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Spectroscopy of Proton Coordination with Ethylenediamine J. Philipp Wagner, David C. McDonald II, and Michael A. Duncan J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b03592 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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The Journal of Physical Chemistry

J. Phys. Chem. A

Spectroscopy of Proton Coordination with Ethylenediamine

J. Philipp Wagner, David C. McDonald II, and Michael A. Duncan* Department of Chemistry, University of Georgia, 140 Cedar Street, Athens, GA, 30602, U.S.A. Email: [email protected]

ABSTRACT Protonated ethylenediamine monomer, dimer and trimer were produced in the gas phase by an electrical discharge/supersonic expansion of argon seeded with ethylenediamine (C2H8N2, en) vapor. Infrared spectra of these ions were measured in the region from 1000 to 4000 cm−1 using laser photodissociation and argon tagging. Computations at the CBS-QB3 level were performed to explore possible isomers and understand the infrared spectra. The protonated monomer exhibits a gauche conformation and an intramolecular hydrogen bond. Its parallel shared proton vibration occurs as a broad band around 2785 cm−1, despite the formally equivalent proton affinities of the two amino groups involved, which usually leads to low frequency bands. The barrier to intramolecular proton transfer is 2.2 kcal mol−1 and does not vanish upon addition of the zero-point energy, unlike the related protonated ammonia dimer. The structure of the dimer is formed by chelation of the monomer's NH3+ group, thereby localizing the excess proton and increasing the frequency of the intramolecular shared proton vibration to 3157 cm−1. Other highly fluxional dimer structures with facile intermolecular proton transfer and concomitant structural reorganization were computed to lie within two kcal mol−1 of 1 ACS Paragon Plus Environment

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the experimentally observed structure. The spectrum of the trimer is rather diffuse and a clear assignment is not possible. However, an isomer with an intramolecular proton transfer like that of the monomer is most consistent with the experimental spectrum.

1. Introduction Proton transfer (PT) reactions lie at the heart of many processes in diverse fields of chemistry, ranging from simple acid-base reactions to the operation of proton exchange membrane fuel cells.1-5 Important areas of research include photosynthesis and ATP synthesis in cell respiration,3,6,7 Brønsted acid catalysis,8 electrochemistry1,3,4 and atmospheric9,10 as well as interstellar chemistry.11-14 Of particular interest is the high proton mobility in acidic aqueous solutions which has been understood in terms of a structural diffusion mechanism over 200 years ago and is inextricably associated with von Grotthuss' name.15-17 Modern computer simulations have helped to understand that the PT mechanism in water is triggered by thermal fluctuations in the hydrogen-bonding network of the surrounding solvent shell in which proton-bound structures (the Eigen,18,19 H3O+(H2O)3, and Zundel,20,21 H2O−H+−OH2, ions) occur as intermediates.17,22,23 The development of new techniques in photodissociation mass spectrometry made the spectroscopy and structure elucidation of such proton-bound dimers24 of well-defined composition possible in the gas phase.25-30 While most of the complexes probed in this way exhibit intermolecular proton sharing,31-48 intramolecular variations of this bonding motif are less well studied.49-54 Here we explore how up to three molecules of the well-known chelating ligand ethylenediamine (en, H2N−C2H4−NH2, Scheme 1a)) commonly used in transition metal chemistry accommodate an excess proton, giving rise to the possibility of both intra- and intermolecular proton binding.

