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Photoelectron Circular Dichroism Spectroscopy in an Orbitally Congested System: The Terpene Endoborneol Gustavo A. Garcia, He´loı¨se Soldi-Lose, and Laurent Nahon* Synchrotron SOLEIL, l’Orme des Merisiers, Saint Aubin BP 48, 91192 Gif sur YVette Cedex, France
Ivan Powis School of Chemistry, UniVersity of Nottingham, NG7 2RD Nottingham, United Kindgom ReceiVed: September 29, 2009; ReVised Manuscript ReceiVed: December 7, 2009
We have measured the photoelectron circular dichroism (PECD) of single enantiomers of endoborneol in the photon region from 9.9 to 23.6 eV by combining circularly polarized synchrotron radiation and a velocity map imaging technique. A photoelectron spectrum and the state-selected fragmentation curves of this terpene were also recorded. Unlike previous case studies, the broad featureless transitions encountered here preclude an orbital by orbital analysis of the PECD, although semiquantitative features of the highest-occupied molecular orbital PECD are identified and compared to full calculations. Despite our inability to further identify individual orbitals experimentally, we show that we are able to unambiguously assign the absolute configuration by comparison with realistic simulated PECD spectra. Furthermore, the calculations predict that for electron kinetic energies above 5 eV; the contributions of individual conformers to the PECD are nearly identical. Should this observation apply to bigger biological systems, the analysis could be greatly simplified by recording high kinetic energy electrons. On the other hand the contributions of the different conformers to the slow electron PECD seem to vary more significantly, and we deduce, within the theoretical limitations, a plausible 1:1:1 distribution of the three identified conformers. Introduction Photoelectron circular dichroism (PECD) has been the subject of many experimental and theoretical studies on small organic chiral systems during the past decade.1 This effect manifests itself as a forward/backward asymmetry, with respect to the light propagation axis, in the angular distribution of photoelectrons emitted upon valence or inner-shell ionization of gas phase enantiomers of a chiral molecule by circularly polarized light (CPL). More precisely, it can be shown2,3 that the general expression for the laboratory-frame photoelectron angular distribution as referred by the angle θ is given by
I(θ) ) 1 + bp1P1(cos θ) + bp2P2(cos θ)
(1)
where Pn refers to the nth-order Legendre polynomial. Equation 1 has been normalized by the total cross-section and applies to pure polarization states of the ionizing light beam and a random orientation of the molecular targets. The parameters b1p and b2p depend upon the photon polarization, p, and the photoionization dynamics of the considered orbital and include both angular momentum coupling terms and electric-dipole photoionization matrix elements. For linear polarization (p ) 0), b10 will necessarily be zero, so that b20 is then just the familiar β anisotropy parameter. In the case of circular polarization states may become nonzero but only in the event that (p ) (1) b{(1} 1 the target molecule is chiral. In this case, the angle θ is measured * To whom correspondence should be sent. Email: Laurent.nahon@ synchrotron-soleil.fr. Phone: +33 (0)1 69 35 96 47. Fax: +33 (0) 1 69 35 94 56.
