Computational Study for the Second-Stage Cracking of the Pyrolysis of Ethylamine: Decomposition of Methanimine, Ethenamine, and Ethanimine

MURDOCH RESEARCH REPOSITORY

This is the author’s final version of the work, as accepted for publication following peer review but without the publisher’s layout or pagination. The definitive version is available at http://dx.doi.org/10.1016/j.comptc.2015.10.032

Almatarneh, M.H., Barhoumi, L., Al-Tayyem, B., Abu-Saleh, A.A- A.A., AL-A’qarbeh, M.M., Abuorabi, F., AlShamaileh, E., Altarawneh, M. and Marashdeh, A. (2016) Computational study for the second-stage cracking of the pyrolysis of ethylamine: Decomposition of methanimine, ethenamine, and ethanimine. Computational and Theoretical Chemistry, 1075. pp. 9-17.

http://researchrepository.murdoch.edu.au/29212/

Accepted Manuscript

Computational Study for the Second-Stage Cracking of the Pyrolysis of Ethyl- amine: Decomposition of Methanimine, Ethenamine, and Ethanimine

Mansour H. Almatarneh, Lina Barhoumi, Ban Tayyem, Abd Al-Aziz A. Abu- Saleh, Marwa M. AL-A’qarbeh, Faten Abuorabi, Ehab AlShamaileh, Mohammednoor Altarawneh, Ali Marashdeh

PII: S2210-271X(15)00438-7 DOI: http://dx.doi.org/10.1016/j.comptc.2015.10.032 Reference: COMPTC 1979

To appear in: Computational & Theoretical Chemistry

Received Date: 1 October 2015 Revised Date: 31 October 2015 Accepted Date: 31 October 2015

Please cite this article as: M.H. Almatarneh, L. Barhoumi, B. Tayyem, A.A-A. Abu-Saleh, M.M. AL-A’qarbeh, F. Abuorabi, E. AlShamaileh, M. Altarawneh, A. Marashdeh, Computational Study for the Second-Stage Cracking of the Pyrolysis of Ethylamine: Decomposition of Methanimine, Ethenamine, and Ethanimine, Computational & Theoretical Chemistry (2015), doi: http://dx.doi.org/10.1016/j.comptc.2015.10.032

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Computational Study for the Second-Stage Cracking of the Pyrolysis of Ethylamine: Decomposition of Methanimine, Ethenamine, and Ethanimine

Mansour H. Almatarneh1*, lina Barhoumi1, Ban Tayyem1, Abd Al-Aziz A. Abu-Saleh1, Marwa M. AL-A'qarbeh1, Faten Abuorabi1, Ehab AlShamaileh1, Mohammednoor Altarawneh2, Ali Marashdeh3

1 Department of Chemistry, University of Jordan, Amman 11942, Jordan. 2School of Engineering and Information Technology, Murdoch University, Perth, Australia. Currently on leave from Chemical Engineering Department, Al-Hussein Bin Talal University, Ma’an, Jordan 3Department of Chemistry, Al-Balqa Applied University, Al-Salt 19117, Jordan

* Corresponding author: Mansour H. Almatarneh, [email protected], (+962) 65355000- Ext. 22177.

Abstract

The mechanism that accounts for the observed experimental activation energy for the decomposition of ethylamine (EA) is still unknown. This paper reports the first detailed study of possible mechanisms for the pyrolysis of second-stage cracking product of ethylamine: the decomposition reaction of methanimine, ethanimine, and aminoethylene. Investigated reactions charactarise either H2 elimination or 1,3-proton shift. These pathways result in the removal of

H2, CH4, and NH3; and the formation of hydrogen cyanide, acetylene, acetonitrile, ethynamine, and ketenimine. The IRC analysis was carried out for all transition state structures to obtain the complete reaction pathways. The stationary points were fully optimized at B3LYP and MP2 levels of theory using the 6-31G(d), 6-31G(2df,p), and 6-31++G(3df,3dp) basis sets. Based on comparing energetic requirements, we find 1,3-proton shift is the most probable pathway for the decomposition of ethanimine. The decomposition reaction of ethenamine was the most plausible reaction with an activation energy of 297 kJ mol−1 calculated at the composite method of G4MP2.

Keywords: Decomposition reaction, Ethylamine, Methanimine, Ethenamine, Ethanimine, Transition state optimization, 1,3-Proton shift

Introduction

Ethylamine (EA), a primary aliphatic amine, is known to be a common starting material for many products including rubber latex, detergents, medicinal preparations, fibers, resins, and organic dyes. The decomposition reaction of EA has been extensively studied experimentally and theoretically [1-21]. This is due to its importance as a surrogate model compound for nitrogen amine contents in biomass [10] and for understanding of the constitution and the molecular evolution of interstellar matter, as being a component of interstellar gases [22]. The decomposition of EA was first investigated by Taylor [3]. It was found that decomposition of EA to be a homogeneous unimolecular reaction by finding no effects for adding the foreign gases hydrogen, nitrogen, and ammonia in temperature range 500-540 °C and at pressures of 50-

400 mmHg [3]. In a following work, Taylor and Ditman [4] reinvestigated the EA decomposition reaction at lighter conditions and concluded that the reaction to be heterogeneous and might go through a chain reaction mechanism at low pressure. In 1975, Lovas et al. [1] proposed a complex two stage cracking steps for EA pyrolysis. They stated that the decomposition reaction can produce nine polar species and three non-polar species, all of these species have been detected except for ethynylamine (CH≡CNH2) and keteneimine (H2C=C=NH) [1].

Methanimine Or methyleneimine (MI), ethenamine or vinylamine (VA), and its tautomer ethanimine or ethyleneimine (EI) are three of the polar products of the first crack routes that undergo further decomposition in the proposed second-crack route [1]. The aforementioned species were found to be major N-carriers during pyrolysis and decomposition of EA. Detailed kinetic models on decomposition of EA included these species [23,24]. (CH2=NH) is the simplest imine compound, it has been observed in the pyrolysis of methylamine, ethylamine (amines in general), and azides as a reactive intermediate [25-33]. A great deal of research has been carried out to assign the microwave, infrared, and electronic spectrum of this moiety [25-

43]. As a reactive species, MI can dissociate by 1,2-elimination of H2 which has been studied both experimentally and theoretically [44-55]. Vinylamine and ethyleneimine are chemicals that widely used in industry; they were first detected as primary pyrolysis product of EA in the gas phase using microwave spectroscopic technique by Lovas et al. [1,2]. They characterized the microwave and infrared spectra of VA and EI two isomers, E- and Z- ethyleneimine in a temperature range from 150 to 900 °C and suggested a non-planar structure of vinylamine

(CH2=CHNH2) [1,2]. Moreover, theoretical studies on VA structure produced the same result [16,18,20,56-60]. Traeger et. al. [12] reported using photoionization mass spectrometry and high-level Ab initio calculations to measure the heat of formation for ethyleneimmonium cation + (CH3CH=NH2 ) and derived an absolute proton affinity for cis (E) and trans (Z) ethyleneimine isomers (CH3CH=NH). Stolkin et al. [7] obtained the first matrix spectra of EI produced by photolysis in low-temperature Argon matrix where EI molecule was produced by the pyrolysis of N-trimethylhexahydro-s-triazine (NTMT). They also reported a set of Ab initio SCF LCAO calculations for both the cis and trans isomer of EI [7]. On the other hand, the gas-phase infrared spectrum of EI produced by the pyrolysis of EA was first obtained by Hashiguchi et al. [11]. Ab initio MO method was used to calculate the force constants of both cis and trans isomers, and

from the microwave and infrared spectra combined with Ab initio MO calculations; they concluded that the cis isomer to be the lower in energy than the trans form by 2.5 kJ mol−1 [11].