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The infrared modes associated with shared protons νsp exhibit a rather complex, highly anharmonic behavior and occur over a broad spectral range. The difference in proton affinity of the two basic moieties that encompass the proton has a crucial impact on the frequency of the shared-proton vibration.37 While a sizeable difference in proton affinity leads to high frequency bands (e.g. in Ar−H+−HOCH3, the protonated methanol-argon complex, at 3330 cm−1),37 small differences result in low energy vibrations (e.g. in the Zundel ion as a doublet around 1000 cm−1).32,34,55 This trend is broken when proton-bound dimers with high dipole moment monomers are involved, because of the formation of unusual structures with the proton closer to the lower proton affinity molecule; these structures are favored by strong ion-dipole interactions.39,56,57 Intramolecular hydrogen bonding has been studied in detail in neutrals like malonaldehyde58-62 and acetylacetone63 and also to some degree in anions of dicarboxylic acids.64,65 In the case of cations, intermolecular proton sharing has been studied in 1,8disubstituted naphthalenes with NH−O and NH−F proton bridges.51,52 Complex band patterns were found in the 2700−3200 cm−1 region in the NH−O systems which could be understood in terms of an anharmonic mixing of the intense NH stretching mode with plethora of overtones and combinations in the background.52 These couplings are one of the reasons for spectral broadening that is often observed in shared proton bands. Such behavior might well be expected for the structurally-similar protonated ethylenediamine molecule studied here. In ethylenediamine two basic amino groups compete for the proton and their relative orientation is constrained by the molecular framework that they are embedded in. Thus, it is instructive to look into a simpler situation in which Brønsted bases with a lone pair on nitrogen can adopt their preferred geometry. An excellent example is protonated ammonia clusters, which have been studied thoroughly with computational chemistry and infrared spectroscopy in 3 ACS Paragon Plus Environment


the gas phase for many years.38,66-74 The protonated ammonia dimer assumes a C3v electronic minimum energy structure with a localized proton, but the barrier for proton transfer is so low that it effectively vanishes when zero-point motion is taken into account, leading to the D3d experimental structure.38,71,75 This is markedly different from the situation in H2O−H+−OH2, which has a single well potential. The shared-proton mode νsp in H3N−H+−NH3 is observed at 374 cm−1 and is thus decidedly lower in energy than that in the analogous protonated water dimer (Zundel ion).71 Larger ammonia clusters tend to form Eigen-like structures, i.e. they constitute ammonium ions with hydrogen bonds to ammonia ligands.70-72,74 The external NH3 molecules display free internal rotation along the (approximately) threefold axis in their vibrational spectra up to the (NH3)7H+ cluster ion.67,68 Naturally, free rotation along the hydrogen-bonding axis cannot be expected for ethylenediamine because of the amino group's incorporation into the molecular framework. Protonated ethylenediamine has been studied previously with mass spectrometry76-79 and computational chemistry.80,81 From proton transfer reactions in the gas phase it is known that diamines display a larger proton affinity than monoamines of comparable size.24,76,78,79 Additionally, large negative entropy changes indicate the formation of cyclic intramolecular hydrogen bonds. The (negative) enthalpy of ring formation −∆H°ring form for ethylenediamine has been estimated to be greater than 9.7 kcal mol−1.76 An early ab initio computational study at the MP3/D95**//HF/6-31G** level found that formation of the intramolecular hydrogen bond in ethylenediamine is favored by 10 kcal mol−1, in good agreement with experiment.80 To the best of our knowledge, the only spectroscopy of protonated en in the gas phase so far is for the monomer complexed with three molecules of water.82 The infrared spectrum is consistent with a cyclic structure despite the solvent's competition for hydrogen bonding. When additional molecules of ethylenediamine interact with a proton, clustering up to the (en)4H+ complex has 4 ACS Paragon Plus Environment

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been observed with high-pressure photoionization mass spectrometry.77 However, little is known about the structures of these larger complexes. This is unfortunate since en has been used as a versatile ligand in transition metal chemistry for many years. The resolution of optical antipodes of the Co3+ ethylenediamine complexes proved pivotal to Alfred Werner establishing his structural ideas of the octahedral configuration, which form the basis for much of modern coordination chemistry.83,84 It is interesting to consider how this bidentate ligand forms complexes with the much smaller proton in contrast to transition metal ions. Here, we investigate this with infrared photodissociation spectroscopy in combination with computational chemistry.