along the direction of the light beam propagation and the first Legendre polynomial term, P1, introduces a cos θ term to the angular distribution. This term is odd with respect to the inversion θfπ - θ and therefore introduces a forward/backward asymmetry to the electron angular distribution. , is antisymmetric with The chiral dichroic parameter, b{(1} 1 the switching of the light helicity (as well as with the exchange of the enantiomer), so that for a given enantiomer PECD is defined by the difference between left- and right-handed CPL
IL(p)+1)(θ) - IR(p)-1)(θ) ) 2b+1 1 cos(θ)
(2)
giving a maximum chiral asymmetry in the forward/backward direction. Equation 1, and hence the PECD effect, stems from the consideration of the differential photoionization cross section and can be derived in the pure-electric dipole approximation.4 Contrast this with the case of an integral measurement such as conventional CD in absorption that only exists because of weaker higher-order radiation-molecule interaction terms. This is why PECD asymmetries are found to be typically 3 orders of magnitude stronger than in absorption CD. Experimentally, asymmetries (2b1) ranging up to more than 20%, usually in the first few electronvolts above threshold have been recorded, both in valence-shell5,6 and core-shell7,8 PECD. The PECD phenomenon arises from an asymmetric electron scattering process off an intrinsically chiral molecular potential. Its intensity varies with the sine of phase differences between scattered outgoing waves of adjacent orbital angular momenta,9 making it sensitive to even small phase shifts induced by changes to the whole molecular structure, even when such changes occur at a site remote from a highly localized initial
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orbital, i.e., whatever the localization of the initial orbital with respect to the chiral center(s). In particular PECD appears to be sensitive to chemical substitution, as observed theoretically on Oxirane derivatives10 and experimentally by a comparison of fenchone and camphor.11 For the same intrinsic reason, PECD is very sensitive to assumed molecular conformation, as has been observed for glycidol12 and alaninol,13 allowing one to deduce, from an experimental/theoretical comparison, information on the sample conformer population. More generally a dual experimental/theoretical approach appears to be crucial for a proper understanding of the PECD process. A significant feature is that this dual approach is, to our knowledge, able successfully to indicate absolute configurationsa major issue in chiralityrelated studiessfor all the molecules that have been studied so far. In this context we present here a complete experimental and theoretical valence-shell PECD study of endoborneol (1). This molecule is closely related to the showcase camphor molecule (II) whose valence PECD has been studied in detail,6,14,15 the two molecules differing only by the presence of a carbonyl group (CdO) in camphor, changed into an hydroxyl (C-OH) group in endoborneol. This difference, as we will see later on, leads to two very important consequences for the endoborneol case: the presence of several conformers and a very poorly structured photoelectron spectrum (PES) in contrast with camphor, which is a very rigid, single conformer molecule, with a quite structured PES due to a very localized highest-occupied molecular orbital (HOMO).
Nontotally rigid endoborneol therefore appears to represent a step toward the large class of floppy organic chiral species possessing several conformers (with very small energetic barriers) with quite delocalized orbitals and whose PESs are consequently very congested and unstructured s unlike most of the molecules whose PECD have been studied so far. One of the aims of the present study is to examine whether, despite this featureless character, it is still possible to get precise information on the photoionization dynamics, the conformer population, and the absolute configuration. We hope to establish the potential scope for extending PECD studies to a wider range of organic chiral species where, as a photoionization-based technique, it in principle offers a universal chiral probe without any limiting selection rules or restricted to chomophorecontaining species, as is the case for conventional absorption CD based upon bound-bound spectroscopic transitions. After the presentation of the experimental and theoretical methods, we will discuss the high-resolution recorded threshold electron spectrum (TPES) in light of calculated ionization potentials (IPs). Then multidimensional binding energy and photon energy-dependent measured and calculated PECD data will be presented and discussed. Experimental Methods Experiments were conducted at the third-generation synchrotron facility, Soleil (St Aubin, France), on the DESIRS Vacuum UltraViolet (VUV) beamline.16 Commercial samples (97% pure,
