# The role of intramolecular hydrogen bonding in photoelectron circular dichroism: the diastereoisomers of 1-amino-2-indanol

Jennifer Dupont, Julien Vincent, Anne Zehnacker, Gustavo Garcia, Laurent

Nahon, Etienne Rouquet, Valéria Lepère, Sebastian Hartweg

► To cite this version:

Jennifer Dupont, Julien Vincent, Anne Zehnacker, Gustavo Garcia, Laurent Nahon, et al.. The role of intramolecular hydrogen bonding in photoelectron circular dichroism: the diastereoisomers of 1-amino-2-indanol. Physical Chemistry Chemical Physics, 2025, 27 (5), pp.2739-2748. 10.1039/d4cp04248d. hal-04966044

HAL Id: hal-04966044

https://hal.science/hal-04966044v1

Submitted on 25 Feb 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Role of intramolecular hydrogen bonding on
photoelectron circular dichroism: the
diastereoisomers of 1-Amino-2-Indanol

Etienne Rouquet, a) b) Jennifer Dupont, a) Julien Vincent, a) Valéria Lepère, a) Anne Zehnacker a) *

Sebastian Hartweg b) Gustavo A. Garcia, b) Laurent Nahon b) *

a) Institut des Sciences Moléculaires d'Orsay (ISMO), CNRS, Université Paris-Saclay,
F-91405 Orsay, France

b) Synchrotron Soleil, L'Orme des Merisiers, St. Aubin BP48, F-91192 Gif sur Yvette,
France

**Corresponding Authors**

* Anne Zehnacker [anne.zehnacker-rentien@universite-paris-saclay.fr](mailto:anne.zehnacker-rentien@universite-paris-saclay.fr)

* Laurent Nahon [laurent.nahon@synchrotron-soleil.fr](mailto:laurent.nahon@synchrotron-soleil.fr)
## ABSTRACT

(1R,2R)-*trans*-aminoindanol and (1S,2R)-*cis*-aminoindanol, denoted as *trans*-Al and *cis*-Al, are diastereoisomer aromatic aminoalcohols differing by the presence of a weak intramolecular hydrogen bond in *cis*-Al, which is absent in *trans*-Al. They also differ by the number of conformers under supersonic jet conditions, one for *trans*-Al and two for *cis*-Al. One-photon and resonance-enhanced two-photon photoelectron circular dichroism (PECD) spectra are obtained for the two molecules. One-photon ionisation above 11 eV results in molecular fragmentation, with very similar fragmentation pathways for *trans*-Al and *cis*-Al. Identical PECD spectra for different fragments allows associating them with reactions happening from the same parent ion within the same electronic state. The shape of the PECD spectra is sensitive to configuration, conformation, and intramolecular hydrogen bonding. Two-photon PECD, being conformer specific, shows that the PECD of the two ring-puckering conformers of *cis*-Al have identical signs but different values.

## Introduction

Chirality is pervasive in Nature and many natural systems show chirality related to their supramolecular structure, like the DNA double helix, or to the presence of one or several stereogenic centres, for example an asymmetric carbon atom, which is substituted by four different substituents. Often, only one enantiomer exists in the biosphere, as it is the case for amino acids or sugars. However, other natural molecules, for example limonene, the terpene with the well-known citrus flavour, exists in both enantiomeric forms in natural essential oils. The same is true for molecules endowed with more than one chiral centre. Proteins are composed of all-L amino acids based upon a single stereogenic centre, but other molecules exist in several diastereoisomers in Nature, for example the cinchona alkaloids. The properties of these molecules containing more than one chiral centre depend on their relative absolute configuration. Most of the time, the properties of the diastereoisomers are drastically different, which allows easy characterisation by spectroscopic techniques for example NMR¹ and separation, e.g. using chromatography. This property is the basis of the resolution techniques for enantiomer separation.² In rare cases, selected properties do not depend much on the absolute configuration of some of the chiral centres. For instance, quinine and quinidine - two out of the 16 possible diastereoisomers for their structure - behave like quasi enantiomers from their solution NMR and vibrational circular dichroism spectra, while their solid-state NMR spectra are very different.³ ⁴

In the past decade, we have extended our gas-phase spectroscopic study of chiral recognition in weakly bound complexes⁵, ⁶ to diastereoisomers, i.e. molecules containing more than one chiral centre. We have focused on cyclic molecules containing two chiral centres of identical or opposite
absolute configuration, in which the steric constraints due to the ring add to those arising from the absolute configurations. In cyclic dipeptides<sup>7-15</sup> or indane derivative,<sup>16, 17</sup> the spectroscopic properties of the two diastereoisomers differ strongly, both in terms of structure and deactivation of the excited state. In particular, (<i>1R,2R</i>)-*trans*-aminoindanol and (<i>1S,2R</i>)-*cis*-aminoindanol or their respective enantiomers (see Figure 1), denoted as *trans*-Al and *cis*-Al hereafter, show marked structural differences. Under jet-cooled conditions, *cis*-Al exists in two conformers related by the five-member ring puckering motion, while *trans*-Al shows a single conformer. The two conformers of *cis*-Al show an intramolecular hydrogen bond while the single conformer of *trans*-Al does not.<sup>16-19</sup> This intramolecular interaction is expected to have pronounced influence on the vibrational spectroscopic properties of *cis*-Al. As such, a downshift of the v(OH) frequency has been observed in *cis*-Al compared to *trans*-Al.<sup>17</sup> Vibrational circular dichroism (VCD), the chiroptical version of IR absorption, is even more sensitive than IR absorption to molecular interactions.<sup>20 21</sup> This sensitivity was demonstrated on the 1-amino-2-indanol diastereoisomers in solution in a strongly interacting solvent.<sup>22, 23</sup>

