Low-Energy Photoelectron Spectrum and Dissociative Photoionization

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Low-Energy Photoelectron Spectrum and Dissociative Photoionization of the Smallest Amides: Formamide and Acetamide Andras Bodi, and Patrick Hemberger J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b10373 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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The Journal of Physical Chemistry

Low-energy Photoelectron Spectrum and Dissociative Photoionization of the Smallest Amides: Formamide and Acetamide

Andras Bodi* and Patrick Hemberger Laboratory for Synchrotron Radiation and Femtochemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland

Abstract The threshold photoelectron spectrum and low-energy dissociative photoionization processes of formamide and acetamide were studied using photoelectron photoion coincidence spectroscopy and vacuum ultraviolet synchrotron radiation. Ab initio calculations and Franck– Condon simulations helped us assign the main vibrational progressions in the spectra and enabled the first conclusive assignment of the first electronically excited states. The adiabatic ̃ + and 𝐀 ̃+ states of formamide (10.236 ± 0.004 eV and 10.643 ± ionization energies to the 𝐗 0.015 eV) and acetamide (9.734 ± 0.008 and 10.282 ± 0.020 eV) have been re-evaluated and spectroscopic transitions were assigned using a Franck–Condon approach. The cationic potential energy surface was explored to rationalize the observed fragmentation patterns and to construct a statistical model, which was fitted to the experimental breakdown diagram. Thermochemical thresholds were measured and calculated for H, CO, and NH2 loss from HCONH2+ as well as for CH3, NH2, CO, HCCO, and NH3 loss from CH3CONH2+. We present the first comprehensive, experimental and theoretical treatise of these fragmentation processes. ̃ + and 𝐀 ̃+ states in The statistical model confirms fast internal conversion between the 𝐗 formamide, as H-transfer in CO loss is shown to take place on the excited state surface. It also explains the five almost simultaneously opening dissociation channels in the acetamide cation quantitatively. The derived 0 K appearance energies have been confirmed by ab initio calculations and by comparison with state-of-the-art thermochemical data, and revise some of the previous results by more than ten times their stated uncertainty.

*

E-Mail: [email protected]

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Introduction Formamide, HCONH2, may have played a role in the onset of both pregenetic and premetabolic processes.1 Hudson et al. proposed an abiotic pathway for adenine and purine synthesis from formamide.2 Jones et al. simulated the effects of galactic cosmic radiation in ultracold ices, and proposed a mechanism for formamide formation from carbon monoxide and ammonia,3 which was recently revisited by Bredehöft in the presence of slow electrons.4 Mendoza et al. detected formamide and isocyanic acid (HNCO) in protostellar regions, and suggested that the former is in fact the hydrogenation product of the latter on icy grain mantles,5 a conclusion with which Lopez-Sepuclre et al. concurred.6 Similarities between interstellar and cometary chemistry, however, suggest that gas phase chemistry could also play an important role in the formation of organic volatiles.7 Formamide is considered to be one of the few starting materials in prebiotic chemistry, which makes its ion chemistry quite interesting also from the point of view of reaction pathways to form biomolecules.8 While formamide is the simplest molecule containing the peptide prototype NH–C=O linkage, the nature of the peptide bond and the smallest chemically representative species is a matter of debate.9 Acetamide, CH3CONH2, has nevertheless been termed the largest interstellar molecule with a peptide bond after the detection of its rotational transitions toward the Sagittarius B2 giant molecular cloud.10 It has also been found along with formamide on the 67P/Churyumov-Gerasimenko comet by Rosetta’s Philae lander more recently.11 Besides its astrochemical relevance, acetamide is also of fundamental interest, because the methyl group has a very low barrier to internal rotation (V3 = 25 cm–1).12 Consequently, the minimum energy geometry and even the planarity of the neutral may be in doubt.9 It is an intriguing question whether and how the virtually free methyl rotor in the neutral affects the photoelectron spectrum (PES). Neither the He II nor the latest He I spectrum show sufficiently resolved vibrational progressions that could be compared with Franck–Condon simulations.13,14 Schwell et al. reported but did not discuss the m/z 59 (i.e., acetamide) photoion mass selected threshold photoelectron spectrum (TPES) up to a photon energy of 10.6 eV, with the best resolved vibrational fine structure so far.15 The electronic structure of neutral molecules and the unimolecular dissociation mechanism of internal energy selected cations can be studied by imaging photoelectron photoion coincidence spectroscopy (PEPICO) using tuneable vacuum ultraviolet radiation.16 Internal energy selection is achieved by photoelectron kinetic energy analysis, which means that the internal energy deposited in the parent ion is set and scanned by tuning the photon energy. The 2 ACS Paragon Plus Environment

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experiment yields the threshold photoelectron spectrum and the breakdown diagram of the sample. Complex dissociation mechanisms can be unveiled based on the latter, as sequential and parallel processes yield differently shaped breakdown curves, characteristic of the internal energy distribution of the intermediate fragment ion or the relative rate constant of the competing dissociation channels, respectively.17,18 In favorable cases, PEPICO experiments may yield sub-kJ·mol–1 thermochemical data, insights into the impact parameter or an accurate barrier to the reverse, association reaction of the fragmentation process observed.19–22 For example, this allowed us to compare the likelihood of the astrochemically relevant association reaction of HCN vs. C2H2 to C8H6+ to form the (iso)quinoline vs. naphthalene cations.23 The photoelectron spectrum of formamide was last studied in detail by ter Steege et al.,24 who reported adiabatic ionization energies to the ground and first excited electronic states of 10.233 ± 0.008 and 10.725 ± 0.020 eV, respectively. Leach et al. confirmed the first value as 10.220 ± 0.005 eV and revised the second to 10.55 eV in the first dissociative photoionization study of formamide.25 For the low-energy range, they also reported appearance energies for H loss, CO loss, and NH2 loss as 11.29 ± 0.01, 11.37 ± 0.02, and 13.11 ± 0.05 eV, respectively, based on photoionization yield curves, and relied heavily on previous mass spectrometric and computational studies in rationalizing the dissociation mechanism.26–28 Intriguingly, CO loss had been reported to take place on the first excited electronic state of the cation. We will show ̃+ state is not isolated and the dissociation is still statistical that while this is indeed the case, the 𝐀 because the ground and first excited states are strongly coupled. Arruda et al. reported the first photoelectron photoion coincidence experiment on the valence photoionization of formamide,29 which, in the absence of internal energy selection by electron kinetic energy analysis, is in fact directly comparable to the previous photoionization mass spectrometry results. Indeed, the reported ionization onset and H loss, CO loss, and NH2 loss appearance energies of 10.14, 11.24, 11.36, and 13.12 eV agree with the Leach et al. values. In contrast to NH2 loss, neither H loss, yielding NH2CO+ at m/z 44, nor CO loss, leading to NH3+ at m/z 17, takes place along a purely attractive bond breaking coordinate. Instead, C–H bond breaking is known to involve a reverse barrier on the order of 0.1 eV, similar in magnitude to H loss in the ethanol cation.30 On the one hand, barriers along H-loss reaction coordinates may be affected by tunneling, which means that the fragmentation may set in below the threshold energy. CO loss, on the other hand, must be preceded by H transfer to nitrogen, which also raises the possibility of a transition state energy in excess of that of the fragments. Tight transition states lead to slow dissociation at threshold, which induces a kinetic shift and a fragment ion appearance energy dependent on the 3 ACS Paragon Plus Environment