Garcia et al. Aldrich) of (1S)-(-)-(Ia) and (1R)-(+)-(Ib) endoborneol were placed in an in-vacuum oven and heated at 145 °C. The resulting vapor was mixed with 0.8 bar of helium and expanded through a 50-µm pinhole. The supersonic expansion was skimmed to form a molecular beam and crossed at a right angle with the vacuum ultraviolet (VUV) photon beam in the ionization chamber, where the ions and electrons produced were accelerated in opposite directions perpendicular to the molecular and photon beams into the Delicious II spectrometer.17 Briefly, this electron/ion spectrometer couples a modified velocity map imaging (VMI) electron analyzer18 with a Wiley-McLaren time of flight (TOF) ion mass spectrometer19 and may be operated in electron-ion coincidence mode. The PECD at each photon energy was measured by recording several photoelectron images for alternate light helicities and then subtracting these to obtain a difference image, which was later treated using the pBasex inversion algorithm20 to recreate the original angular distribution of the difference. The full procedure and precautions to minimize purely instrumental effects has been described previously.6 The apparatus is capable of imaging electrons with kinetic energies of up to 17 eV, with 5% resolving power, E/∆E, for the fastest electrons.18 In addition Delicious II was also employed for the spectroscopic and fragmentation studies of endoborneol, since it allows the detection of threshold electrons in coincidence with mass spectra with sub-millielectronvolt resolutions.17 In this work we were able to record threshold PES (TPES) and threshold photoelectron photoion coincidence (TPEPICO) spectra, probing the ion’s spectroscopy and state-selected fragmentation with a modest resolution of 50 meV. The experimental setup is permanently installed in one of the two monochromatised branches available in the DESIRS beamline. This undulator-based beamline delivered left- and right-CPL over the whole experimental energy range with circular polarization values of |S3| g 0.9, as accurately measured by a dedicated VUV polarimeter21 just upstream from the sample. For energies below 15 eV, a gas filter22 was filled with 0.25 mbar of argon to avoid high harmonics generated by the undulator that would be transmitted by the grating. We selected a low-dispersion (200 gr/mm), high-flux grating (∼1012 ph/s/ 0.1% bandwidth) mounted on the beamline’s 6.65 m Eagle offplane monochromator,23 offering a tunable resolving power of up to 4000. In practice, for the PECD measurements the resolution is limited by the kinetic energy (KE) resolution of the spectrometer in the AR-PEPICO mode and therefore the entrance and exit slits were simply adjusted to avoid saturation of the detectors. For the spectroscopy studies (TPES and TPEPICO), the slits were set to provide a photon resolution of 11 meV around 10 eV. Computational Methods The analyses presented below treat three distinct conformers of endoborneol, corresponding to rotation of the hydroxyl group about the C-O bond axis. Preliminary calculations gave clear indications that for any given method the relative energetic ordering was very basis-set dependent; calculations were therefore performed using the very accurate complete basis set (CBS)24 and Gaussian-225 methods in Gaussian 0326 to assess the likely energy differences, and hence thermal populations, of these conformers. The results for the 0 K energies are summarized in Table 1. Other properties were calculated at the MP2/6-31G(p,d) optimized geometries for each of these conformers, including: (i) vertical ionization energies for the various valence orbitals
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TABLE 1: Calculated 0 K Absolute Energiesa, Relative Energies*, and HOCC Dihedral Angles† for Conformers of Endoborneol C1
C2
C3
gauche-
anti
gauche+
-466.234329 -5.38 -466.216861 2.31 -466.195913 2.29 -73.8°
-466.238788 -17.09 -466.217649 0.24 -466.196646 0.37 +173.2°
-466.232278
method CBS-Qd CBS-Q(B3LYP)d G2(MP2)e MP2/6-31G(p,d)
E (0 K) ∆E (0 K) E (0 K) ∆E (0 K) E (0 K) ∆E (0 K) ∠H-O-C-C*
-1
-466.217740 -466.196786 +68.27°
Absolute 0 K energies in Hartrees. * Energies relative to C3 conformer, in kJ mol . Dihedral angles ∠HOCC* are given for the S enantiomer Ia, where C* refers to the asymmetric C atom. Note that for the R enantiomer Ib the sense of rotation will be reversed and hence the labels gauche+ and gauche- will also be exchanged. d Reference 24. e ref 25. a
Figure 1. TPES and mass-selected TPEPICO yields for endoborneol (154 amu) recorded with an effective 50-meV resolution. Also included are TPEPICO yield curves for the principal fragment species. Calculated OVGF/cc-pVDZ vertical ionization energies for the conformers C1-C3 of endoborneol are indicated at top of the figure.