Keteneimine is an unstable isomer of acetonitrile. It was firstly synthesized by Staudinger and Hauser in 1919 [61]; it has been assigned as one of the chemical components of the interstellar medium [62] and its spectrum has been obtained [63-70]. Moreover, theoretical studies to obtain the optimized geometries of ketenimines and the substituent effects on the stability of ketenimines have been reported [71-80]. Keteneimine is a transient tautomer of the stable acetonitrile and a reactive intermediate with sp-hybridized central carbon atom; it can undergo electrocyclic ring closure, nucleophilic addition, radical addition, and other reactions [81]. Ethynylamine is another unstable isomer of acetonitrile, its vibrational frequencies and rotational constants were firstly predicted by Saebø et al. [82] using Ab initio MO calculations in 1984. While the first experimental generation of ethynylamine was reported in 1986 by Baar et al. [83] and later in 1988 by Wentrup et al. [84]. Ab initio calculations predicted keteneimine to be more stable than ethynamine by by 25-67 kJ mol−1 [77,85].

Recently, Almatarneh et al. [21] reported a theoretical study for the mechanisms of the decomposition reaction of EA. They were interested in the first stage-cracking pattern postulated for the decomposition of EA. Four pathways were proposed for the decomposition of EA. The decomposition reaction involved the removal of H2, NH3, CH4, and the formation of methanimine (methyleneimine, MI), ethenamine (vinylamine, VA), and ethanimine (ethyleneimine, EI) by elimination and 1,3-proton shift. They found that the activation energy for the most favorable pathway is 281 KJ/mol which was higher than the expected experimental value. In this work, we focused on the second-stage cracking of the pyrolysis of EA that suggested by Lovas et al. [1]. Investigated pathways include either H2 elimination or 1,3-proton shift. These pathways lead to the formation of hydrogen cyanide, acetylene, acetonitrile, ethynamine, and ketenimine via the removal of H2, CH4, and NH3. Despite recent aforementioned progress, the underlying mechanism for the decomposition of ethylamine is still unknown. The most intriguing question is related to the extent in which the primary decomposition products of EA catalyse the degradation of the parent EA. In our previous work on the unimolecular decomposition of EA, we found that the initial decomposition channel corresponds to fission of the C-N bond. Out fitted pressure-dependent reaction rate constants

agrees well with analogous experimental measurements behind reflected shock tube [86]. To this end, we report in this study a detailed account of possible mechanisms for the parallel decomposition of MI, VA, and EI. Results obtained in this study will be instrumental to understand the overall decomposition of alkyl amines and the likelihood of the competitions between unimolecular- and bimolecular-derived routes.

2. Computational method

All calculations were performed using the program Gaussian09 [87] package. The geometries of all reactants, transition states, and products were fully optimized at B3LYP, M06-2X [88], and MP2 levels of theory using the 6-31G(d) basis set. Both extended 6-31G(2df,p) and 6- 31++G(3df,3dp) basis sets at B3LYP level of theory have also been utilized to examine the effect of diffuse functions and to confirm the reliability of the 6-31G(d) basis set. Single point energies were determined using G4MP2 composite method. The principle reason for employing different levels of theory is to assure that the relative stabilities remain independent of the basis set. All reaction pathways have been verified using Intrinsic Reaction Coordinate (IRC) analysis of all proposed transition states. The last structures of the IRC in both directions were further optimized in order to positively identify their corresponding minima.

3. Results and Discussions Scheme 1 depicts reactants, transition states, and products for second-stage decomposition reaction pathways of EA. The second-stage cracking of the pyrolysis of EA can follow seven possible pathways designated as denoted by pathways (A, B1→B3, C1→ C3). Pathways B1 and C3 are initiated by 1,3-proton shifts while pathways A , B2, B3, C1, and C2 are initiated by 1,2 elimination were investigated. All pathways involved one-step mechanism. The activation energies, the enthalpies of activation and Gibbs free energies of activation for the proposed mechanisms at various levels of theory are listed in Tables 1, 3, and 5. The energies, the enthalpies, and Gibbs free energies for this reaction at various levels of theory are listed in Tables 2, 4, and 6. Optimized geometries for the reactants, transition states, and products involved in all investigated pathways (A , B1→B3, C1→ C3) are shown in Fig. 1, and their relative energies are given in Figs. 2→4.

3.1. The Decomposition of Methanimine Intermediate One possible pathway has been proposed for the decomposition of methanimine which is a one- step mechanism, see Scheme 1. The optimized geometries for the reactant, transition state, and product involved in pathway A along with their relative energies are given in Figs. 1 and 2. The reaction was initiated by 1,2-elimination to produce H2 and a hydrogen cyanide. It is worth noting that there was a significant decrease in the ∠CNH angle from 125° in the reactant (RA) to

55° in the transition state (TSA). The NH and CH bonds increased significantly in the transition state. The NH and CH bonds increased from 1.02Å and 1.09Å in the reactant to 1.41Å and 1.49Å in the transition state, respectively (with largest value of 1.55Å at MP2 level of theory). As can be seen in Table 1, the activation energies for pathway A were 396 and 404 kJ mol−1 at B3LYP/6-31++G(3df,3dp) and G4MP2 levels of theory, respectively. Table 2 shows the energies, enthalpies, and Gibbs free energies for the decomposition reaction of methanimine in kJ mol−1 for pathway A. The values of enthalpies and Gibbs free energies indicate that the reaction is endothermic and endergonic (non- spontaneous) at all levels of theory, except at MP2 level of theory where the Gibbs free energies are slightly negative. From Table 1, the activation energies for the decomposition of methanimine at G4MP2 level differ by no more than 8 kJ mol−1 from the B3LYP/6-31++G(3df,3dp) level of theory, and no more than 3 kJ mol−1 from the less expensive method of B3LYP/6-31G(2df,p). The activation energies at the more modern functional, M06-2X [88], is 405 kJ mol−1, and differ by no more than 2 and 4 kJ mol−1 from G4MP2 and the B3LYP/6-31G(2df,p) levels of theory, respectively.