experimental time scale. Never before have these phenomena been addressed explicitly in formamide, which means that reported appearance energies may or may not correspond to an energy threshold in the fragmenting ion. This warrants revisiting the low-energy dissociative photoionization channels in an internal energy selected, threshold photoionization experiment capable of rate constant measurements, such as the Imaging Photoelectron Photoion Coincidence (iPEPICO) experiment31 at the VUV beamline of the Swiss Light Source.32,33 This enables us to address tunneling effects and kinetic as well as competitive shifts quantitatively and to establish physically well-defined threshold energies and relate them to the appearance energy of the fragment ions. Furthermore, in the discussion of the CO loss, Leach et al. drew significantly on the quantum chemical calculations carried out in the Terlouw group in the mid1990’s.27,28 Now, more than a quarter century later, even standard composite quantum chemical approaches are expected to outperform cutting-edge approaches from yesteryear in terms of precision and reliability. Thus, it is worthwhile to revisit Terlouw’s insights into the ion chemistry of formamide to confirm or revise them as necessary. Watanabe et al. reported the ionization energy of acetamide based on photoionization mass spectrometry.34 The isomerization dynamics of the acetamide cation has been addressed in several works both computationally and experimentally, by electron and chemical ionization and ion cyclotron resonance experiments also combined by tandem mass spectrometry.35,36 Schröder et al. pointed out that understanding the keto/enol tautomerization in the cation is essential to understanding the complex dissociative photoionization mechanism of acetamide, which yields five fragment ions in an energy window of about 0.5 eV.37 They devised a barrier height titration method using tuneable synchrotron radiation and chemical monitoring, and reported an enol reactivity onset of 10.42 ± 0.05 eV. This is lower than the reported lowest dissociative photoionization threshold to form NH4+ at m/z 18 and HCCO at 10.76 ± 0.07 eV37 or to form CH2NH3+ and CO at 10.71 ± 0.03 eV,15 suggesting that enol chemistry is accessible at the onset of dissociative photoionization. In fact, G2(MP2) calculations by Mourgues et al. suggested that the transition state to NH4+ production lies at the hydrogen transfer step from the hydroxyl group in the enol to the amine group.36 However, the dissociative photoionization of acetamide was first studied only recently in detail by Schwell et al. in a synchrotron-based coincidence photoionization experiment.15 They published appearance energies for low- and high-energy fragments, and reduced the error bars significantly. However, in the absence of rate constant measurements and internal energy selection, they could only surmise the physical significance of the observed fragmentation thresholds. 4 ACS Paragon Plus Environment

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Experimental and Computational methods Acetamide and formamide were purchased from Sigma–Aldrich. Their vapor was introduced into the iPEPICO endstation31 at the vacuum ultraviolet VUV beamline32,33 of the Swiss Light Source at room temperature through a high-throughput needle valve and a 6 mm outer diameter Teflon tube. Synchrotron radiation was collimated, dispersed by a 600 lines/mm laminar grating, and focused onto the 200 μm exit slit in a differentially pumped gas filter filled with 10 mbar of a Ne:Ar:Kr mixture to suppress higher order radiation above 14 eV. Monochromatic VUV radiation entered the ionization chamber with a bandwidth of 3 meV and ionized the sample at a pressure of 2 × 10–6 mbar. Photoelectrons and -ions were extracted from the ionization volume in a constant 120 V cm–1 field in opposite directions. In the iPEPICO experiment, photoelectrons are velocity map imaged onto a Roentdek DLD-40 delay line detector with a close to 1 meV resolution at threshold, i.e., zero kinetic energy. Coincident ion time-of-flight (TOF) analysis is achieved by using the electrons as start signal and accelerating the ions in a 55-mm first acceleration region, followed by a 10-mm second acceleration region, in which they reach their final potential of 1740 V. After passing a 550 mm drift region, they are detected using a 40 mm Jordan TOF detector in space focusing conditions. False coincidences do not correspond to electron/ion pairs emanating from the same single ionization event. The false coincidence background corresponds to a baseline in the mass spectrum,38 which can be subtracted. Close to zero kinetic energy electrons are imaged onto a central spot on the detector, and can be selected as start signal to obtain mass spectra. Kinetic energy electrons with negligible off-axis momentum are also imaged onto this spot. The resulting hot electron contamination can be subtracted based on a ring region around the central spot to obtain the true threshold photoelectron or photoionization mass spectrum.39,40 The long first acceleration region and the low electric field lead to typical cation residence times of several µs. When metastable ions dissociate on this time scale, this yields asymmetrically broadened daughter ion peak shapes toward higher TOF, from which rate information can be extracted and the rate curve modelled using statistical theory to quantify the kinetic shift.19 When the TOF offset of the daughter ion is comparable to the instrumental peak width, e.g., in the case of H loss, the center-of-gravity shift of the daughter peak has been shown to be an equally good measure of the dissociation rates.41 In formamide and acetamide, all fragment ion peaks were observed to be symmetric, and their center does not shift as a function 5 ACS Paragon Plus Environment