obtained using the outer valence Green’s function (OVGF) electron propagator method27 with a cc-pVDZ basis and (ii) photoionization dipole matrix elements as a function of electron kinetic energies. These latter quantities were obtained using the CMS-XR method, implemented as previously described.28 For the construction of the neutral molecule XR potential the optimized atomic radii of Takai et al.29 were selected and used with angular basis of spherical harmonic functions extending to lmax ) 1, 3, 6 for, respectively, the H atom regions, the C and O atom regions, and the asymptotic outer region. When calculating final state continuum electron functions from the already converged neutral potential the corresponding angular basis set size was increased to lmax ) 3, 6, 18. The photoionization cross sections and b1 chiral distribution parameters were subsequently calculated from the energy-dependent dipole matrix elements.3 Results In Figure 1 we present the outer valence region of the 50meV resolution TPES of endoborneol, believed to span the ionization of several of the outermost valence orbitals. The TPES can, however, be seen to be relatively featureless. This is in contrast with the reasonably structured valence PES of the chemically similar camphor6 (II) and fenchone11 molecules; especially, the endoborneol spectra lack a clearly defined HOMO
†
band as possessed by these other molecules. Also shown on this figure are the OVGF/cc-pVDZ calculated vertical ionization energies, marking where corresponding features for the outer endoborneol orbital ionizations might be expected. This level of calculation worked well for the camphor and fenchone cases.6,11 While, with guidance from these calculations, one may discern slight suggestions of corresponding underlying features in the TPES it is clear that it will not prove readily possible to extract details, such as angular distribution parameters, attributable to individual orbitals from such congested spectra. This will be even more so when observing fast electrons, with reduced resolution, using the VMI technique. Also shown in Figure 1 are the TPEPICO mass-resolved ion spectra. The masses 136 and 139 amu could not be separated with the applied extraction field due perhaps to their velocity component along the detector’s axis. We can readily assign these masses to the loss of water and methyl respectively. The possible permutations on the rest of the fragments are too great to provide a confident assignment on the fragment’s structure, except perhaps mass 110 amu (-C2H4O), which may be produced as a loss of either vinyl alcohol or acetaldehyde. At 9.7 eV the parent ion stops being the most abundant mass and the fragmentation onset begins at around 9.4 eV, where the OVGF method predicts the first electronic state peaks. Our principal series of VMI measurements, spanning photon energies in the range 9.9 to 23.6 eV, were made with the S-(-) enantiomer. Figure 2 shows typical data (PES and PECD) extracted from a series of VMI electron images recorded with left and right CPL at a photon energy of 20 eV. Again the broad PES (i.e., the total energy dependent cross-section) is relatively featureless, certainly at the more modest resolution achieved by the VMI technique (AR-PEPICO mode). The PECD reveals somewhat more structure, with a series of minima in the parameter curve hinting at the underlying corresponding b{(1} 1 changes in orbital contribution to the overall ionization yield. A second, limited set of measurements using the R-(+) enantiomer were acquired for a few selected photon energies during a second experimental run at the same beamline, several parameter curve for the R months later. The hν ) 20 eV b{(1} 1 enantiomer is also shown in Figure 2. The R enantiomer PECD mimics that of the S enantiomer but with the theoretically expected sign reversal, confirming the molecular origin of the asymmetry. The absolute magnitude of the asymmetry observed for this second enantiomer does, however, appear somewhat attenuated. We can discount this being due to any reduction in the degree of SR polarization since this was checked by polarimetry measurements prior to both series of measurements and found to be constant (S3 g 0.9) at each energy. Rather we attribute it
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Figure 4. Isosurface representations showing the HOMO orbitals of three endoborneol conformers.
Figure 2. PECD for enantiomers of endoborneol recorded at a photon energy of 20 eV. The PES obtained from the same experimental velocity mapped image is also shown on the same ionization energy scale (with arbitrary intensity).
Figure 3. Calculated PECD curves for the outermost orbital of the three conformers of S-endoborneol. Also included are mean experimental b{(1} values extracted from the nominal HOMO ionization 1 region (around 9.7-10 eV), plotted as a function of photon energy.
