predicted by the EHMO spectrum of the full structure. Thus there are experimentally detectable effects associated with the interaction of two C==€--c-O units. This EHMO study suggests propenal could be almost as useful for understanding the core spectra of 3 as consideration of the
Distinguishing Quinoid and Benzenoid Structures
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The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6609
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fully delocalized electronic structure of p-benzoquinone. (4) Fully Delocrllzed p-Benzoquinone Structure. Since this is the basis for the EHMO discussion (section 3) it is not further elaborated except to note that all spectral features have been explained within this approach. What is it then that allows us to say the spectrum of 3 characterizes a quinoid structure while that of 1 or 2 is that of a benzenoid structure? In a sense observation of two strong ?r* resonances is one characteristic feature. EHMO studies of 3,6dmethylene- 1,4cyclohexadiene(not shown) show this also occurs in an all-carbon quinoid structure, one where there is negligible splitting of the C 1s ionization energies. However the prominent character of the second ?r* resonance in 3 is mainly associated with the chemical shift caused by the heteroatom (cf. the appearance of two ?r* resonances of almost equal intensity in the C 1s spectrum of 2). The fact that the second peak is more intense than the fmt is associated both with the overlap of two chemically shifted spectra and with the characteristics of the two lower ?r* orbitals of 3. The most characteristic feature of the quinoid structure is the red shift of the lowest energy ?r* resonance which leads to the appearance of the first ?r* resonance below 284 eV. This in turn can be directly related to the loss of aromatic stabilization. It is exactly this aspect of the C 1s and N 1s energy loss spectra of polyemeraldine that was used to argue for the presence of quinoid rings at neutral P H . ~Similarly the lowest energy transition in 2,1,3-benzothiadiazole occurs at 284.3 eV, about 1 eV below the lowest energy C 1s ?r* transitions in corresponding benzenoid S-N ring compound^.^ If the basis for distinguishing benzenoid and quinoid structures relies on changes in the LUMO energy, then one might expect that other spectroscopies sensitive to the LUMO energy would have a similar potential for distinguishing quinoid and benzenoid structures. Thus comparison of low energy electron transmission (ETS) spectra of p - b e n ~ o q u i n o n e ~ and ~ ~ *hydroquinone ~* (or phenol) should, and does, show a similar shift (see Figure 10). Although the ETS of hydroquinone has not been reported, the lowest energy anion state of pbenzoquinone22*464s is so stabilized relative to that of benzene26or phenol25that pbenzoquinone has a negative electron affinity. One might expect that valence excitation spectra would also be sensitive. However the lowest energy blg(?r) 2b2,(a*) transition in 3 occurs at 4.5 eVZ2s2'while that of 2 occurs at 4.26 eV,48 the opposite sense to that expected if the shift in LUMO energy between quinoid and benzenoid structure was the dominant factor. It appears that valence
-
-
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Figure 10. Correlation of the energies of the occupied x and unoccupied x* MOs of 1,2, and 3 derived from core excitation (this work), electron and photoelectron spectroscopy.5b53The transmission ~pectroscopy,~~*~~ ETS values of hydroquinone have not bccn reported, so dashed lines are used to indicate their approximate position. Similarly the ISEELS energies indicated with open circles are estimated positions for transitions for which direct experimental evidence is lacking. electronic spectroscopy is complicated both by concomitant shifts in HOMO energies and by extensive configuration interaction associated with a density of excited states which is much greater than that in core excitation. Can the same effect be seen in the occupied valence orbitals and spectroscopiessensitive primarily to them? According to the pairing there should be a mirror relationship between the energies of occupied ?r and unoccupied ?r* orbitals. This would predict that the "HOMO of 3 should be higher in energy (Le. lower IP) than that of 1 or 2. In fact the energy of the uHOMO of 3, as detected by photoelectron spectro~copy~*~' decreases relative to the uHOMO of 2,52 l?3and ben~ene~~-i.e.the IP of 3 is higher than in 1 or 2. Thus the shift of the uHOMO energy is in the same direction as that of the T * energy ~ (Figure ~ 10) ~ rather ~ than in the opposite direction as expected from the pairing theorem. Clearly the pairing concept is inapplicable in these compounds. The shift in the uHOMO energy between 2 and 3 is smaller than that in core excitation. A greater sensitivity of unoccupied rather than occupied orbital energies to benzenoid/quinoid distinctions was also found in work on S-N ring compound^.^ 4.2. C O M ~ C ~to~Organic O ~ S Conductors. The present study offers strong support to the proposed interpretation of the pH dependence of the C 1s and N 1s spectra of polyemeraldine in terms of a quinoidal structural transformation? In addition to identification of the red-shifted a* resonance as characteristic of quinoid structures, the present results support the observation of a much stronger effect of quinoidal distortion on the N 1s than the C 1s spectrum (see Figure 1). In a similar fashion there is much greater difference between the 0 1s than the C 1s spectra of 3 and (1,2) (Figures 2 and 3). In order to better quantify the extent of quinoid distortion from measured core spectra of polyemeraldine, it would be useful to carry out model compound studies for the analogous C=N, imine-like quinoid system. The species which would be the direct counterparts to 1,2, and 3 would be aniline, 1,4-diaminobenzene, and 3,6-diimine- 1,4-cyclohexadiene. The question of the relative stability of quinoid versus
6610 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
benzenoid structures is frequently encountered in the area of conducting aromatic polymers. Wallnofer et aLS4have recently used quantum total energy calculations to address this question in a number of species including isothionaphthalene, where the ground-state structure remains in dispute.6~~~ The C 1s and S 1s spectra of isothionaphthalene, compared to those of thiopheneS5 and n a ~ h t h a l e n e could , ~ ~ be used to investigate this point experimentally. 5. Summary
Oscillator strength spectra for C 1s and 0 1s excitation of gaseous phenol, hydroquinone, and p-benzoquinone have been reported for the first time. Core excitation features which can be used to distinguish quinoid and benzenoid structures have been identified. Assignments for all spectral features have been proposed through a combination of analysis of systematic trends and comparison to EHMO results. On the basis of this analysis revisions have been suggested to recently reported assignments for the C 1s NEXAFS spectrum of solid phenol.’ This study amply illustrates the utility of EHMO-EICVOM calculations for aiding the interpretation of core excitation spectra. Such simple calculations, in conjunction with the preparation of spectra based on the results, reveal the effects of core hole relaxation and can be useful for exploring the connection between core excitation spectra, electronic structure, and bonding. Acknowledgment. Funding for this research was provided by NSERC (Canada), who have also provided a postgraduate scholarship for J.T.F. We thank Alex Wen and Tolek Tyliszczak for operation of the spectrometer and some assistance with data analysis, Alec Bain for providing the purified phenol sample, and Carlo Meali for generously providing the orbital viewing programs. Discussions with Roald Hoffmann (Cornell) concerning the EHMO results are gratefully acknowledged. Registry No. p-Benzoquinone, 106-51-4; hydroquinone, 123-31-9; phenol, 108-95-2;benzene, 71-43-2;1,4-cyclohexanedione, 637-88-7.
References and Notes (1) Hitchcock, A. P. Phys. Scr. 1990, T31, 159.
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