Inner-Shell Spectroscopy of Benzaldehyde ... - ACS Publications

J . Phys. Chem. 1992, 96, 8736-8750

8736

Inner-Shell Spectroscopy of Benzaldehyde, Terephthalaldehyde, Ethyl Benzoate, Terephthaloyl Chloride, and Phosgene: Models for Core Excitatlon of Poly(ethylene terephthalate) A. P. Hitchcock,* S. G. Urquhart, Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada U S 4Ml

and E. G.Rightor Texas Polymer Centre, B- 1225, Dow Chemical USA, Freeport, Texas 77541 (Received: March 9, 1992; In Final Form: July 8, 1992) Oscillator strengths for C 1s and 0 1s excitation of terephthalaldehyde, benzaldehyde, and ethyl benzoate and C Is, 0 Is, and C1 2p excitation of terephthaloyl chloride and phosgene have been derived from electron energy loss spectra recorded under scatteringconditions where electric dipole transitions dominate. Extended Hiickel molecular orbital (EHMO) calculations were used to aid interpretation of the core excitation spectra of these multifunctional organic molecules. The experimental molecular spectra and EHMO calculations were used to estimate the C 1s and 0 1s core excitation spectra of poly(ethy1ene terephthalate) (PET). Comparisons to the C 1s and 0 1s energy loss spectra of PET recorded in an electron microscope have demonstrated that gas-phase spectra and theoretical studies of molecular analogues are useful in interpreting the core excitation spectra of polymers.

1. Introduction

Electronic states involving excitation and ionisation of inner-shell (core) electrons are a localized and rather sensitive probe of the environment in the region of the core ho1e.l For this reason core excitation spectroscopies are finding increasing use in the characterization of a wide variety of materials including semiconductors, catalysts, polymers, etc. The core excitation spectra of polymers may be studied by electron energy loss (EELS)24 or by X-ray absorption (NEXAFS) The former method has been applied in transmission electron microscopy for years,8 but its application to polymers has often been thwarted by the high electron beam damage associated with the long recording times required for inefficient serial detectors. The development of commercial parallel-detection electron energy loss spectrometers has significantly extended the range of applications and has improved the ability of EELS to provide useful chemical inf~rmation.~ Since parallel-EELS has only recently been used for low-dose studies of polymers,I0 the understanding of relationships between recorded spectral features and molecular structure is at an early stage of development. Characterization of these relationships is extremely important for the successful application of core excitation spectrosoopy to high spatial resolution studies of polymer interfaces, dispersed phases, and additives. Both experimental studies and quantum chemical calculations can be used to explore the relationship between core excitation spectral features and the geometric and electronic structure of polymers. Experimental approaches include fingerprinting through comparisons to the core spectra of homopolymers and comparison of polymer spectra with spectra of the monomer or suitable small molecule analogues. This latter approach has the advantage that the geometry of the molecule is generally known and its electronic structure can be calculated. In principle core excitation spectra can be predicted via ab initio or semiempirical quantum chemical calculations. However relatively few such calculations have been reported, mostly for very small, monofunctional species,'I and there have only been a few studies of the reliability of such calculations.12J3Recently, extended Hiickel molecular orbital (EHMO) calculations have been applied successfully to studies of the core excitation spectra of organometallic complexes.l"16 While EHMO was chosen in those applications since it is the only widely available semiempirical package which can handle all elements of the periodic table, a wide range of semiempirical or ab initio approaches could in principle be applied to studies of organic small molecules and polymers. The present work explores the applicability of EHMO to studies of core spectra of reasonably complex organic molecules. Certainly there are much more sophisticated quantum

chemical packages, but EHMO adequately reflects the topological aspects of molecular electronic structure (i.e., symmetry and dependence on bond lengths and angles), and it was a convenient choice since programs and procedures for relating EHMO results to core excitation spectra had been developed in earlier studies.Iel6 An in-depth study of the applicability of EHMO to conjugated organic molecules, which provides insight into various choices in relating EHMO output and experiment, has been published elsewhere. I Although the literature contains X-ray absorption and EEL spectroscopic studies on numerous small molecules," the spectra of analogues of complicated polymers such as poly(ethy1ene terephthalate) (PET) were not available prior to this study. A preliminary presentation of this work has been made,l* and a companion paper, which discusses more fully the polymer spectra and their relationship to the molecular spectra, is presented e1se~here.I~ In the present work spectrastructure relationships are explored through experimental and EHMO studies of a series of related small molecules. PET polymer spectra are then compared to simulations obtained from appropriate combinationsof the small-molecule spectra and to predictions from extended Huckel molecular orbital theory. The small molecule data include the C 1s and 0 1s spectra of benzaldehyde, C6H5CH0 (BA); terephthalaldehyde, p(HCO)C6H,(HCO) (TA); ethyl benzoate, C6H5C02C2H5 (EC); and the C Is, 0 Is, and C1 2p spectra of terephthaloyl chloride, ~ - ( ~ ~ o c ) c ~(TC). ~ ~ The ( c Co IS, ~ O~ IS, ) and C12p spectra of phosgene, ClzCO (PH), were also recorded in order to account for the acyl chloride functional group in relating the spectra of TC to those of PET. (Note that the indicated two letter codes are used for convenience throughout the text.) The spectra are reported on absolute oscillator strength scales using a previously developed conversion scheme.20 EgertonE has emphasized the utility of absolute oscillator strength scales as a means of removing instrumental dependent aspects of energy loss spectra and thus facilitating comparisons of data from different laboratories. The spectra of the small molecules are tentatively assigned with the aid of extended Hiickel molecular orbital (EHMO) calculations21 in which the equivalent ionic core virtual orbital model (EICVOM)22is used to predict core excitation spectra. The molecular orbital descriptions provided by these calculationsemphasize the need to go beyond simple building block modelsz3when dealing with conjugated systems. The utility of using experimental spectra of small molecule analogues and calculations to assist analysis of polymer EELS is illustrated by comparison with parallel-EELS spectra of PET. Three different linear combina-

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0022-365419212096-8736%03.00/0 0 1992 American Chemical Society