to reduced signal-to-noise obtaining during the brief second measurement period, most likely a result of some misalignment of the nozzle with respect to the skimmer due to time constraints. The increased background signal from unfocused, thermal gas does not affect the shape of the PECD curve, which is after all a difference measurement, but does affect its normalization to the mean signal.6 Such a misalignment that could affect the VMI performances, especially resolution, could also be at the origin of some slight differences in some features observed on the two PECD curves. parameters for ionization of the HOMO of Calculated b{(1} 1 the three S-enantiomer conformers are presented in Figure 3. It is immediately apparent that the predicted PECD for photoionization of this outermost orbital is very different for each conformer. The C2 anti conformer in particular shows negative parameter in the low and high excursions in its predicted b{(1} 1 KE part of the spectrum, where the two gauche conformers have positive going peaks. This can be partially rationalized by reference to the pictorial representation of the HOMO in these conformers, shown as Figure 4. The different orientation of the nominally nonbonding O p orbital in the anti conformation
restricts its interaction with adjacent C-C σ bonds, giving a very different character to the C2 HOMO. As noted, the HOMO ionization does not appear as a distinct PES band in endoborneol so that, unlike for camphor6 and fenchone,11 experimental PECD values are not readily extracted for comparison. Nevertheless, the calculated OVGF ionization energies (Figure 1) suggest that photoelectrons emitted with kinetic energies corresponding to 9.7 to 10 eV binding energy will come predominantly from the HOMO. Mean experimental values formed in the energetic region between 9.7 and b{(1} 1 10.0 eV are presented for comparison with the calculated HOMO PECD curves in Figure 3. A number of strong caveats are applied. Electron extraction conditions are varied at each photon energy to ensure the image size always approximately fills the imaging detector’s active area, and so the outer HOMO region of the spectra always fall at essentially fixed detector radius. But the VMI resolving power, E/∆E, at any given radial position is constant so that the absolute resolution,∆E, must worsen with higher electron kinetic energy, E. Consequently, at higher photon energies, where the emitted HOMO electrons will be faster, there can be expected greater “contamination” of the supposed HOMO region with overlapping ionization from adjacent orbitals. Second, the steeply rising total cross-section in this region will lead to a skewed average, and may induce some degree of irreproducibility. The rather higher predicted HOMO IP for the C2 conformer also suggests that the PECD sampling may have some bias against this conformer’s HOMO ionization. Bearing in mind these caveats, one can discern an acceptable correspondence between the sampled experimental data points and at least the C1 and C3 conformer curves in Figure 3. values Figure 5 provides an overview of the calculated b{(1} 1 for each conformer of S-endoborneol. The PECD for each individual orbital appears as a vertical line in this figure, with value. Reading across horizontally at a a color-coded b{(1} 1 given photon energy suggests an idealized PECD spectrum. Each sampled point is plotted with a size proportional to the estimated cross section to convey some impression of significance of each point for an overall spectrum. Distinct differences between orbitals, between conformers, and between energies can be seen from this figure. Discussion Rather than pursue an orbital by orbital analysis, we have attempted an overall simulation of the PECD curves to facilitate a comparison between theory and experiment for this system on the basis of the calculations appearing in Figure 5. Stick photoelectron spectra are first obtained by combining, at each photon energy, calculated orbital cross sections. These are naturally obtained in the CMS-XR program as a function of electron kinetic energy and are converted to a binding energy scale by addition of the relevant ionization energy. For this
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Figure 6. Calculated and experimental PECD spectra for S-endoborneol. Calculated individual conformer curves are plotted as broken lines, and their 1:1:1 average is plotted as a solid line.
Figure 5. Calculated PECD for conformers of S-endoborneol as a function of photon (hν) and binding energy (BE). Calculations for individual orbitals then appear as vertical traces. At each plotted sample point on the hν vs BE grid the relative photoionization cross-section is represented by the size and b{(1} values are color encoded. 1