3.2. The Decomposition of Ethanimine Intermediate Three decomposition pathways of ethanimine were investigated as shown in Scheme 1 and Fig. 1. In pathway B1 (Fig. 3), the transfer occurred from the imino group to the methyl group to produce CH4 and HCN (hydrogen cyanide) through 1,3-proton shift. The structure of ethanimine obtained from IRC calculations on the transition state was found to be Z-ethanimine conformer for pathway B1 and E conformer for pathways B2 and B3. Comparing energies of the two conformers indicate that the E conformer is marginally more stable in the range of (1.2−4.8 kJ mol−1). It is worth mentioning that there was a significant decrease in the ∠CNH angle from 111° in the reactant to 56.6° in the transition state (TSB1) and for ∠CCN decreased from 128° to 118° in TSB1. The CC single bond and NH bond increased significantly from 1.50Å to 1. 96Å and 6

1.02Å to 1.39Å, respectively. Decreasing the ∠CCN and ∠CNH angles result in a significant angle strain of the transition state, and thus accounts for the high energy barrier. The activation energies for pathway B1 were 392 and 402 kJ mol−1 at B3LYP/6-31++G(3df,3dp) and G4MP2 levels of theory, respectively, see Table 3. Meanwhile, pathways B2 and B3 involved a 1,2-elimination of hydrogen. In pathway B2, the elimination step occurred at CN bond to afford CH3CN (acetonitrile) and H2. Transition state (TSB2) geometry showed significant decrease in the ∠CNH angle from 111° in reactant to 58° in TSB2. The ∠NCH angle is also decreased from 122° in the reactant (R ) to 114° in the TS ; in B2 B2 which it accounts in angle strain in the four-membered ring transition state. For pathway B3, the

1,2-elimination of hydrogen occurred at the CC bond to form H2C=C=NH (ketenimine) and H2.

The major changes in the geometry of the transition state (TSB3) were in ∠CCN, ∠C(2)C(1)H and 2 3 ∠C(1)C(2)H angles (where C(2):Sp carbon and C(1): Sp carbon). The ∠CCN, C(2)C(1)H and

∠C(1)C(2)H angles were changed from 122°, 116°, and 110° in the reactant to 139°, 100°, and 58° in the TSB3. The activation energies for pathway B2 were 379 and 387 kJ mol−1 at B3LYP/6- 31++G(3df,3dp) and G4MP2 levels of theory, respectively. While for pathway B3, the barrier was 381 kJ mol−1 and 396 kJ mol−1 at B3LYP/6-31++G(3df,3dp) and G4MP2 levels of theory, respectively, see Table 3. Pathways B2 and B3 signify 1,2 hydrogen elimination. The energy barrier for pathway B2 has a lower activation energy than Pathway B3. Pathway B2 is the most favorable among the other postulated pathways for the decomposition of ethanimine since it has the lowest activation energy at all calculated levels of theory. The activation energies for pathways B1 and B2 obtained at all level of theory were lower than theoretical values computed by Arenas et al. [89] (444 kJ mol−1 for pathway B1 and 431 kJ mol−1 for pathway B2). The relative energies for the decomposition pathways for ethanimine (B1→B3) at all studied levels of theory are shown in Fig.3. For pathways B2 and B3, it may be assumed that hydrogen abstraction from nitrogen and carbon sites should occur equally as CH and NH bond strengths are almost equal. The barriers for these proposed mechanisms were different depending on the site where the abstraction occurs. Table 4 shows the enthalpies and Gibbs free energies for the decomposition reaction of ethanimine in kJ mol−1 for pathways B1, B2 and B3. It can be seen from Table 4 that the decomposition reaction of ethanimine is endothermic and endergonic (i.e. non-spontaneous 7

reaction) at all levels of theory for pathways B2 and B3, while endothermic ( except for MP2/6- 31G(d) and MP2/6-31+G(d) levels of theory) and exergonic (i.e. spontaneous) for pathway B1. Hydrogen elimination from CN bond is the most probable pathway in the proposed mechanism. It has a lower activation energy over the 1,3-proton shift step (pathway B1) and hydrogen elimination from CC bond. As we can see, hydrogen abstraction from C and N atoms (pathway B2) is the most probable primary step in the proposed mechanism for the decomposition reaction. The comparison of the relative energies for the three pathways at G4MP2 level of theory is given in Fig. 3. It is worth mentioning that the activation energies at G4MP2 level of theory differ by no more than 15 kJ mol−1 from the ones calculated at DFT functionals. While at M06-2X/6-31G(2df,P) level of theory is no more than 7 kJ mol−1 for the decomposition of ethanimine.

3.3. The Decomposition of Ethenamine Intermediate The decomposition of ethenamine (aminoethylene) can follow several possible pathways designated as pathways C1→C3. The geometries for the reactants, intermediates, transition states, and products involved in pathways C1→C3 are shown in Fig. 1 and scheme 1, and their relative energies are given in Fig. 4. The structure of ethenamine obtained from IRC calculations on the transition state was found to involve two conformers; planer and pyramidal ethenamine. Planer ethenamine was the reactant for both pathways C1 and C3. However, pathway C2 was initiated by pyramidal ethenamine conformer. The value of energy of the two conformers obtained at B3LYP/6- 31++G(3df,3dp) level of theory implied that the planer conformer is more stable by 27.4 kJ mol−1 in agreement with a previous study by Miranda et.al [60]. For pathways C1 and C2, 1,2- elimination of hydrogen was the initiating step. In pathway C1, the hydrogen elimination occurred at CN bond to produce H2C=C=NH (ketenimine) and H2. Transition state

(TSC1) geometry showed significant difference at different levels of theory. NH bond increased from 1.01 Å in the reactant to 1.18 Å at B3LYP/6- 31++G(3df,3dp), 1.36Å at G4MP2, and 1.53Å at MP2/6-31+G(d) in the transition state. Meanwhile, CH bond showed an increase from 1.08Å to 1.82Å at B3LYP/6- 31++G(3df,3dp), 1.50Å at G4MP2, 1.33Å at MP2/6-31+G(d), and 1.94 Å at B3LYP/6-31+G(d) level of theory for the transition state. A similar variation was noticed for the bond angles. The ∠CNH and ∠NCH angles were decreased from 116° and 113° in the reactant to 92° and 81° at B3LYP/6- 31++G(3df,3dp), 74° and 94° at MP2/6-31+G(d), and 8

97° and 79° at B3LYP/6-31+G(d) level of theory. The significant decrease in these angles account in the angle strain in the four membered ring transition state and high energy barrier for pathway C1 of 418 kJ mol−1 at B3LYP/6-31++G(3df,3dp) level of theory (433 kJ mol−1 at G4MP2 level of theory), see Table 5.

