Rotamers and Migration: Investigating the Dissociative

Article pubs.acs.org/JPCA

Rotamers and Migration: Investigating the Dissociative Photoionization of Ethylenediamine Giel Muller,† Krisztina Voronova,‡ Bálint Sztáray,*,‡ and Giovanni Meloni*,† †

University of San Francisco, San Francisco, California 94117, United States University of the Pacific, Stockton, California 95211, United States

‡

S Supporting Information *

ABSTRACT: The unimolecular dissociation of energyselected ethylenediamine cations was studied by threshold photoelectron photoion coincidence spectroscopy (TPEPICO) in the photon energy range of 8.60−12.50 eV. Modeling the breakdown diagram and time-of-flight distributions with rigid activated complex RRKM theory yielded 0 K appearance energies for eight dissociation channels, leading to NH2CHCH2+• at 9.120 ± 0.010 eV, CH3C(NH2)2+ at 9.200 ± 0.012 eV, NH2CHCH3+ at 9.34 ± 0.08 eV, CH2NH2+ at 9.449 ± 0.025 eV, CH2NH3+ at 9.8 ± 0.1 eV, c-C2H4NH2+ at 10.1 ± 0.1 eV, CH3NHCHCH2+ at 10.2 ± 0.1 eV, and the reappearance of CH2NH2+ at 10.2 ± 0.1 eV. The CBS-QB3calculated pathways highlighted the influence of intramolecular hydrogen attractions on the dissociation processes, presenting novel isomers and low-energy van der Waals intermediates that led to fragments in good agreement with experimental results. While most of the dissociation channels take place through reverse barriers, the 0 K heat of formation of •CH2NH2 was determined to be 147.6 ± 3.7 kJ mol−1, in excellent agreement with literature, and the 0 K heat of formation of CH2NH3+ at 844 ± 10 kJ mol−1 is the first experimentally measured value available and is in good agreement with theory.

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borane to form innovative lubricant and fuel additives.7 In general, many biodiesel blends with low cetane numbers present longer ignition delays and accelerate the formation of oil sludge. These engine deposits lead to greater release of smoke and exhaust, underlining the importance of effective fuel dispersant-detergents. In an effort to better understand the combustion processes pertaining to EN polyamides and chelated complexes, it is necessary to determine precise thermochemical data of neutral ethylenediamine and its ionic species in the gas phase. Threshold photoelectron photoion coincidence spectroscopy (TPEPICO) is an effective technique in determining highly accurate dissociative photoionization onsets, and to explore dissociation dynamics, branching ratios, and energy partitioning among the product channels of internal energy-selected cations. Accurate thermochemical parameters of the neutral and ionic species, which are important in combustion and atmospheric models, can be ascertained using the data gathered from TPEPICO measurements.8−15 Previous experiments and computations on EN neutral provided some insight into understanding unimolecular dissociation on the cationic potential energy surface. Radom

INTRODUCTION The effects of climate change are widely observed and greenhouse gases are a large contributing component. As of 2013, CO2 emissions in the US resulting from liquid fuel combustion for transportation generated 26% of all greenhouse gases.1,2 Recent initiatives have focused on reducing emissions by researching novel technologies and alternative energy sources such as electric, hybrid, and biodiesel combustion. Biodiesel has the potential to be a carbon-neutral source of energy, contingent upon both the blend of the fuel and its impact upon the engine in which it is burned. Many small amine-containing compounds, such as hydrazine and its methyl derivatives, are used extensively in various industrial arenas, including rocket fuel combustion processes.3,4 Ethylenediamine, the smallest polyethylene amine, possesses two terminal amino groups that enable the compound to readily form imidazolidines, succinimides, chelates, and polyamides. This versatility accounts for its many uses, ranging from the production of fungicides to biofuels. The potential roles of ethylenediamine (EN) in fuel combustion are extensive. Tang et al.5 studied the catalytic activity of EN in the transesterification process of rapeseed oil to generate biofuels, while fuel corporations have been incorporating EN to improve the cetane number in diesel fuels for internal combustion engines.6 EN has recently been shown to enhance motor performance when coupled with © XXXX American Chemical Society

Received: April 6, 2016 Revised: May 13, 2016

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DOI: 10.1021/acs.jpca.6b03516 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A et al.16 investigated the internal rotation in 1,2-disubstituted C− C bonds, reporting low-energy conformations that are stabilized by intramolecular hydrogen bonding involving the amine moieties. Two gauche rotational isomers (rotamers) were presented and the lowest-energy structure, which was calculated to be in more than 95% abundance, agreed with previous electron diffraction studies by Yokozeki et al.17,18 Over two decades later, Kazerouni et al.19 conducted an ab initio investigation into the various conformations of ethylenediamine at the HF/6-31G* level.20 Three low-energy configurations were presented, including an anti form of EN with no symmetry, which was determined to be less stable than the two gauche forms. The lowest-energy gauche form was stabilized with hydrogen bonding, while in the other gauche structure, the nitrogen lone pairs were opposite of one another. Alsenoy et al.21 calculated 10 conformations of EN using the FORCE method22,23 with the 4-21G basis set,24 and Lee et al.25 carried out Møller−Plesset perturbation (MP2) calculations with the 6-31G** basis set to explain the structure and conformational energies. Krest’yaninov et al.,26 guided by Weinhold’s27 view of hydrogen bonding as charge transfer between orbitals, determined the most stable gauche form to have weak intramolecular hydrogen bonding characteristics. The behavior of the EN radical cation is relatively unexplored, however Yue and Baer’s28 DFT and PEPICO study on ethylene glycol provides insights to possible hydrogen bonding interactions and isomerizations in ethylenediamine photodissociation. An EN photoionization experiment by Wei et al.29 utilized computational analysis at the G3 level to explain the appearance of the observed fragments. Of the observed fragments up to 12.5 eV (m/z = 30, 43, 44, and 59), only the m/z = 30 pathway had excellent agreement between experimental and computational results. This served as a motivation to revisit the dissociation of the ethylenediamine ion via TPEPICO, using the analysis by Wei et al.29 as a starting point for the current investigation. With TPEPICO and computational analysis we further characterize the rotation, H-migration, and functional group migration processes involved in EN dissociative photoionization, as these are of potential interest to the scientific community30−33 and can be applied, for example, to combustion diagnostics.15