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of energy. Therefore, only the breakdown diagram yields fragmentation branching ratios, i.e., relative rate information. Threshold photoionization mass spectra can be reduced by plotting the fractional parent and daughter ion abundances as a function of photon energy in the breakdown diagram. At room temperature, the internal energy of the neutral may also be used in the dissociation, meaning that the breakdown diagram will be broadened to lower photon energies with respect to the threshold, effectively measuring the internal energy distribution of the sample.42,43 If multiple dissociation channels compete, the relative daughter ion abundances in the breakdown diagram are characteristic of the branching ratios, i.e., of the ratios of the dissociation rates. Sequential dissociation reactions have not been observed in formamide and acetamide below a photon energy of 13.5 eV. In the statistical model,19 the internal energy distribution of the neutral, the density of states of the parent ion and the number of states of the transition states are calculated based on rotational constants and harmonic vibrational frequencies to obtain an ab initio rate curve and, based on ion optics parameters, emulate the experimental results. Compared with methanol and the methylperoxy radical,43,44 acetamide has an internal methyl rotor with an even lower barrier, which was handled as a Pitzer rotor45 in calculating its internal energy distribution. We have used two statistical approaches to calculate the dissociation rates, the rigid activated complex Rice–Ramsperger–Kassel–Marcus (RAC-RRKM) and the simplified statistical adiabatic channel models (SSACM). The same rate equation is employed in both approaches: 𝑘(𝐸) =

𝜎𝑁‡ (𝐸−𝐸0 ) ℎ𝜌(𝐸)

,

(1)

where 𝜎 is the symmetry number of the dissociation, 𝑁 ‡ (𝐸 − 𝐸0 ) the number of states of the transition state at excess energy 𝐸 − 𝐸0 , ℎ Planck’s constant and 𝜌(𝐸) the density of states of the fragmenting ion at internal energy 𝐸.46 RRKM and SSACM differ in how they address the number of states of the transition state, i.e., the size of the bottleneck between the reactant and the products in phase space. In RRKM, the number of states is evaluated at a transition state geometry, which can correspond to a saddle point on the potential energy surface in the case of a real transition state, or to an arbitrary geometry along the bond dissociation coordinate in question. The rate curve is then adjusted by scaling the 2–5 transitional frequencies by a common factor. In contrast, SSACM is an extension of phase space theory, in which the products are assumed to represent the bottleneck. The anisotropy of the potential energy surface is accounted for by an energy-dependent rigidity factor, which is used to scale the transition 6 ACS Paragon Plus Environment

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state number of states.47 The SSACM approach has been shown to be superior to the RRKM extrapolation to threshold when the dissociation takes places along a purely attractive potential energy curve, i.e., without a saddle point.48,49 Therefore, we used RRKM to calculate the rate curves in dissociations taking place via a well-defined activated complex and SSACM to model them in dissociations with a rate determining step at a purely attractive region of the potential energy surface in the absence of a true transition state. Quantum chemical calculations support the data analysis in several ways. The statistical model is based on rovibrational densities and numbers of states, which are based on ab initio frequency calculations. Only a minute part of the potential energy surface (PES) can be explored in all but the smallest molecules. As illustrated by the previously missed formaldehyde-loss channel in neutral dimethyl methylphosphonate,50,51 and by the counterintuitive dissociative photoionization mechanism of dimethyl carbonate, which yields larger fragments with increasing internal energy,52 experimental data are an essential guide to the exploration of the PES, even in small systems. Nevertheless, once a feasible mechanism is found, advanced computational chemistry approaches can confirm the experimental results, and may even be comparably or even more accurate if large kinetic or competitive shifts complicate the experimental data analysis. The potential energy surface of the cation and the methyl internal rotation in the neutral acetamide were explored using density functional theory at the B3LYP/6-311++G(d,p) level of theory and Gaussian 16.53 The total 0 K energy of the stationary points were then refined using the G4 and W1U(sc) composite methods for transition states,54–56 and, additionally, CBSAPNO for minimum geometries.57 DFT and Hartree–Fock calculations erroneously yield ̃+ (𝐴ʺ) minimum of both the formamide and excited state energies and wave functions at the 𝐀 acetamide cations, which allowed us to obtain the energetics of the first excited state using composite methods, as well. We have also addressed the methyl internal rotation in acetamide and the question of the minimum energy geometry by further coupled cluster calculations, up to CCSD(T)/cc-pVQZ for valence and CCSD/aug-cc-pCVTZ for core correlation, including Douglas–Kroll–Hess 2nd order scalar relativistic corrections at the all electron CCSD/MTSmall level,58 as well as diagonal Born–Oppenheimer corrections at the CCSD/cc-pVDZ level59 using Q-Chem 4.3 and CFOUR60,61 in addiction to Gaussian 16. Interpreting photoelectron spectra to extract ionization energies and rationalize the vibrational fine structure is made possible by Franck–Condon simulations, in which the nuclear wave function overlap is calculated between the neutral and the ion vibrational states. This may 7 ACS Paragon Plus Environment


even apply to broad and barely structured Franck–Condon envelopes, as was recently shown for the methyl peroxy radical,43 and can identify overlapping electronic states as, e.g., in dichloromethane.62 The first two electronic states overlap in the first TPES band of both formamide and acetamide, and, thanks to harmonic Franck–Condon calculations, we will be ̃+ state.63 The consistency of B3LYP/6able to revisit their ionization energies to the 𝐀 311++G(d,p) results was confirmed by ωB97X-D/def2-TZVPP and MP2/def2-TZVPP frequency calculations.