2. Methods 2.1 Experimental Protonated ethylenediamine clusters were produced with a pulsed electrical discharge in a supersonic expansion of argon seeded with ethylenediamine vapor. Argon tagged complexes of the stoichiometry (C2H8N2)nH+Ar with n = 1−3 were mass-selected in a custom-made reflectron time-of-flight mass spectrometer30 and their infrared transitions were probed in the range of 1000−4000 cm−1 with a Nd:YAG-pumped infrared OPO/OPA laser system (LaserVision). Radiation in the 1000−2150 cm−1 region was generated in a second stage of difference frequency generation utilizing a silver gallium selenide crystal. The infrared spectra were obtained by monitoring the fragment ion yield from argon loss with respect to the photon energy of the laser.

2.2 Computational Possible structures of protonated ethylenediamine and its clusters were explored and optimized at the B3LYP/6-311G(2d,d,p) level of theory. Frequency computations assured that 5 ACS Paragon Plus Environment


all reported minima display none, and transition states exactly one, imaginary mode. For comparison to experimental infrared spectra, harmonic vibrational frequencies were corrected with an empirical scaling factor of 0.971 that was derived from the known infrared spectrum of neutral methylamine.85 The reported energies are ∆H0 values given in kcal mol–1 obtained with the CBS-QB3 composite quantum chemistry method as implemented in the Gaussian09 program package.86,87 Although this method has been criticized recently,88,89 the energies obtained are likely to be more reliable than those from ordinary DFT computations, and higher level treatments would be challenging for the largest structures encountered in this study.

3. Results and Discussion In agreement with a previous mass spectrometric study77 we found reasonable signal levels for the clustering of up to at least four molecules of ethylenediamine around a proton (see mass spectrum in Figure S1). Although tagging with argon makes the measurement of photodissociation spectra possible in the infrared, it also leads to mass coincidences in the higher range of protonated ethylenediamine clusters. For example, the protonated en trimer is mass coincident with the triply-argon-tagged monomer at our resolution, which complicates the assignment of the larger clusters. Nevertheless, the yield of the protonated en pentamer is rather small. To understand how the structures of protonated ethylenediamine clusters evolve upon the addition of further ligands, we examine the IR spectra and compare them to theoretical predictions of spectra for conceivable isomers.

3.1 Monomer. The infrared spectrum obtained for the argon-tagged protonated ethylenediamine monomer is displayed in the upper black trace of Figure 1. The most notable feature in the 6 ACS Paragon Plus Environment

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spectrum is the intense diffuse band between 2400 and 3300 cm–1 that peaks around 2785 cm–1. Two additional sharper structures at 2973 and 2995 cm–1 are contained within the overall absorption. This band is reminiscent of the situation in 1,8-disubstituted naphthalenes with an intramolecular NH−O proton bridge.52 Thus, the band might well correspond to the NH–N stretching that is substantially broadened from anharmonic effects. Two strong bands at 3306 and 3367 cm–1, together with a weak one at 3433 cm–1, are observed to the blue of the broad feature. These peaks are rather sharp and in the region expected for free N–H stretches. Further absorptions are found in the fingerprint region between 1300 and 1650 cm–1, the most prominent of which occur at 1595 and 1398 cm–1. To better understand the spectrum we performed CBSQB3 computations of possible structures, which are discussed later. Because protonation of a flexible bidentate ligand can have a significant effect on its conformational preferences, we first considered neutral ethylenediamine. The open anti-en conformer is 0.9 kcal mol–1 less stable the cyclic gauche-en structure (Scheme 1a)). In agreement with the computed energetics, the gauche isomer was found to be the most abundant species in a gas phase electron diffraction study of neutral ethylenediamine.90 Upon protonation the preference for the cyclic intramolecular hydrogen-bound structure is strongly increased and the gauche-en-H+ conformer is 9.3 kcal mol–1 lower in energy than the anti-en-H+ structure (Scheme 1b)). The two conformers are connected via a transition state (Figure S8) that is 12.0 kcal mol−1 higher in energy than gauche-en-H+. Again, this is in good agreement with previous computations and gas phase thermochemical measurements.76,80 The small excess proton in gauche-en-H+ is localized on one of the two available amino groups as opposed to, for instance, group 13 metals which form symmetric complexes with the bidentate ligand.91 In contrast to the protonated ammonia dimer,75 the barrier for intramolecular proton transfer is small and does not vanish when harmonic zero-point energy is added within the employed level 7 ACS Paragon Plus Environment