Core Excitation Spectra of PET Analogues tions of the small molecule C l a spectra and EHMO calculations of dimethyl terephthalate (MT) are explored as estimates of the C 1s spectrum of PET. 2. Experimental Section A. Gas-PhaseEELS of Model Compounds. The inner shell electron energy loss spectra (ISEELS) were recorded on the McMaster gas-phase spectrometer,' operated under inelastic scattering conditions under which electric dipole transitions dominate. The final electron energy was 2.5 keV, the scattering angle was 2 O , and the resolution was 0.6-eV fwhm. The samples were all obtained commercially (from Aldrich, except for phosgene which was obtained from Matheson) and used without further purification. The least volatile sample, terephthalaldehyde (mp 116 "C), was introduced directly into the collision cell of the spectrometer, which was heated to 40 OC. The spectra of the other species were obtained by sampling the equilibrium vapor pressure at room temperature or that achieved with an externally heated inlet. The absolute energy scales were established by recording the spectrum of a stable mixture of each compound with C02, using the accurate values previously determined for the sharp 1s ?r* features in C 0 2 as reference standards.24 The as-recorded spectra were converted to absolute optical oscillator strengths using procedures discussed and tested previously.20 This involved subtraction of a background generated by extrapolation of the preedge region through a fit of the experimental data to the function, a(E - b)C(a, b, c are fitted parameters); correction for kinematic factors connecting energy loss and ph~toabsorption;~~ and normalization at 25 eV above the ionization threshold to the atomic oscillator strengthz6appropriate for the number of atoms of that type in the molecule. Absolute intensity scales are needed to estimate the polymer spectrum through the addition of appropriate combinations of the small molecule spectra. E. Extended HBckel Molecular Orbital Calculations. Semiempirical extended Hiickel molecular orbital (EHMO) calculations have been used to assist spectral assignments by investigating the unoccupied MOs in the core excited molecules. EHMO calculations were carried out for the core edges of all molecules except for the C1 2p spectra. The fixed molecular geometries used in the calculation were ground-state experimental geometries taken from the literature (where available) or estimated from similar species.27 The energies and exponents for the basis atomic orbitals were the default values of the EHMO program.21 Figure 1 sketches EHMO energies and wave functions of T * unoccupied molecular orbitals (UMOs) of terephthalaldehyde (TA)in order to illustrate several aspects of the application of EHMO to core excitation spectra. In general the virtual orbital character in the core excited states differs from that in the ground state because the valence electron distribution relaxes in the presence of a localized core hole. EHMO calculations, carried out in the equivalent ionic core virtual orbital model (EICVOM),ZZ have been found to account for the relaxation induced by the core hole and to give satisfactory predictions of core excitation spectra of organic" and organometallic compound^.'^-'^ The effect of core hole relaxation on the A* UMOs of TA, as predicted by the EHMO-EICVOM results, is illustrated in Figure 1. There are systematic changes in orbital energy and spatial distribution as the core hole is introduced and placed at chemically inequivalent sites. For example, the curve sketched under the plot of the ground-state ?r* UMOs is the expected C ls(C0) spectrum ifthe UMOs had the energies and C 2p(CO) density found at the rightmost carbonyl carbon in the ground state. A complex spectrum with significant r* intensity over a range of energies would be expected ifthe ?r* orbitals in the core excited states were similar to those in the ground state. However the core hole strongly stabilizes the LUMO which lowers its energy and localizes it on the core excited carbonyl (see the curve sketched under the plot of the C ls(C0) UMOs). The higher energy ?r* UMOs are either unaffected or experience an "antilocalization" whereby the C 2p density is shifted away from the core hole relative to its distribution in the ground state. Note that the intensities (areas) of the spectral features labeled a-d in these two plots are directly

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The Journal of Physical Chemistry, Vole 96, No. 22, 1992 8737 given by the A 0 coefficients on the atoms in the MO sketches with the corresponding labels. The core hole relaxation effects vary considerably from location to location in the same molecule and among different molecules. EHMO calculationscarried out in the EICVOM approximation appear to be sensitive to these changes. This is essential in order to reliably predict core excitation spectra. Core excitation spectra were generated from the EHMO results using procedures described previ~usly.'~-''As an example, the spectrum predicted by extended Hiickel for C 1s excitation of the carbonyl carbon of TA (one of three chemically inequivalent carbons) is the sum of 21 Gaussian lines corresponding to excitations from C ls(C0) to each of the 21 unoccupied MOs in a minimal basis set description of CHOC6H4NH0(Le,, the basis set is modified by placing a nitrogen atom at one of the aldehyde groups to represent core excitation at that carbon). The position of each line is given by the energy of the virtual MO. The width of each line increases from 1 to 12 eV as the orbital energy increases (the values used for constructing each simulation are given in footnotes to the tables summarizing the EHMO results). In principle it should be possible to develop an a priori scheme for selecting line widths since there is known to be a systematic relationship between line width and core excitation energy relative to the IP for continuum states. However, for this work the experimentally observed line widths have been used as a guide to selecting fmed line widths in given ranges of orbital energies. The full spectral range was covered by four or five such fmed-line-width sections, with gradually increasing line widths. The peak area is given by Z$(N 2p), where c(N 2p) is the LCAO coefficient, which is the contribution of the N 2p A 0 to the virtual MO in CHOC6H4NH0. A similar calculation was carried out for C ls(C-H) and C ls(C-R) excitations in order to account for C 1s excitation at the other two chemically inequivalent carbons in TA [the C-H carbons on either side of the CHO group are formally chemically inequivalent but the predicted spectra at these sites are very similar (see Table VI)]. Figure 1 presents the predicted shape of the w* signal for each of the C 1s component spectra of TA as well as that of the 0 Is spectrum, on the EHMO orbital energy scale. The EHMO prediction of the complete C 1s spectrum of TA was then constructed by a 2:2:4 weighted sum of the C ls(C=O), C ls(C-R), and C ls(C-H) component spectra. In principle, if EHMO-EICVOM was accurate, the orbital energies would be equal to the experimental term values relative to the appropriate C 1s IP. In fact, the EHMO-EICVOM orbital energies are found to be systematically lower than the observed term values by 4-5 eV. Since EHMO does not provide any quantitative information concerning the core hole energy and since comparison on an absolute energy scale is more informative than on a relative energy scale, appropriate energy scales for the EHMO spectra have been derived as outlined below. Two procedures were explored for setting the absolute energy scale. In the first procedure, the zero of each component EHMO spectrum was assigned to the measured or estimated IP for that particular carbon. All components were then added and the sum was rigidly shifted to provide alignment with the lowest energy T* feature in the experimental spectrum. A limitation of this approach is that measured ionization potentials were not available for many of these species and thus they had to be estimated from those of related species. The second procedure (which was eventually used in generating most of the EHMO spectra presented in this paper) was to set absolute energy scales for each EHMO component spectrum on an individual basis, by setting the energy of a particular transition (usually that corresponding to excitation to the ?rSLUMO) to that of the experimental feature so assigned. This procedure gave component spectra shifted by up to 1 eV relative to those derived using the previously described IP alignment procedure. Full details of the transformation from the EHMO orbital energy to C 1s excitation energy scales are given in footnotes to the tables reporting the EHMO results. Core excitation spectra were generated from EHMO calculations for all of the species in a similar manner, with appropriate consideration of the contribution from each chemically inequivalent

Hitchcock et al.

8738 The Journal of Physical Chemistry, Vol. 96, No. 22, 1992

I

g round state

CH

c 1s CR

0 1s

co

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Orbital Enargy (aV) Figure 1. Sketchesof the unoccupied r* molecular orbitals of terephthalaldehyde (TA) calculated by EHMO for the ground state, C ls(CH), C ls(CR), C Is(CO), and 0 1s states. The radii of the circles about individual atoms are proportional to the LCAO MO coefficients (taken from the complete charge matrix of the EHMO output]') and thus the areas of the circles on the cure excited atom are proportional to the expected core excitation intensity, Le., these give the areas of the peaks in the component spectra plotted underneath each MO scheme. These component spectra are combined in the correct ratio, summed with an atomic oscillator strength continuum and aligned to the experimental spectrum to form the final EHMO spectrum.

atom in each case. An oscillator strength scale for the EHMO calculated spectra was established by matching the integrated intensity of a well-defined transition (typically the lowest energy 1s T* transition) to that observed experimentally. In addition to the core virtual valence excitations, ionization continua are also included in the EHMO spectra by adding an appropriate multiple of the atomic continuum oscillator strength spectrumz6 (Figure 3 gives graphical examples of the buildup of the complete EHMO spectra from these components). Within the ability of EHMO to reproduce the electronic structure of these molecules and possible errors associated with incorrect geometries, this approach should be able to predict all spectral features except oneelectron excitations to Rydberg states and two-electron (double excitation, shake-up, and shake-off) transitions. In summary, we emphasize that the EHMO calculations provide only relative energies and intensities of the core valence excitations. To facilitate comparison of experiment and calculation, experimental results are used to establish absolute energy and intensity scales and peak widths. C. ParaUeCEELS of PET. Parallel-detection EELS was performed with a Gatan Model 666 spectrometer on a JEOL 2000FX transmission electron microsoope (TEM) equipped with