Results and Discussion Threshold photoelectron spectra. The TPES of formamide and acetamide are shown in Figures 1 and 2, respectively, together with the results of harmonic Franck–Condon (FC) calculations. The B3LYP/6-311++G(d,p) and MP2/def2-TZVPP FC spectra for formamide are both characterized by a strong origin transition and an approximately 0.2 eV progression, due to two vibrations at 1610 and 1680 cm–1, corresponding to combinations of the NH2 bend and the NCO asymmetric stretching vibrations. Nevertheless, the agreement of the DFT-based FC simulation is somewhat better with the experiments than that of the MP2-based calculation. The calculated adiabatic ionization energy to the ground electronic state ([1a] in Fig. 1) is 10.211, 10.256, and 10.252 eV at the G4, CBS-APNO, and W1U(sc) levels of theory, respectively. The Stark shift in a continuous 120 V cm–1 extraction field is 8.3 meV.64,65 Although the TPES of small halogenated hydrocarbons has been found to be unaffected by such a broadening,66 the spectra of most organics is influenced by the continuous extraction field as expected.67 Therefore, the Stark shift has been accounted for to obtain the experimental value of 10.236 ± 0.004 meV. Leach et al. reported 10.220 eV,25 ter Steege et al. 10.233 eV24 as the adiabatic ̃ + (𝐴ʹ) electronic energy is 0.27 eV ionization energy to the ground state ion. Although the 𝐗 ̃+ (𝐴ʺ) energy at the 𝐀 ̃+ (𝐴ʺ) minimum energy geometry (B3LYP/6-311++G(d,p) below the 𝐀 TD-DFT value, see structure [1b] and Figure 4 for the σ and π type neutral molecular orbitals, which correspond to ionization to these states), the wave function can be trapped in 𝐴ʺ symmetry, which allows us to employ electronic ground state computational approaches to ̃+ (𝐴ʺ) state energy and photoelectron spectrum. The computed adiabatic energy address the 𝐀 to the first electronically excited state of formamide is 10.674, 10.640, and 10.674 eV at the G4, CBS-APNO, and W1U(sc) levels of theory, respectively. While the origin transition is predicted to be the most intense by the Franck–Condon simulation (see Fig. 1), both B3LYP and MP2 ground state FC calculations predict a ground state band in this energy range, as well, at somewhat less than half the intensity of the first two dominant peaks in the TPES. However, 8 ACS Paragon Plus Environment

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the experimental TPES is faithfully reproduced by the FC calculation at higher energies of the ̃+ (𝐴ʺ) band if the origin band spectrum is shifted to 10.643 ± 0.015 eV, and assuming that the 𝐀 ̃+ ← 𝐗 ̃ and 𝐀 ̃+ ← 𝐗 ̃ transitions. The 𝐀 ̃+ peak at 10.63 eV is the result of the superposition of 𝐗 state spectrum is characterized by vibronic transitions in the ion’s OCN bend mode at 550 cm– l

and the NH2(CHO) umbrella mode at 960 cm–1. The latter also arises in combination with

excitations in the neutral molecule, because it has a much lower energy at 230 cm–1 and is already excited at room temperature. Based on the shape of the photoionization spectrum, Leach et al. suggested an ionization energy to the first excited state of 10.55 eV.25 The corresponding value reported by ter Steege et al. was 10.725 ± 0.020 eV24 vs. the value reported by Siegbahn et al. at 10.699 or, alternatively, at 10.477 eV.68 Among these studied, Siegbahn et al. in fact ̃+ state progression, but correctly identified the peak at 10.699 eV as the second peak in the 𝐀 could not determine the fundamental energy correctly and overestimated it by ca. 0.2 eV. This assignment history of the second ionization energy of formamide serves to illustrate how difficult it is to identify vibrational progressions in photoelectron spectra of even small molecules without modeling the band. If a rovibrationally resolved high-resolution spectrum is lacking, it is almost impossible to determine which spectral feature belongs to which state per se, and to assign overlapping peaks belonging to different electronic states. The acetamide TPES, as shown in Figure 2 together with FC simulations based on B3LYP/6-311++G(d,p), MP2/def2-TZVPP, and ωB97X-D/def2-TZVPP calculations, is more congested than that of formamide, due to the larger number of FC active normal modes, in particular the methyl rotation coordinate. In addition to the NH2 bend and NCO asymmetric ̃ + (𝐴ʹ) state spectrum stretch combinations at 1620 and 1690 cm–1, the coarse structure of the 𝐗 of acetamide is also characterized by the 370 cm–1 CH3 rocking mode. The modelled multiplet fine structure of the first band is characteristic of the methyl internal rotation upon ionization, and has recently been observed in the high-resolution electronic spectrum of the o-xylyl radical, as well.69 In the neutral minimum geometry at the B3LYP/6-311++G(d,p) level of theory, one methyl hydrogen is anti-periplanar with the oxygen atom [2b]. In the ground state cation, a methyl hydrogen is synperiplanar with the oxygen atom (see below), [2a] in Fig. 2, which involves a 30° rotation of the methyl group. Similar to the o-xylyl case, in lieu of an exact treatment of the methyl internal rotation, we accept some added uncertainty as far as the origin transition is concerned. Based on maximizing the overlap of the first peak in the B3LYP, MP2, and ωB97X-D FC calculations and taking into account the Stark shift, the experimental acetamide ionization energy is 9.730, 9.738, and 9.734 eV, respectively, i.e., IEad = 9.734 ± 9 ACS Paragon Plus Environment