electrons to be focused onto concentric rings with radii as a function of their velocity perpendicular to the extraction axis. This way, the contribution of energetic (“hot”) electrons contaminating the center detector for threshold electrons was eliminated by multiplying the hot electron signal (outer ring) by an experimentally determined factor and subtracting this from the signal at the center detector.34 In the linear time-of-flight (LinTOF) setup, the ions were mass-analyzed with Wiley−McLaren38 space-focusing geometry and accelerated to 100 eV in the first 5 cm long acceleration region, then rapidly accelerated to 260 eV in a short second acceleration region. Thereafter, ions drifted across a 34 cm field-free region after which they were detected by a Chevron stack of multichannel plates detector. Electron detection served as the start signal and its corresponding ion hit served as the stop signal and the photoelectron-photoion coincidences yielding the time-of-flight spectra were acquired with a Cronologic HPTDC card, utilizing the multistart/multistop coincidence acquisition scheme, as previously described.39 Using the individual center and off-center electron detector time-of-flight spectra, an optimized factor of 0.1834,40 was used to subtract coincidences from hot electrons to yield TOF spectra in which each coincidence is coming from a threshold photoelectron. These TOF spectra were also subjected to background subtraction by integrating signal in time-of-flight areas where no peaks are present. The low-field, relatively long extraction region is essential to obtain dissociation rate information, which is manifested experimentally in asymmetric daughter-ion TOF peak shapes. As a side effect, the peak widths (especially in the case of slow dissociations) are too wide for baseline separation of fragments with 1 m/z difference. In the case of fast dissociations where the daughter-ion peaks are symmetrical, the deconvolution was achieved by fitting Gaussians using the IGOR41 multipeak fitting tools. Slow dissociations are more complicated due to asymmetric peaks; however, they can be isolated if the adjacent peak corresponds to that of a fast dissociation process as was the case in this investigation. Ion signals were corrected for the presence of the 13C and 15N isotopes (Supporting Information, Figure S1). Slow dissociations, indicated by asymmetric TOF peaks, when some of the parent ions dissociate in the drift region, lead to the so-called kinetic shift.42,43 Therefore, to extract accurate 0 K appearance energies, the TOF peak shapes also need to be modeled to extract the unimolecular dissociation rates by our custom MiniPEPICO40 software, using RRKM theory. This analysis needs rotational constants and vibrational frequencies, therefore quantum mechanical calculations were carried out at the B3LYP/6-31G(d) level of theory44−46 with the Gaussian 0947 suite for use in PEPICO analysis. Energetic calculations were conducted with CBS-QB3, and in some cases the G348 method was also utilized for comparison to literature results. Transition states with one negative imaginary frequency were visually inspected using GaussView49 and intrinsic reaction coordinate (IRC)50,51 calculations were performed to verify direct relationships between reactants and products. In the PEPICO analysis, the computed harmonic vibrational frequencies of the transitional modes were scaled to fit the experimental data.40

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EXPERIMENTAL SECTION Time-of-flight mass spectra of energy-selected EN cations were collected on the custom-built TPEPICO spectrometer located at the University of the Pacific. The apparatus has been described in detail and only a brief overview is given here.14,34,35 Ethylenediamine was purchased from SigmaAldrich (≥99% purity) and its vapor was effusively introduced into the ionization chamber at room temperature. The EN molecules were ionized with vacuum ultraviolet light generated from a hydrogen discharge lamp operating at approximately 1.0−1.5 Torr H2 pressure and dispersed by a 1 m normal incidence monochromator. The photon energy scale was calibrated using the hydrogen Lyman-α and Lyman-β resonance lines and the resolution was 8 meV at 10 eV. Photoelectrons were extracted through a 6.75 mm long 20 V cm−1 static electric field and were accelerated under velocity map imaging (VMI) conditions into a 13 cm drift tube set to 77 V. A mask at the end of the drift region contained a 1.4 mm aperture for a center and a 2 × 8 mm opening for an off-center Channeltron electron detector. Velocity map imaging (VMI), first introduced for TPEPICO by the Baer group,36,37 allows B


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The Journal of Physical Chemistry A