From the electronic spectroscopy point of view, the first electronic transition of indane derivatives in the near UV involves the π electrons located on the aromatic ring and is not very sensitive to the presence of the intramolecular H-bond: the S<sub>1</sub>←S<sub>0</sub> transitions of all jet-cooled indane derivatives are located in the same energy range, within a few hundred cm<sup>-1</sup>.<sup>16, 24-27</sup> In contrast, H-bonding results in an electronic population transfer from the lone pair n<sub>N</sub> of the amino nitrogen to the σ* orbital localised on the OH bond. This in turn is expected to influence the ionisation properties and the photoelectron circular dichroism (PECD), defined as a forward-backward asymmetry of the angular distribution of the photoelectrons ejected from a chiral molecule by circularly polarised light (CPL).<sup>28</sup> PECD has proved to be very sensitive to both structural and conformational isomerism as well as molecular interactions.<sup>29-33</sup> Its sensitivity to intramolecular hydrogen bond has been exemplified by the amino acid proline.<sup>32</sup> Usually, the one-VUV-photon ionisation scheme<sup>34</sup> used in the studies mentioned above does not allow probing selectively the different conformers of a flexible molecule.<sup>31, 32, 35-43 44, 45</sup> A few exceptions have been reported. For example, a large difference in ionisation energy, clearly resolved in the Photoelectron Spectrum (PES), allows selection of a given conformer, as reported for proline.<sup>32</sup> Modifying the ratio between conformers is also possible by changing the cooling conditions.<sup>33</sup> In the line of PECD experiments based on ns lasers,<sup>46, 47</sup> we have recently developed a conformer-specific PECD scheme, which rests on 2-photon resonance-enhanced ionisation. The S<sub>0</sub>-S<sub>1</sub> resonance condition and the narrow bandwidth of the ns laser used ensure the conformer selection. This scheme was successfully applied to jet-cooled 1-indanol, which shows two
conformers related by a ring puckering motion.⁴⁸ We have shown that the PECD of the two conformers excited in their S₁ states are opposite in sign and differ in magnitude.

Here, we extend such studies to more complex indane derivatives, namely, trans -Al and cis -Al. The complexity of this system arises from the possible coexistence of isomers resulting from the ring inversion together with the possible intramolecular hydrogen bond, a case reminiscent of that of proline.³² We combine a one-photon VUV ionisation at the Synchrotron SOLEIL and the two-photon scheme that we recently developed. Using the one-photon ionisation over a wide energy range (~4 eV above the ionisation energy) allows ionising different orbitals, in particular those located on the amino or hydroxyl substituents, and assessing the effect of the electron kinetic energy on the PECD. In contrast, ionisation by the laser-based 2-photon scheme is limited to the highest occupied molecular orbitals (HOMO and HOMO-1) but has the advantage to provide conformer-specific PECD. By combining these complementary experimental approaches with quantum chemical calculations, we aim to shed light on the role of the puckering motion and the intramolecular hydrogen bond on the chiroptical properties of cis -Al and trans -Al as measured by PECD.

Experimental and theoretical methods

(1R,2S)-(+)-cis-1-amino-2-indanol and (1S,2R)-(-)-cis-1-amino-2-indanol were purchased from Sigma Aldrich corporation and were used without further purification (99% chemical purity), while (1R,2R)- (-)-trans-1-amino-2-indanol, and (1S,2S)-(+)-trans-1-amino-2-indanol were purchased from Tokyo Chemical Industry corporation and were used without further purification (97% chemical purity).

1. Experimental methods

PECD is manifested by a forward/backward asymmetry, along the light propagation axis, in the angular distribution of the photoelectrons after ionisation of a chiral molecule by circularly polarised light. In the electric dipole approximation, the intensity of photoelectrons ejected from a randomly oriented molecule is written as:

$$
I^{\{p\}}(\theta) = \frac{l_{tot}}{4\pi} \left( 1 + \sum_{n=1}^{2l} b_n^{\{p\}} P_n(\cos \theta) \right)
$$

where P_n is the n^th order Legendre polynomial, l is the number of photons in the ionisation process, θ is the angle between light propagation direction and the photoelectron emission axis. p is characteristic of the light polarisation (p=0 for linear polarisation, p=+1 for left circular polarisation (LCP) and p=-1 for right circular polarisation (RCP)). I_tot is the total number of emitted electrons and
$\frac{I_{tot}}{4\pi} = b_0$. Note that the dependence on the electron kinetic energy (eKE) of $b_n$ and $I_{tot}$ has been removed for clarity, and that the photoelectron spectrum is merely $b_0(eKE)$.

For a two-photon ionisation process, this equation becomes:

$$I^{\{p\}}(\theta) = \frac{I_{tot}}{4\pi} \left( 1 + b_{1}^{\{p\}} P_{1}(\cos \theta) + b_{2}^{\{p\}} P_{2}(\cos \theta) + b_{3}^{\{p\}} P_{3}(\cos \theta) + b_{4}^{\{p\}} P_{4}(\cos \theta) \right)$$

while only the first three terms exist for one-photon ionisation. The even terms $\frac{I_{tot}}{4\pi} = b_0, b_2^{\{p\}}$ and $b_4^{\{p\}}$ correspond to the PES, the anisotropy coefficient, and the usually negligible $b_4^{\{p\}}$ term,⁴⁹ which do not depend on chirality. The PECD is related to the odd terms, which are non-zero only for chiral molecules and for circular polarisation. It is expressed as PECD = $2b_1^{\{1\}}$ for one-photon ionisation and PECD = $2b_1^{\{1\}} - \frac{1}{2}b_3^{\{1\}}$ for a two-photon process.⁵⁰ Note that high order terms $n > 2$ appear because of the alignment induced by the first photon. However, this effect is expected to be negligible in the nanosecond regime due to fast rotational dephasing as we observed on 1-indanol,⁴⁸ although slight departures from this behaviour have been evidenced.⁴⁶

The one-photon ionisation experiments were performed on the variable-polarisation VUV-undulator based DESIRS beamline⁵¹ of the synchrotron SOLEIL, using the SAPHIRS setup⁵² equipped with the DELICIOUS III spectrometer.⁵³ Briefly, the samples were heated to 110°C for trans-Al and 100°C for cis-Al to increase their vapour pressure, and swept by 0.6 bar of Helium through a 200 µm nozzle. The resulting molecular beam crossed the CPL synchrotron radiation perpendicularly. After photoionisation, ions and electrons were extracted and accelerated by a series of grid-less electrodes in opposite directions along the axis perpendicular to both the light and supersonic jet axes. They were detected in coincidence by two position-sensitive detectors (PSD) coupled to a velocity map imaging spectrometer (VMI) for the electrons and a modified Wiley-MacLaren time-of-flight (TOF) spectrometer for the ions. The photoelectron images were mass selected at the mass of the parent or of different fragments and analysed with Abel-inversion via the pBasex software.⁵⁴ The error was estimated assuming a Poisson distribution of the image pixel intensities and propagating the Poisson standard deviation through the image inversion transformation.