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0.008 eV, in agreement with the PEPICO result of Schwell et al., 9.71 ± 0.02 eV,15 and superseding past determinations as reviewed by Schwell et al. The computed values of 9.703, 9.736, and 9.753 eV using the G4, CBS-APNO, and W1U(sc) composite methods, respectively, are also in excellent agreement with the experimental result. As will be discussed below, the methyl torsional change upon ionization is less than certain, but the good agreement between experiment and simulation suggests that the remaining modes are well reproduced by assuming [2a] and [2b] to be minima in the cation and in the neutral, respectively, while the methyl rotation has little influence on the overall appearance of the TPES. If the acetamide cation geometry optimization is started at the neutral optimized B3LYP methyl group orientation, the wave function does not leave the 𝐴ʺ symmetry and the calculation ̃+ (𝐴ʺ) electronic state (see [2b]). Thus, here again, we can use ground state yields the 𝐀 computational approaches to calculate the energetics and the Hessian matrix to simulate the photoelectron spectrum. G4, CBS-APNO, and W1U(sc) composite methods yield an excited state ionization energy of 10.256, 10.283, and 10.284 eV, respectively. In addition to the methyl internal rotation, bond lengths also change, as summarized in Table 1. These results help match ̃+ (𝐴ʺ) state to the experimental peak observed at the first peak in the FC simulation of the 𝐀 10.28 eV. The most intense transitions belong to the excitation of the O rocking and NH2CCH3 bending modes at 520 and 410 cm–1, respectively, and the modeling yields a tentative acetamide ̃+ (𝐴ʺ) ionization energy of 10.287 ± 0.010 eV. Schwell et al. estimated this quantity as 10.1 𝐀 eV, and it is evident from the TPES and the modeling that one has to rely heavily on the ab ̃+ ← initio ionization energy as well as the Franck–Condon simulations in assigning the origin 𝐀 ̃ transition in acetamide. 𝐗 Table 1. Bond lengths between heavy atoms in formamide as well as acetamide neutral and cationic states, optimized at the B3LYP/cc-pVTZ+d level of theory.

R(C=O) ̃(𝐴ʹ) 𝐗 ̃ + (𝐴ʹ) 𝐗 ̃+ (𝐴ʺ) 𝐀

1.21 1.26 1.25

Formamide R(C–N) 1.36 1.29 1.34

R(C–H) /Å 1.11 1.10 1.09

R(C=O)

Acetamide R(C–N)

1.22 1.26 1.28


1.36 1.30 1.33

R(C–C) 1.52 1.51 1.48

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Acetamide structure. Demaison et al. discussed the lack of planarity of the prototypical peptide linkage in detail and suggested that formamide was an imperfect example because of its undisputed planarity. The acetamide minimum geometry, on the other hand, was found to be a perpendicular structure, with the methyl rotation between [2a] and [2b] and a slightly pyramidal nitrogen center.9 Be that as it may, the orientation of the methyl group at the minimum energy geometry changes significantly upon ionization. The 60° methyl rotation in the ionization of the t-butyl and the methyl peroxy radicals was shown to affect the Franck– Condon envelope significantly, complicating the assignment of the origin transition.43,70 This prompted us to revisit this issue briefly. By varying the methyl torsional angle, a non-symmetric and two symmetric structures were optimized both in the cation and in the neutral at the CCSD/cc-pVTZ level of theory. The electronic energy was lowest for the non-symmetric structure, confirming the non-planar global minimum on the potential energy surface. This was also in accord with cheaper, M06-2X/def2-TZVPP results. We further evaluated the electronic energy at the CCSD/cc-pVTZ optimized geometries by accounting for core correlation at the CCSD/cc-p(C)VTZ level, including approximate triples contributions at the CCSD(T)/ccpVQZ level, as well as relativistic corrections as implemented in the W1U method and diagonal Born–Oppenheimer corrections at the CCSD/cc-pVDZ level of theory. The results are summarized in Table 2. Structure [2a] is then found to be 1.5 meV and [2b] 2.3 meV less stable than the non-symmetric optimum in the neutral, which can be compared with the W1-computed relative electronic energies at the CCSD-optimized geometries of –2.1 and 0.1 meV, respectively. In other words, the W1 composite method predicts the symmetric [2a] structure to be the overall minimum on the PES, but both approaches suggest that the methyl group is essentially a free rotor. The relative energies for the [2a] and [2b] conformers relative to the non-symmetrical one in the cation are 1.1 and 6.2, as well as 8.6 and –1.7 meV using the same coupled-cluster and W1U approaches, respectively, indicating a less free rotor but a larger uncertainty in the calculated energies. The picture changes fundamentally for the neutral when the zero-point corrections are added after removing the methyl torsional mode. As shown in Table 2, the global minimum then appears to be the synperiplanar [2a] structure by a significant margin. These results suggest that the total energy may in fact be minimal at a CS geometry, but a fundamentally non-Born–Oppenheimer approach may be needed to settle this question reassuringly.



Table 2. Relative conformational energies in acetamide neutrals and cations with or without zero-point energy corrections using our coupled-cluster (CC) approach or the single-point W1 composite method calculations at the CCSD-optimized geometries.

Non-symmetrical [2a] [2b]

Neutral / meV w/o ZPE with ZPE CC W1 CC W1 0.0 0.0 0.0 0.0 2.3 0.1 –13.8 –16.0 1.5 –2.1 –10.9 –14.4

Cation / meV w/o ZPE with ZPE CC W1 CC W1 0.0 0.0 0.0 0.0 6.2 –1.7 8.3 0.3 1.1 8.6 7.6 15.1

Dissociative ionization. Reaction energy profiles and ab initio input are necessary to model the breakdown diagram and understand the significance of the obtained threshold energies. In the case of formamide, three parallel dissociation channels open up below a photon energy of 13.5 eV. The lowest energy channel is H loss, which is quickly outcompeted by the loss of 28 amu, most likely CO, to yield the ammonia cation at m/z 17. At energies above 11.7 eV, H loss starts to rise again, and a new channel at m/z 29 appears at 12.5 eV, assigned to the formation of HCO+. As reviewed by Leach et al., the C–H bond is broken in H loss, which is associated with a reverse barrier as the O=C=N bond angle changes to 180°.25 Ruttink et al. calculated a 191 meV reverse barrier and measured it to be 130 meV. Their experimental value is corroborated by our computed result of 140 meV (Figure 4). There being general agreement about the chemical identity of the fragments, we refer the reader to the discussion by Leach et al. on how to rule out alternative, isobaric fragment ions.25 The highest energy channel in the observed photon energy range, the loss of NH2, takes place along a purely attractive potential. The calculated threshold, 12.42 eV, is much lower than the experimental value of Leach et al., 13.11 ± 0.05 eV,25 or the electron ionization value of 13.70 eV.71 This will be further discussed below in the light of the statistical model. Last, our calculations confirm the Ruttink et al. ̃ + (𝐴ʹ) state is mechanism for the production of NH3+ at m/z 17: The H-transfer barrier on the 𝐗 ̃+ (𝐴ʺ) state instead, where it too high at 12.38 eV, and hydrogen migration takes place on the 𝐀 is allowed at already 11.50 eV, only 50 meV higher than the H-loss barrier. The energy diagram shown in Figure 4 and the computed rotational constants and vibrational frequencies form the starting point of the statistical model fit to the experimental data (see below). As seen in the acetamide breakdown diagram in Figure 5 and pointed out by Schröder et al.,37 five parallel dissociative ionization channels open up in the 10.7–11.2 eV photon energy 12 ACS Paragon Plus Environment