For pathway C2, the 1,2-elimination of hydrogen occurred at the CC bond to form HC≡C-NH2

(ethynamine) and H2. The major changes in the geometry of the transition state (TSC2) were in

CCN, C(2)C(1)H and C(1)C(2)H angles. The ∠CCN, ∠C(2)C(1)H, and ∠C(1)C(2)H angles was changed from 122°, 120°, and 121° in reactant to 151°, 104°, and 61.4° in TS . Also, a significant C2 increase in the bond length occurred for C(2)H and C(1)H. An increase from 1.08Å and 1.09Å in reactant to 1.50Å and 1.52Å for C(2)H and C(1)H, respectively, at B3LYP/6-31++G(3df,3dp) level of theory. The geometry of the transition state at MP2/6-31+G(d) level of theory has a noticeable difference in the bond angle and length values from other levels of theory. The values of ∠CCN, and ∠C(1)C(2)H angles were 145° and 38.4°; meanwhile the bond length values for

C(2)H and C(1)H were 2.14Å and 1.37Å for the transition state at MP2/6-31+G(d) level of theory. As shown in Table 5, the activation energies for pathway C2 were 458 and 481 kJ mol−1 at B3LYP/6-31++G(3df,3dp) and G4MP2 levels of theory, respectively, indicated an angle strain in the four membered ring transition state. Both pathways C1 and C2 initiated by 1,2 hydrogen elimination, and as CH and NH bond strengths are almost the same, it may be assumed that hydrogen abstraction from nitrogen and carbon sites should occur equally. Similar to what we mentioned earlier for the decomposition of ethanimine, the abstraction from NH bond (C1) has the lower activation energy compared to CH bond (C2). For pathway C3, the ethenamine reactant was planar as revealed from IRC calculations. The reaction initiated by hydrogen transfer, via a 1,3-proton shift, from the methylene group to the amine group to produce acetylene and ammonia. A significant decrease occurred in the

∠C(1)C(2)H angle from 122° in the reactant to 71.8° in the transition state (TSC3). The ∠CCN angle decreased from 127° to 105° in the TSC3. The C(2)H and NH bonds increased significantly from 1.08Å and 1.39Å to 1.61Å and 1.51Å at B3LYP/6-31++G(3df,3dp) level, respectively. It is worth mentioning that the conformation of the reactant for pathway C3 differs at all the levels of theory used in this study. Planar ethenamine conformer was the optimized reactant at B3LYP/6- 31G(d), B3LYP/6-31G(2df,p), and B3LYP/6-31++G(3df,3dp) levels of theory. Meanwhile, the pyramidal conformer was the optimized one at MP2/6-31G(d), MP2/6-31+G(d), and B3LYP/6- 9

31+G(d). This can be explained by the effect of the employed level of theory on the stability of the conformer, noting that the geometry of amnio group in the transition state is pyramidal at all levels of theory. The activation energies for pathway C3 were 301 and 297 kJ mol−1 at B3LYP/6-31++G(3df,3dp) and G4MP2 levels of theory, respectively. A comparison of the activation energies for the three pathways at G4MP2 level of theory can be seen in Fig. 4. As can be seen in Table 5 and Fig. 4, pathway C3 has the lowest energy barrier at all levels of theory and is the most favorable decomposition route for ethenamine over C1 and C2. The relative energies, enthalpies, and Gibbs free energies for the decomposition reaction of ethenamine in kJ mol−1 for pathways C1→ C3 are given in Table 6. It can be seen that the decomposition of ethenamine is endothermic and endergonic (non-spontaneous reaction) at all levels of theory for the three proposed pathways. As we can see, hydrogen elimination from C and N atoms is the most probable primary step in the proposed mechanism for the decomposition reaction. However, the barriers for these mechanisms were different depending on the site where the abstraction occurs. Our study found that the keteneimine to be more stable than ethynamine by 40 kJ mol−1 at B3LYP/6- 31++G(3df,3dp) level of theory which is in an agreement to previous findings [77,85].

For the decomposition of ethenamine, the activation energies at G4MP2 level of theory differ by no more than 2-23 kJ mol−1 from the ones calculated at DFT functionals. While the difference in activation energy at M06-2X/6-31G(2df,p) from G4MP2 and B3LYP/6-31G(2df,p) levels of theory is no more than 13 and 16 kJ mol−1. It is worth noting that the activation energies at B3LYP/6-31G(2df,p) differ by no more than 1-8 kJ mol−1 from the values of B3LYP/6- 31++G(3df,3dp) level of theory for the decomposition of methanimine, ethanamine, and ethenamine. Therefore, it is more convenient to use B3LYP/6-31G(2df,p) level of theory which is the least computationally expensive method for providing reliable energetics for such system.

Conclusion The second-stage cracking mechanisms of ethylamine, decomposition of mathanimine, ethanimine, and ethenimine, were investigated using DFT calculations. Seven pathways for the decomposition reaction were investigated. The IRC analysis was carried out for all transition state structures to obtain the complete reaction pathways. We found that the hydrogen elimination from CN bond is the most probable pathway compared to the elimination from CC

bond. Pathway B2, where it involved a 1,3-proton shift, is the most favorable among the other postulated pathways for the decomposition of ethanimine since it has the lowest activation energy at all calculated levels of theory. For the decomposition reaction of ethanimine, C3 is the most plausible reaction with an activation energy of 303, 301, and 297 kJ mol−1 at B3LYP/6- 31G(2df,p), B3LYP/6-31++G(3df,3dp), and G4MP2 levels of theory, respectively. A complete characterization of the overall decomposition mechanisms calls for a consideration of unimolecular fission of N-H fissions and bimolecular reactions with the title species with the O/H radical pool.

Acknowledgment M. H. Almatarneh is grateful to the Atlantic Computational Excellence Network (ACENET) for computer time.

References:

1. F.J. Lovas, F.O. Clark, E. Tiemann, Pyrolysis of Ethylamine. I. Microwave Spectrum and Molecular Constants of Vinylamine, J. Chem. Phys. 62 (1975) 1925–1931. 2. F.J. Lovas, R.D. Suenram, D.R. Johnson, F.O. Clark, E. Tiemann, Pyrolysis of

Ethylamine. II. Synthesis and Microwave Spectrum of Ethylidenimine (CH3CH=NH), J. Chem. Phys. 72 (1980) 4964–4972. 3. H.A. Taylor, The Decomposition of Ethylamine. A Unimolecular Reaction, J. Phys. Chem. 34 (1930) 2761–2770. 4. H.A. Taylor, J.G. Ditman, The Decomposition of Ethylamine and Diethylhydrazine, J. Chem. Phys. 4 (1936) 212–218. 5. D.E. Gardin, G.A. Somorjai, Vibrational Spectra and Thermal Decomposition of Methylamine and Ethylamhe on Ni(III), J. Phys. Chem. 96 (1992) 9424–9431. 6. A. Lucassen, K. Zhang, J. Warkentin, K. Moshammer, P. Glarborg, P. Marshall, K. Kohse-Höinghaus, Fuel-nitrogen Conversion in the Combustion of Small Amines Using Dimethylamine and Ethylamine as Biomass-Related Model Fuels, Combustion and Flame 159 (2012) 2254–2279.