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a lanthanum hexaboride filament (LaBs), as described in greater detail else~here.'~ The TEM was operated at 100 kV accelerating voltage with the filament desaturated. Pellets of PET from Goodyear Tire and Rubber Company were microtomed at room temperature on a Reichert-Jung Ultracut E. The sections were placed on top of copper grids modified with holey carbon. During data collection the sample was maintained at -170 "C to minimize beam damage. Spectra were obtained at the lowest possible electron dose (160 e-/nm2), on fresh areas over a hole in the support film. Studies of the dose dependence of the core spectrum indicate that significant changes occur at doses above 200 e - / r ~ n ~ . ' ~ The collection angle was 5.6 mrad. Selected area electron diffraction mode was used to collect the post specimen illumination (camera length 30 cm) and position it over the spectrometer. The width of the zero loss peak was typically 1.1 eV fwhm. 3. Results and Discussion 3.1. C 1s Spectra of Molecular Analogues. 3.1.1. Benraldehyde, Terephthalaldehyde, and Ethyl Benzoate. The C 1s oscillator strength spectra of benzene (taken from earlier work2*), BA, TA, and EB are plotted on a common energy scale in Figure 2. The hatched lines indicate the ionization potentials (IPS) as

The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 8739

Core Excitation Spectra of PET Analogues TABLE I: Energies, Term Values, and P r o term energy (iO.1 eV) TCH 285.07” 5.4 286.0 (sh) 287.7 2.8 289.2 1.3 290.3 (br) 290.5b 291.5b 293.3b 293.6 -3.1 295.4 (8) 301 (1) -9.5 ~

~~

~~

w Assignments for Features in the C 1s Spectrum of values (eV) TCR Tco CH asc4(5a”) _ _ 5.5 3.8 5.6 **c-R( 19a’) a * (2Oa’) 1.2 3.0 IP

Benzaldehyde (BA) assignments CR

co

T * ~ < (5a”) 19a’)

n*c-0(5a”)

a*(20a’)

n*C-R( 19a’)

IP IP -2.1

U*CC(l)‘

U*CC(l)‘

u*cc(2,3)‘

,J*CC(2,3)‘

-2.1

U*C,O

-8.5

-.

‘Calibration: -5.67 ( 5 ) relative to the C 1s T* resonance of C 0 2 (290.74 eV). *IPS from XPS.29 The quoted single C H IP (290.74 eV) has been divided into a CH component (290.5 (3) eV) and a CR component (291.5 (3) eV). c u * ~ ~ ( 1 , 2 ,refer 3 ) to u* resonances identified in the C 1s continuum of benzene.28 TABLE II: Energies, Term Values, and Proposed Assignments for Features in the C 1s Spectrum of Terephthaladehyde (TA) term values (eV) assignments energy (iO.l eV) TCH TCR Tco CH CR co 284.67‘ 5.8 ~*~--(20a’) 285.8 4.7 5.7 ~*~~(2la’) ~*~~(20a’) 288.2 5.1 ~*~=0(+)(20a’) 35 288.8 1.7 2.7 4.5 r*c-R(+)(23a’) 3P 290.3 1.2 3.0 ~*~..~(+)(23a’) **,,(-)(2la’) 290.56 IP 291.5b IP 292.0 (4) (sh) 1.3 4P 293.3b IP o*cc(l IC 293.6 -3.1 -2.1 u*cc(l )‘ 294.7 (8) -1.4 U*C4 302 (1) -11.5 -10.5 u*cc(2,3)‘ U*CC(2,3)‘

”Calibration: -6.07 (3) eV relative to C02. bIPs of TA assumed identical to those of BA. cu*cc(1,2,3) refer to u* resonances identified in the C 1s continuum of TABLE III: Energies, Term Values, and Proposed Assignments for Features in the C 1s Spectrum of Ethyl Benzoate (EB) assignment (final orbital) term value (eV) energy (f0.1 e v ) TCH TCR TCH, Tc-0 Tc-0 CH CR CH3 C-0

285.00“ 287.5 (sh) 288.2 289.6 290.5b 290.7 291.5‘ 292.5d 293.3 293.0‘ 294.58 296.5 303 (1)

5.5 3.0 2.3

-2.8

5.5 3.4

3.3 1.9

6.3 2.9

2.3

0.8

1.8

a*c-c(30a) 3P 4p IP

-0.3

**c-c(30a) **c=c(3W

3.8

4P u*c +6.0 eV. dThe 17a, and 18a, orbitals have mixed character in the species with the C-H carbon replaced by a nitrogen. In all other cases (TA and other species) the spatial distribution of the orbital is similar to that of the ground state except for enhancement of the density at the core excited atom in selected orbitals, due to the effect of relaxation in response to the localised core hole. See Figure 1 for sketches of these T * UMOs.

4

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C . 0

H' Figure 4. Orbital correlation diagram for the unoccupied r* orbitals of benzene, benzaldehyde, terephthalaldehyde, and formaldehyde, based on EHMO calculations for the ground state of these species. The radius of each circle is proportional to the 2p, A 0 coefficient on that atom in the indicated MO.

the A* UMOs of benzene, benzaldehyde, terephthalaldehyde, and formaldehyde is outlined in a correlation diagram for the ground-state A* orbitals in Figure 4. The correlation diagram for the ground-state A* UMOs of benzene, ethyl benzoate, and

methyl formate is presented in Figure 5 . The EHMO spectra and the orbital diagrams provide a basis for the spectral interpretation discussed below. BA, TA, and EB are expected to have strong C ls(C0) T *

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Hitchcock et al.


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Figure 5. Orbital correlation diagram for the unoccupied r * orbitals of benzene, ethyl benzoate, and methyl formate, based on EHMO calculations for the ground state of these species.

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transitions. These should occur above the C ls(C-H) r*nng features on account of the higher C l s ( C 4 ) IP. The peaks around 288 eV in BA and EB and those at 288 and 290 eV in TA are assigned to C ls(C-0) excitations to orbitals of primarily r*c-o character. If there were negligible interaction between the aldehyde groups at each end of the molecule, one might have expected a single C ls(C0) T * transition ~ in TA with twice the intensity of that of BA. However, this is not observed. Instead, as clearly indicated by the EHMO results, there is significant interaction between the two aldehyde groups in TA mediated through conjugation with the rlli MOs. This results in two r*m levels in TA (see Figures 1 and with C l s ( C 4 ) excitations to them occurring at 288.2 and 290.3 eV according to the proposed assignment. Both peaks are relatively sharp and of similar intensity. The EHMO results (Figure 3, Table V) predict only a transition in either EB or BA but two single C ls(C0) r*C ls(C0) **-transitions in TA, separated by 1.8 eV, which is attributed to the r*co(+)/r*co(-) splitting. The corresponding X * ~ ~ ( + ) / T * ~ ~ (splitting - ) in terephthaloyl chloride, TC is 1.4 eV (see below). The observation of two rlc0resonances in TA and TC is a breakdown of the "building block" model, similar in character to those noted in other conjugated molecule^.^' Note that assignment of a specific spatial character to any of the low energy u* MOs is an approximation, the validity of which depends on the location of the core hole. In the above discussion use of the labels, and ?r*c4 suggests two different orbitals with distinct spatial character. However, the EHMO results indicate these are in fact the same LUMO, but that the spatial distribution changes with the location of the core hole so that either a or r*c4 character is emphasized. The changes in r* UMO character, and in a few cases extensive orbital mixing, is well illustrated for TA in Figure 1. A number of other shoulders and weak peaks are observed in the 288-292-eV region in the C 1s spectra of BA, TA, and EB. These are attributed to C 1s u * ~ - C~ ls(ring) , r*, and C 1s Rydberg transitions, as outlined in Tables 1-111. In some cases specific Rydberg states are suggested on the basis of their term values and expected quantum defects.32 The C Is continuum in the spectrum of BA, TA, and EB (Figure 2) is dominated by two features, a relatively sharp resonance at 293 eV and a broader, structured resonance peaking around 303 eV. A similar pattern of features has been observed in the C 1s spectra of all benzenoid specie^.^^^^^ On the basis of an M S - X a calculation,28these features have been attributed to quasi-bound states arising from C 1s promotions to the u*cc orbitals of benzene. On the basis of ab initio independent particle and CI quantum calculations, Schwarz et al." have suggested that the prominent 293-eV feature is a state of mixed character, with both C lsring 2 ~ * ( b *and ~ ) ( l ~ - ~ , r - ' doubly , r * ~ )excited configurations contributing strongly. This interpretation has been

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Core Excitation Spectra of PET Analogues

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300 Energy (eV)

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Figure 6. C 1s oscillator strength spectra of phosgene and terephthaloyl chloride (set caption to Figure 2 for experimental conditions). The bottom trace is a simulation of the terephthaloyl chloride spectrum. It is the sum of two spectra of benzene (in a 1:2 ratio, one shifted by 0.6 eV to account for the chemical shift in the two types of ring carbon atoms) and twice the spectrum of phosgene, shifted -1.3 eV to account for the chemical shift between the C 1s IP of ClCOCl and Ph-COCI.