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MODELING PEPICO spectroscopy is a powerful tool to determine accurate dissociative photoionization onsets, which can then be used to arrive at valuable thermochemical information. The MiniPEPICO program, described in detail by Sztáray, Bodi, and Baer,40 calculates the necessary density and number of states functions, and internal energy distributions to model the socalled breakdown diagram, which represents fractional ion abundances as a function of the photon energy. These modeled breakdown diagrams can be directly compared or fitted to the experimental fractional abundances. This serves both as a helpful guide in understanding the parallel and consecutive dissociation pathways as well as the model that fits the experimental data yielding the experimental appearance energies. For slow dissociations, the MiniPEPICO40 program can model the dissociation rates either with Rigidd Activated Complex (RAC−) RRKM theory,52−54 variational transition state theory (VTST),55 or the simplified statistical adiabatic channel model (SSACM).56 These rate constants as a function of internal energy (E) are all given by k(E) =

σN ⧧(E − Eo) ℏρ(E)

central carbon bond from cis to trans orientations, and it was found that small barriers distinguish multiple configurations. At each minimum the energy of the structure was determined using CBS-QB3, and a following potential energy surface was scanned where the NH2 group rotates on a fixed bond with the aim to observe the effect of nitrogen−hydrogen positioning in each form. Several of the neutral geometries and energies have been made available in Figure S2 in the Supporting Information. The lowest-energy conformation found is geometrically similar to that presented in the previous photodissociation study of ethylenediamine by Wei et al.29 using G3, however the C−C and C−N bonds are slightly elongated in the current structure. For direct comparison, the CBS-QB3 energy of the structure found using B3LYP/6-31G(d) was subsequently optimized with G3, confirming a marginally lowerenergy structure of molecular EN than had previously been reported (Figure 1). The conformation is stabilized by intramolecular hydrogen bonding in the gauche state, in agreement with previous studies.16−18

(1)

⧧

where N (E − E0) is the number of states of the transition state (TS) at excess energy above the dissociation barrier E0, ρ(E) is the density of states of the molecule, ℏ is Planck’s constant, and σ corresponds to the symmetry number of the TS. The three statistical rate models coded into MiniPEPICO (RAC-RRKM, VTST, SSACM) differ on their treatment of the transition state number of states function. As mentioned above, quantum-chemical calculations provide the energy distributions, number and density of states functions that are used in the PEPICO analysis. Loose transition states (those with no reverse barrier) represent a special challenge and, as a start, transition state structures are estimated from constrained optimizations in which a bond length (roughly corresponding to the reaction coordinate) is scanned and frozen. The experimental appearance energies (AE) of the fragments are obtained once the best fit is reached between the experimental and modeled breakdown diagrams and, in the case of slow dissociations, the TOF spectra. Combining these appearance energies with known heats of formation of the neutral parents and fragments, it is possible to arrive at the heats of formation of the daughter ions for those channels that proceed without reverse activation barriers.

Figure 1. Lowest-energy conformation of neutral ethylenediamine and energy in Hartrees as calculated by CBS-QB3 (left) and G3 (right). The CBS-QB3 neutral total energy is used to determine the relative energies of all stationary points on the potential energy surface and the G3 value can be directly compared with the previous G3 optimization calculations.29 Bond lengths are provided in angstroms (Å) and angles in degrees.

Cationic EN. Bond lengths, angles, and rotations were scanned to search for rotamers in the cationic state and three minima were observed with small rotational barriers, in agreement with Wei et al.29 However, the geometries and energies of these structures differ from previous results and are thus provided in Figure 2. Structure [1] was found to be the highest energy cationic rotamer and the orientation is in best agreement with the neutral. The energy difference between the neutral structure to cation [1] is calculated at 8.57 eV using CBS-QB3 (8.55 with G3), while the relative energies for conformers [3] and [5] are 8.20 and 8.18 eV, respectively. The calculated ionization energy of EN using rotamer [1] is also in good agreement with 8.54 eV calculated by Wei et al.29 using G3 and 8.6 eV determined by Kimura et al.62 (value obtained from the NIST Web site)63 through HeI photoelectron spectroscopy. Several studies have reported ionic isomerization pathways prior to dissociation.32,33 In light of the work by Yue and Baer28 with ethylene glycol and Wei et al.29 with EN, the quantum chemical calculations regarding the rotational barriers (from [1] to [5] in Figure 2) and known isomerization processes were necessary for the current PEPICO investigation. A specific lowenergy isomer was reported to be involved in the m/z = 43 and 44 formation channels of EN photodissociation29 and was reproduced with CBS-QB3. The potential energy surface detailing the initial rotations and isomerization are shown in

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COMPUTATIONAL OVERVIEW Stationary points are found by using the Synchronous TransitGuided Quasi-Newton (STQN) method57,58 at the B3LYP/631G(d) level/basis set. Subsequent CBS-QB3 calculations are conducted to determine the relative energies, and are accompanied by a mean absolute deviation (MAD) from experiments of ±0.05 eV.46,59−61 Stationary points are denoted by [x] with transition states distinguishable by [x]⧧; x corresponds to the appropriate step along the potential energy surface. Neutral EN. Intramolecular hydrogen bonding and stereoelectric effects are known to contribute to the various rotamers of ethylenediamine in the neutral state.16,26 Potential energy surfaces have been scanned at the B3LYP/6-31G(d) level of theory to evaluate how the NH2 moiety rotates around the C


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Figure 2. B3LYP/6-31G(d) optimized geometries of the three cationic ethylenediamine conformers involved in the photodissociative processes. Bond lengths are provided in angstroms (Å) and angles in degrees.