The two-photon experiments were performed at ISMO using a resonance-enhanced two-photon ionisation (RE2PI) scheme coupled with a tailor-made VMI spectrometer (Photek). Helium at a pressure of 2 bar, seeded with trans-Al or cis-Al, was expanded in a pulsed supersonic expansion. The temperature was 95°C and 85°C for trans-Al or cis-Al, respectively. $S_1 \leftarrow S_0$ spectra were recorded using one-colour resonance-enhanced two-photon ionisation (RE2PI) monitoring the electron current as a function of the laser wavelength.⁴⁸ The resonance condition allows conformer selection by setting the excitation laser on its specific $S_1 \leftarrow S_0$ transition. Ionisation happens via absorption of a
second photon originating from the same circularly-polarised laser pulse, provided by a frequency-
doubled dye laser (Sirah equipped with Coumarin 540A) pumped by the second harmonic of a NdYAG
(Surelite III – Continuum, repetition rate: 10 Hz). The frequency-doubled dye laser was first separated
from the visible by a MgF₂ Rochon prism, resulting in a linearly polarised UV laser (horizontal
polarisation). The polarisation of the UV laser was then made circular by a quarter-wave plate
mounted on a computer-controlled motor. The rate of circular polarisation was measured by a
second quarter-wave plate and an additional Rochon prism used as a polarisation analyser, following
the published procedure, and was between 97 and 99%.⁵⁵ The electrons produced were directed
onto a PSD consisting of microchannel plates, a phosphor screen and a CCD camera. The
photoelectron VMI images were processed as described above. Two different homemade acquisition
programmes were used for recording the images. Each of them is based on centroid and photon-
counting options, allowing for the estimation of the error bars; they differ by the efficiency of the
centroiding. Indeed, the first version of the software allowed working only with low-intensity
electron signal to prevent loss of signal in the central region of the image due to saturation effects.
This version was used for *trans*-Al, and the acquisition time was of the order of 12 to 18 hours for
one photon energy. The second software was much more efficient, which made it possible to work
with high signal intensity and thus to reduce the acquisition time to 1 to 4 hours. It was used for *cis*-
Al, and gave results comparable to those obtained with the first acquisition programme.

**2. Theoretical methods**

The structures of jet-cooled *trans*-Al and *cis*-Al were determined before, by means of double
resonance spectroscopy combined with DFT calculations.<sup>16, 17</sup> The previously determined structures
were optimised again at the MP2/6-31++g(d,p) level of theory and the electronic densities were
calculated at the same level using the Gaussian software, version G16 B01.<sup>56</sup> The electronic
isodensity of the HOMO and HOMO-1 was plotted with the VMD software (isodensity value of
0.04).<sup>57</sup> The orbital energies were calculated with the outer valence Green's function (OVGF) method
and cc-pVTZ basis set on the MP2 geometries.<sup>36, 58</sup>

**Results and Discussion**

**1. Theoretical results**

The most stable calculated structures of *trans*- and *cis*-Al are shown in Figure 1. *Trans*-Al presents a
single conformation with both the OH and NH₂ groups in equatorial position, as expected by the
minimisation of the 1,3-diaxial interactions. The v(OH) stretch frequency is 3616 cm⁻¹, typical of a
free OH with no interaction between the two substituents. At the opposite, the *cis*-Al
diastereoisomer exists in two quasi-isoenergetic conformers, *cis*-Al₁ and *cis*-Al₁₁, the latter being less
stable by 0.24 kcal/mol at the MP2/6-31++g(d,p) level of theory (ZPE-corrected electronic energy).

The *cis*-Al₁ conformer has the OH group in equatorial position and the NH₂ group in axial position, while it is the opposite for *cis*-Al₁₁. These two conformers are connected by a ring puckering motion like in the previously studied 1-indanol.²⁷ Both conformers of *cis*-Al show an intramolecular OH...N hydrogen bond as manifested by the downshift of the v(OH) stretch frequency (3459 and 3467 cm⁻¹ for *cis*-Al₁ and *cis*-Al₁₁, respectively). The energy difference between the two conformers of *cis*-Al is within the calculation error and the two of them are expected to be nearly equally populated in our typical experimental conditions.

The electron densities and the ionisation energies for each orbital are shown in Figure 2. For both the HOMO and HOMO-1, the electron densities are mostly localised on the benzene ring and show limited hyperconjugation with the C₃H σ orbitals. Among the three systems, *cis*-Al₁ stands out by the fact that the nitrogen lone pair slightly contributes to the electronic density of the HOMO and HOMO-1. It should be noted that it is the only molecule with axial NH₂. The HOMO-2 of *trans*-Al is localised on the amino group with a major contribution of the nitrogen lone pair. For *cis*-Al, the HOMO-2 also corresponds mainly to the nitrogen lone pair nN but it shows a slight density on the oxygen lone pair nO, which is more marked for *cis*-Al₁ than *cis*-Al₁₁. The HOMO-3 shows major contribution of the nO in *trans*-Al with only limited density on nN. As noted for the HOMO-2, the mixing in the HOMO-3 between nN and nO is larger in *cis*-Al₁, with additional contribution of the allocyclic σ bonds for *cis*-Al₁. The HOMO-4 corresponds to σ orbitals in the three systems.

Besides showing the largest mixing between nN, nO, and σ orbitals, *cis*-Al₁ stands out by showing slightly higher ionisation energies for the HOMO and HOMO-1, while those of *cis*-Al₁₁ and *trans*-Al are almost identical. The HOMO-2 ionisation energy is higher in *cis*-Al than in *trans*-Al, due to the OH...N hydrogen bond, which stabilizes the lone pair of nitrogen, while it is the opposite for HOMO-3. The largest differences between the three systems appear for the HOMO-2 and HOMO-3, due to different couplings between the nN and nO lone pairs, which is much sensitive to hydrogen bonding.