Figure 7. Comparison of the C 1s spectra of phosgene and terephthaloyl chloride with that predicted by EHMO calculations within the EICVOM model. The upper calculated spectra are based on calculationsusing CI 3d orbitals while the lower calculated spectra are based on calculations without Cl 3d orbitals. The orbital energies, cocficients, and details of how the energy, oscillator strength scales and peak widths were established are presented in Tables I X and X.

t

supported by the absence of polarization dependence in NEXAFS studies of monolayers of oriented benzene on some surfaces. However, the EHMO calculation of benzene predicts a separation of 4.0 eV between the r*( lezu)and r*( 1bzJ resonances (Table IV), supporting assignment of the 289-eV peak, 3.7 eV above the .*(e?,) resonance, to the (C 1s-’,?r*(ba)) state, in agreement with the MS-Xa calculations.28 In addition to the u*cc features, the near continuum of BA, TA, and EB should also contain contributions from u*c4 (in all three species) as well as an additional u*c-o resonance in EB. Weak, broad shoulders interspersed among the normal benzene ring continuum shape resonances have been so assigned (see Tables I-111), with guidance from the empirical correlation of bond lengths and resonance energie~.’~ 3.1.2. Phosgene and Terephthaloyl Chloride. The C 1s spectra U 9 L l’* of phosgene (PH) and terephthaloylchloride (TC) are presented in Figure 6 while the energies, term values, and proposed assignments are given in Table I11 for these species. The C 1s spectrum of phosgene is a classic example of the potential barrier effect.36 It is dominated by three sharp discrete features, and 1’ relative to the other molecules discussed, its C 1s continuum is very weak at threshold. The discrete features are attributed to C 1s excitations to the 7r*(3b2),~ * ~ - ~ ( + ) (and 5 b~~*)~ ~ ~ ( - ) ( 6 a , ) Figure 8. Orbital correlation diagram for the unoccupied r* orbitals of virtual orbitals respectively (C,, symmetry assignments for benzene, terephthaloyl chloride, and phosgene based on EHMO calcuphosgene in the y z plane). The 1.9-eV separation of the features lations for the ground state of these species. assigned to the C 1s u * ~ - ( + ) and C 1s u * ~ ~ ~transitions (-) is quite large, but not unexpected given the proximity of the two block picture is approximately applicable. The major difference C-Cl bonds. This is analogous to, but much smaller than, the occurs around 291 eV where C l s ( C 4 ) u * ~ ~ ( + / and - ) splitting of the u * ~and u*, orbitals in CO2.’’ The C 1s spectrum C l s ( C 4 ) u * ~ ~ ~ ( + transitions /-) are expected. Since the of TC is similar to that of TA, which is not surprising given the estimated spectrum cannot account for interaction of the two similarity in structure and the fact that only two of the carbons ?r*c=o and u*c-cI orbitals on opposite sides of the ring, it is not of TC are attached to chlorine. Figure 6 also includes an estimate surprising to find deviations from the measured spectrum of TC of the TC spectrum generated by adding twice the PH spectrum in this region. (shifted by -1.3 eV to account for the difference in chemical shift The EHMO C 1s spectra of PH and TC are compared to the between a carbon attached to one versus two C1 atoms) to that experimental spectra in Figure 7, while EHMO numerical results of two spectra of benzene, one with a 2/3 weighting on the original for selected orbitals are presented in Tables IX and X. Figure 8 illustrates the relationship of the u* UMOs of benzene, PH and energy scale (corresponding to C ls(C-H) excitation in TC), the other with a weighting shifted -0.6 eV to position the main TC through a ground state orbital correlation diagram. Com?r* peak at 284.7 eV (corresponding to C ls(C-R) r* excitation parison of the r* UMOs of TC with those for TA (Figure 1) in TC). In general the estimated and measured spectra are in indicates considerable similarity. The EHMO calculations have agreement, particularly in the r* region, suggesting a building been carried out both with and without use of C1 3d orbitals. In

c

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Hitchcock et al.

there is a u*c4 resonance at 304.5 eV, at the energy expected from the bond length c ~ r r e l a t i o n .The ~ ~ EHMO calculation of PH (Figure 7) predicts a single u * ~ + continuum resonance at an energy in reasonable agreement with experiment. The C 1s continuum of TC is more complex, as expected from the larger number of (I* orbitals. There is relatively poor agreement between the EHMO calculation and experiment in the continuum region of TC. Note that for most of the ring compounds the EHMO continua generally predict the correct number of strong features but predict them with too high an intensity and at an energy relative to the A* resonances below that found experimentally. In a recent systematic study of u* orbital energies Lindholm et al.I3 also found that the core u* energies predicted by a reparametrized CNDO program occurred at too low a relative Terephthaloyl Chloride (TC) energy. assignment (final orbital) term value (eV) c The most intense feature in the C 1s spectrum of TC is assigned (*O.l eV) TCH TCR Tco CH CR co to the overlap of C ls(CH) **(16bz) transitions, corresponding 284.7' 6.1 ~*(16bJ to the lower energy principal component, and C ls(CR) A*T*(16b2) 285.6 sh 6.2 (16b2) transitions, attributed to the intense, poorly resolved, 287.4 7.4 **c=o(+) (16b2) high-energy shoulder at 285.6 eV. The overall peak is much 288.7 2.1 5.9 r*(17b2) **c=o(-) broader than the corresponding feature in benzene because of the 290.3 1.5 3.5 u*c-ccl(+) @I) contributions from the chemical shift between C ls(CH) and C 290.F IP ls(CR), which is estimated to be 1.0 eV. At the same time the 291.gC IP 293.1 -2.3 -1.3 1.7 u*c-c(l) u*cz(l) PCxI(-) (al) C ls(CR) chemical shift appears to be less in TC than in TA since 294.U IP peaks are observed in TA whereas two well-resolved C 1s 296.2 -5.4 u*c-c(2) only a shoulder is found in TC. The second peak in the spectrum 297.8 sh -6.0 u*c-c(2) of TC is attributed to C ls(COC1) A * ~ ~ (excitations. +) This 303 (2) br -12 -11 -8 ~ ' ~ ~ (u*c4(3) 3 ) uIC+ peak occurs 1.3 eV below the corresponding A*,-+ resonance in Calibration: phosgene: -2.20 (5) eV; terephthaloyl chloride: -6.04 (4) PH, whereas the C 1s IPS of the carbonyl carbons in these two relative to C02. 61P from XPS.29 cIPs estimated from other species: CH species differ by 1.9 eV. The 0.6 eV difference between the from benzene (290.3); C-R from monosubstituted benzenes with electron excitation and ionization energies reflects half of the A * ~ ~ ( + ) / withdrawing ligands (Bz-F 292.5, Bz-OH 292.0; Bz-NO, 292.1); CO from A * ~ ~ (splitting -) in TC, which is quite large (as in TA) because Z-BuCOCI (294.76).29 of the relatively strong interaction between the two localized many applications of EHMO, particularly to photoelectron orbitals arising through conjugation of the r*coand the ring A * ~ spectroscopy, the 3d orbitals of third-row elements are generally molecular orbitals. On the basis of a comparison to the EHMO not included. The comparison between experimental and EHMO results (Figure 7), the C ls(C0) A * ~ ~ (transition -) is believed spectra of PH (Figure 7) clearly shows that it is important to to be the weak peak at 288.7 eV. It is masked in TC because of include C1 3d orbitals when applying EHMO to core excitation. overlap with the C ls(C0) u*cxI transitions. The peaks at 290.3 and 293 eV in TC are assigned to C lsEHMO predicts a separation of 3.1 eV (without C1 3d) and 0.8 eV (with C1 3d) for the two features which correlate with those (C-0) and C l s ( C 4 ) u * ~ < ~ ( -transitions. ) The comparison between the measured C 1s spectrum of TC and and C 1s u * ~ ~ ~excitations ( - ) attributed to C 1s that simulated from the spectra of benzene and phosgene (Figure in PH. When the EHMO calculation is carried out with C1 3d 6) graphically illustrates the difference between the u*ccI regions orbitals there is reasonably good agreement between EHMO and of TC and PH. The two sharp C 1s u*c-cI features of PH are experiment with regard to the u*ccl energy relative to the A* prominent in the estimated spectrum, but there is a poor match energy as well as in the relative intensities of the two u*cxl to the experimental spectrum of TC in this region. However, the resonances. Since uscxI resonances are a much smaller comtwo PH-derived peaks are centered about the 290.3-eV feature ponent of the C 1s spectrum of TC, the difference between the in TC. This feature appears to contain most of the C 1s u*~-,-, EHMO spectrum calculated with and without C1 3d orbitals is smaller than in PH. However, the C ls(C=O) component oscillator strength in TC. The EHMO calculations of TC indicate there is extensive interaction and delocalization in both the u and spectrum of TC calculated with and without C13d orbitals (Figure 7) differ in ways parallel to the changes observed in the two PH A orbitals. However, when the interacting groups are widely calculations. In particular orbitals of and u * ~ ~ ~ ( - )spaced as is the case of the two A * Cor~ u*cxI orbitals, the character in TC shift to lower energy with the inclusion of the localized core hole selectively enhances excitations to the lower energy orbital of the two orbitals which arise from the A*-(+/-) C1 3d orbitals. and pairwise interaction. The C 1s spectrum of PH exhibits two weak features just below The continuum region above 293 eV in TC consists of the the IP, which are attributed to Rydberg transitions, consistent expected benzene-like features with an additional peak at 296 eV, with their absence in the EHMO predicted spectrum. In addition TABLE VIII: Energies, Term Values, and Proposed Assignments for Features in the C Is Spectra of Phosgene (PH) and Terephthaloyl Chloride (TC) Phosgene (PH) E (kO.1 eV) T (ev) assignment 8.0 **c=o (3bJ 28a.w 290.7 6.1 U*c-cl(+) (5b1) 292.6 4.2 u*c-c~(-) (6a1) 294.3 2.5 3P 1.5 4P 295.3 296.75b IP -0.1 double excitation 296.8 300 (1) -3.3 2e-7.8 U * C (7aJ ~ 304.5 (8)