of theory. The computed barrier height associated with the C2-symmetric TS1-en-H+ is 2.2 kcal mol–1. Given the short distance the proton travels and the low energy required, tunneling effects are likely to be important for the interconversion of the two equivalent gauche structures in Scheme 1c). It is important to notice that the C–C bond in gauche-en-H+ represents a chiral axis giving rise to two isoenergetic enantiomers (not taking into account parity violation). While a left-handed helix leads to the gauche– or λ isomer, a right-handed helix yields the gauche+ or δ isomer as depicted in the Newman projections in Scheme 1d). The racemization reaction is facile and the associated transition state TS2-en-H+ is 2.9 kcal mol–1 higher in energy than both of the enantiomers. Intramolecular proton transfer via TS1-en-H+ does not change the helicity of the diamine. While this property is less important for the monomer, it becomes important for the dimer and higher oligomers because it gives rise to the possibility of diastereoisomers which are different in energy and exhibit different vibrational spectra. To denote these isomers we prefer to use the λ/δ nomenclature that is rather common in inorganic chemistry.84 The infrared spectra of the gauche and anti isomers of protonated ethylenediamine predicted by theory (scaled harmonic) are plotted as the blue and red traces in Figure 1, respectively. It is immediately clear that only the lowest energy gauche-en-H+ structure provides a good match for the experimental photodissociation spectrum. Indeed, the broad spectral feature around 2785 cm–1 can be assigned to the NH–N vibration which corresponds to the parallel intramolecular shared proton mode νsp (||). It is likely that an anharmonic coupling mechanism similar to the one found in 1,8-naphthalenes is at work here, causing the observed broadening of this band.52 The analogous mode in the trans-en-H+ conformer would be the symmetric stretching of the ammonium group that suffers a strong red-shift upon intramolecular hydrogen bond formation of about 500 cm–1 (Figure 1). Although the two amino groups 8 ACS Paragon Plus Environment

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formally display the same proton affinity for symmetry reasons, the intramolecular shared proton vibration is found at a higher frequency than expected from the intermolecular trend.37 This also holds true for the comparison to the protonated ammonia dimer, in which this mode is found below 400 cm–1.71 The molecular scaffold seems to prevent a geometrical arrangement that would facilitate more favorable hydrogen bonding and its spectroscopic consequences (the intramolecular hydrogen bond in gauche-en-H+ displays an angle of 124°, which is far from linear). The two narrower features at 2973 and 2995 cm–1 within this diffuse hydrogen bonding band could correspond to C–H stretching vibrations νCH. Although they have very low predicted intensity, the high laser power in this region together with the described anharmonic effects52 could account for their increased strength. The three sharp bands at 3306, 3367 and 3433 cm–1 correspond to the free N–H stretching vibrations of both the NH2 and the NH3+ groups. The highest energy band at 3433 cm–1 is

assigned to the asymmetric stretching mode of the NH2 group . The peak at 3367 cm−1 ,

originates from two vibrational modes, namely the symmetric NH2 stretching and the

asymmetric NH3+ stretching . The lowest energy band of this group at 3306 cm–1 is ,

assigned to the stretching mode that is symmetric with respect to the two free N-H oscillators of the ammonium group NH3+. However, it is asymmetric with respect to the entire NH3+ moiety since the group’s symmetric stretch corresponds to the broad, parallel shared proton mode. We

refer to this vibration as the symmetric mode because the two non-H-bound hydrogens ,

experience the largest displacements and occur in phase. To account for the relative intensities observed, we multiplied the computed intensities of the free N–H stretches by a factor of two. The scaling becomes unnecessary once argon tagging is explicitly treated in the computations. We found that complexation of the rare gas atom occurs preferentially at the four free N–H 9 ACS Paragon Plus Environment