Figure 3. Breakdown diagram of internal energy-selected ethylenediamine cations. Experimental data represented by symbols and solid lines correspond to the best fit of the RRKM modeling of the data.

Figure S3 and the significant barrier 0.95 eV (9.13 eV relative to the neutral) leading to the isomer agrees with the saddle point calculated at 9.16 eV by Wei et al.29 using G3. The rotations can be accessed at photon energies just slightly higher (0.02 eV) than the ionization energy of EN, which makes them energetically favorable pathways prior to photodissociation.

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RESULTS AND DISCUSSION Time-of-flight spectra of energy-selected ethylenediamine ions were collected in the photon energy range of 8.60−12.50 eV at room temperature. The breakdown diagram (Figure 3) represents the observed fragments CH 4 N + , CH 5 N +• , C2H5N+•, C2H6N+, C2H7N2+ at m/z = 30, 31, 43, 44, and 59, respectively, as well as their MiniPEPICO-extracted appearance energies. The experimental TOF spectra revealed all peaks to be symmetric, indicating fast dissociation processes, except the first channel leading to a daughter ion with m/z = 43 (Figure 4). TOF peaks corresponding to m/z = 30, 44, and 59 reappear at higher energies, evidenced in the breakdown diagram (Figure 3). Each dissociation pathway is outlined below in order of appearance as shown in the breakdown diagram (Figure 3). A simplified potential energy surface diagram is depicted in Figure 5, and the corresponding experimental reaction rates are shown in Figure 6. For the full potential energy surface diagram, the structures of the ions, intermediates, TSs, and their respective geometries see Figures S4−S5 and Tables S1−2 in Supporting Information.

Figure 4. Time-of-flight (TOF) mass spectra of ethylenediamine (NH2CH2CH2NH2+•) and the CH3C(NH2)2+, NH2CHCH3+, and NH2CHCH2+• fragment ions (m/z = 60, 59, 44, and 43 respectively).

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DISSOCIATION CHANNELS In the current study, nine dissociation channels were investigated by quantum chemical calculations, eight of which were able to be modeled (Figure 3). The ninth channel was excluded from the modeling due to the low abundance and technical limitations of the MiniPEPICO program. This channel will be outlined in limited detail following the descriptions of the modeled pathways. Additional computational analysis regarding the identification of the structures is provided in Supporting Information. D


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Figure 5. A simplified CBS-QB3 calculated potential energy surface diagram for the photodissociation channels R1−R8 of the ethylenediamine cation with the relative energies at 0 K for the stationary points (for structures of the ions, intermediates and TSs see Figure S5). Only energies of initial rotamers, highest energy barriers, and associated products are shown. Dotted lines connote indirect transitions as steps have been removed for clarity; solid lines show direct transitions.

Figure 6. RRKM rate constants plotted as a function of EN ion internal energy as determined by modeling of TPEPICO data (for structures of the ions, intermediates, and TSs see Figure S5).

(1) m/z = 43, NH2CHCH2+•. Reaction 1 (R1) leads to the resonance-stabilized ethenamine ion [11]: NH 2CH 2CH 2NH 2+• → NH 2CHCH 2+• + NH3

There are two parallel channels corresponding to this TOF, evidenced by the increasing fractional abundance, disappearance, and subsequent reappearance at higher energy as shown in the breakdown diagram (Figure 3). In this case, the parent and the daughter ion peak shapes are not baseline separated, due to the low field and long extraction region, which are essential to obtain unimolecular rate information. However, the kinetic information hidden by the m/z = 60 peak could be extracted from another parallel channel, namely, the formation of the m/z = 43, since the first order dissociation rate belongs to the fragmentation of the parent ion. The PEPICO photoionization onset of structure [18] at 9.200 ± 0.012 eV is in good agreement with the highest barrier of 9.13 eV calculated using CBS-QB3 and the higher-energy, multistep process helps explain why this fragment appears at energies greater than R1. This dissociation channel differs from the original m/z = 59 formation outlined by Wei et al.,29 in which the experimental onset of m/z = 59 was reported at 9.06 eV and the G3 barrier for H-loss from a central carbon was calculated at 0.99 eV above their IE of 8.54 eV. Tunneling was suggested to play a role in their early detection of this fragment. Our CBS-QB3

[11] (R1)

The low-energy, resonance-stabilized ethenamine cation and neutral ammonia are formed via a series of NH2-migrations, in which the functional group “walks” the molecule prior to dissociation as a result of H-interactions with nitrogen. Overall, this dissociation pathway is the most energetically favorable and is thus the first to occur and the most dominant within the low photon energy range (Figure 3). This channel is identical to that proposed by Wei et al.29 and the extracted 0 K appearance energy of 9.120 ± 0.010 eV matches both CBS-QB3 (9.13 eV) and G3 (9.16 eV)29 theoretical expectations well. (2) m/z = 59, CH3C(NH2)2+. The 1-aminoethaniminium ion [18] is the second fragment to form via the H-migration barrier [6]⧧ and consists of a hydrogen loss after a multistep process of H and NH2-migrations (R2). NH 2CH 2CH 2NH 2+• → CH3C(NH 2)2+ + •H