7. I. Stolkin, T.K. Ha, H.H. Gunthard, N-methylmethyleneimine and Ethylideneimine: Gas- and Matrix-Infrared Spectra, AB Initio Calculations and Thermodynamic Properties, Chem. Phys. 21 (1977) 327–347. 8. U. Mioc, N. Petranovic, Ethylamine Behavior of 3A Zeolite Surface, J. Phys. Chem. 79 (1975) 1476–1478. 9. F. Jordan, Nonempirical Molecular Orbital Calculations on the Electronic Structures, + Preferred Geometries, and Relative Stabilities of Some C2H6N Isomeric Ions, J. Phys. Chem. 80 (1976) 76–82.

10. S. Li, D.F. Davidson, R.K. Hanson, Shock Tube Study of Ethylamine Pyrolysis and Oxidation, Combustion and Flame 161 (2014) 2512–2518. 11. Hashiguchi, K., Hamada, Y., Tsuboi, M., Koga, Y., Kondo, S. Pyrolysis of Amines: Infrared Spectrum of Ethylideneimine, J. Mol. Spectrosc. 105 (1984) 81–92. + 12. J.C. Traeger, Z.A. Harvey, Heat of Formation for the CH3CH=NH2 Cation by Photoionization Mass Spectrometry, J. Phys. Chem. 110 (2006) 8542–8547. 13. Y. Hamada, K. Hashiguchi, A.Y. Hirakawa, M. Tsuboi, M. Nakata, M. Tasumi, S. Kato, K. Morokuma, Vibrational analysis of Ethylamines: Trans and Gauche Forms, J. Mol. Spectrosc, 102 (1983) 123–147. 14. Y. Hamada, N. Sato, M. Tsuboi, Amino Wagging and Torsion Vibrations of Vinylamine, J. Mol. Spectrosc. 124 (1987) 172–178. 15. Y. Hamada, M. Tsuboi, K. Yamanouchi, K. Kuchitzu, Molecular Structural of the Gauche and Trans Conformers of Ethylamine as Studies by Gas Electron Diffraction, J. Mol. Struct. 146 (1986) 253–262. 16. K. Müller, I.D. Brown, Enamines I, Vinyl Amine, a Theoretical Study of its Structure, Electrostatic Potential, and Proton Affinity, Helv. Chim. Acta 61 (1978) 1407–1418. 17. D. Zeroka, J.O. Jensen, A.C. Samuels, Rotation/ Inversion Study of the Amino Group in Ethylamine, J. Phys. Chem. A. 102 (1998) 6571–6579.

18. H.M. Badawi, Structural stability and vibrational analysis of aminoethylene CH2=CH-

NH2 and aminoketene O=C=CH-NH2, J. Mol. Struct. (Theochem) 726 (2005) 253–260.

19. D. Zeroka, J.O. Jensen, A.C. Samuels, Infrared Spectra of some Isotopomers of Ethylamine and the Ethylammonium Ion: a Theoretical Study, J. Mol. Struct. (Theochem) 456 (1999) 119–126. 20. R.A. Eades, D.A. Weil, M.R. Ellenberger, W.E. Farneth, D.A. Dixon, C.H.Jr. Douglass, Electronic Structure of Vinylamine. Proton Affinity and Conformational Analysis, J. Am. Chem. Soc. 103 (1981) 5372–5377. 21. M.H. Almatarneh, M. Altarawneh, R.A. Poirier, I.A. Saraireh, A High Level Ab Initio, DFT, and RRKM Calculations for the Unimolecular Decomposition Reaction of

Ethylamine, J. Comput. Sci. 5 (2014) 568–575. 22. G. Danger, J.-B. Bossa, P. De Marcellus, F. Borget, F. Duvernay, P. Theulé, T. Chiavassa, L. d’Hendecourt, Experimental Investigation of Nitrile Formation From VUV Photochemistry of Interstellar Ices Analogs: Acetonitrile and Amino Acetonitrile, Astronomy & Astrophysics (A&A) 525 (2011) A30. 23. S. Li, D.F. Davidson, R.K. Hanson, Shock Tube Sudy of Ethylamine Pyrolysis and Oxidation, Combustion and Flame 161 (2014) 2512–2518 24. A. Lucassen, K. Zhang, J. Warkentin, K. Moshammer, P. Glarborg, P. Marshall, K. Kohse-Höinghaus, Fuel-Nitrogen Conversion in the Combustion of Small Amines Using Dimethylamine and Ethylamine as Biomass-Related Model Fuels, Combustion and Flame 59 (2012) 2254–2279. 25. D.R. Johnson, F.J. Lovas, Microwave Detection of the Molecular Transient

Methyleneimine (CH2=NH), Chem. Phys. Lett. 15 (1972) 65–68. 26. R. Pearson, F.J. Lovas, Microwave Spectrum and Molecular Structure of Methylenimine (CH2NH), J. Chem. Phys. 66 (1977) 4149–4156. 27. J.B. Peel, G.D. Willett, Photoelectron Spectrum of Methylenimine by Spectrum Stripping, J. Chem. Soc. Faraday Trans. 71 (1975) 1799–1804. 28. Y. Hamada, K. Hashiguchi, M. Tsuboi, Y. Koga, S. J. Kondo, Pyrolysis of Amines: Infrared Spectrum of Methyleneimine, J. Mol. Spectros. 20 (1984) 70–80. 29. J.M. Dyke, A.P. Groves, A. Morris, J.S. Ogden, A.A. Dias, A.M.S. Oliveira, A.M.C. Moutinho, Study of the Thermal Decomposition of 2-azidoacetic Acid by Photoelectron and Matrix Isolation Infrared Spectroscopy, J. Am. Chem. Soc. 119 (1997) 6883–6887.

30. J.M. Dyke, A.P. Groves, A. Morris, J.S. Ogden, A.A. Dias, A.M.S. Olivereira, A.M.C. Moutinho, A Study of the Thermal Decomposition of Azidoacetone by Photoelectron and Matrix Isolation Spectroscopy, J. Phys. Chem. A 103 (1999), 8239–8245. 31. H. Bock, R. Dammel, Gas-Phase Pyrolyses of Alkyl Azides: Experimental Evidence for Chemical Activation, J. Am. Chem. Soc. 110 (1988), 5261–5269. 32. N. Hooper, L.J. Beeching, A. Morris, J.S. Ogden, A.A. Dias, M.L. Costa, M.T. Barros, M.H. Cabral, A.M.C. Moutinho, A Study of the Thermal Decomposition of 2- Azidoethanol and 2-azidoethyl Acetate by Ultraviolet Photoelectron Spectroscopy and

Matrix Isolation Infrared Spectroscopy, J. Phys. Chem. A 106 (2002) 9968–9975. 33. H. Bock, R. Dammel, The Pyrolysis of Azides in the Gas Phase, Angew. Chem., Int. Ed. Engl. 26 (1987) 504–526. 34. P.D. Godfrey, R.D. Brown, B.J. Robinson, M.W. Sinclair, Discovery of Interstellar Methanimine (Formaldimine), Astrophys, Lett. 13 (1973) 119–121. 35. J.E. Dickens, W.M. Irvine, C.H. DeVries, M. Ohishi, Hydrogenation of Interstellar