bonds. The spectra of the optimized argon tagged isomers are displayed in Figure S28. While the position of the bands is not strongly affected, their relative intensities change noticeably in favor of the (formerly) free N–H stretches. Multiple argon-tag isomers have to be considered to account for the observed intensity pattern. The binding energy of argon is quite low, computed at approximately 650 cm–1. Because of the small vibrational shifts and the complexity introduced by the multiple binding positions, we decided to neglect the argon in further computations. The fingerprint region shows some deviations in the observed intensities from the spectrum predicted for the gauche-en-H+ isomer. The intense band at 1595 cm–1 and the weaker one at 1641 cm–1 match up with the predicted position of three H–N–H deformation modes. One of the vibrations also involves the motion of the shared proton out of the N–H–N plane. It has been found previously that shared proton vibrations often occur as intense features in the spectrum that may not be described well by simple harmonic frequency computations.37 Thus, we assign the peak at 1595 cm–1 to the out-of-plane perpendicular shared-proton mode (νsp (⊥) o.o.p.). The band at 1398 cm–1 with medium intensity is likely to be the second perpendicular shared-proton mode within the N–H–N plane (νsp (⊥) i.p.). The infrared transition is predicted with strong intensity and can also be described as the umbrella vibration of the NH3+ group. Other computed transitions with weaker intensity below 1100 cm–1 are not observed in the experiment. This might be caused by the low laser powers available in this region. Overall, the infrared spectrum of protonated ethylenediamine monomer is consistent with the single gauche-en-H+ conformer. The parallel shared proton mode occurs as one single intense feature that is spread out over several hundred wavenumbers and has highest intensity around 2785 cm–1. The two perpendicular νsp are found as the most intense bands in the


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fingerprint region at 1595 and 1398 cm–1. Having established the general structure and spectroscopy of the monomer, we focus next on the dimer.

3.2 Dimer. The spectrum of the protonated ethylenediamine dimer in the upper black trace of Figure 2 appears to be similar to that for the monomer, but with greater spectral complexity. Once again, there is a broad transition over several hundred wavenumbers with the highest intensity near 2781 cm–1, entailing sharper features at 2900, 2944, 2959 and 3005 cm–1. Three additional sharp features occur to the blue of the diffuse band in positions similar to bands of the monomer, but with different relative intensities. The lowest energy peak at 3321 cm–1 differs most in position (+15 cm−1) and displays weak intensity, while the higher frequency peaks at 3378 and 3433 cm–1 have medium intensity. Based on the assignments for the monomer, this pattern suggests a higher relative abundance of free NH2 vibrations, as is to be expected. The most intense peak in the spectrum at 3157 cm−1 is accompanied by two satellite bands at 3091 and 3247 cm–1. This feature is completely absent in the spectrum of the monomer and indicates the emergence of a new structural motif. The fingerprint region is dominated by a continuous absorption between 1450−1680 cm−1 with strong peaks at 1602 and 1629 cm−1 and a medium intensity peak at 1549 cm−1. Weaker bands in this region occur at 1322, 1367, 1468 and 1668 cm−1. Again, to make sense of this spectrum, we used computations to explore possible structures. Our computational strategy for en2H+ isomers is based on the finding that the monomer prefers a closed, cyclic structure. Further amino groups are most likely to gather around the excess proton, which can only be achieved when all involved diamines exhibit gauche conformations. Thus, it seems reasonable to start from the gauche-en-H+ monomer structure and 11 ACS Paragon Plus Environment