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range. These are, in order of appearance, CO loss yielding CH2NH3+, HCCO loss leading to NH4+, CH3 loss producing NH2CO+, NH2 loss forming CH3CO+, and NH3 loss resulting in CH2CO+. Until now, the most detailed ab initio potential energy surface describing these channels was presented by Mourgues et al. at the G2(MP2) level of theory, although without the CO-loss channel.36 Our results, shown in Figure 6, include the CO-loss pathway and offer a more detailed view of the stationary points and an improved accuracy. Nonetheless, they generally confirm the G2(MP2) results of Mourgues et al. In particular, we calculate the barrier to enol formation in the cation to be 10.880 eV above the neutral, which can be compared to 10.892 eV, the value of Mourgues et al. However, Schröder et al. reported enolic reactivity at already 10.42 ± 0.05 eV.37 This discrepancy in the appearance of enol reactivity and the COloss appearance energy could in part be due to tunneling. However, a second H-transfer isomerization step was found to involve the most energetic transition state to HCCO and CO loss, and the experimental appearance energies agree very well with the calculations without considering tunneling, which calls into question its role in the first H-transfer step, too. Therefore, tunneling is proposed not to play a significant role in the dissociative photoionization of either system (see also below). Statistical model. Based on the computed reaction energy curves, we created a combined SSACM/RRKM model to fit the breakdown diagram of formamide and acetamide. SSACM was used in the absence of a rate determining saddle point along the dissociation coordinate. This is the case for NH2 loss in formamide as well as CH3 and NH2 loss in acetamide ions. H and CO loss in the formamide cation is clearly associated with an activated complex, the harmonic vibrational frequencies of which were used to calculate the transition states’ number of states in the RRKM approach. While the fact that the fractional abundance of NH3+ quickly overtakes that of NH2CO+ together with the computational result that the former takes place on the excited electronic state may be interpreted as a sign for isolated state behavior, the breakdown diagram of formamide (Figure 3) is very well reproduced by the statistical model using a single transition state for each channel. As the rate curves show, H loss is much slower than CO loss at first, and dominates only below the CO-loss threshold. As the energy is increased, the H-loss rate constants rise more rapidly, which is responsible for its slowly rising abundance above 11.7 eV. This also means that the formamide cation dissociation mechanism passes the duck test72 of statisticality,52 i.e., the branching ratios are determined by the relative phase space volume at the transition state for H and CO losses. Consequently, coupling between the ground and first excited electronic states is strong enough so that both transition states are equally accessible irrespective of the original ion state. The NH3-, HCCO- and CO-loss 13 ACS Paragon Plus Environment


processes from the acetamide ion take place after the system surpasses the [8]‡ transition state. This is a similar situation to the recently studied dissociative photoionization of adamantane,73 in which the reactive flux branched after surpassing a high-lying transition state to yield lower lying fragments. However, the ketene cation at m/z 42, formed by NH3 loss, lies higher than the transition state, albeit it is the least intense channel in the studied energy range. We assume that these three channels may be described by an RRKM model with nearly identical thresholds to NH4+ and CH2NH3+ formation. In the energy range of CH2CO+ production, the common rate determining step of the three channels will still be at the [8]‡ activated complex, which is why we also employ RRKM to model this process. Ruttink et al. reported that, with respect to the alternative, CO-loss product, D loss from DCONH2+ yielded a ca. 40 times lower fragment abundance than H loss from HCONH2+ or HCOND2+.28 Similar to the dissociative photoionization of acetone, in which methane loss is quantitatively suppressed by deuteration,74 the missing D-loss signal may be due to suppressed tunneling across the barrier, an explanation also preferred by Ruttink et al. However, an experimental and a theoretical argument speak against large tunneling contributions: (1) Tunneling rates must rise to 104–105 s–1 in order to contribute to the fragment ion signal significantly on the time scale of the mass analysis. Yet we have not observed a center-ofgravity shift in the H-loss fragment ion, which implies no kinetic shift and a fast dissociation at threshold. In H-tunneling in the ethanol cation across a small H-loss reverse barrier, such a kinetic shift was readily observed,30 which indicates experimentally that tunneling is not a major contribution to H loss in formamide. (2) Furthermore, the calculated critical frequency is rather low at 517 cm–1 (B3LYP/cc-pVTZ+d result), and the corresponding normal mode also involves the linearization of the O=C–N bond angle, which means that the geometry change at the transition state geometry affects heavy atoms, which are much less likely to tunnel. D loss may also be suppressed because of a zero-point energy shift. The CO loss energetics is unaffected by deuteration but H-loss is shifted to higher energies by ca. 0.082 eV (B3LYP/cc-pVTZ+d result), which means that the energy window in which H loss is competitive with CO loss is mostly closed upon deuteration of the carbon hydrogen. Indeed, by raising the H-loss transition state energy by this much, the H-loss abundance can be quantitatively suppressed at low energies, confirming that tunneling effects are not needed to explain the suppressed D-loss channel in the dissociative photoionization of deuterated formamide.


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Table 3. Experimental and computed ionization and 0 K appearance energies (E0). Thermochemical dissociative photoionization (∆rHo0K) energies are also given for comparison where relevant.

