Volume 30, Issue 20 e202400038
Research Article
Open Access

Organic Molecules Mimic Alkali Metals Enabling Spontaneous Harpoon Reactions with Halogens

Dr. Wenjin Cao

Dr. Wenjin Cao

Physical Sciences Division, Pacific Northwest National Laboratory, 902 Battelle Boulevard, P. O. Box 999, MS J7-10, Richland, WA, 99352 USA

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Dr. Xue-Bin Wang

Corresponding Author

Dr. Xue-Bin Wang

Physical Sciences Division, Pacific Northwest National Laboratory, 902 Battelle Boulevard, P. O. Box 999, MS J7-10, Richland, WA, 99352 USA

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First published: 29 January 2024

Graphical Abstract

Octamethylcalix[4]pyrrole (omC4P) behaves like alkali metals and reacts with halogens (X) via the harpoon mechanism to form charge-separated omC4P+ ⋅ X complexes.harpoon reactionlong range electron transferphotoelectron spectroscopy

Abstract

The harpoon mechanism has been a milestone in molecular reaction dynamics. Until now, the entity from which electron harpooning occurs has been either alkali metal atoms or non-metallic analogs in their excited states. In this work, we demonstrate that a common organic molecule, octamethylcalix[4] pyrrole (omC4P), behaves just like alkali metal atoms, enabling the formation of charge-separated ionic bonding complexes with halogens omC4P+ ⋅ X (X=F−I, SCN) via the harpoon mechanism. Their electronic structures and chemical bonding were determined by cryogenic photoelectron spectroscopy of the corresponding anions and confirmed by theoretical analyses. The omC4P+ ⋅ X could be visualized to form from the reactants omC4P+X via electron harpooning from omC4P to X at a distance defined by the energy difference between the ionization potential of omC4P and electron affinity of X.

Introduction

First proposed by Polanyi in 1920, the harpoon mechanism describes chemical reactions via long-range electron transfer (ET) that create charge–separated ion pairs to prompt the Columbic interionic attraction between two initially neutral reactants.1-3 It predicts ET occurring at a distance Rx(Å)≅e2E0=14.4/ΔE0(eV) that is inversely proportional to the endoergicity ΔE0 (the difference between ionization energy and electron affinity of the two reactants).1 The harpoon reactions have become a classic prototype for long–range ETs that play a central role in chemistry and biochemistry.4-6 Alkali–metals with a single ns1 valence electron serve as an ideal candidate of the electron donor (D) in a harpoon reaction, as they are able to efficiently donate the electron to an electronegative partner (A) at a certain collision rate. As seen in multiple crossed molecular beam studies on reactions of alkali metals with halogen containing molecules, larger than normal cross sections have been observed due to the long–range ET via the harpoon mechanism, leading to the formation of a stable ionic complex.7-12 Though rare gas atoms and organic molecules have also been found to harpoon electrons to iodine molecules under such a mechanism in a series of pump–probe investigations conducted by the Zewail group and others, they require photoexcitation to initiate the ET reaction; the charge-separated complexes formed are also in excited states that finally relax back to weakly bound ground states.13-15 To date, translational motion induced reactions between D and A via the harpoon mechanism that form a stable ionic complex are still limited to those with alkali metals as electron donors. Is there any candidate other than alkali metals that reacts with halogens via such a mechanism to form a stable ionic complex? An alternative method to find an answer to this question beyond the beam experiments is to probe the bonding nature of the reaction products between the chosen molecules and halogens by photodetaching the corresponding anions utilizing negative ion photoelectron spectroscopy (NIPES).16-18 Despite a broad range of molecular systems investigated with this method, no trace of charge-separated ground state complexes has been found, with all prior complexes being composed of neutral halogens and ligands that act as solvents.19-23

Octamethylcalix[4] pyrrole (omC4P) is a popular and versatile organic host molecule and binds to various charged or neutral species in bulk environments.24-27 In this work, we report gas-phase NIPES studies on a set of host–guest anionic complexes omC4P ⋅ X (X=F, Cl, Br, I, and SCN), in which the host neutral and guest anion interacts with each other via multiple hydrogen bonds.24, 28 Photodetaching these anionic complexes allows us to access them in their neutral charge state. Both spectroscopic profiles and theoretical modellings suggest the dominant detachment channel rising from ionizing the host omC4P to enable formation of charge-separated ionic complexes omC4P+ ⋅ X as the most stable neutral configuration. The formation of these ionic complexes can be visualized by electron harpooning from omC4P to X at a critical distance Rx upon approaching each other, in which omC4P behaves like an alkali metal atom. The observation of alkali–metal–like organics have important implications in material syntheses and biological systems where various types of organic molecules and ET processes broadly exist.