Molecules: a Survey for Methylenimine (CH2NH), Astrophys. J. 479 (1997) 307–312. 36. D.E. Milligan, Infrared Spectroscopic Study of the Photolysis of Methyl Azide and

Methyl-d3 Azide in Solid Argon and Carbon Dioxide, J. Chem. Phys. 35 (1961) 1491– 1497. 37. M.E. Jacox, D.E. Milligan, The Infrared Spectrum of Methylenimine, J. Mol. Spectrosc. 56 (1975) 333–356. 38. C.B. Moore, G.C. Pimentel, T.D. Goldfarb, Matrix Photolysis Products of Diazomethane: Methyleneimine and Hydrogen Cyanide, J. Chem. Phys. 43 (1965) 63– 70. 39. G. Duxbury, H. Kato, M.L. Le Lerre, Laser Stark and Interferometric Studies of Thioformaldehyde and Methyleneimine, Faraday Discuss. Chem. Soc. 71 (1981) 97–110. 40. L. Halonen, G. Duxbury, High Resolution Infrared Spectrum of Methyleneimine,

CH2NH, in the 3μm Region, J. Chem. Phys. 83 (1985) 2091–2096. 41. L. Halonen, G. Duxbury, The Fourier Transform Infrared Spectrum of Methyleneimine in the 10μm Region, J. Chem. Phys. 83 (1985)2078–2090.

42. L. Halonen, G . Duxbury, Fourier Transform Infrared Spectrum of CH2NH: the ν1 Band, Chem. Phys. Lett. 118 (1985) 246–251. 14

43. A. Teslja, B. Nizamov, P.J. Dagdigian, The Electronic Spectrum of Methyleneimine, J. Phys. Chem. A, 108 (2004) 4433–4439. 44. M.T. Nguyen, D. Sengupta, T.K. Ha, Another Look at the Decomposition of Methyl Azide and Methanimine: How Is HCN Formed, J. Phys. Chem. 100 (1996) 6499–6503. 45. R. Sumathi, Dissociation and Isomerization Reactions of Formaldimine on the Ground and Excited State Surface, J. Mol. Struct. (Theochem) 364 (1996) 97–106. 46. M.T. Nguyen, J. Rademakers, J.M.L. Martin, Concerning the Heats of Formation of the [C, H ,N]+. Radical Cations, Chem. Phys. Lett. 221 (1994) 149–155. 3 47. J. Roithova, D. Schröder, H. Schwarz, Unimolecular Fragmentation of CH3NH2: Towards a Mechanistic Description of HCN Formation, Eur. J. Org. Chem. 2005 (2005) 3304– 3313. 48. J.F. Arenas, J.I. Marcos, J.C. Otero, A. Sanchez-Galvez, J. Soto, Nitrenes as Intermediates in the Thermal Decomposition of Aliphatic Azides, J. Chem. Phys. 111 (1999) 551-561. 49. J. Zhou, H.B. Schlegel, Ab Initio Classical Trajectory Study of the Dissociation of n+ Neutral and Positively Charged Methanimine (CH2NH n=0-2), J. Phys. Chem. A 113 (2009) 9958–9964. 50. C. Larson, Y.Y. Ji, P. Samartzis, A.M. Wodtke, S.H. Lee, J.J.M. Lin, C. Chaudhuri, T.T. Ching, Collision-Free Photochemistry of Methylazide: Observation of Unimolecular Decomposition of Singlet Methylnitrene, J. Chem. Phys. 125 (2006) 133302.

51. R. Sumathi, M.T.A Nguyen, Theoretical Study of the CH2N System: Reactions in both Lowest Lying Doublet and Quartet States, J. Phys. Chem. A 102 (1998) 8013–8020. 52. H. Tachikawa, T. Iyama, T. Fukuzumi, Decomposition Dynamics of Interstellar HCNH: Ab-Initio MO and RRKM Studies, Astronomy & Astrophysics (A&A) 397 (2003) 1–6. 53. A. Metropoulos, D.L.A Thompson, Quantum Chemistry Study of the Dissociation and Isomerization Reactions of Methylene Amidogene, J. Mol. Struct. (Theochem) 822 (2007) 125–132. 54. Y. Tang, C.J. Nielsen, A Systematic Theoretical Study of Imines Formation From the

Atmospheric Reactions of RnNH2-n with O2 and NO2 (R=CH3 and CH3CH2, n=1 and 2), Atmos. Environ. 55 (2012) 185–189.

55. C. Pouchan, K. Zaki, Ab Initio Configuration Interaction Determination of the Overtone Vibrations of Methyleneimine in the Region 2800–3200 cm-1, J. Chem. Phys. 107 (1997) 342–345. 56. J.D. Dill, A. Greenberg, J.F. Liebman, Substituent Effects on Strain Energies, J. Am. Chem. Soc. 101 (1979) 6814–6826. 57. S. Saebø, L. Radom, The structure of Vinylamine, J. Mol. Struct. (Theochem) 89 (1982) 227–233. 58. B.J. Smith, L. Radom, Ethynamine: the Remarkable Acid-Strengthening and Base-

Weakening Effect of the Acetylenic Linkage. A Comparison with Ethenamine and Methylamine, J. Am. Chem. Soc. 114 (1992) 36–41. 59. A.A. Zavitsas, D.W. Rogers, N. Matsunaga, Remote Substituent Effects on Allylic and Benzylic Bond Dissociation Energies. Effects on Stabilization of Parent Molecules and Radicals, J. Org. Chem. 72 (2007) 7091–7101. 60. M.S. Miranda, J.C.G. Esteves da Silva, A. Castillo, A.T. Frank, A. Greer, J.A. Brown, B.C. Davis, J.F. Liebman Amino, Ammonio and Aminioethenes: a Theoretical Study of their Structure and Energetics, J. Phys. Org. Chem. 26 (2013) 613–625. 61. H. Staudinger, J. Meyer, Über Neue Organische Phosphorverbindungen III. Phosphinmethylenderivate und Phosphinimine, Helv. Chim. Acta. 2 (1919) 635–646.

62. F.J. Lovas, J.M. Hollis, A.J. Remijan, P.R. Jewell, Detection of Ketenimine (CH2CNH) in Sagittarius b2(n) Hot Cores, Astrophys. J. 645 (2006) L137–L140. 63. M. Rodler, R.D. Brown, P.D. Godfrey, L.M. Tack, Generation, Microwave Spectrum and Dipole Moment of Ketenimine, Chem. Phys. Lett. 110 (1984) 447–451. 64. M. Rodler, R.D. Brown, P.D. Godfrey, B. Kleibömer. The Rotation-Inversion Spectrum

of Ketenimine, H2C=C=NH, J. Mol. Spectrosc. 118 (1986) 267–276. 65. H.W. Kroto, G.Y. Matti, R.J. Suffolk, J.D. Watts, M. Rittby, R.J. Bartlett, Photoelectron Spectroscopic and Theoretical Study of Ketene Imine, CH,=C=NH, and Ketene N-

Methylimine, CH2=C=NCH3, J. Am. Chem. Soc. 112 (1990) 3779–3784.