## 2. One-photon ionisation results

### Mass spectra and fragmentation pathways

The mass spectra obtained at different photon energies between 9.0 and 13.5 eV are shown in Figure S1 of the Supplementary Information (SI). The fragmentation pathways are identical for both diastereoisomers, with the same fragments and similar relative yields as a function of the photon energy. As will be seen later in the analysis of the threshold photoelectron-photoion coincidence (TPEPICO) spectra, minor differences are observed in the relative intensities of some of the fragments, which cannot be detected at this scale. The parent and its ¹³C-containing isotopologue are observed at *m/z* 149 and 150. The main fragments observed correspond to the loss of a hydrogen
atom at *m/z* 148, as already observed in 1-indanol, the loss of NH₃ at *m/z* 132 and the loss of water at *m/z* 131. The fragments at *m/z* 104 and *m/z* 106 correspond to the C₁-C₂ bond cleavage in alpha position to the amino group, accompanied by hydrogen atom or proton transfer, which is frequent in amines and amino alcohols.⁵⁹,⁶⁰

## One-photon PES and PECD spectra at 11 eV

The PES recorded at 11 eV and summed on all masses (parent and all the fragments) are shown in Figure 3a. They do not show notable differences between *trans*- and *cis*-Al, apart from a slightly different ionisation onset and slight variations in the relative intensities. The largest difference appears between 9.5 and 10.2 eV, where the PES of *cis*-Al is more intense than that of *trans*-Al. In addition, a weak shoulder is observed at the ionisation onset of *cis*-Al, suggesting the contribution of the two conformers, which have slightly different calculated ionisation energies. Comparison with the calculated energies, indicated as black arrows in Figure 3a, allows dividing the spectrum in four regions, corresponding to the ionisation of the first four orbitals. The PES filtered at the mass of the parent (Figure 3b) shows intensity only in the regions of the HOMO and HOMO-1, which means that this energy range is below the fragmentation threshold. That filtered at *m/z* 148 (Figure 3c) shows a single peak in the region of the HOMO-2. A single peak is also observed in the PES filtered at *m/z* 122, slightly shifted to higher energy (Figure 3d). These results suggest that the *m/z* 148 and *m/z* 122 arise from fragmentation after ionisation of the HOMO-2. The appearance threshold of *m/z* 122 is slightly higher than that of *m/z* 148 because the former results from secondary fragmentation of the latter. Finally, the PES filtered at *m/z* 132 (Figure 3e) shows a weak peak in the region of the HOMO-2 followed by a more intense peak in that of the HOMO-3, while those filtered at *m/z* 106 (Figure 3f) and 104 (Figure 3g) show a single peak in the region of the HOMO-3. These results suggest that the *m/z* 132, 106 and 104 arise from fragmentation after ionisation of the HOMO-3, *m/z* 106 and 104 being secondary fragments of *m/z* 132. This hypothesis is verified by comparing the PECD spectra of the observed fragments: the 1-photon PECD spectra filtered at *m/z* 148 and *m/z* 122 are identical, which confirms that the processes leading to these two fragments take place within the same cationic state accessed via removal of an electron in the HOMO-2 orbital. The same is true for those filtered at *m/z* 132, 106 and 104 which are produced after ionisation of the HOMO-3 orbital. Were this not the case, the PECD spectra would be different for the different fragments observed, which would prove that the fragments are not related to the same orbitals, as was seen for serine⁴² and proline.³² Note that at the onset of the HOMO-3 band, the PECD of *trans*-Al is different from that at the centre of the band, and closer to that of the HOMO-2 band. This could be due to a breakdown of the Franck-Condon principle, as seen in other systems,⁶¹⁻⁶³ or the fact that the PECD at any given
energy will carry contributions from all contributing states $PECD(eKE) = \frac{\sum_i I_{tot}^i(eKE) PECD^i(eKE)}{\sum_i I_{tot}^i(eKE)}$, where $I_{tot}^i$ and $PECD^i$ are the ionisation cross-section and PECD of the $i^{th}$ orbital.³⁶

For the ionisation of the HOMO (Figure 3a and 3b), the PECD is maximum at 8.5 eV with a value of 15% for *trans*-Al while it is 6% for *cis*-Al. It then changes sign at around 9.4 eV for *trans*-Al and 9.0 eV for *cis*-Al. This suggests that the PECD of the HOMO and HOMO-1, which are both π aromatic orbitals, have the same sign in *trans*-Al but opposite sign in *cis*-Al. This is not unexpected since the PECD is known to be very sensitive to initial (electronic) and final (scattering) effects and therefore might be the result of the slight differences in the shape of the orbitals mentioned earlier, differences in the scattering potential of the ion, or the existence of two contributing conformers for *cis*-Al. Ionisation of the HOMO-2 (Figure 3d) leads to the formation of *m/z* 148 and results in PECD of identical signs for (*1R*,2*R*)-*trans*-Al and (*1R*,2*S*)-*cis*-Al, as if only the absolute configuration of the amino-bearing carbon mattered. However, they differ in magnitude, that of *cis*-Al reaching 7% while that of *trans*-Al being close to zero. Again, differences between the isosurfaces of the HOMO-2 shown in Figure 2 could contribute to this different behaviour as the mixing between the nitrogen and oxygen lone pairs is different in *trans*-Al and *cis*-Al. This is for the initial state effect, on top of which slight differences in the molecular potential between the two diastereoisomers might also affect the scattering process (final state) leading to different PECD magnitudes. The PECD resulting from the ionisation of the HOMO-3 (Figure 3e-g) is of identical sign and similar magnitude for (*1R*,2*R*)-*trans*-Al and (*1R*,2*S*)-*cis*-Al, despite the differences in the orbitals of the two molecules.