[18] (R2) E


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The Journal of Physical Chemistry A NH 2CH 2CH 2NH 2+• → c−C2H4NH 2+ + •NH 2

calculations for the same process yielded a barrier at 9.56 eV, in good agreement with the computational results of Wei et al.,29 however not in experimental agreement with either study. Since the N−H bond is generally known to have greater strength than the C−H bond, it is therefore reasonable that m/z = 59 formation from H-loss at the amine site is calculated to require larger energy regardless of the resulting cationic conformer. It is determined that the R2 dissociation pathway must involve isomerization and does not arrive from direct H-loss from any of the initial EN ion conformers. (3) m/z = 44, NH2CHCH3+. The ethylidenimmonium ion, [19], is the proposed cationic structure formed along with the neutral •NH2, from a van der Waals product complex [14]: NH 2CH 2CH 2NH 2+• → NH 2CHCH3+ + •NH 2

(R6)

The CBS-QB3 dissociation limit is calculated to be 10.10 eV [24]. The second TPEPICO appearance of m/z = 44 at 10.1 ± 0.1 eV is in excellent agreement with this calculated barrier. The percent abundance of m/z = 44 is low at a maximum of 2.5%, due to competition from the two subsequent channels at slightly higher energies. The appearance energy of this fragment has not been previously reported from EN dissociation. (7) m/z = 59, CH3NHCHNH2+. The second m/z = 59 appearance (R7) above 10.2 eV involves relatively low energy rotational barriers and rotamers, [29]⧧ to [32]: NH 2CH 2CH 2NH 2+• → CH3NHCHNH 2+ + •H

[19]

The experimental 0 K appearance energy of this channel is 10.2 ± 0.1 eV, again in great accord with the CBS-QB3 calculated barrier at 10.23 eV. (8) m/z = 30, CH2NH2+. The methylenimmonium ion [37] is formed again as in R4, however with a higher energy neutral fragment:

The relative energy of the products involved in R3 is calculated at 8.98 eV, which is higher than the significant energies involved in both the R1 and R2 channels (8.41 and 8.76 eV, respectively). This may provide one explanation as to why this fragment is the third to appear in the current study, though they all share the same H-transfer barrier [6]⧧ with a theoretical dissociative threshold at 9.13 eV. The m/z = 44 TOF band is buried by the dominant asymmetric m/z = 43 TOF peak up to ∼9.3 eV, after which deconvolution of the peaks is possible. As shown in the breakdown diagram (Figure 3), the experimental appearance energy of m/z = 44 is far extrapolated by RRKM modeling, leading to a larger-than-usual uncertainty. The TPEPICO experimental appearance energy for this molecular dissociation pathway is found to be 9.34 ± 0.08 eV, in significant disagreement with the previous dissociative photoionization study on EN, reporting the AE at 8.90 ± 0.03 eV29 for the same channel. (4) m/z = 30, CH2NH2+. The breakdown diagram in Figure 4 shows R4 to be the most dominant channel in EN dissociation within the 8.60−12.50 eV photon energy range, forming the methylenimmonium ion (CH2NH2+) and the neutral aminomethyl fragment (•CH2NH2), shown together as structure [20]:

NH 2CH 2CH 2NH 2+• → CH 2NH 2+ + CH3NH•

[37] (R8)

The TPEPICO appearance energy is reported at 10.2 ± 0.1 eV as compared to the CBS-QB3 barrier of 10.23 eV. (9) m/z = 30, CH2NH2+. The ninth dissociation channel (R9) appears above 12.1 eV in the breakdown diagram, where a discrepancy in the modeling exists as the abundance of m/z = 30 increases and m/z = 59 decreases. Wei et al.29 reported a second m/z = 30 onset at 12.80 ± 0.06 eV and presented a mechanism showing the consecutive dissociation pathway of their proposed m/z = 59 structure to form CH2NH2+, the ylide + NH2CH−, and •H without barrier. The G3 energy sum of these products was reported at 12.57 eV. As discussed above regarding R2, Wei et al.’s29 energetic calculations for the m/z = 59 structure were not in agreement with the experimental appearance energies of m/z = 59 in either study. In addition, the CBS-QB3 energy (12.58 eV) of the m/z = 30 channel products from this structure seems too high to agree with the earlier appearance detected in this study around 12.2 eV (Figure 3). An alternate pathway is outlined in Figure S6, where m/z = 30 results from consecutive dissociation of an isomer of m/z = 59. The associated structures and energies are provided in Figure S7. Calculations at the CBS-QB3 level show that structure [34] possesses sufficient internal energy at 12.21 eV to undergo a 1,3-hydrogen shift to yield a higher-energy m/z = 59 isomer, which freely dissociates to CH2NH2+ and NHCH2 without a reverse barrier. This value is the highest-energy saddle point in the potential energy surface for the pathway leading to the third m/z = 30 appearance (Figure 3). The 12.21 eV barrier is in agreement with the approximate experimental appearance of CH2NH2+ around 12.2 eV and the disappearance of m/z = 59 within the same photon energy range. The intramolecular nitrogen−hydrogen attractions play a profound role on the dissociation dynamics of the EN ion. Stable van der Waals complex intermediates are involved in five of the eight modeled channels, and the low energies of these structures incentivize their formations ([10], [14], [22], and [27]). In most cases, these structures dissociate to product fragments without any further reverse barriers, with the