chelate the center of the excess charge, i.e. the ammonium group NH3+, with the second diamine ligand. This approach yields isomer A-en2-H+ with one intramolecular and two intermolecular proton bridges, as depicted in Scheme 2. The additional coordination by the second en ligand localizes the excess proton on the NH3+ group, and the intramolecular proton transfer that was facile for the monomer does not exist anymore in the dimer. All attempts to localize an A'-en2H+ structure (Scheme 2), which would result from a chelation of the non-protonated amino group, converged into the A-en2-H+ minimum. A second approach to search for low-energy minima of the dimer started from an optimization of a D2-symmetric stationary point with a four-fold coordination of the proton (Figure S13). The structure turned out to be a third-order saddle point which could be relaxed in C2-symmetry along its imaginary modes resulting in the transition structures TS3-H+ and TS4H+ (Scheme 2). These figure-eight shaped transition states correspond to an intermolecular proton transfer reaction between two equivalent minima, which were obtained from further relaxation along the remaining imaginary modes and confirmed by IRC computations. The minimum energy structures B-en2-H+ and C-en2-H+ eventually obtained both display two intermolecular hydrogen bonds, while one N−H of the ammonium functional group is uncoordinated. Surprisingly, the structures are only 1.5 and 1.8 kcal mol−1 higher in energy than the A-en2-H+ isomer and can be reached from the latter via simple C−N bond rotations that are associated with barriers of 3.0 and 4.0 kcal mol−1, respectively (Figures S21 and S22). Their barriers to intermolecular proton transfer are low, amounting to only 0.8 and 0.4 kcal mol−1 respectively. Migration of the proton goes along with a simultaneous opening and closing of the cluster on opposite ends. Thus, PT induces structural changes in the surrounding coordination environment, which is somewhat complimentary to the current picture of the Grotthuss mechanism in which thermally induced structural changes facilitate the proton transfer. One 12 ACS Paragon Plus Environment

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might envision that apt substitution of ethylenediamine might stabilize structures B and C over A, giving access to these highly interesting, fluctional92 dimer structures. Scaled harmonic infrared spectra of the A-en2-H+, B-en2-H+ and C-en2-H+ structures are presented as the blue, red and green traces in Figure 2, respectively. Only isomer A-en2-H+is able to account for the strongest observed band in the experimental IR spectrum at 3157 cm−1. We conclude that this lowest energy structure is the main contributor to the spectrum, while the other two isomers cannot be ruled out completely. This intense band corresponds to the parallel type, intramolecular shared proton mode (||), which was observed as the broad feature

around 2785 cm−1 in the monomer. Thus, the coordination of the ammonium group by the second ethylenediamine increases its proton affinity substantially, resulting in the significant blue shift of approximately 370 cm−1 for the shared proton vibration. This means that the broad absorption at 2781 cm−1 in the protonated dimer has a different origin than the similar broad band in the monomer. The continuous absorption lines up well with the symmetric and asymmetric intermolecular parallel shared proton modes of isomer A-en2-H+, although contributions from isomers B-en2-H+ and C-en2-H+ cannot be ruled out due to the diffuse nature of the spectrum in this region. The sharper features within the band at 2900, 2944, 2959 and 3005 cm−1 might be caused by various C−H stretching modes or result from more complex anharmonic couplings. The highest energy bands at 3433, 3378 and 3321 cm−1 can also be accounted for only by considering structure A-en2-H+; the other isomers cannot reproduce the observed intensity ratio. The band at 3433 cm−1 is assigned to the free asymmetric stretching of the NH2 group of the protonated ethylenediamine unit of A-en2-H+. The band at 3378 cm−1 that is shaded on the red edge probably has its origin in the two asymmetric stretching modes of the non-protonated en's amino groups. The shoulder at lower energy corresponds to the symmetric stretching of the 13 ACS Paragon Plus Environment


amino group of the protonated ethylenediamine unit. In-phase and out-of-phase symmetric NH2 stretches of both amino groups of the non-protonated ethylenediamine contribute to the band at 3321 cm−1. The absorptions near 1600 cm−1 are much broader than in the spectrum of the monomer and thus more difficult to assign. Five H−N−H deformation modes are predicted in this region by our scaled, harmonic frequency computation. Two of these vibrations exhibit perpendicular shared-proton character, explaining the broadening and high intensity in the photodissociation spectrum. We assign the highest intensity bands at 1602 and 1629 cm−1 as perpendicular shared proton modes caused by both intra- and intermolecular proton binding. The peak at 1549 cm−1 in the experiment is attributed to the umbrella mode of the ammonium group. This other perpendicular shared proton mode is blue-shifted by 151 cm−1 compared to the monomer as already seen before for the (||) band. This is caused by the increased localization of the