### Threshold photoelectron photoion coincidence (TPEPICO) and threshold photoelectron spectra (TPES) up to 11 eV

The threshold photoelectron spectrum (TPES) is shown in Figure 4. It shows the same trend as the sum of the PES, with a slightly lower ionisation onset for *trans*-Al, and the same difference in intensity between 9.5 and 10.5 eV. Figure 4 shows the threshold photoelectron-photoion coincidence (TPEPICO) spectrum recorded in coincidence with the mass of the parent *m/z* 149 and its ¹³C containing counterpart at *m/z* 150. The other spectra shown in Figure 4 are the sum of the TPEPICO spectra recorded in coincidence with the fragments showing the same PECD spectrum, i.e. *m/z* 148 and 122 on the one hand and *m/z* 132, 106, 104 on the other hand. The spectra for individual masses are shown in Figure S2 of the SI. The three summed TPEPICO spectra show different trends that confirm the conclusions drawn from the PECD spectra: the TPEPICO spectrum in coincidence with the mass of the parent reproduces well the first two convoluted bands of the TPES, with onsets at 8.4 eV and 8.5 eV for *trans*-Al and *cis*-Al, respectively, and maxima around 8.8 and 9.2 eV. By comparison with the calculated orbital energies listed in Figure 2, these two bands can be
safely assigned to the HOMO and HOMO-1 orbitals. Second, the TPEPICO summed on the *m/z* 148 and *m/z* 122 fragments, compared with the PES and the calculated orbital energy at 9.51, 9.66 and 9.89 eV for *trans*-Al, *cis*-Al₁ and *cis*-Al₁₁ respectively, suggests that these fragments result from fragmentation after ionisation of the HOMO-2. It should be noted that the fragmentation in HOMO-2 is slightly more efficient in *cis*-Al than *trans*-Al. Finally, the TPEPICO spectrum summed on the *m/z* 132, 106 and 104 fragments corresponds well with the calculated energies for the HOMO-3.

### One-photon PES and PECD spectra at 12 eV

As PECD strongly depends on the kinetic energy of the ejected electron, the PECD spectra were also recorded at $h\nu = 12$ eV. They are shown in Figure 5 for one enantiomer of each diastereoisomer. They are very different from the PECD spectra at 11 eV, as expected from the kinetic energy dependence. For both diastereoisomers, the maximum PECD is around 10% at the ionisation onset at 8.6 eV. This intense PECD corresponds to the ionisation from the HOMO orbital, which as mentioned before is a $\pi$ orbital localised on the aromatic ring far from the chiral centres. The PECD at 12 eV for the HOMO-3 is very weak for both diastereoisomers unlike that observed at 11 eV. Finally, the PECD of the HOMO-4 are observed around 3%.

Moreover, an unexpected specular relationship is visible in some regions of the PECD spectra between the two diastereoisomers (*1R*,*2R*)-*trans*-Al and (*1S*,*2R*)-*cis*-Al, for example in the region of the HOMO-4 in which they show quasi opposite PECD spectra, although they are not enantiomers. This suggests that the configuration of the OH group does not intervene in defining the PECD since only the chirality of the NH₂-bearing carbon changes between the two diastereoisomers. This could happen if the five-member ring opens up in the excited state of the ion, or if hydrogen loss from the C₂ carbon site induces loss of chirality of the latter, and if non Franck-Condon behaviour take place in the system, as previously observed in methyloxirane.⁶³ However, for ionisation at 13.5 eV, the PECD spectra for the HOMO-4 are not mirror image to each other, although of opposite sign (see Figure S3). Therefore, we can offer no clear-cut explanation for this surprising behaviour, which is beyond the scope of this article.

### 3. Two-photon PECD spectra

In order to separately study the influence of the hydrogen bond and that of conformational isomerism, we have recorded the PECD of *trans* and *cis*-Al using a two-photon ionisation scheme. The resonance-enhanced multi-photon ionisation (REMPI) spectra of the molecules recorded by monitoring the electron current are shown in Figure S4 of the SI. They are identical to those previously obtained by monitoring the ions.¹⁷
The PECD was recorded by setting the laser on the origin transition of *trans*-Al and that of the two conformers of *cis*-Al. The results are shown in Figure 6. The PECD shown in Figure 6 corresponds to 2b₁-0.5b₃. However, b₃ is negligible here and 2b₁-0.5b₃ = 2b₁ within the error bars, as it was already the case for 1-indanol.⁴⁸ It should be noted that only the HOMO and the onset of HOMO-1 can be ionised at this 2-photon energy and the shape of the PES is similar for *trans*-Al and *cis*-Al. The ionisation threshold is however slightly lower for *cis*-Al₁ than *cis*-Al₁, which reproduces well the calculations. The obtained PECD spectra show excellent mirroring between the two enantiomers of *trans*-Al and *cis*-Al₁, which validates our experimental approach. Although not as good as for *trans*-Al and *cis*-Al₁, the mirroring is also acceptable for *cis*-Al₁. The PECD depends on both conformation and configuration. That of (*R*,*R*)-*trans*-Al indeed reaches a maximum (3%) at 8.6 eV then changes sign, reaching a second extremum (-5%) at 9.0 eV. In contrast, that of (*S*,*R*)-*cis*-Al₁ is positive at the onset with a maximum of 4%, goes down to zero and reaches then a second positive maximum of similar intensity. That of (*S*,*R*)-*cis*-Al₁ is also positive with a slightly oscillating value over the whole energy region.

Previous studies on the related molecule 1-indanol have evidenced the role of the puckering motion in shaping the PECD spectra.⁴⁸ We have therefore recorded the PECD of both *trans*-Al and *cis*-Al₁ excited in the vibronic band assigned to the excitation of the puckering motion, which is clearly distinguishable in the REMPI spectrum. The obtained PECD spectra are shown in Figure S5 of the SI and show no modification compared to the origin. In the systems studied there, excitation of the puckering motion does not influence the PECD.