[20] (R4)

These fragments are generated by the bisection of the lowestenergy conformer of the EN ion [5], and the experimental 0 K appearance energy of 9.449 ± 0.025 eV is in excellent agreement with the CBS-QB3 energy of 9.49 eV. The AE reported by Wei et al.29 is 9.30 ± 0.03 eV. (5) m/z = 31, CH2NH3+•. The appearance of this cationic fragment is observable at slightly higher energies than that of m/z = 30 in R4. The methyleneammonium ion, structure [23], is expected to be the primary contributor through CH2NH-loss, and the overall channel is represented by reaction R5: NH 2CH 2CH 2NH 2+• → CH 2NH3+• + CH 2NH

[36] (R7)

(R3)

NH 2CH 2CH 2NH 2+• → CH 2NH 2+ + •CH 2NH 2

[28]

[23] (R5)

CH2NH3+

The RRKM model of ion from EN is in good agreement with experimental results, and the photoionization onset is reported here at 9.8 ± 0.1 eV. No appearance energy of this ion has been previously reported from EN dissociation. (6) m/z = 44, c-C2H4NH2+. The reappearance of the m/z = 44 corresponds to the formation of cyclic aziridinium ion [28] from NH2-migration and ultimate NH2-loss: F


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The Journal of Physical Chemistry A Table 1. Auxiliary and Derived Thermochemical Dataa species NH2CH2CH2NH2 m/z = 60 NH2CHCH2+• m/z = 43 CH3C(NH2)2+ m/z = 59 NH2CHCH3+ m/z = 44 CH2NH2+ m/z = 30

CH2NH3+ m/z = 31

c-C2H4NH2+ m/z = 44 CH3NHCHCH2+ m/z = 59 NH3 H • NH2 • CH2NH2

NHCH2

ΔfHo0

AE

11.2 ± 0.6 9.120 ± 0.010e 8.85f 9.13 ± 0.05c 9.200 ± 0.012e 9.06f 9.13 ± 0.05c 9.34 ± 0.08e 8.90f 9.13 ± 0.05c 9.449 ± 0.025e 9.30f 9.49c 10.2 ± 0.1e 10.23 ± 0.05c 9.8 ± 0.1e 9.85 ± 0.05c

10.1 ± 0.1e 10.10 ± 0.05c 10.2 ± 0.1e 10.23 ± 0.05c

ΔfHo298

Ho298 − Ho0

−17.0 ± 0.6 −19.2 ± 2.7d

16.42c

683.8 ± 1.4g

665.1 ± 1.4g 657 ± 9h

13.148c

762.3 ± 1I 764.0 ± 1.8j

750.3 ± 1I 752.1 ± 1.8j

10.324c

860 ± 10e

844 ± 10e 848.0 ± 0.9d 855k 832.6l 724 ± 9m

12.813c

−45.558 ± 0.030j 217.998 ± 0.000j 186.05 ± 0.15j 147.6 ± 3.7e 150.1 ± 0.6d 148.74 ± 1.01j 88.701 ± 2.09o 89.0 ± 0.4d

10.045n 6.197n 9.929n 11.450c

b

743 ± 9m

−38.565 ± 0.030j 216.034 ± 0.000j 188.94 ± 0.15j 160.6 ± 2.7e 159.63 ± 1.01j 96.64 ± 2.09o

b

11.203c

10.142c

The PEPICO experimental appearance energies in eV, enthalpies of formation (ΔfHo0, ΔfHo298) and heat content functions (Ho298 − Ho0) in kJ mol−1. Our experimental derived values are in bold. bGood and Moore.70 cCBS-QB3 calculated thermochemical values. dIsodemic reaction networks. eThis work. fWei et al.29 gTraeger et al.95 hLossing et al.80 IBodi et al.75 jActive Thermochemical Tables (ATcT).85−87 kSana at al.83 l Bouma et al.92 mSolka et al.96 nChase, M. W.97 oOliveira et al.81 a

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THERMOCHEMISTRY Auxiliary thermochemical data and the results of this work are summarized in Table 1. The enthalpy of formation of liquid EN was reported in 1900 by Berthelot69 at −5.82 kcal mol−1 (−24.4 kJ mol−1). Seventy years later, however, Good and Moore70 determined the heat of formation of condensed-phase EN at −15.06 ± 0.13 kcal mol−1 (−63.0 ± 0.5 kJ mol−1) through oxygen-bomb combustion calorimetry and used enthalpies of vaporization to determine the gas-phase standard enthalpy at −17.0 ± 0.6 kJ mol−1. Verevkin et al.71 evaluated consistent heats of vaporization of EN obtained from other studies72,73 and used Good and Moore’s ΔfHo298(l) to arrive at ΔfHo298(g) = −17.4 ± 0.6 kJ mol−1. This value differed from Verevkin et al.’s71 G4 calculated value (−15.3 kJ mol−1) using their most stable hydrogen-bonded conformer. The study of Burkey et al.74 on the heats of formation of α-aminoalkyl radicals utilized the calculated heat of formation of EN at −18.0 kJ mol−1 as prescribed by Benson and co-workers’ additivity contributions, however no error bar was provided. An isodesmic reaction network14,65,75 was used to obtain a reliable value for the heat of formation of neutral ethylenediamine, where the heats of formation of ethylene glycol, methylamine, methanol, butane, and ethane are all exper-