Results and Discussion

Figure 1 shows the NIPE spectra of a set of complexes of halide or pseudo halide ions bound to an omC4P molecule (omC4P ⋅ X, X=F−I, SCN) recorded at cryogenic conditions using 157 nm (7.866 eV) photons. All spectra consist of multiple bands over a wide electron binding energy (EBE) range without the respective X spectral signature near the detachment threshold region, completely opposite to the expectation that such spectra would have assumed the isolated X characters albeit shifted to higher EBE.29 Moreover, these spectra exhibit overall similarities in spectral profile, i. e., three major bands I, II, and III, with I and II separated by ~0.6 eV and III located ~1.1 eV further to the blue. Band II is also coupled with a shoulder on the higher EBE side (labeled as II′ in Figure 1a) for omC4P ⋅ F, omC4P ⋅ Cl, and omC4P ⋅ Br. Similar vertical detachment energies (VDEs) of 3.91, 3.97, 4.00, 4.10, and 4.17 eV, with a 0.26-eV spread, are determined for X=F, Cl, Br, I, and SCN, respectively, from the maximum of each band I (Table 1). Despite these similarities, for omC4P ⋅ Cl, omC4P ⋅ I, and omC4P ⋅ SCN, an outstanding feature (labeled with* in Figure 1a) is, however, present at 6.43, 5.07, and 5.40 eV, respectively, much higher than the lowest EBE band I.

Details are in the caption following the image

T=20 K NIPE spectra (red trace) and simulated density of state (DOS) spectra (gray trace) based on computed high–lying MOs (a), as well as deconvolutions of NIPE spectra (red trace) into contributions from omC4P host (green dotted lines) and guest ions (blue trace) respectively (b). The computed geometries are also shown as insets. The MOs dominated by omC4P host and guest ions are represented with green and blue sticks with alternating lengths corresponding to MO degeneracies. The gray dotted lines are adapted spectra of isolated anions from refs 34, each with a purple arrow and number indicating the shift in EBE.

Table 1. Computed bond distance (R, in Å) between X and adjacent H atoms, experimental (expt.) and calculated (calc.) VDEs (in eV), ΔEBEs (in eV), and calc. host-guest dissociation energies (Des, in eV), as well as association constants (Kas, in M−1), of various omC4P ⋅ X (X=F, Cl, Br, I, or SCN) complexes. All calculations were carried out with M06-2X−D3/maug-cc-pVTZ(−PP).

X=

R

VDE

ΔEBE[b]

calc. De

expt.

calc.

anion

neutral

Ka[d]

F

1.74

3.91

4.01

>3.8[c]

4.26

3.55

17170

Cl

2.30

3.97

4.10

2.82

2.82

2.36

350

Br

2.49

4.00

4.12

2.42

2.51

1.75

10

I

2.77

4.10

4.18

2.01

2.15

1.03

<10

SCN

2.04[a]

4.17

4.30

1.85

2.37

1.68

  • [a] Measured from the N atom of SCN. [b] Determined by differences between EBEs of the first band (*) attributed from the guest ion and VDEs of the isolated anions, which are taken from refs 33 for F/Br, Cl, I, and SCN, respectively. [c] Estimated based on the effective detection limit of ~7.2 eV and electron affinity of F atom of 3.4 eV (ref 30). [d] From ref [24].

In a conventional scheme (as indicated by the red arrow in Figure 2), the difference between the VDE of a complex and that of the corresponding isolated anion, denoted as ΔVDE, serves as an effective measurement for the dissociation energy (De) of the anionic complex since that of neutral complex is much smaller and can be neglected.29 Given the VDE of 3.40/3.61/3.36/3.06/3.55 eV for isolated F/Cl/Br/I/SCN,30-33 ΔVDEs of these complexes, i. e., 0.51, 0.36, 0.64, 1.04, and 0.62 eV, exhibit no correlation to Des when compared to either computed values or previously measured association constants (Table 1). Considering the overall similar spectral profile and VDE for each omC4P ⋅ X complex, a different scheme is indicated. Specifically, the dominant photodetachment channel is not from the guest ion, but from the omC4P host moiety instead, leading to the formation of a charge-separated ionic complex omC4P+ ⋅ X. Photodetachment to such a ground state (as indicated by the blue arrow in Figure 2) leads to an apparent VDE that is no longer related to electron affinity (EA) of the guest X but rather associated with the ionization energy (IE) of the omC4P host molecule. Such a hypothesis is supported by the observation of similar spectral patterns that dominate NIPE spectra of various omC4P ⋅ X complexes since they all have photodetachment taking place from the host molecule. On the other hand, the extra band labeled with an asterisk * (Figure 1a) could be attributed to the transition to the neutral excited state in the omC4P ⋅ X configuration, whose EBE difference compared to the VDE of isolated X then truly represents the effective host-guest De within the anionic complex.