purpose we have preferred the OVGF calculated ionization energies over those of the CMS-XR program itself, believing them to be more accurate. The stick spectrum is then convoluted with Gaussian functions described by a kinetic energy dependent width, modeled on the established resolution function of the VMI spectrometer, This has approximately a square root dependence on the ratio F ) E/Vrep, where Vrep is the applied repeller (extraction) potential.18 A similar procedure is followed values weighted (i.e., combination of discrete calculated b{(1} 1 by cross-section, followed by convolution with energy-dependent width functions) to obtain a continuous PECD spectrum. In principle the experimental peak width contributed by each orbital ionization should consist of a vibrational envelope and an instrumental resolution function. Here instrumental resolution is straightforwardly estimated from the known characteristics and calibrated performance of the DELICIOUS VMI spectrometer,18 and in the absence of further information it is normal and reasonable to take this instrumental function to be a Gaussian (normal distribution). It is much less obvious that the vibrational envelope would be Gaussian, and its width would
in any case vary from orbital to orbital depending on the orbital bonding character. So aside from knowing that the vibrational/ rotational width is nonzero we are, in practice, rather ignorant of both its shape and width. However, at higher photon energies the VMI resolution degrades (i.e., the instrumental width increases), and so in the convolution of instrumental and vibrational envelopes, the instrumental Gaussian should eventually dominate, so that uncertainty about the vibrational shape has minimal impact. But at lower photon energies the vibrational envelope probably dominates the instrumental contribution, maximizing our ignorance, and producing much greater uncertainties in the simulation since the CMS-XR calculations do not consider vibrations. In accord with these expectations, it is found that the above simulation procedure generates acceptable results in comparison with the experimental PES at higher photon energies (hν g 18 eV), but is increasingly less convincing below this. Similar observations pertain to the simulated PECD spectra. We therefore now focus on comparison of experimental and simulated PECD spectra only at these higher photon energies. The simulated PECD spectra presented in Figure 6 show good overall agreement with the corresponding S enantiomer experimental curves, in particular matching in sign and magnitude the negative going peak at an ionization energy of ∼11.5 eV, and tracking the changing amplitude of this broad feature as the photon energy increases. As theoretically anticipated, and seen in Figure 2, the sign of this peak would be reversed for the R enantiomer, thus the close match that is demonstrated unambiguously corroborates that the absolute configuration of the sample is the same as that adopted (i.e., S) for the CMSXR calculation. It is notable that in this ionization region, and at these photon energies, i.e., electron kinetic energies >8 eV,
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the three conformers are predicted to behave very similarly, albeit the C3 curves may be in very slightly poorer agreement with the experimental PECD. However, at higher ionization energies the conformer predictions in Figure 6 diverge markedly from each other, much as already noted in the data in Figure 3. Generalizing, it would appear that for low electron kinetic energy (e5 eV) the predicted PECD is strongly conformer dependent but is much less so for faster electrons. Such a behavior, already observed, for example in glycidol12 is in accord with the intuition that slow scattered outgoing electrons would be more sensitive to the molecular potentialsand hence the molecular structure or conformationsthan faster electrons. But it is interesting to speculate that in the event of any shape resonant trapping of the outgoing electron, the enhanced electron-molecule interaction expected in the continuum scattering would (re)introduce a pronounced conformer dependence at a resonance energy. All the experimental data here of course represents an average taken over the conformer population present in the molecular beam. The two variants of the CBS calculations (Table 1) give conflicting ordering for the stability of conformers C1-C3, and while the CBS-Q(B3LYP) and G2(MP2) calculations are in apparently good agreement, it should be recognized that the indicated energy differences are at the very limit of accuracy expected of these calculations (mean absolute errors ∼4 kJ mol-1).24 When it is also recognized that there is some uncertainty over the exact molecular beam temperature or indeed whether a Boltzmann conformer distribution is obtained in the supersonic expansion, it seems most reasonable to assume an equal conformer population in the absence of any stronger counter indications. Consequently, equally weighted 1:1:1 averages of the -C3 conformer curves are also included in Figure 6. These demonstrate an across the board improved agreement with experiment over that achieved by the curves for any individual conformer alone, though most notably at ionization energies greater than 16 eV where we noted a greater conformer dependence. In the same manner, Figure 3, showing the selected HOMO region PECD, includes an equally weighted average of the three calculated PECD curves. It is seen again seen that this leads to a distinct improvement of the theory-experiment agreement (not withstanding the caveats concerning the reliability of the experimental