exception of R2 where additional steps are required for H-loss. The functional group migration processes in NH2- and NH3loss channels are the most emblematic of the influence of hydrogen attractions in EN photodissociation. Interestingly, CH2NH2+, •CH2NH2, and CH3NH• are linked in the context of methylamine, a known interstellar molecule.64 Studies on the photodissociation of methylamine (CH3NH2) describe CH2NH2+ formation and its importance in Titan’s ionosphere.65,66 It has also been proposed elsewhere67 that upon exposure to cosmic rays, methylamine can form both • CH2NH2 and CH3NH• fragments, which are presented in the EN photodissociation channels R4 and R8, respectively, to form CH2NH2+. The interconversion barrier between the two isomers was calculated by Knowles et al.67,68 at 1.83 eV (from • CH2NH2) to ascertain the stability of these radicals as they relate to possible amino acid precursors in space. This 1,2hydrogen transition loosely resembles the 1,2-hydrogen shift from structure [32] to the EN+• isomer [34], suggesting potential future interest in EN and [34] in astrophysical research. G


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formation of the CH2NH2+ ion to be 750.3 ± 1 kJ mol−1, which was used in the determination of the neutral methylamine radical fragment. The 298 K heat of formation of the •CH2NH2 fragment is provided in the Active Thermochemical Tables (ATcT)85−87 at 148 ± 1 kJ mol−1. Aside from this, current thermochemical data of •CH2NH2 is believed to be limited to Burkey et al.’s74 calculated ΔfHo298 at 151 kJ mol−1 using available experimental values. An isodesmic reaction is used to calculate the heat of formation of •CH2NH2 using theoretical calculations and experimental values referenced in Table 1.

imentally well-known and listed in Table S3. Averages and combined errors were determined as described in a previous TPEPICO thermochemical investigation.76 HOCH 2CH 2OH + 2CH3NH 2 → NH 2CH 2CH 2NH 2 + 2CH3OH Δf H o298[NH 2CH 2CH 2NH 2] = −18.8 ± 0.9 kJ mol−1 (2)

CH3CH 2CH 2CH3 + 2CH3NH 2 → NH 2CH 2CH 2NH 2 + 2CH3CH3

•

CH3 + CH3NH 2 → CH4 + •CH 2NH 2

Δf H o298[NH 2CH 2CH 2NH 2] = −19.7 ± 0.9 kJ mol−1

Δf H o298[•CH 2NH 2] = 150.1 ± 0.6 kJ mol−1

(3)

(7)

ΔfHo298 [•CH2NH2]

The TPEPICO experimental = 147.6 ± 3.7 kJ mol−1 is in excellent agreement with the isodesmic, experimental, and literature values and serves as a good calibration for the experimental heat of formation of CH2NH3+. The R5 channel produces the CH2NH3+• ion and the neutral CH2NH fragment. A wide range of values are reported in the literature88,89 for the heat of formation of the neutral fragment, from 69 kJ mol−1 by Peerboom et al.90 to 110.46 kJ mol−1 by DeFrees et al.91 In 2001, Oliveira et al.81 aimed to reduce the uncertainty by using the W2 thermochemical method, reporting the theoretical standard enthalpy at 21.1 ± 0.5 kcal mol−1 (88.7 ± 2.1 kJ mol−1), and the ATcT85−87 reports the value at 88.7 ± 0.98 kJ mol−1, as well. An isodesmic reaction was explored to determine a CBSQB3, theoretical-experimental hybrid heat of formation of CH2NH, for which the known literature values for pertinent species are provided in Table S3:

CH 2CHCH3 + 2CH3NH 2 → NH 2CH 2CH 2NH 2 + CH 2CH 2 + CH4 Δf H o298[NH 2CH 2CH 2NH 2] = −19.7 ± 1.0 kJ mol−1 (4)

CH3CH3 + 2CH3NH 2 → NH 2CH 2CH 2NH 2 + 2CH4 Δf H o298[NH 2CH 2CH 2NH 2] = −19.1 ± 0.8 kJ mol−1 (5)

CH3NHCH3 + CH3CH3 + NH3 → NH 2CH 2CH 2NH 2 + 2CH4 Δf H o298[NH 2CH 2CH 2NH 2] = −19.0 ± 0.7 kJ mol−1 (6)

With the use of this method, the average Δ f H 298 [NH2CH2CH2NH2] = −19.2 kJ mol−1 with an uncertainty of ± 2.7 kJ mol−1 was calculated and used in conjunction with TPEPICO to determine the reverse barriers and heats of formation of photodissociative fragments. Appearance energies only correspond to the thermochemical limit in the absence of a reverse barrier. Available gas-phase heats of formation of fragments are presented in Table 1 and these literature ΔfHo298 were used, along with that of the parent ethylenediamine, to determine the ΔrHo298 for each channel. This value was then converted to ΔrHo0 using thermal enthalpy values provided in the literature77 or CBS-QB3 calculated values (thermal correction to enthalpy).78 While thermochemical data are scarce for several fragments, the heats of formation for both m/z = 44 (NH2CHCH3+ and c-C2H4NH2+) are available in the literature. The reverse barriers for R3 and R6 channels were calculated as the difference between the 0 K appearance energies involving barriers and the ΔrHo0 using experimental ΔfHo298, corrected to 0 K, and are reported as 0.39 ± 0.08 and 0.52 ± 0.14 eV (37 ± 8 and 50 ± 14 kJ mol−1), respectively. The R4 channel yields the methylenimmonium ion fragment [20], CH 2 NH 2 + , and the neutral methylamine radical, • CH2NH2. Previous experiments have been published on methylamine (CH3NH2)65,66 and ethylamine cation79 dissociative photoionization, where the CH2NH2+ ion fragment is formed via H-loss and CH3-loss, respectively. Many high-level theoretical and experimental values for the heat of formation of this ion are available in the literature43,75,80−84 and a few are listed in Table 1. Bodi et al.65 conducted TPEPICO experiments on primary amines and determined the heat of o