.

-

-

I

-

~~

-

-

-

-

-

-

-

-

-

-

TABLE I X EHMO Results for Selected Orbitals of Phosgene4 (PH) eV [ D 2 ( N 2p)l C 1s excited states CI 3d no CI 3d 11.53 (0.417) 10.27 (0.812) 10.84 (0.049) 1.39 (1.052) 10.02 (0.125) -1.76 (0.720) 7.32 (0.229) -21.5 (0.222) -20.1 (0.786) -e,

(C2")

sYm 3b2 5b1 6a,

char

** U*C-cl(+)

a*c-c1(-)

ir*/CI 3db 7a1

U*C4

ground CI 3d 10.52 10.06 9.44 5.48 -36.5

state no CI 3d 7.70 -6.23 -12.36 -34.8

eV [ D 2 ( F 2p)l 0 Is excited states CI 3d no CI 3d -10.71 (0.034) -8.02 (0.126) 10.09 (0.004) -6.07 (0.064) 9.46 (0.051) -12.78 (0.011) 5.67 (0.029) -31.77 (0.103) -30.22 (0.522) -e,

OThe EHMO calculated (single component) spectrum was aligned at the ir* resonance energy of 288.74 eV. The individual line widths for the C Is spectrum were set to 1.0 eV fwhm for c < -5.0 eV; 3.0 eV for -5.0 < t < 0 eV; 6.0 eV for 0 < t < 4.0 eV; 15.0 eV for 4.0 < c < 20.0 eV; and 20.0 eV for t > +20.0 eV. For the 0 1s spectrum they were set to 1.5 eV fwhm for t < -5.0 eV; 3.0 eV for -5.0 < c < 0 eV; 5.0 eV for 0 < t < 4.0 eV; 8.0 eV for 4.0 < t < 10.0 eV; 12 eV for 10.0 < c < 20.0 eV and 15.0 eV for > +20.0 eV. the C 1s excited state this orbital is (0.64*CZp, -0).25*02p,-0.39Cl13d,, +0.50C113d,, - 0.39C1,3dX, + 0.50C1,3dY,J. The molecule lies in the x,y plane with the C=O bond along the x axis.



+d, 530

,

,

540

,

,

550

,

,I 560

Energy (eV)

Figure 9. 0 1s oscillator strength spectra of aldehydes (benzaldehyde, terephthalaldehyde), esters (methyl formate, ethyl benzoate), and acyl chlorides (phosgene and terephthaloyl chloride). See Figure 2 for experimental details.

d

which may be the counterpart to that observed at 297 eV in PH which is tentatively attributed to a double excitation, perhaps involving simultaneous 1s a* and a a* promotions, since all of the expected one-electron features have been accounted for. The well-known two-electron excitation in CO, which occurs somewhat higher, around 302 eV,3* is of similar intensity relative resonance as the 297-eV feature in PH. to the 3.2. 0 1s Spectra. The 0 1s spectra of BA, TA, methyl formate,j9 EB, PH, and TC are plotted in Figure 9 organized according to functional groups (aldehydes, esters, and acyl chlorides). The energies, term values, and proposed assignments are summarized in Table XI. 0 1s spectra are generally noiser than C 1s spectra because of the decrease in inelastic scattering cross sections with increasing energy loss. In addition there are generally fewer oxygen than carbon atoms. The presentation of the 0 1s spectra in Figure 9 clearly illustrates the potential of core excitation spectra for speciation-i.e. identification of functional groups from the characteristic positions and relative intensities of spectral features. All of the spectra are dominated by the low-energy 0 1s a*cv feature around 531 eV, and all exhibit a broad, high-energy continuum feature around 545 eV ascribed to the u*cv resonance, consistent with each species containing a carbonyl group. At the same time, spectra of species within the same functional group are much more similar than spectra of species belonging to different groups. The clear qualitative differences found among the different functional groups can be related in a simple manner to the differences in their structure. The 0 1s spectra of esters have a large near-continuum intensity peaking around 541 eV, which is a strong resonance, associated with the C-O single bond. This feature is not observed in the spectra of aldehydes and acyl chlorides, since these species do not contain a C-0 single bond. The 0 1s spectra of the acyl chlorides have a relatively strong discrete feature around 535 eV which is attributed to "charge-transfer" type excitations to orbitals of uscx, character. These are detected even at the 0 atom because there are small 0 2px and 0 2p,, contributions to the orbitals. As with the C Is spectra, 0 1s spectra generated from the EHMO results are in good agreement with experiment. The EHMO 0 1s excitation spectra are presented in Figure 10, while the orbital energies and relevant coefficients for selected orbitals are summarized in Tables IV-VI and IX.The excellent agreement with regard to the spectral shapes in the preedge region indicates that even simple EHMO calculationsare capable of reproducing the essential aspects of the electronic structure relevant to the strongest core excitation features. In particular, EHMO COKectly predicts the strong near continuum resonance in the esters associated with the presence of C-O single bonds, a feature which has no counterpart in the aldehyde or acyl chloride spectra since these species only contain C-0 double bonds. The second rel-

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h


Hitchcock et al.