66. E. Jacox, D.E. Milligan, Infrared Study of the Reactions of CH2 and NH with C2H2 and

C2H4 in Solid Argon, J. Am. Chem. Soc. 85 (1963) 278–282.

67. M.E. Jacox, Matrix Isolation Study of the Interaction of Excited Argon Atoms with Methyl Cyanide. Vibrational and Electronic Spectra of Ketenimine, Chem. Phys. 43 (1979) 157–172. 68. M.K. Bane, C.D. Thompson, E.G. Robertson, D.R.T. Appadooc, D. McNaughton, High- resolution FTIR Spectroscopy of the ν8 and Coriolis Perturbation Allowed ν12 Bands of Ketenimine, Phys. Chem. Chem. Phys. 13 (2011) 6793–6798. 69. M.K. Bane, C.D. Thompson, E.G. Robertson, D.R.T. Appadooc, D. McNaughton, High- Resolution Fourier-Transform Infrared Spectroscopy of the ν6 and Coriolis Perturbation

Allowed ν 10 Modes of Ketenimine, J. Chem. Phys. 135 (2011) 224306. 70. M.K. Bane, C.D. Thompson, E.G. Robertson, D.R.T. Appadooc, D. McNaughton, High- Resolution Fourier-Transform Infrared Spectroscopy of the Coriolis Coupled Ground State and ν7 Mode of Ketenimine, J. Chem. Phys. 134 (2011) 234306. 71. K. Sung, Substituent Effects on Stability of Ketenimines, J. Chem. Soc. Perkin Trans. 2, (1999) 1169–1174 72. J. Kaneti, M.T. Nguyen, Theoretical Study of Ketenimine: Geometry, Electronic Properties, Force Constants and Barriers to Inversion and Rotation, J. Mol. Struct. (Theochem) 87 (1982) 205–210. 73. T.K. Ha, M.T. Nguyen, An Ab Initio SCF and CI Study of Ketene Imine, J. Mol. Struct. (Theochem) 87 (1982) 355–364. 74. S. Lacombe, G. Pfister-Guillouzo, Detection in the Gas Phase of Unstable Ketenimines, J. Chem. Soc. Faraday Trans. 2, 85 (1989) 741–745. 75. M.T. Nguyen, Ab initio SCF Study of the Molecular Structures and Relative Stabilities of + the C2H4N Cation Isomers, J. Chem. Soc., Perkin Trans. 2, (1984) 1401–1405. 76. E. Sonveaux, J.M. André, J. Delhalle, J.G. Fripiat, Theoretical Study of the Electronic Structure of Ketene, Ketenimine, Keteniminium Ion and Related Cumulenes. Evaluation of a 1,2 Dipolar Model, Bull. Soc. Chim. Belges. 94 (1985) 831–847. 77. R.D. Brown, E.H.N. Rice, M. Rodler, Ab Initio Studies of the Structures and Force Fields of Ketenimine and Related Molecules, Chem. Phys. 99 (1985) 347–356. 78. G.V. Shustov, A.V. Kachanov, S.N. Denisenko, R.G. Kostyanovskii, Asymmetric nitrogen.76. Quantum Chemical Study of Geometry and Configurational Stability of Ketenimines, Russian Chemical Bulletin 41 (1992) 2028–2033. 17

79. M.T. Nguyen, A.F. Hegart, The Reaction Pathway for the Hydration of Ketenimine by Water Dimer. An Ab Initio Study, J. Mol. Struct. (Theochem) 93 (1983) 329–332. 80. D. Tahmassebi, Substituent Effects on 13C Chemical Shifts of Ketenimines: a GIAO/HF and DFT Study, Magn. Reson. Chem. 41 (2003) 273–277. 81. M. Alajarin, M. Marin-Luna, A. Vidal, Recent Highlights in Ketenimine Chemistry, Eur. J. Org. Chem. 2012 (2012) 5637–5653. 82. S. Saebø, L. Farnell, N.V. Riggs, L. Radom, Molecular Structure, Rotational, Constants, and Vibrational Frequencies for Ethynamine (NH -C =CH): A Possible Interstellar 2 Molecule, J. Am. Chem. Soc. 106 (1984) 5047–5051. 83. B.V. Baar, W. Koch, C.B. Lebnlla, J.K. Terlouw, T. Weiske, H. Schwarz, Aminoacetylene and Its Mono- and Dication- Identification of Potentially Interstellar Molecules, Angew. Chem., Int. Ed. Engl. 25 (1986) 827–828. 84. C. Wentrup, H. Briehl, P. Lorencak, U.J. Vogelbacher, H.-W. Winter, A. Maquestiau, R.

Flammang, Primary Ethynamines (HC=CNH2, PhC=CNH2), Aminopropadienone

(H2NCH=C=C=O), and Imidoyketene (HN=CHCH=C=O). Preparation and Identification of Molecules of Cosmochemical Interest, J. Am. Chem. Soc. 110 (1988) 1337–1343. 85. A.C. Hopkinson, M.H. Lien, K. Yates, I.G. Csizmadia, A Non-Empirical Molecular

Orbital Study of Valence Tautomers of C2H3N, Int. J. Quantum Chem. 12 (1977) 355– 368. 86. S. Li, D.F. Davidson, R.K. Hanson, Shock Tube Study of Ethylamine Pyrolysis and Oxidation, US National Combustion Meeting (2013) Paper No. 070RK–0075. 87. M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery, Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G. A. Voth, P. Salvador, J.J. 18

Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian, Inc., Wallingford CT, Gaussian 09, Revision D.01, 2013. 88. (a) Y. Zhao, Y., D.G. Truhlar, The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06 Functionals and Twelve Other Functionals, Theor. Chem. Acc. 120 (2008) 215-241. (b) Y. Zhao, D.G. Truhlar, Density Functionals with Broad Applicability in Chemistry, Acc. Chem. Res. 41 (2008) 157-167.

89. J.F. Arenas, J.I. Marcos, I. López-Tocón, J.C. Otero, J. Soto, Potential-Energy Surfaces Related to the Thermal Decomposition of Ethyl Azide: The Role of Intersystem Crossings, J. Chem. Phys. 113 (2000) 2282–2289.

Figures Captions: Fig.1: Optimized Geometries for the Reactants, Transition States, and Products Involved in all Investigated Pathways (A, B1→B3, C1→ C3). Fig.2: Optimized Structures for the Decomposition of Methanimine at B3LYP/6- 31++G(3df,3dp) level of Theory and their Relative Energies at Different Levels of Theory for Pathway A. Fig.3: A Comparison of the Activation Energies for the Decomposition of Ethanimine (kJ mol−1) at G4MP2 Level of Theory for Pathways B1→B3. Fig.4: A Comparison of the Activation Energies for the Decomposition of Ethenamine (in kJ mol−1) at G4MP2 Level of Theory for Pathways C1→ C3.

Scheme 1: Possible mechanisms for the pyrolysis of second-stage cracking products of ethylamine: the decomposition reaction of methanimine, ethanimine and aminoethylene.