HNCO + CH 2CH 2 → CH 2NH + CH 2CO Δf H o298[CH 2NH] = 89.0 ± 0.4 kJ mol−1

(8)

The derived heat of formation from the isodesmic reaction is in close agreement with the reported values.81,85 Thermochemical data regarding the CH2NH3+• ion are much less available in the literature than that of the CH2NH neutral. In 1983, Bouma et al.92 revealed results of ab initio calculations that placed the heat of formation of CH2NH3+• at 199 kcal mol−1 (832.6 kJ mol−1). Holmes et al.93 measured the heat of formation of CH2NH3+• through collision induced dissociation mass spectrometry and determined the total m/z = 31 signal to be the 13C and 15N isotopologue of m/z = 30 up to 10.9 eV, concluding the heat of formation must be higher than 229 kcal mol−1 (958 kJ mol−1), though no specific value was reported. This is in contrast with the most recently provided gas-phase enthalpy of formation83 at 855 kJ mol−1, which was determined using MO methods at the MP4/6-31+G(2df,p) level.94 An isodesmic reaction network was created to establish another baseline for comparison of the enthalpy of formation of CH2NH3+• ion, using literature thermochemical data available in Table S3. •

CH 2NH 2 + NH4 + → CH 2NH3+• + NH3

Δf H o298[CH 2NH3+•] = 848.6 ± 1.1 kJ mol−1

(9)

•

CH 2NH 2 + H 2NO+ → CH 2NH3+• + HNO

Δf H o298[CH 2NH3+•] = 847.2 ± 1.4 kJ mol−1 H

(10)



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The resulting ΔfHo298 [CH2NH3+•] values were averaged, and the average value was determined to be 848.0 ± 0.9 kJ mol−1, falling between the two previously reported theoretical values. The experimental 0 K appearance energy of 9.8 ± 0.1 eV from the current TPEPICO experiment was used along with literature values for the EN parent and the neutral •CH2NH2 to arrive at the enthalpy of formation of the CH2NH3+• ion. We report the experimental ΔfHo298 [CH2NH3+•] at 844 ± 10 kJ mol−1 from ethylenediamine photodissociation, in serendipitous agreement with the theoretical value from the reaction network calculations.

ACKNOWLEDGMENTS This work has been funded by the National Science Foundation (CHE-1266407). The authors would like to thank the University of San Francisco faculty development fund for financial assistance, professors Claire Castro and William Karney for their support and use of the USF chemistry computer cluster, and Krisztián G. Torma for assistance in TOF peak deconvolution.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b03516. Full potential energy surface diagram, the structures of the ions, intermediates, TSs, and their respective geometries (PDF)

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REFERENCES

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CONCLUSIONS Threshold photoelectron photoion coincidence spectroscopy experiments were conducted on ethylenediamine in the 8.60− 12.50 eV photon energy range, where eight dissociation channels were modeled: NH 3 -loss, H-loss, NH 2 -loss, CH2NH2-loss, CH2NH-loss, a reappearing NH2-loss, a second H-loss, and CH3NH-loss. An additional pathway was observed: a consecutive dissociation involving H-loss followed by CH2NH-loss. The ninth dissociation was not modeled due to low fractional ion abundance. Eight of the nine channels involved initial rotational barriers leading to the low-energy anticonformation of the EN cationic state. The NH2- and NH3loss pathways involved functional group migrations due to intramolecular hydrogen attractions, highlighting the influence of these interactions on the photodissociation processes. The H-loss pathways involve several rearrangements, including Hmigrations and functional group migrations prior to dissociation. CH2NH2+ is the dominant product of EN dissociation within the scanned photon energy range and forms neutral • CH2NH2 via C−C bond cleavage at lower energy, while the ion is again formed at higher energy after H-migration to form the CH3NH neutral radical fragment. The CH2NH-loss pathway yields the CH2NH3+ ion, where the •CH2NH2 fragment rotates to form a hydrogen-bonding stabilized van der Waals complex that facilitates the transfer of an H to form the proposed products. Several of these fragments had not been detected in ethylenediamine dissociation. Isodesmic reactions and networks were used to calculate the heats of formation of ethylenediamine, •CH2NH2, CH2NH3+•, and CH2NH to validate the use of literature thermochemical data as anchors in the determination of TPEPICO-derived heats of formation of •CH2NH2 and CH2NH3+. The 0 and 298 K heats of formation are reported in addition to the reverse barriers for both NH2-loss channels.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: bSztáray@pacific.edu. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. I


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