## Conclusion

From the present study we can clearly infer that the PECD spectra of the diastereoisomers of 1-amino-2-indanol are sensitive to configuration, conformation, and intramolecular hydrogen bonding. Moreover, the PECD is strongly dependent upon the ionised orbital, as observed in most of the systems previously studied. For *trans*-Al, with a one-photon ionisation at an energy of 11 eV, the PECD highest value corresponds to the ionisation of the HOMO orbital. The PECD of the HOMO-1, although weaker, has the same sign as that of the HOMO. In contrast, the PECD spectrum of *cis*-Al at the same photon energy and in the same experimental conditions shows a sign inversion between the HOMO and the HOMO-1. The different shape of the HOMO and HOMO-1 PECD between *trans* and *cis*-Al shows that the influence of the intramolecular interaction is visible on the π orbitals far from the hydrogen bond and from the chiral centres. The sensitivity of the HOMO and HOMO-1 contrast with the behaviour of the HOMO-2. As the HOMO-2 is mainly the lone pair of the nitrogen involved as an acceptor in the H-bond, one could expect that the corresponding PECD is very
different in *trans*-Al and *cis*-Al. However, the PECD resulting from the ionisation of the HOMO-2 is weak, whatever the energy. At higher photon energy (12 and 13.5 eV), a surprising "quasi-enantiomer" behaviour between the two diastereoisomers is observed for the ionisation of the HOMO-4. Ionisation at lower photon energy (~ 9.2 eV) with a two-photon process allows for conformer-selective PECD measurements. At this energy, for *trans*-Al, the PECD of the HOMO and the HOMO-1 are of different sign, which contrasts to *cis*-Al. The two conformers of *cis*-Al show PECD spectra of identical signs but different shapes. Excitation of the puckering motion in the S<sub>1</sub> state does not influence the shape of the PECD spectra. The study reported here also shows that PECD can be used as a tool for disentangling the fragmentation processes. Indeed, the fragments arising from fragmentation in the same orbital show identical PECD, which helps interpret the observed fragmentation patterns.

## Acknowledgment

AZ and LN thank the financial support of the projects CONF-PECD and FLEX-PECD by the program "Investissements d'Avenir LabEx PALM" (ANR-10-LABX-0039-PALM). The authors are grateful to the SOLEIL general staff for providing synchrotron beam under proposal No. 20200234. We acknowledge the computing centre MésoLUM managed by ISMO (UMR8214) and LPGP (UMR8578), Université Paris-Saclay (France). This work was supported by state funding from the ANR under the France 2030 program, with reference ANR-23-EXLU-0004, PEPR LUMA TORNADO.
References

1. J. M. Seco, E. Quiñoá and R. Riguera, *Tetrahedron-Asymmetry*, 2001, **12**, 2915-2925.

2. J. M. Will, A. Behrens, M. Macke, C. D. Quarles, Jr. and U. Karst, *Analytical Chemistry*, 2021, **93**, 878-885.

3. V. P. Nicu, S. R. Domingos, B. H. Strudwick, A. M. Brouwer and W. J. Buma, *Chemistry-a European Journal*, 2016, **22**, 704-715.

4. L. Lodhi, J. P. Yadav, T. Yamazaki, N. T. Duong, S. L. Poojary, K. K. Dey, Y. Nishiyama and M. Ghosh, *Journal of Physical Chemistry C*, 2022, **126**, 17291-17305.

5. A. Zehnacker and M. A. Suhm, *Angewandte Chemie-International Edition*, 2008, **47**, 6970-6992.

6. A. Zehnacker, *International Reviews in Physical Chemistry*, 2014, **33**, 151-207.

7. K. Yoshizawa, K. Hirata, S.-i. Ishiuchi, M. Fujii and A. Zehnacker, *ChemPhysChem*, 2023, **24**.

8. K. Yoshizawa, K. Hirata, S.-i. Ishiuchi, M. Fujii and A. Zehnacker, *Journal of Physical Chemistry A*, 2022, **126**, 6387-6394.

9. A. F. Perez-Mellor, R. Spezia and A. Zehnacker, *Symmetry-Basel*, 2022, **14**, 679.

10. K. Le Barbu-Debus, A. Perez-Mellor, V. Lepere and A. Zehnacker, *Physical Chemistry Chemical Physics*, 2022, **24**, 19783-19791.

11. J. Dupont, R. Guillot, V. Lepère and A. Zehnacker, *Journal of Molecular Structure*, 2022, **1262**, 133059.

12. A. Pérez-Mellor, K. Le Barbu-Debus, V. Lepere, I. Alata, R. Spezia and A. Zehnacker, *The European Physical Journal D*, 2021, **75**, 165.

13. A. Pérez-Mellor, I. Alata, V. Lepère, R. Spezia and A. Zehnacker-Rentien, *International Journal of Mass Spectrometry*, 2021, **465**, 116590.

14. A. Pérez-Mellor, I. Alata, V. Lepère and A. Zehnacker, *The Journal of Physical Chemistry B*, 2019, **123**, 6023-6033.

15. F. Ben Nasr, I. Alata, D. Scuderi, V. Lepere, V. Brenner, N. E. Jaïdane and A. Zehnacker, *Physical Chemistry Chemical Physics*, 2019, **21**, 15439-15451.

16. K. Le Barbu-Debus, F. Lahmani, A. Zehnacker-Rentien and N. Guchhait, *Chemical Physics Letters*, 2006, **422**, 218-225.

17. A. Bouchet, J. Klyne, G. Piani, O. Dopfer and A. Zehnacker, *Physical Chemistry Chemical Physics*, 2015, **17**, 25809-25821.

18. K. Le Barbu-Debus, M. Broquier, A. Mahjoub and A. Zehnacker-Rentien, *Physical Chemistry Chemical Physics*, 2009, **11**, 7589-7598.

19. K. Le Barbu-Debus, N. Guchhait and A. Zehnacker-Rentien, *Physical Chemistry Chemical Physics*, 2007, **9**, 4465-4471.

20. C. Merten, *Physical chemistry chemical physics : PCCP*, 2023, DOI: 10.1039/d3cp03408a.

21. A. Bouchet, T. Brotin, D. Cavagnat and T. Buffeteau, *Chemistry-a European Journal*, 2010, **16**, 4507-4518.

22. K. Le Barbu-Debus and A. Zehnacker, *Chirality*, 2021, **33**, 858-874.

23. K. Le Barbu-Debus, J. Bowles, S. Jähnigen, C. Clavaguéra, F. Calvo, R. Vuilleumier and A. Zehnacker, *Physical Chemistry Chemical Physics*, 2020, **22**, 26047-26068.

24. D. Scuderi, A. Paladini, M. Satta, D. Catone, S. Piccirillo, M. Speranza and A. G. Guidoni, *Physical Chemistry Chemical Physics*, 2002, **4**, 4999-5003.