excess proton and concomitant stronger N−H bonds and higher force constants. The intensity pattern of observed and predicted bands for isomer A-en2-H+ at lower energy agrees quite well, but their character is difficult to describe representing complex mixtures of bending-type motions. From the preceding analysis it is evident that structure A-en2-H+ is the main experimentally observed species. However, the structure considered in Figure 2 represents only the (δ,δ) isomer of A-en2-H+. While its (λ,λ) enantiomer exhibits an identical infrared spectrum, the (λ,δ) diastereoisomer can have different physical behavior towards achiral infrared light. In the same way, the latter displays an IR spectrum identical to its (δ,λ) enantiomer. We have considered these possible stereoisomers in Figure S29. However, we find that the deviation in the spectra between the diastereoisomers is extremely small and the difference in energy is only


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about a hundredth of a kcal mol−1 in favor of the structures with a mixed helicity. Still, these small differences may account for some of the observed broadening.

3.3 Trimer. The spectrum of the protonated ethylenediamine trimer as depicted in the upper black trace in Figure 3 features hardly any structure over the range in which the cluster displays photodissociation. Most notable is an approximately 1000 cm–1 wide band centered at around 3000 cm–1. Features at 2893, 2932, 2958 and 3048 cm–1 stand out from this broad continuous absorption similar to what was seen before for the smaller clusters. One might envision that many isomeric species contribute to this spectrum. A sharper band of medium intensity at 3382 cm−1 with two weak peaks at higher frequency next to it appears distinct from the diffuse absorption. A weak band at 1596 cm–1 is the only other significant detail in the spectrum. Assignment of this spectrum is challenging and computations can only go so far as to rule out unreasonable isomers. For the structural elucidation of the protonated trimer it seems reasonable to start with the assigned main species of the dimer. The A-en2-H+ complex comprises three NH2 groups that can be chelated when adding the third ethylenediamine ligand. Because the center of charge (NH3+) is already coordinatively saturated, there is no clear choice for an obvious binding site. Complexing the amino group of the protonated en unit in the observed dimer structure yields structure A-en3-H+ (Figure 3). This structure is interesting because the possibility of an intramolecular proton transfer is restored in the species. The barrier for migration of the proton to the adjacent amino group is computed to be 3.5 kcal mol–1 (Figure S26) and thus only slightly higher than in the monomer. When one of the NH2 groups of the non-protonated en unit in Aen2-H+ gets chelated, structure B-en3-H+ results. This complex is 2.0 kcal mol–1 more stable than 15 ACS Paragon Plus Environment


A-en3-H+ and does not enable the monomer-like proton transfer. However, the coordinatively stabilized NH2 group becomes a better proton acceptor and moving the proton over to it yields structure C-en3-H+. This structure is 2.6 kcal mol–1 more stable than B-en3-H+ and makes the emergence of the latter less likely. Of course these structures are not the only ones possible, but they are carefully chosen based on insights gained from studying monomer and dimer. The issue of possible diastereoisomerism still applies to all of the computed clusters, complicating the analysis even further. It is encouraging that the spectrum predicted for structure A-en3-H+ (Figure 3) has at least some similarity to the patterns in the experiment. The most intense computed band with high parallel shared proton character lines up well with the center of the broad absorption around 3000 cm–1. The computations further predict a distinct feature that corresponds to stretching vibrations of the chelated amino group resembling the band at 3382 cm–1. However, the absolute position of this vibration is well removed from the band in the experiment. The experimental position is similar to those for the previously observed free NH2 stretches, which are hardly visible in Figure 3 for the isomers taken into account computationally. The band at 1596 cm–1 corresponds to H–N–H deformation modes with perpendicular shared proton character. The position is close to that found in the monomer at 1595 cm–1, strengthening the assignment to a monomer-like structure such as A-en3-H+. For even lower energy bands, photodissociation might not be competitive anymore with intramolecular vibrational redistribution processes, preventing their observation. Based on spectral comparison, it is reasonable that structure A-en3-H+ at least partly accounts for the observed infrared spectrum. The most intense bands with parallel shared proton motion of isomers B-en3-H+ and C-en3-H+ are predicted much lower in the infrared, making the occurrence of these structures less plausible. However, it is disconcerting that these isomers are 16 ACS Paragon Plus Environment