Formamide ̃ + HCONH2+ 𝐗 ̃+ HCONH2+ 𝐀 NH2CO+ + H NH3+ + CO HCO+ + NH2 Acetamide ̃ + CH3CONH2+ 𝐗 ̃+ CH3CONH2+ 𝐀 NH2CO+ + CH3 CH3CO+ + NH2 CH2NH3+ + CO NH4+ + HCCO CH2CO+ + NH3

E0 / eV This work Expt. Calc. 10.236 10.240

± 0.004

10.643 11.421 11.452 12.385

10.663 11.454 11.499 12.425

0.015 0.066 0.094 0.080

9.734 10.282 11.091 11.183

9.731 10.274 11.028 11.162

0.008 0.020 0.126 0.042

10.982

10.981 0.002

11.138

11.130 0.016

Thermochem. Literature ∆rHo0K

12.400

AE / eV Literature 10.220 ± 0.005,a 10.233 ± 0.008,b 10.03,c 10.14c a 10.55, 10.725 ± 0.020b 11.29 ± 0.01,a 11.28,c 11.24c 11.37 ± 0.02a, 11.36c a 13.11 ± 0.05, 12.44,c 13.12,c 13.70d

11.054 11.184

11.03–11.07

9.71 ± 0.02,e 9.68 ± 0.03f 10.10e 11.6,d 10.88 ± 0.03,e 11.00 ± 0.04f 11.7,d 11.19 ± 0.05,e 11.24 ± 0.05f 10.71 ± 0.03,e 10.77 ± 0.05f 10.77 ± 0.03,e 10.76 ± 0.07f 11.25 ± 0.05,e 11.13 ± 0.07f

This work’s calculations are average G4, CBS-APNO and W1U(sc) results for minima and average G4 and W1U(sc) results for transition states. Literature values were taken from aLeach et al.,25 bter Steege et al.,24 cArruda et al.,29, dLoudon and Webb,71 eSchwell et al.,15 and fSchröder et al.37

The measured and calculated appearance energies are compared with literature results in Table 3. The uncertainty is defined as double the difference between the experimental and computational result, which is probably a conservative estimate in most cases, except for the H-transfer barrier in the acetamide cation at 10.98 eV and the ammonia-loss appearance energy of 11.13–11.14 eV, as experiment and theory appear to agree serendipitously in these two cases. Furthermore, the error bars on the ionization energies refer to the results of the Franck–Condon simulations. The physical meaning of the previously reported dissociative photoionization appearance energies is often poorly defined, and in the absence of a quantitative treatise of the thermal and competitive shifts, they appear to be phenomenological thresholds for observing the reported fragment ions. In certain cases, they are nonetheless used to derive thermochemistry, or, alternatively, thermochemical thresholds are invoked to determine reverse barrier heights. While the results are often qualitatively and sometimes quantitatively correct, this is partly due to fortuitous error cancellation. On the other hand, the reported appearance energies for H or NH2 loss from the formamide cation are off by 14 times the given error bar, 15 ACS Paragon Plus Environment


which underlines the importance of modeling thermal and competitive shifts quantitatively. Overall, our agreement is best with the thresholds reported by Schröder et al. for acetamide.37 Whenever appearance energies correspond to the dissociative photoionization energy and not to the energy of the activated complex, they can be used to derive or to confirm ancillary thermochemical data. In the following, we used W1U(sc) calculated thermal enthalpies and the known elemental thermal enthalpies75 to convert room temperature heats of formation to 0 K when needed, and report 0 K values throughout. The enthalpy of formation of formamide is known at –179.7 ± 0.4 kJ mol–1,76 that of the formyl cation and amidogen are listed in the Active Thermochemical Tables (ATcT)77,78 as 827.802 ± 0.099 and 188.91 ± 0.12 kJ mol–1, respectively. Thus, the calculated appearance energy of HCO+, the only dissociative photoionization process of formamide without a reverse barrier, is established as 12.400 ± 0.004 eV, which agrees with the experimental result of 12.385 ± 0.080 eV. The acetamide heat of formation79 is converted to –223.02 ± 0.78 kJ mol–1 at 0 K. In its dissociative photoionization, three fragmentation processes are proposed to take place without a reverse barrier: NH2, NH3 and CH3 loss. The 0 K enthalpies of formation of CH3CO+ and CH2CO+ are readily available in ATcT. As is isocyanic acid, HNCO at –115.85 ± 0.85 kJ mol–1, for which we also consider two literature values at –117.6 ± 0.8 and –116.2 ± 1.4 kJ mol–1,80,81 combined with the W1Ucalculated protonation energy of –718.14 kJ mol–1 to yield H2NCO+, the product of CH3 loss in the acetamide cation. The dissociative photoionization energies for CH3CO+, CH2CO+ and H2NCO+ can thus be derived as 11.184 ± 0.011, 11.054 ± 0.008, and 11.03–11.07 eV, respectively, which are also given in Table 3. The generally good agreement between model, computations and thermochemistry corroborate the proposed mechanisms. Finally, the carbonyl group coordinates to an amine and a methyl group in acetamide, which means it is intriguing to compare the dissociative photoionization mechanism of acetamide with that of acetone74 and urea,81 containing two methyl and two amine groups, respectively. In the lowest energy channel of acetone, a hydrogen tunnels across the methyl groups slowly to form a methane neutral and the CH2CO+ cation. The analogous channel yields the NH3 neutral fragment in acetamide and takes place over the enol tautomer. Direct methyl loss becomes dominant in acetone at slightly higher energies and is also a major channel in acetamide, as is NH2 loss. In the urea cation, H-transfer is fast and yields isocyanic acid and an ammonia cation at the thermochemical threshold, a channel slowly outcompeted by direct NH2 loss 0.6 eV higher in energy. Thus, all three samples exhibit direct bond breaking as well as H-transfer initiated dissociation channels, which account for three out of five parallel acetamide cation 16 ACS Paragon Plus Environment

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decomposition channels. Additionally, the NH2 group in acetamide readily accepts further hydrogens/protons, which makes NH4+ and NH3CH2+ formation energetically favored, opening up the remaining two decay channels.