25. A. Das, K. K. Mahato, S. S. Panja and T. Chakraborty, *Journal of Chemical Physics*, 2003, **119**, 2523-2530.

26. K. Le Barbu-Debus, F. Lahmani, A. Zehnacker-Rentien and N. Guchhait, *Physical Chemistry Chemical Physics*, 2006, **8**, 1001-1006.
27. K. Le Barbu-Debus, F. Lahmani, A. Zehnacker-Rentien, N. Guchhait, S. S. Panja and T. Chakraborty, *Journal of Chemical Physics*, 2006, **125**, 174305.

28. I. Powis, in *Advances in Chemical Physics*, Vol 138, ed. S. A. Rice, 2008, vol. 138, pp. 267-329.

29. L. Nahon, L. Nag, G. A. Garcia, I. Myrgorodska, U. Meierhenrich, S. Beaulieu, V. Wanie, V. Blanchet, R. Geneaux and I. Powis, *Physical Chemistry Chemical Physics*, 2016, **18**, 12696-12706.

30. S. Turchini, *Journal of Physics-Condensed Matter*, 2017, **29**, 503001.

31. S. Daly, M. Tia, G. A. Garcia, L. Nahon and I. Powis, *Angewandte Chemie-International Edition*, 2016, **55**, 11054-11058.

32. R. Hadidi, D. K. Božanić, H. Ganjitabar, G. A. Garcia, I. Powis and L. Nahon, *Communications Chemistry*, 2021, **4**, 72.

33. J. Dupont, V. Lepere, A. Zehnacker, S. Hartweg, G. A. Garcia and L. Nahon, *The Journal of Physical Chemistry letters*, 2022, **13**, 2313-2320.

34. L. Nahon, G. A. Garcia and I. Powis, *Journal of Electron Spectroscopy and Related Phenomena*, 2015, **204**, 322-334.

35. A. Giardini, D. Catone, S. Stranges, M. Satta, M. Tacconi, S. Piccirillo, S. Turchini, N. Zema, G. Contini, T. Prosperi, P. Decleva, D. Di Tommaso, G. Fronzoni, M. Stener, A. Filippi and M. Speranza, *ChemPhysChem*, 2005, **6**, 1164-1168.

36. G. A. Garcia, L. Nahon, C. J. Harding and I. Powis, *Physical Chemistry Chemical Physics*, 2008, **10**, 1628-1639.

37. S. Turchini, D. Catone, G. Contini, N. Zema, S. Irrera, M. Stener, D. Di Tommaso, P. Decleva and T. Prosperi, *ChemPhysChem*, 2009, **10**, 1839-1846.

38. G. A. Garcia, H. Soldi-Lose, L. Nahon and I. Powis, *Journal of Physical Chemistry A*, 2010, **114**, 847-853.

39. S. Turchini, D. Catone, N. Zema, G. Contini, T. Prosperi, P. Decleva, M. Stener, F. Rondino, S. Piccirillo, K. C. Prince and M. Speranza, *ChemPhysChem*, 2013, **14**, 1723-1732.

40. M. Tia, B. C. de Miranda, S. Daly, F. Gaie-Levrel, G. A. Garcia, L. Nahon and I. Powis, *Journal of Physical Chemistry A*, 2014, **118**, 2765-2779.

41. D. Catone, S. Turchini, G. Contini, T. Prosperi, M. Stener, P. Decleva and N. Zema, *Chemical Physics*, 2017, **482**, 294-302.

42. S. Hartweg, G. A. Garcia, D. K. Bozanic and L. Nahon, *Journal of Physical Chemistry Letters*, 2021, **12**, 2385-2393.

43. J. Triptow, A. Fielicke, G. Meijer and M. Green, *Angewandte Chemie-International Edition*, 2023, **62**, e202212020.

44. C. J. Harding, E. Mikajlo, I. Powis, S. Barth, S. Joshi, V. Ulrich and U. Hergenhahn, *Journal of Chemical Physics*, 2005, **123**.

45. I. Powis, C. J. Harding, S. Barth, S. Joshi, V. Ulrich and U. Hergenhahn, *Physical Review A*, 2008, **78**.

46. A. Kastner, G. Koumarianou, P. Glodic, P. Samartzis, N. Ladda, S. T. Ranecky, T. Ring, V. Sudheendran, C. Witte, H. Braun, H. G. Lee, A. Senftleben, R. Berger, G. B. Park, T. Schafer and T. Baumert, *Physical Chemistry Chemical Physics*, 2020, **22**, 7404-7411.

47. S. T. Ranecky, G. B. Park, P. C. Samartzis, I. C. Giannakidis, D. Schwarzer, A. Senftleben, T. Baumert and T. Schaefer, *Physical Chemistry Chemical Physics*, 2022, **24**, 2758-2761.

48. E. Rouquet, J. Dupont, V. Lepere, G. A. Garcia, L. Nahon and A. Zehnacker, *Angewandte Chemie (International ed. in English)*, 2024, DOI: 10.1002/anie.202401423, e202401423-e202401423.

49. A. Comby, S. Beaulieu, M. Boggio-Pasqua, D. Descamps, F. Legare, L. Nahon, S. Petit, B. Pons, B. Fabre, Y. Mairesse and V. Blanchett, *Journal of Physical Chemistry Letters*, 2016, **7**, 4514-4519.

50. C. S. Lehmann, N. B. Ram, I. Powis and M. H. M. Janssen, *Journal of Chemical Physics*, 2013, **139**, 234307.
51. L. Nahon, N. de Oliveira, G. A. Garcia, J. F. Gil, B. Pilette, O. Marcouille, B. Lagarde and F. Polack, *Journal of Synchrotron Radiation*, 2012, **19**, 508-520.

52. X. Tang, G. A. Garcia, J.-F. Gil and L. Nahon, *Review of Scientific Instruments*, 2015, **86**.

53. G. A. Garcia, B. K. C. de Miranda, M. Tia, S. Daly and L. Nahon, *Review of Scientific Instruments*, 2013, **84**, 053112-.

54. G. A. Garcia, L. Nahon and I. Powis, *Review of Scientific Instruments*, 2004, **75**, 4989-4996.

55. B. Schaefer, E. Collett, R. Smyth, D. Barrett and B. Fraher, *American Journal of Physics*, 2007, **75**, 163-168.

56. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, Williams, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. J. Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, T. A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman and D. J. Fox, *Journal*, 2016.