TABLE XI: Energies, Term Values, and Proposed Assignments for Features in the 0 Is Spectra of Benuldebyde, Terepbtbrlndebyde, Ethyl Benzoate. Metbvl Formate. Phneae. - . a d Terephtbaloyl Chloride benzaldehyde (BA) E (AO.1 eV) T (eV) 533.6 536.5 (w) 537.4 537.7 539.0 544 (2)

4.1 1.2 0.3 -1.3 -6

terephthalaldehyde (TA) E (f0.1eV) T (eV) 533.5 536.1 537.7 538.8 544 (2)

assignment (final orbital)

4.2 1.6

3s 4P 5P

IPb

-0.6 -6

2eU * M

ethyl benzoate (EB) E (f0.l eV) T(C0) 53 1.5" 6.5 534.3 3.7 537.4 0.6 538.0"

methyl formatec (MF) assignment T(OR) E (f0.l eV) T(C0) T(OR) co OR 532.Ib 6.4 if* 5.2 535.1 3.8 4.8 3s if* 2.1 536.9 1.6 3.0 RYd 3P 538.56 IP(C=o) 539.8 0.1 RYd 539.9 539.96 IP(O-R) 530.8 -1.3 541 ( I ) -1.1 U*C4 -8.5 U*C-O 546.3 -8.3 547 (1) phosgene (PH) terephthaloyl chloride (TC) E (fO.l eV) T (ev) assignment E (fO.l eV) T (ev) assignment 531.5" 8.2 if* 53 1.3" 7.6 if*C-O(+) 534.4 sh 5.3 3s 534.4 4.5 if*c-O(-) 4.0 3P, u*c-d+) 535.7 535.7 3.2 3P, U*C_CI(+) 537.7 1.2 4P 5 39.72' 538.9e IP 540.5 (3) -0.7 U*C-C,(+) 540.7 (3) -1.8 u*c-Cd-) 546 ( I ) -6 Q*C-O 545 (1) -6 U*C-o(+) 549 ( I ) -10 .*c-O(-) "Calibration: BA -4.39 (5); TA -4.8 (2); EB -3.92 (5) MF -3.28 (5); PH: -3.9 (2) eV TC: -4.2 ( I ) eV relative to 0 1s if* of C 0 2 (535.4 eV). bFrom XPS.29 cTaken from previous work.39 dEstimated from the measured IPS of related specks: (IP(C=O), IP(0R)) values are CH3COOH (538.3, 540.5); HCOOCH, (538.5, 539.9); CH3COOEt (537.8, 539.2). 'From XPSZ9(PH) and estimated from values for similar species for TC ((CH3),CCOCI 538.8 eV).

-.

530

540

550

Figure 10. EHMO calculated 0 Is spectra of benzaldehyde, terephthalaldehyde, methyl formate, ethyl benzoate, phosgene, and terephthaloyl chloride. The latter two calculations were carried out with CI 3d orbitals in the basis set. The carbonyl and ether components of the ester spectra are indicated. The orbital energies, coefficients and details of how the energy, oscillator strength scales and peak widths were established are presented in Tablex V-VII, IX, and X.

-

atively intense 0 1s u*ccI peak in the spectra of PH and TC is also reproduced by EHMO, although it occurs at a reasonable relative energy only when C1 3d orbitals are included in the calculation. 3.3. CI 2p Spectra. The C1 2p oscillator strength spectra of PH and TC are compared in Figure 11, while the energies, term values, and proposed assignments are listed in Table XII. The discrete re@on of the C12p spectra is less sharply structured than that of the C 1s and 0 1s spectra of the same species. This is attributed in part to overlap of two manifolds of transitions separated by the 1.6-eV C 1 2 ~ ~ , ~ 2pIl2 - C l spin-orbit splittingw and

Energy (eV)

Figure 11. C12p oscillator strength spectra of phosgene and terephthaloyl chloride (see caption to Figure 2 for experimental conditions).

in part to the fact that C12p excitation occurs to the 3s, 4s, and 3d components of the virtual orbitals which tend to be rather small since the C13p AOs are the major contributor to low-lying virtual valence orbitals. Even so there are differences between the C1 2p spectra of PH and TC spectra which may reflect the expected differences between the u*ccI orbitals in PH and TC. In par-



TABLE XII: Energies,Term Values, and Proposed Assignments for Features in the CI 2p Spectra of Phosgene and Terephtsrbyl Chloride

c1 2p terephthaloyl chloride

phosgene

E (10.1 eV) 200.8” 202.3 (sh) 202.8 (sh) 203.8 204.4 (sh) 206.1 (sh) 207.4b 208.56 209.1 215 (2) br 221 (2) br

-

E (i0.3 eV) 271.5 278.0

TIl2

T3/2

6.6 6.2

E ( i 0 . l eV) 201.3‘ 202.9

T3/2

assignment

TI12

4.8

312

112

U*C-Cl(+)

4.3

4.6

U*C-Cl(+) U*C-Ck)

4.7

O*C-Ck)

3.0

RYd 2.4

-1.7 -10

T (eV) 7.0 0.5

-0.6 -9

205.2 (sh) 206.1b 207.2c 209.2 216 (2) 224 (2)

0.9

2.0

-3.1 -10

-2.0 -9

IP

Cl 2s (Phosgene Only; Spectrum Not Shown) assignment E (* 0.3 eV) ** 278.5 RYd 284 ( i )

-

RYd RYd

RYd IP double excitation? CI 3d CI 3d

T (ev)

assignment IP (estimated)

-6.

‘Calibration: phosgene: -87.9 (1) eV relative to the C 1s R* feature of same species. Terephthaloyl chloride: -83.4 (2) eV relative to the C 1s r* feature of same species. bIPs from XPS.29 ‘C1 2pjI2 IP estimated from those of related species: (t-BuCOCI = 206.0; CH3CI = 206.3).

ticular the two peaks around 201 and 204 eV in PH are assigned to a*cxl(+) and u * ~ ~ , (features -) and those at 201 and 203 eV to the corresponding features in TC. The narrower u * ~ ~ , (separation -) is consistent with a smaller interaction between the more widely separated C-Cl bonds in TC. The strong, broad structure in the near C1 2p continuum of each species is attributed to transitions to final states with large C13d character which give rise to a delayed continuum maximum through the centrifugal barrier associated with the 1 = 2 final angular mom e n t ~ ml . ~ 4. C Is and 0 1s Excitation of Poly(ethy1ene terephthalate): Comparison to Spectra of Small-Molecule A~logues The utility of core excitation studies of small molecule analogues to assist analysis of the core spectra of polymers is illustrated by comparison of these experimental and theoretical studies with a complimentary study of core excitation of poly(ethy1ene terephthalate) (PET). The C 1s and 0 1s energy loss spectra of a commercial sample of PET, recorded by parallel-EELS, are presented in Figure 12, in comparison with estimates of the C 1s and 0 1s spectra derived from the small-molecule results. Three different linear combinations of the C 1s spectra have been explored as estimates of the C 1s spectra of PET (central panel of Figure 12), while the 0 1s spectrum has been compared to that of ethyl benzoate (right-hand panel of Figure 12). The chemical basis of the three C 1s spectral simulations is indicated in the left panel of Figure 12. Solely on the basis of the similarity of molecular and elemental formula, simulation A would be expected to provide the best fit to the experimental data, followed by simulation B, and then simulation C. The structure constructed via simulation A differs from the repeat unit of PET only in terms of having three additional C-H bonds (whose characteristic resonances would occur around 289 eV and likely be masked by resonances) and one additional 0-H bond (silent in the C 1s region). However simulation A does not contain a parasubstituted unsaturated substituent, which can be involved in long-range orbital interactions through conjugation. Such interactions have been shown through the studies of TA and TC to give rise to spectral features beyond those seen in monosubstituted benzenes (BA, EB). For this reason simulations B and C are worth considering, even though they do not match the molecular or elemental formula as well as simulation A. All three simulations clearly indicate that the discrete region of the C 1s spectrum of the polymer should be dominated by two clearly separated sharp resonances of approximately ?rSrlng and u*co character, while the continuum will be benzene-like. Two low-lying ?r* resonances do dominate the carbon edge region of the PET spectrum. The splitting between the first and