Details are in the caption following the image

Schematic diagrams showing the energy profile of photodetaching an anion (X) bound to a ligand (L). In a conventional scheme when photodetachment takes place from the anionic part (indicated by a red arrow), the electron affinity (EA) difference between X and L ⋅ X complex measures dissociation energy (De) in L ⋅ X since De(L ⋅ X) is negligible. When photodetachment take place from the ligand L (indicated by a blue arrow), the experimental VDE of L ⋅ X or approximately EA of L+ ⋅ X is equal to De(L ⋅ X)+IE (L)−De(L+ ⋅ X).

To verify such an assignment, theoretical computations have been carried out at the M06-2X−D3/maug-cc-pVTZ level of theory to locate optimized geometries of various omC4P ⋅ X, to examine their charge distributions, molecular orbitals (MOs), and to calculate their energetics. Through extensive previous studies, it is known that though isolated omC4P prefers to adopt a 1,3-alternative conformation in which the N−H from adjacent pyrrole moieties point in opposite directions, the cone conformation with all four N−H bonds pointing at the same focal point on the C4 molecular axis becomes dominant when binding to a guest ion to facilitate N−H⋯X hydrogen bond interactions.24 All anions are found to be bound on the principal axis of the omC4P cone, forming a highly symmetric C4v complex with four identical N−H⋯X bonds (See insets of Figure 1 and Figure S1 for different views). For the linear SCN anion, it aligns perfectly with the C4 axis with the N atom pointing towards the omC4P pocket to bind with the four N−H bonds. The N−H⋯X bond distances are found to increase along the halide series from 1.74 Å for omC4P ⋅ F, to 2.30 Å for omC4P ⋅ Cl, to 2.49 Å for omC4P ⋅ Br, and to 2.77 Å for omC4P ⋅ I (Table 1). The calculated bond distances here match excellently with those in a recent study by Weber and coworkers,28 with a trend inversely correlated with the previously measured stability constants (Table 1),24 both indicating the strongest interaction to F and weakest to I for the host. The calculated bond distance for omC4P ⋅ SCN is 2.04 Å, only longer than that in omC4P ⋅ F while shorter than those with all other halide ions (Table 1). At this level of theory, the predicted VDEs for all complexes match well with the observed values, where theoretical values are consistently ~0.1 eV higher (Table 1), demonstrating trustworthy predicted conformations for subsequent MO analyses.

For all five complexes, the HOMOs are found to be dominated by contributions from the omC4P host molecule (represented by green sticks in Figure 1a). The highest MO attributed to the F is 4.20 eV deeper in energy compared to the HOMO. Considering the experimental VDE of 3.91 eV for omC4P ⋅ F, such a feature is expected to possess an approximate EBE of ~8.1 eV, which is out of the photo energy limit of 7.866 eV produced from the 157 nm laser beam (Figure S2). Based on computed energies of high-lying MOs, the simulated density of states (DOS) spectrum for the omC4P ⋅ F complex has nicely reproduced the observed four–band spectral profile from the experimental NIPE spectrum (Figure 1a). For omC4P ⋅ Cl, omC4P ⋅ Br, omC4P ⋅ I, and omC4P ⋅ SCN, their first MO attributed to the guest ion lies at 2.28, 1.62, 0.94, and 1.03 eV below the HOMO (represented by blue sticks in Figure 1a), corresponding to the features at EBEs of ~6.25, 5.62, 5.07, and 5.40 eV, respectively. These predicted EBEs match well with those features marked by an “*” for omC4P ⋅ Cl, omC4P ⋅ I, and omC4P ⋅ SCN. For omC4P ⋅ Br, the predicted feature at 5.62 eV overlaps with the strongest band III from ionizing omC4P and is therefore buried underneath this broad band. Combining the contributions from the MOs of both omC4P host and guest ions, the simulated DOS spectrum of each complex reaches an excellent agreement with the corresponding NIPE spectrum (Figure 1a). It is worth noting that in all these complexes, contributions from the omC4P host share quite similar patterns in the number of MOs, their intervals, and relative intensities, resulting in a common four-band spectral feature, whereas contribution from the guest ion is then distinguished based on the remaining features.