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lower in energy. This suggests that better computations of the relative energies are needed or that different isomers need to be taken into account. Furthermore, the efficient formation of the less stable complex A-en3-H+ could conceivably be accounted for by kinetic effects in the discharge/supersonic expansion. Such effects have been seen before in the case of the protonated formaldehyde dimer.48 Unfortunately, an unambiguous assignment is not possible for the protonated trimer and multiple structures are likely to be relevant.

4. Conclusion and Outlook From this study, we can conclude that ethylenediamine coordinates a proton in a unique way that is fundamentally different from this ligand's behavior towards larger transition metal ions. Structures and spectroscopy of these complexes are also significantly different from those of protonated ammonia clusters due to the constraints introduced by the hydrocarbon scaffold. The infrared spectrum of the monomer is consistent with a single gauche conformer displaying an intramolecular proton bond. Despite the equal proton affinity of the amino groups, the parallel shared proton band is found as a very broad feature at high energy around 2785 cm−1. Intramolecular proton transfer is expected to be facile since it is associated with a low barrier and short traveling distance. Adding a second ethylenediamine to the protonated monomer leads to chelation of the NH3+ group, suppressing the swift proton transfer and shifting the intramolecular shared proton vibration to the blue by 370 cm−1. Other isomers of the dimer with facile proton transfer and concomitant substantial structural rearrangements are found within 2 kcal mol−1 of the observed dimer structure. It is not possible to assign a structure to the trimer because of its diffuse infrared spectrum. However, structure A-en3-H+, in which the intramolecular proton transfer found in the monomer is restored, is most consistent with the experiment.



Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website. This data includes the mass spectrum, optimized geometries, unscaled harmonic frequencies and CBS-QB3 energies, comparison of experimental spectra to one another, computed spectra of argon tagged monomers, computed spectra of stereoisomers of the dimer, and the full citations for references 87 and 89.

Author Information Corresponding Author *E-mail: [email protected] ORCID J. Philipp Wagner: 0000-0002-1433-0292 Michael A. Duncan: 0000-0003-4836-106X

Acknowledgments We gratefully acknowledge support for this work by the National Science Foundation (M.A.D. grant CHE-1464708). J.P.W. acknowledges the Alexander von Humboldt Foundation for a Feodor Lynen Postdoctoral Fellowship.


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Figure Captions

Figure 1. Comparison of the experimental photodissociation infrared spectrum of the argon tagged protonated ethylenediamine monomer to scaled, harmonic spectra of the gauche-en-H+ and anti-en-H+ isomers computed at the CBS-QB3 level.

Scheme 1. Relevant conformers of neutral and protonated ethylenediamine and their interconversion at the CBS-QB3 level of theory. Their relative energies are given in kcal mol−1. The gauche-anti isomerism of neutral and protonated ethylenediamine is displayed in sections a) and b), respectively. Section c) shows the intramolecular proton transfer and the racemization process via internal rotation of the chiral axis is displayed in d).

Figure 2. Comparison of the experimental photodissociation infrared spectrum of the argon tagged protonated ethylenediamine dimer to scaled, harmonic spectra of the most relevant located isomers computed at the CBS-QB3 level.

Scheme 2. Relevant isomers of the protonated ethylenediamine dimer and their internal proton transfer reactions. Realtive energies were computed with the CBS-QB3 method and are given in kcal mol−1.

Figure 3. Comparison of the experimental photodissociation infrared spectrum of the argon tagged protonated ethylenediamine trimer to scaled, harmonic spectra of the most relevant located isomers computed at the CBS-QB3 level. 30 ACS Paragon Plus Environment

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Figure 1.



Scheme 1.


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Figure 2.



Scheme 2.


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Figure 3.



ToC Graphic:


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