-

530 540 550 Energy Loss (eV)

Figure 12. Parallel-EELS spectrum of ply(ethylene terephthalate) (PET) in the region of the C 1s and 0 1s edges. The underlying valence and plural scattering continuum has been subtracted by extrapolation of the prcedge signal. Three simulations of the C 1s oscillator strength spectrum of the repeat unit of PET are also plotted. These are constructed from linear combinations of the small molecule spectra as indicated in the left-hand panel. The simulated spectra were smoothed to lower the effective resolution to that of the 1.1-eV resolution of the parallel-EELS experiment as determined from the width of the elastic peak.

second transitions (3.2 eV) is identical to that between the first two (major) peaks of EB and very close to that observed for TA (3.5 eV). The breadth of the first transition for PET is such that it could encompass several T* components, as seen for TA, although from the C 1s spectrum of EB, the C ls(CR) rIrrin transition in an ester such as PET is expected to be shifted seven! eV to higher energy. A twepeaked r* structure has been observed in the C 1s energy loss spectra of phenylene oxide polymers and was attributed to chemical shifts between the C ls(CH) and C ls(C0) carbons? While the agreement between the experimental PET spectrum and simulations from the molecular spectra is reasonable, there are some significant differences. The ?r* features in the PET spectrum are weaker relative to the C 1s continuum and are less well defined. There is a feature at 291 eV which has no counterpart in simulation A and, at best, a shifted counterpart (the third peak at 290 eV) in simulations B and C. Several experimental aspects of the parallel-EELS measurement could explain some of these differences. On the basis of the method of Egerton et

-


the inelastic mean free path for 100-keV electrons in PET is estimated to be about 130 nm. The region of the PET sample examined had a thickness of approximately one mean free path, and thus plural scattering contributions could exist. Yet the 291-eV feature persisted even in thinner samples and through deconvolution of the spectra of thicker samples (see below). These results suggest that the 291 eV feature is not associated with plural scattering. The sample was at -170 OC during acquisition which may have caused condensation of some hydrocarbons on the sample. Radiation damage is another concern.I9 This tends to reduce organic compounds, creating unsaturated centers which have associated A* resonance^.^^ Thus the smaller **/continuum intensity ratio in PET relative to the molecular analogues is not consistent with extensive radiation damage effects. In fact at larger doses the PET parallel-EELS spectrum develops additional intensity at 287.5 eV, in the region of the resonance, rather than at 285 eV, the region of resonance^.'^ This suggests the ester functional group is more radiation sensitive than the aromatic ring. The intensity of ?r* resonances relative to that of the continuum is frequently much smaller in polymer than in small molecule core excitation spectra. In part this may be associated with long-range interactions between units. However there are recent examples of X-ray absorption spectra of polymers where the peak height of A* resonances is 2-3 times larger than the continuum,6VMrather similar to that found in the small-molecule analogues. While core r* resonances appear to be somewhat broader and less intense in the spectra of condensed solids than in those of free molecules and in longer-chain versus shorter-chain polymers,6 presumably due to intermolecular or intrachain interactions, the difference is more dramatic here than in previous studies. Several factors could be involved in the changes in the ?r* region: a lower resolution of the parallel-EELS spectrometer than that estimated from the width of the ndoss peak; contributions from hydrocarbon contamination and/or from impurities in the commercial PET sample; electron beam damage effects (which could distort spectral shape); and plural scattering. The latter should not affect the first 5 eV of C 1s excitation structure but will broaden and possibly add structure in the higher portions of the spectrum. The third feature at 291 eV in the parallel EELS C 1s spectrum of PET does not have a counterpart in simulation A but does have one in simulations B and C. This feature had somewhat different relative intensities in spectra recorded at different places in the same sample and in different samples. On this basis, it is possible this feature is a structured plural scattering signal, perhaps corresponding to a combined C ls(CH) plus *(HOMO) **(LUMO) transition, since PET has a u ?r* valence band transition around 6.8 eV. Yet, deconvolution of the PET C 1s spectrum using the low-loss data does not remove this feature. Alternatively the 291-eV peak could be a C 1s ?y* excitation feature characteristic of the interaction of two ?T*~+, orbitals on opposite sides of the ring, similar to the r*c-osplitting seen in TA and TC. The molecular simulations are consistent with this interpretation. Simulation A, based on EB which does not contain two carbonyl groups, does not exhibit a feature corresponding to the 291-eV PET feature. However, both simulation B (based on TA)and simulation C (based on TC) exhibit a corresponding peak although it is located somewhat lower in energy. In addition, a structure is observed at an energy similar to this 291 eV PET feature in the EHMO calculation of dimethyl terephthalate, a molecule which is a close match to the PET structural repeat unit (see below). The existence and assignment of the 291-eV feature in PET is being considered further through comparison with a recent NEXAFS study of PET.4S The 0 1s spectrum of PET is also shown in Figure 12 along with that of EB, which has been broadened to match the resolution of the parallel-EELS spectrometer, as measured by the 1.1-eV fwhm of the elastic peak. Ethyl benzoate might be expected to be a good model for the 0 1s spectrum of PET since it has exactly the same local structure around the oxygen atoms. However the additional p-C0,Et group in PET could lead to a a*co(+)/

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--

Hitchcock et al. I

I

I

0-K

PET oxpt.

i

C-R

c-0

-CH 3

I . . . . I . . . . I . , 530 540 550 Energy Loss (eV> Figure 13. EHMO calculated C 1s and 0 1s spectra of dimethyl terephthalate ((C6H4)-1,4-(C02Me),) compared to the experimental parallel-EELS spectra of PET. See Table XI11 for numerical details.

*Ic splitting, o(as -found ) in the experimental and EHMO studies of TA and TC. In this regard EB may not be a satisfactory model for the 0 1s spectrum of PET. The lowest energy ? T * ~ +resonance in the 0 1s spectrum of PET occurs at a position similar to that of the 0 1s ?r*c4 resonance in EB. However, it is considerably broader and weaker relative to the continuum than the 0 1s ?r* resonance in EB. This may reflect a distortion by plural scattering of the “discrete”-to-continuumratio in favor of the continuum, as seems to occur in the C 1s spectrum. Deconvolution of the parallel-EELS spectra does enhance the intensity of the ** features relative to the intensity of the far continuum but it makes little difference to the relative intensity of the u* and a* resonances. Another factor giving rise to broadening of the fmt feature in the 0 1s spectrum of PET could be contributions from 0 1s a*co(+) and 0 1s rSCO(-) transitions. However this interpretation is inconsistent with the small molecule results since the widths of the main 0 1s a* resonance of TA and BA and of TC and PH are similar (see Figure 10). The comparison with molecular analogues of PET clearly aids the assignment of the PET spectral features. In particular it identifies the two resolved C 1s features as transitions to virtual orbitals (bands) of mainly T * (284.7 ~ eV) and aZCO (287.9 eV) character. Furthermore the comparison between experiment and simulation suggests that there may be other effects, such as band broadening or additional unresolved transitions since the PET T* features are considerably broader than those in the molecular simulations even after the latter have been broadened to match the 1.1 eV experimental parallel-EELS resolution. This work demonstrates the reasonableness of modeling the core excitation spectra of polymers with those of appropriate small molecules, an approach that has been found useful in studies of simpler condensed phase systems.6*28 EHMO calculations of dimethyl terephthalate, C6H4-1,4-(COZCH3)2 (MT), have been carried out since this species is very similar to the PET monomeric structure. The C 1s and 0 1s spectra predicted from these results are presented in Figure 13 in comparison to the parallel-EELS spectra of PET. The energies and coefficients for selected orbitals of MT are given in Table XIII. An orbital correlation diagram for the ground state r* UMOs of benzene, dimethyl terephthalate and methyl formate is presented in Figure 14. The EHMO spectrum of MT was constructed by aligning specific peaks in the individual CH, CR, CO, and CH3 C 1s component spectra at energies considered “reasonable” from the simulation A spectrum (see footnote of Table XIII). This estimation is in good agreement with that