Based on the above analysis, we deconvoluted each recorded NIPE spectrum into two components: the omC4P host and the anion guest. Since the omC4P ⋅ F spectrum is solely contributed from ionizing omC4P, it serves as an ideal reference to differentiate the guest anion signatures from the omC4P host contribution in other complexes (Figure 1b). For omC4P ⋅ Cl, the deconvoluted spectrum exhibits a single band at 6.43 eV derived from detaching Cl. Note that although there exists a small spin–orbit coupling (SOC) splitting of 0.11 eV for the Cl atom as seen from the isolated Cl NIPE spectrum,34 a single SOC-unresolved band feature is often observed in Cl containing complexes.35-37 Though no apparent Br feature is visible in the omC4P ⋅ Br spectrum, deconvoluting it by subtracting the omC4P features reveals an extra band maximizing at 5.78 eV (labeled by “a” on Figure 1b). In addition, there is also a shoulder at 6.25 eV (labeled by “b” on Figure 1b) with a 0.47 eV interval to a, paralleling to the SOC splitting of 0.46 eV of atomic Br.34 The EBEs of these two peaks correlate well to the computed Br-based MO energies (cf. Figure 1a). For omC4P ⋅ I, the extra band * at 5.07 eV is dominated by the contribution from the I moiety (labeled by “a” on Figure 1b), which is coupled by another band at 6.07 eV (labeled by “b” on Figure 1b). The interval of 1.00 eV between a and b also matches well with that of 0.94 eV observed from isolated I.34 Finally, the deconvoluted SCN spectrum is composed of a single band at 5.40 eV, which possesses a nearly identical spectral shape to the previously recorded SCN−[33] while blue shifted by 1.85 eV (Figure 1b). The experimental EBE increase (ΔEBE) for each X due to its complexation with omC4P is thus determined, which descends with halide size and is in good accord with the calculated omC4P-halide interaction energy in the anionic charge state (Table 1). The corresponding neutral complexes possess charge-separated ground state configurations of omC4P+ ⋅ X, with the omC4P ⋅ X configuration lying much higher in energy (cf. the energy scheme in Figure 2). Consequently, substantial Des were predicted for all neutral complexes as well (Table 1).

Parallelly, subsequent charge and electrostatic potential (ESP) analyses on the neutral complexes support the ionic bonding nature between omC4P and X, which remain in a charge-separated omC4P+ ⋅ X state. The computed ESP indicates substantial negative charge on the guest halogens or SCN, while the methyl groups of the omC4P host are positively charged (Figure 3). Upon photodetaching omC4P ⋅ X, Mulliken charges of the omC4P host are seen to change from negative to positive, while those on X remain nearly the same, i. e, −0.28/−0.72→+0.71/−0.71, −0.26/−0.74→+0.70/−0.70, −0.29/−0.71→+0.67/−0.67, −0.30/−0.70→+0.62/−0.62, and −0.40/−0.60→+0.57/−0.57 for charges on host/guest moieties with X=F, Cl, Br, I, and SCN, respectively (Figure 3 and Table S1), unravelling the emitted electron from ionizing the host omC4P molecule (see Table S1 and Figure S3 for charge and ESP change upon photodetachment). The ground state for the neutral complexes is a charge–separated omC4P+ ⋅ X, while the conventional omC4P ⋅ X configuration is an excited state.

Details are in the caption following the image

Electrostatic potential (ESP) plotted on the neutral molecular surface of omC4P ⋅ X (X=F−I and SCN). The numbers on top of each complex are the computed Mulliken charges of the guest halogen atoms or SCN molecule.