data points that were expressed above). A clear inference from these observations is that the experimental sample is not dominated by any single conformer, and that essentially equal populations exist under the conditions of our experiment. Conclusions The vibronic congestion observed in the spectroscopic and PECD measurements of endoborneol renders the analysis of the data more complicated due to the impossibility of assigning discrete orbitals to be compared with the calculations. The difficulty is compounded for PECD at energies close to threshold, where spectrometer resolution is sufficient to require explicit consideration of the band vibrational envelopes. However at higher photon energies the vibrational widths become negligible in comparison to the resolution, and we demonstrate that we are then able to satisfactorily match experimental and simulated PECD spectra. The good agreement with the theory allows us to unambiguously define the absolute configuration of the enantiomers, even in the case of a spectrally congested system such as endoborneol. Furthermore we observe that, for this system at least, there are relatively small differences
Garcia et al. in the predicted conformer PECD as long as the electron kinetic energy exceeds ∼5 eV. For these faster electrons, a confident assignment of absolute configuration in no way depends upon assuming knowledge of feasible conformer populations in the experimental sample. If subsequently found to hold more generally, this conclusion might be of significance for the study of bigger biomolecular systems, since it means that even with a significant undetermined conformer distribution, we would be able to assign absolute configuration. Conversely, at lower electron kinetic energies, conformer dependence of the predicted PECD is very much greater. An equally weighted average over the three identified conformers provides better agreement with experiment in such cases than does any individual conformer. Though the comparisons so achieved remain far from perfect, the relative improvement is sufficient to indicate that the anticipated 1:1:1 conformer population is a priori highly plausible in this system. Acknowledgment. We thank the general staff of Soleil for running the facility. We are also in debt to Jean-Francois Gil, assistant-engineer of DESIRS, for technical help on SAPHIRS. Gaussian 03 calculations for the endoborneol conformers were made with support from the EPSRC National Service for Computational Chemistry Software. References and Notes (1) Powis, I. Photoelectron Circular Dichroism in Chiral Molecules. In AdV. Chem. Phys.; Light, J. C., Ed.; Wiley: New York, 2008; Vol. 138, pp 267. (2) Ritchie, B. Phys. ReV. A 1976, 14, 359. (3) Powis, I. J. Chem. Phys. 2000, 112, 301. (4) Ritchie, B. Phys. ReV. A 1976, 13, 1411. (5) Stranges, S.; Turchini, S.; Alagia, M.; Alberti, G.; Contini, G.; Decleva, P.; Fronzoni, G.; Stener, M.; Zema, N.; Prosperi, T. J. Chem. Phys. 2005, 122, 244303. (6) Nahon, L.; Garcia, G. A.; Harding, C. J.; Mikajlo, E. A.; Powis, I. J. Chem. Phys. 2006, 125, 114309. (7) Hergenhahn, U.; Rennie, E. E.; Kugeler, O.; Marburger, S.; Lischke, T.; Powis, I.; Garcia, G. J. Chem. Phys. 2004, 120, 4553. (8) Ulrich, V.; Barth, S.; Joshi, S.; Hergenhahn, U.; Mikajlo, E. A.; Harding, C. J.; Powis, I. J. Phys. Chem. A 2008, 112, 3544. (9) Harding, C. J.; Powis, I. J. Chem. Phys. 2006, 125, 234306. (10) Stener, M.; Fronzoni, G.; Di Tommaso, D.; Decleva, P. J. Chem. Phys. 2004, 120, 3284. (11) Powis, I.; Harding, C. J.; Garcia, G. A.; Nahon, L. ChemPhysChem 2008, 9, 475. (12) Garcia, G. A.; Nahon, L.; Harding, C. J.; Powis, I. Phys. Chem. Chem. Phys. 2008, 10, 1628. (13) Turchini, S.; Catone, D.; Contini, G.; Zema, N.; Irrera, S.; Stener, M.; Tommaso, D. D.; Decleva, P.; Prosperi, T. ChemPhysChem 2009, 10, 1839. (14) Garcia, G. A.; Nahon, L.; Lebech, M.; Houver, J. C.; Dowek, D.; Powis, I. J. Chem. Phys. 2003, 119, 8781. (15) Lischke, T.; Bo¨wering, N.; Schmidtke, B.; Mu¨ller, N.; Khalil, T.; Heinzmann, U. Phys. ReV. A 2004, 70, 22507. (16) http://www.synchrotron-soleil.fr/portal/page/portal/Recherche/ LignesLumiere/DESIRS. (17) Garcia, G. A.; Soldi-Lose, H.; Nahon, L. ReV. Sci. Instrum. 2009, 80, 023102. (18) Garcia, G. A.; Nahon, L.; Harding, C. J.; Mikajlo, E. A.; Powis, I. ReV. Sci. Instrum. 2005, 76, 053302. (19) Wiley, W. C.; Maclaren, I. H. ReV. Sci. Instrum. 1955, 26, 1150. (20) Garcia, G. A.; Nahon, L.; Powis, I. ReV. Sci. Instrum. 2004, 75, 4989. (21) Nahon, L.; Alcaraz, C. Appl. Opt. 2004, 43, 1024. (22) Mercier, B.; Compin, M.; Prevost, C.; Bellec, G.; Thissen, R.; Dutuit, O.; Nahon, L. J. Vac. Sci. Tech. A 2000, 18, 2533. (23) Nahon, L.; Alcaraz, C.; Marlats, J. L.; Lagarde, B.; Polack, F.; Thissen, R.; Lepe`re, D.; Ito, K. ReV. Sci. Instrum. 2001, 72, 1320. (24) Montgomery, J. A.; Frisch, M. J.; Ochterski, J. W.; Petersson, G. A. J. Chem. Phys. 1999, 110, 2822. (25) Curtiss, L. A.; Raghavachari, K.; Trucks, G. W.; Pople, J. A. J. Chem. Phys. 1991, 94, 7221. (26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
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