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TABLE XIII: EHMO Results for Selected Orbitals of Dimethyl Terephthalate (MT)“ ground -e, eV (Zc*(N 2p) (C,) sym char state C ls(C=O) C ls(C-R) C ls(C-H) 1Pb.C 294.5 292.1 290.7 28a’ rsc0(+) 10.15 11.16 (0.462) 10.46 (0.099) 10.40 (0.099) 31a’ T * ~ ~ ( - ) 8.71 9.39 (0.077) 8.73 (0.004) 9.33 (0.133) 32a’ r*,ing 8.34 8.34 (0.0) 8.34 (0.0) 8.62 (0.007) 33a’ r*cR(+) 6.30 6.80 (0.044) 7.52 (0.180) 6.42 (0.012) 35a’ r*CR(-) 3.97 4.14 (0.012) 4.64 (0.036) 4.51 (0.042)

C ls(CH2) 290.3 10.14 (0) 8.71 (0)

-t, eV (Z?(F 2p) 0 ls(C=O) 0 Is(0Me)

538.2

6.31 (0)

10.26 (0.021) 8.83 (0.019) 8.34 (0) 6.36 (0.008)

3.97 (0)

3.99 (0.002)

8.34 (0)

539.9 10.21 (0.010) 8.78 (0.010) 8.34 (0) 6.34 (0.004) 3.98 (0.001)

“The EHMO C 1s spectrum was initially constructed using line widths of 1.0 eV fwhm for c < -4.0 eV; 3.0 eV for -4.0 < < 0 eV; 5.0 eV for 0 < t < 4.0 eV; 8.0 eV for 4.0 < t < 10.0 eV; and 12.0 eV for t > +10.0 eV. The 0 Is widths were the same except the minimum width was 1.5 eV.

The full EHM spectrum was then additionally smoothed (to perhaps the equivalent of 2 eV Gaussians for the r* resonances) in order to reflect the lower parallel-EELS resolution. bTheseIPSwere derived from the solid-state PET with the C Is(C0) IP aligned to that of EB. ‘The EHMO component spectra were placed on absolute scales by aligning the following peaks: C ls(CH) r*(28a’) @ 285.1 eV; C ls(CR r*(28a’) @I 285.1 eV; C ls(C0) r*(28a’) @ 288.3 eV; C Is(0Me) r’(28a’) @ 293.0 eV: 0 ls(C0) *(28a’) @ 531.0 eV; C Is(0R) r*(28a’) @ 532.2 eV.

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core excitation spectra can be predicted a priori without calibration of the calculation via comparisons of calculated and experimental spectra of model compounds. 5. Summary

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Figure 14. Orbital correlation diagram for the unoccupied ?r* orbitals of benzene, dimethyl terephthalate and methyl formate based on EHMO calculations for the ground state of these species. generated by aligning the zeros of the individual component spectra with the experimental C 1s IPSof solid PET6 and then aligning the first transition of the sum with that of the PET parallel-EELS C 1s spectrum. A larger broadening has been used in generating the EHMO calculated spectrum of MT than in the other EHMO spectra in order to better reflect the apparent experimental resolution of the parallel-EELS spectrum of PET. Overall there is reasonable agreement between the EHMO calculation of MT and the parallel-EELS spectrum of PET. One feature of interest is that, relative to the other small molecule analogue simulations, the MT calculation predicts higher energy C 1s r* structures in the region of the 291-eV feature observed in PET. This supports attribution of the 291-eV feature to a splitting of the r z C dresonance associated with cross-ring interaction. Similarly EHMO predicts a second 0 1s a* component in MT around 534 eV. This may explain the signal observed between the r * and the u* resonance in the experimental 0 1s spectrum of PET (Figure 12). Recently better resolved parallel EELS spectra of PET have been obtained4’ which show many of the fine structures predicted by the small molecule analogue simulations and the EHMO calculation of MT. In addition, the experimental ISEELS spectra of MT have been recorded and are in good agreement with the EHMO results. These spectra will be reported in a future publication which will also compare the X-ray absorption and parallel EELS spectra of PET.45 It is unlikely that a direct evaluation of the PET spectrum by comparison to EHMO calculations of MT would have been successful without first exploring the application of EHMO to the five molecular analogue species which have been the main focus of this work. In the future it will be of interest to see if polymer

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The inner-shell spectra of five molecules (12 edges), most of whose chemical structures are related to poly(ethy1ene terephthalate), have been recorded for the first time and analyzed through spectral comparison and with the aid of EHMO calculations. The utility of EHMO for assisting the interpretation of the core excitation spectra of complex molecules has been extensively explored and amply demonstrated. With further work on deriving schemes for theoretical evaluation of core level chemical shifts, it would appear EHMO (and other semiempirical quantum chemical programs) could have good predictive value in the area of core excitation spectroscopy. Perhaps this is not surprising since core excitation samples the unoccupied orbitals in the region just around the nuclei. This aspect of molecular electronic structure is easier to represent with simpler theories than more complex properties such as valence excitation or dipole moments, which are dependent on delicate balances of several competing effects over the whole molecular structure. It would be of considerable interest to continue the development of EHMO and/or other quantum chemical treatments to a level where they could be used to predict the core spectra of a complete polymer chain. Such studies would be useful to explore the consequences of different interchain interactions on core excitation spectra. Already there has been some work in this regard3A and much more has been carried out in the application of extended Hiickel techniques to the prediction of the valence band structure of polymers.48 Various combinations of the molecular spectra have been used to model the C 1s spectrum of PET. Although the experimental PET spectrum recorded by parallel-EELS exhibits discrete features at energies similar to those of the dominant features of the spectra simulated from the small molecule analogues, significant discrepancies remain.I9 While some of these differences may reflect inadequacies of the choice of small molecule analogues (cf. the question regarding the 291 eV feature in PET), it remains possible that some of the differences are artifacts of the application of EELS to polymers. This is another reason why comparison with experimental and calculated spectra of molecular analogues has been valuable. Once the experimental procedures needed to obtain reliable core spectra of polymers are satisfactorily identified, it should be possible to use comparisons between molecular simulations and experimental core excitation spectra of polymers to study differences between monomer and polymer systems and thus obtain insight into spatially extended aspects of the electronic structure of the polymer. A good understanding of the core spectra of homopolymers is critical to the extension of core excitation spectroscopy to studies of more complicated systems such as segmented copolymers and polymer blendsaM This work demonstrates the viability of using molecular analogues to approximate and thus better understand the spectra of larger systems compased of similar structural components. The localized character of core


excitation is the key to the viability of this approach. Acknowledgment. This research has been supported by Dow Chemical U.S.A.and the Natural Sciences and Engineering Research Council of Canada. We acknowledge the patient sample preparation work of Gene Young and helpful discussions with B. Colegrove (Dox-TX) . Registry No. C12C0, 75-44-5; ClCOC6H4-p-CoC1, 1oO-20-9; PhCHO, 100-52-7; p-CHOC,H4CH0, 623-27-8; PhC02Et, 93-89-0; HC02Me, 107-31-3.

References aod Notes (1) Hitchcock, A. P. Phys. Scr. 1990, T31, 159.

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