Conclusions The threshold photoelectron spectrum and the dissociative photoionization of formamide and acetamide have been measured and analyzed. We could model the TPES with the help of Franck–Condon simulations, and identify the main vibrational progressions as well as a definitive adiabatic ionization energy to the ground and first electronic excited state of both species. The latter values differed significantly from previous, in part tentative, assignments. In this context, we discuss the almost free methyl rotor in acetamide and the question of its planarity. Furthermore, given the correct starting geometry, electronic structure calculations may be trapped in the first electronically excited state of the cation, which allowed us to report accurate ionization energy calculations using composite methods, as well. With the help of ab initio exploration of the cationic potential energy surface and by fitting a statistical model to the experimental breakdown diagram, we are presenting the first self-consistent description of the low-energy fragmentation mechanism of formamide and acetamide cations. The experimental 0 K appearance energies are further corroborated by ancillary thermochemical data. The fragmentation mechanism of formamide is peculiar, because CO loss takes place on ̃+ (𝐴ʺ) state. Yet the breakdown diagram is well reproduced by a the electronically excited 𝐀 statistical model, which suggests that this state is not “isolated,” but rather couples very strongly with the ground cationic state, and both H- and CO-loss transition states are accessible independent of the original cation electronic state. In the case of acetamide, a surprising number of fragmentation channels compete, which is rationalized in terms of the potential energy surface.

Acknowledgements This project was funded by the Swiss Federal Office of Energy (SFOE, Contract Number SI/501269-01). The PEPICO experiments were carried out at the VUV beamline of the Swiss Light Source. We wish to thank Patrick Ascher (PSI) for technical support.



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Page 22 of 29

Page 23 of 29

1.29 Å

[1a]

1.35 Å

[1b]

TPES / arb. unit

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


̃+(𝐴ʺ) 𝐀 B3LYP/6-311++G(d,p)

MP2/def2-TZVPP

B3LYP/6-311++G(d,p)

10.1

10.2

10.3

10.4

10.5 10.6 hν / eV

10.7

10.8

10.9

11.0

Figure 1. Formamide TPES in black shown together with two ground state and an excited state FC calculations. The FC calculations were carried out at room temperature and the line transitions have been convoluted with a 40 cm–1 (4.9 meV) full width at half maximum (FWHM) Gaussian to facilitate the comparison with experiment. The dominant transitions in the B3LYP/6-311++G(d,p) FC calculations are also shown as stick spectra. N –C bond energies in the ground [1a] and first excited [1b] cation electronic states are also given. 23 ACS Paragon Plus Environment


[2b] [2a] ̃+(𝐴ʺ) 𝐀 B3LYP/6-311++G(d,p)

TPES / arb. unit

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 24 of 29

expt. ωB97X-D/ def2-TZVPP

MP2/def2-TZVPP

B3LYP/6-311++G(d,p)

9.6

9.7

9.8

9.9

10.0

10.1 hν / eV

10.2

10.3

10.4

10.5

10.6

Figure 2. Acetamide TPES in black shown together with three ground state and an excited state FC calculations. The FC calculations were carried out at room temperature and the line transitions have been convoluted with a 40 cm–1 FWHM Gaussian. The dominant transitions in the B3LYP/6311++G(d,p) FC calculations are also shown as stick spectra. Acetamide structures with a methyl hydrogen synperiplanar [2a] and antiperiplanar [2b] with the oxygen atom are also shown. See text for a detailed description of the methyl orientation in the minimum. 24 ACS Paragon Plus Environment

Page 25 of 29

1.1

17

lg (k/s–1)

HCONH2+ m/z = 45

14

NH3+ + CO m/z = 17

0.9

11 8

11.4

Fractional ion abundance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


11.8

12.2

E / eV

12.6

0.7

0.5 NH2CO+ + H m/z = 44

0.3

0.1

HCO+ + NH2 m/z = 29

-0.1 11.1

11.3

11.5

11.7

11.9

12.1

12.3 12.5 hν / eV

12.7

12.9

13.1

13.3

13.5

Figure 3. Formamide experimental breakdown diagram (dots) shown together with the statistical model (continuous lines). Arrows indicate the derived 0 K threshold energies for dissociative photoionization. The inset show the model dissociation rate constant curves as a function of energy above the neutral. 25 ACS Paragon Plus Environment


‡

+ NH2

12.5

12.38

12.42 m/z = 29

12.0 ‡

11.5

E / eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 26 of 29

+H 11.31 m/z = 44

11.45 11.50 ‡

11.0

[1b] [1a]

10.66

10.5

10.0

̃+(𝐴ʺ) 𝐀 NH3+ + CO

IC 10.08

10.24 ̃𝐗 +(𝐴ʹ) m/z = 45

10.46 m/z = 17

Figure 4. Stationary points driving the fragmentation of the ion on the formamide potential energy surface. Minimum energies are average G4, CBSAPNO and W1U(sc) values, transition state energies are G4 and W1U(sc) averages. The neutral HOMO and HOMO–1, corresponding to ionization ̃ + and 𝐀 ̃+ states, respectively, are also shown. to the 𝐗 26 ACS Paragon Plus Environment

Page 27 of 29

1.1

CH3CONH2+ m/z = 59

0.9

Fractional ion abundance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


CH2NH3+ + CO m/z = 31

0.7

NH2CO+ + CH3 m/z = 44

0.5 NH4+ + HCCO m/z = 18

0.3

CH2CO+ + NH3 m/z = 42 0.1

-0.1 10.6

10.7

10.8

10.9

11.0

11.1

11.2 11.3 hν / eV

11.4

11.5

11.6

11.7

11.8

Figure 5. Acetamide experimental breakdown diagram (dots) shown together with the statistical model (continuous lines). Arrows indicate the derived 0 K threshold energies for dissociative photoionization. 27 ACS Paragon Plus Environment


‡

m/z = 42

m/z = 43

11.75 [5]‡

m/z = 44

+ NH2 [3]

11.25

+ NH3

‡ ‡

[4]

10.75

[8]‡

[6]‡

‡

10.25

‡

[12]‡

[13]

‡

[2a]

+ CO m/z = 31

[9]

+ CO

9.25

8.75

+ HCCO [11]

‡

[6]

9.75

m/z = 18

[10]

+ CH3

E / eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 28 of 29

‡

[7]

Figure 6. Stationary points driving the fragmentation of the acetamide cation on the potential energy surface. Minimum energies are average G4, CBS-APNO and W1U(sc) values, transition state energies are G4 and W1U(sc) averages. 28 ACS Paragon Plus Environment

Page 29 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


TOC Graphic


Low-Energy Photoelectron Spectrum and Dissociative Photoionization

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