It is illustrative to think of the scenario of prying apart the charge-separated omC4P+ ⋅ X complex. At the asymptotic limit, the much higher IE of the closed-shell omC4P molecule (6.35 eV at CASSCF (21,11)/cc-pVTZ) compared to the EAs of halogens or SCN (ranging from 3.61 eV for Cl to 3.06 eV for I) makes the charge-separated omC4P++X channel well above the neutral omC4P+X in energy. Charge recombination must occur along the lowest dissociation path of omC4P+ ⋅ X→omC4P+X, or equivalently, electron harpooning from omC4P to X takes place upon two partners approaching each other at a critical point. To quantitatively illustrate this electron harpooning that generates omC4P+ ⋅ X, one-dimensional potential energy scans were carried out within the CASSCF (21,11) framework for the X=Cl and I cases, which possess the largest and smallest EA of 3.61 and 3.06 eV31, 32 among all systems studied herein (Figure 4). An active space containing 21 electrons (16 from omC4P and the other 5 from halogen) occupying the 11 high-lying MOs (3 halogen p orbitals and 8 highest-lying omC4P MOs) that heavily mix with each other (see Figure 1) were chosen. At a sufficient separation distance of ~20 Å, the neutral omC4P+X asymptotic is calculated to be more stable by 2.75 and 3.29 eV for X=Cl and I, which serve as a good estimation for their ΔE0 (IEomC4P−EAX). When the guest X approaches the host omC4P, the total energy of the omC4P+X channel remains largely flat, but quickly closes the energy gap with respect to the charge-separated omC4P++X curve that is progressively stabilized due to appreciable Coulombic attractions. At a critical distance Rx, these two curves cross, indicating that electron harpooning from omC4P to X occurs to produce the more stable omC4P+ ⋅ X charge-separated state. The distance between X atom and adjacent N−H group (Rx) at which the energy crossing occurs is predicted to be 4.1 Å and 3.6 Å for X=Cl and I respectively. After taking electron correlations from DLPNO–NEVPT2 calculations into consideration, nearly identical Rxs of 4.1 Å and 3.7 Å for X=Cl and I are determined (Table S2), demonstrating reasonable accuracy in CASSCF calculations to qualitatively predicting Rxs of various complexes. The determined Rx values are then found inversely proportional to their ΔE0 (cf. Rx=5.2 and 4.4 Å using Rx(Å)≅e2E0=14.4/ΔE0(eV)). Hereby the organic omC4P molecule behaves just like an alkali metal atom, e. g., the Na with a similar IE, being able to efficiently donate an electron to a vicinity halogen to form ionic charge–separated complexes via the harpoon mechanism.

Details are in the caption following the image

One–dimensional potential energy curves for neutral and charge–separated states of omC4P+X (X=Cl and I). The solid and dash lines represent the energy profiles (E, in hartrees) of the charge–separated and neutral states for X=Cl (in green) and I (in violet) with respect to distances (R, in Å) between X and adjacent N−H group based on CASSCF (21,11) calculations.

Conclusions

Electron photodetachment on a series of anionic host-guest complexes of omC4P ⋅ X (X=F−I or SCN) allows us to systematically examine the electronic structures and bonding properties of the corresponding complexes at the neutral charge state. The similarity in the spectral profile and electron binding energy manifests the lowest energy structure of all complexes being in the charge-separated omC4P+ ⋅ X configuration. In contrast, the conventional neutral form of omC4P ⋅ X is an excited dimer (excimer). Theoretical analyses indicate the lowest electron detachment channel is to ionize the neutral omC4P host molecule, resulting in substantial dissociation energies for these ionic non-metallic organic host-guest complexes. Formations of these complexes can be envisioned to undergo thermal collisions between omC4P and X, followed by electron harpooning from omC4P to X at a distance (Rx(Å)≅e2E0) inversely proportional to the endoergicity ΔE0 (IPomC4P−EAX). One-dimensional CASSCF potential energy curve calculations confirm the curve crossing and charge transfer along the reaction coordinate. Accordingly, we demonstrate that the organic omC4P molecule can behave like an alkali metal atom enabling harpoon reactions with halogens. Given the fact that various similar types of organic molecules broadly exist in chemical syntheses and biological functioning processes, the current finding that they can act like alkali metals to effectively initiate long-range ET reactions may provide crucial yet missing links to better understand the complexity of biological ET reactions and novel ionic chemical bond formations in synthetical material sciences.

Supporting Information

The authors have cited additional references within the Supporting Information.38-55

Acknowledgments

This work was supported by U. S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, Condensed Phase and Interfacial Molecular Science program, FWP 16248.

    Conflict of interests

    The authors declare no conflict of interest.

    Data Availability Statement

    The data that support the findings of this study are available in the supplementary material of this article.