57. W. Humphrey, A. Dalke and K. Schulten, *J. Molec. Graphics*, 1996, **14**, 33-38.

58. W. Vonniessen, J. Schirmer and L. S. Cederbaum, *Computer Physics Reports*, 1984, **1**, 57-125.

59. P. Butz, R. T. Kroemer, N. A. Macleod, E. G. Robertson and J. P. Simons, *Journal of Physical Chemistry A*, 2001, **105**, 1050-1056.

60. A. G. Martínez, E. T. Vilar, A. G. Fraile, R. Martínez-Alvarez, S. D. Cerero and P. Martínez-Ruiz, *Rapid Communications in Mass Spectrometry*, 2005, **19**, 1005-1010.

61. H. Ganjitabar, G. A. Garcia, L. Nahon and I. Powis, *Journal of Chemical Physics*, 2020, **153**, 034302.

62. M. M. R. Fanood, H. Ganjitabar, G. A. Garcia, L. Nahon, S. Turchini and I. Powis, *ChemPhysChem*, 2018, **19**, 921-933.

63. G. A. Garcia, L. Nahon, S. Daly and I. Powis, *Nature Communications*, 2013, **4**, 2132.
<table><caption>Table 1: Vertical and zero-point-corrected ionisation energy of the studied systems.</caption><thead><tr><td>System</td><td>Vertical ionisation energy (eV)</td><td>Zero-point corrected adiabatic ionisation energy (eV)</td></tr></thead><tbody><tr><td>trans-Al</td><td>8.94</td><td>8.71</td></tr><tr><td>cis-Al<sub>1</sub></td><td>9.17</td><td>8.99</td></tr><tr><td>cis-Al<sub>2</sub></td><td>9.01</td><td>8.77</td></tr></tbody></table>
<figure><img src="image_2.png" /><figcaption>Figure 1: Skeletal formula and atoms numbering (top) together with most stable conformers (bottom) of (1R,2R)-trans-Al (left) and (1S,2R)-cis-Al (right). The asymmetric carbons are marked by an asterisk.</figcaption></figure>
<table><thead><tr><th></th><th>trans-Al</th><th>cis-Al<sub>I</sub></th><th>cis-Al<sub>II</sub></th></tr></thead><tbody><tr><td>HOMO (π)</td><td>8,68 eV</td><td>8,78 eV</td><td>8,64 eV</td></tr><tr><td>HOMO-1 (π)</td><td>9,03 eV</td><td>9,10 eV</td><td>9,00 eV</td></tr><tr><td>HOMO-2 (n<sub>N</sub>)</td><td>9,51 eV</td><td>9,66 eV</td><td>9,89 eV</td></tr><tr><td>HOMO-3 (n<sub>O</sub>)</td><td>10,65 eV</td><td>10,34 eV</td><td>10,32 eV</td></tr><tr><td>HOMO-4 (σ)</td><td>11,46 eV</td><td>11,44 eV</td><td>11,36 eV</td></tr></tbody></table>

Figure 2: Localisation of the five highest occupied molecular orbitals. The isosurfaces of the electronic density are calculated at the MP2/6-31++g(d,p) level and plotted with an isovalue of 0.04. The blue and red colours correspond to positive and negative values, respectively. The orbital energies are calculated at the OVGF/cc-pTVZ level on the optimized MP2 geometries.
<figure><img src="image_3.png" /><figcaption>Figure 3: PES spectra (black line) and PECD spectra (red and blue lines with error bars) obtained by 1-photon ionisation of *trans*-Al (left) and *cis*-Al (right) for a photon energy of 11.0 eV. Spectra obtained a) by summing all fragments, at the mass of b) the parent *m/z* 149+150, c) the fragment *m/z* 148, d) the fragment *m/z* 122, e) the fragment *m/z* 132, f) the fragment *m/z* 106, and g) the fragment *m/z* 104. The black arrows indicate the calculated energy of the HOMO, HOMO-1, HOMO-2 and HOMO-3 orbitals (see Figure 2). The colour code is blue for (*1R*,*2R*)-*trans*-Al and (*1S*,*2R*)-*cis*-Al and red for (*1S*,*2S*)-*trans*-Al and (*1R*,*2S*)-*cis*-Al.</figcaption></figure>
<figure><img src="image_4.png" /></figure>

Figure 4: TPES and TPEPICO spectra of a) *trans*-Al and b) *cis*-Al recorded between 8.3 and 11 eV. TPES (black) and TPEPICO spectra for the parent at *m/z* 149 +150 (blue), the sum of the fragments *m/z* 148+122 (red) and the sum of *m/z* 132+106+104 (yellow). As expected, the TPES corresponds to the sum of all TPEPICO spectra. The dashed lines correspond to the orbital energies calculated for *trans*-Al (black), *cis*-Al<sub>I</sub> (green) and *cis*-Al<sub>II</sub> (purple). The X, A, B, C and D labels correspond to the experimental values of the orbital energies from which the electron is ejected, namely, HOMO to HOMO-3.
<figure><img src="image_5.png" /><figcaption>Figure 5: PES (solid line) and PECD spectra (line with error bars) of (1R,2R)-trans-Al (purple) and (1S,2R)-cis-Al (orange) for a photon energy of 12.0 eV. The spectra are obtained by summing the PECD recorded at all masses. The grey zones indicate the energy range of the HOMO, HOMO-1, HOMO-2 and HOMO-3 orbitals of both diastereoisomers.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 6: Photoelectron spectra (full line) and PECD (circles) for the two enantiomers of *trans*-Al and *cis*-Al excited at their respective transition origin in a 1C-R2PI ionisation scheme. The total ionisation energy is 9.182 eV for *trans*-Al, 9.155 for *cis*-Al, and for *cis*-Al<sub>II</sub>. (1S, 2S)-*trans*-Al and (1S, 2R)-*cis*-Al are shown in red and (1R, 2R)-*trans*-Al and (1R, 2S)-*cis*-Al in blue. The PECD is defined as 2b<sub>1</sub>-0.5 b<sub>3</sub> (see text). Inserts are corresponding raw (left half) and Abel inverted (right half) difference images for the two diastereoisomers.</figcaption></figure>