Fig.1: Optimized Geometries for the Reactants, Transition States, and Products Involved in all Investigated Pathways (A, B1→B3, C1→C3). 21

Fig.2: Optimized Structures for the Decomposition of Methanimine at B3LYP/6- 31++G(3df,3dp) level of Theory and their Relative Energies at Different Levels of Theory for Pathway A.

Fig.3: A Comparison of the Activation Energies for the Decomposition of Ethanimine (kJ mol−1) at G4MP2 Level of Theory for Pathways B1→B3.

500 481 Pathway C1

433 Pathway C2

400 Pathway C3

297 300

200

163 Relative Energy (kJ/mol) Energy Relative 121 100 110

0 0

Fig.4: A Comparison of the Activation Energies for the Decomposition of Ethenamine (in kJ mol−1) at G4MP2 Level of Theory for Pathways C1→ C3.

Table 1: Activation Energies, Enthalpies of Activation, and Gibbs Free Energies of Activation for the Decomposition of Methanimine (in kJ mol-1) at 298.15 K (Pathway A)

B3LYP/ B3LYP/ B3LYP/ B3LYP/6 M06-2X/ M06-2X/ MP2/ MP2/ 6-31G 6-31+G 6-31G -311++G 6-31G 6-31G G4MP2 6-31G 6-31+G (d) (d) (2df,p) (3df,3pd) (d) (2df,p) (d) (d) ∆E‡, TSA 418 417 399 396 428 405 404 437 434 ∆H‡, TSA 418 418 400 397 428 406 405 437 435 ∆G‡, TSA 417 416 399 395 427 405 403 437 434 ∆S‡, TSA 0.005 0.007 0.004 0.004 0.005 0.004 0.004 0.003 0.003

Table 2: Relative Energies, Enthalpies, and Gibbs Free Energies for the Decomposition of Methanimine (in kJ mol-1) at 298.15 K (Pathway A).

B3LYP/ B3LYP/ B3LYP/ B3LYP/6 M06-2X/ M06-2X/ MP2/ MP2/ 6-31G 6-31+G 6-31G -311++G 6-31G 6-31G G4MP2 6-31G 6-31+G (d) (d) (2df,p) (3df,3pd) (d) (2df,p) (d) (d) ∆E, TSA 43 45 44 44 44 43 31 -4 16 ∆H, TSA 51 53 52 51 49 47 39 3 8 ∆G, TSA 32 33.1 33 32 37 36 20 -16 -5 ∆S, TSA 0.064 0.065 0.062 0.064 0.038 0.037 0.063 0.062 0.046

Table 3: Activation Energies, Enthalpies of Activation, and Gibbs Free Energies of Activation for the Decomposition of Ethanimine (in kJ mol-1) at 298.15 K (Pathways B1→B3).

B3LYP/ B3LYP/ B3LYP/ B3LYP/6 M06-2X/ M06-2X/ MP2/ MP2/ 6-31G 6-31+G 6-31G -311++G 6-31G 6-31G G4MP2 6-31G 6-31+G (d) (d) (2df,p) (3df,3pd) (d) (2df,p) (d) (d) ∆E‡, TSB1 409 407 395 392 414 398 402 424 420 ∆H‡, TSB1 410 408 396 394 416 399 403 426 421 ∆G‡, TSB1 407 405 394 391 414 397 401 423 419 ∆S‡, TSB1 0.009 0.009 0.009 0.009 0.007 0.007 0.009 0.008 0.008 ∆E‡, TSB2 398 398 381 379 408 387 387 415 412 ∆H‡, TSB2 399 399 382 380 409 388 387 416 413 ∆G‡, TSB2 398 398 381 379 407 387 386 415 412 ∆S‡, TSB2 0.004 0.005 0.004 0.004 0.004 0.003 0.004 0.003 0.004 ∆E‡, TSB3 409 405 389 381 425 403 396 442 433 ∆H‡, TSB3 408 404 389 381 425 402 396 441 432 ∆G‡, TSB3 410 406 390 382 426 404 397 443 434 ∆S‡, TSB3 -0.004 -0.004 -0.004 -0.003 -0.005 -0.005 -0.004 -0.005 -0.005

Table 4: Relative Energies, Enthalpies, and Gibbs Free Energies for the Decomposition of Ethanimine (in kJ mol-1) at 298.15 K (Pathways B1→B3).

B3LYP/ B3LYP/ B3LYP/ B3LYP/6 M06-2X/ M06-2X/ MP2/ MP2/ 6-31G 6-31+G 6-31G -311++G 6-31G 6-31G G4MP2 6-31G 6-31+G (d) (d) (2df,p) (3df,3pd) (d) (2df,p) (d) (d) ∆E, TSB1 9 9 13 7 13 16 4 -26 -22 ∆H, TSB1 18 18 20.8 16 21 24 12 -18 -13 ∆G, TSB1 -15 -16 -8 -17 -2 -1 -17 -55 -54 ∆S, TSB1 0.111 0.112 0.095 0.111 0.079 0.083 0.096 0.124 0.140 ∆E, TSB2 25 30 27 122 35 33 23 -12 -4 ∆H, TSB2 31 39 33 130 40 39 29 -4 4 ∆G, TSB2 16 14 17.0 101 25 24 13 -33 -19 ∆S, TSB2 0.051 0.084 0.052 0.098 0.052 0.050 0.053 0.096 0.078 ∆E, TSB3 136 137 124 122 153 138 136.4 141 141 ∆H, TSB3 144 145 131 130 159 143 143.8 148 147 ∆G, TSB3 123 121 112 101 146 131 124.1 128 131 ∆S, TSB3 0.070 0.081 0.066 0.098 0.044 0.043 0.066 0.069 0.054

Table 5: Activation Energies, Enthalpies of Activation, and Gibbs Free Energies of Activation for the Decomposition of Ethenamine (in kJ mol-1) at 298.15 K (Pathways C1→ C3).

B3LYP/ B3LYP/ B3LYP/ B3LYP/6 M06-2X/ M06-2X/ MP2/ MP2/ 6-31G 6-31+G 6-31G -311++G 6-31G 6-31G G4MP2 6-31G 6-31+G (d) (d) (2df,p) (3df,3pd) (d) (2df,p) (d) (d) ∆E‡, TSC1 425 423 417 418 447 433 433 456 454 ∆H‡, TSC1 426 425 418 420 447 434 434 457 454 ∆G‡, TSC1 424 420 416 417 446 432 432 456 453 ‡ ∆S , TSC1 0.008 0.016 0.006 0.013 0.003 0.006 0.006 0.003 0.003 ∆E‡, TSC2 478 479 459 458 492 469 481 528 491 ∆H‡, TSC2 480 481 461 461 495 471 484 530 493 ∆G‡, TSC2 476 477 457 455 491 467 480 526 490 ∆S‡, TSC2 0.013 0.014 0.013 0.021 0.012 0.014 0.013 0.014 0.009 ∆E‡, TSC3 309 277 303 301 309 303 297 294 285 ∆H‡, TSC3 308 277 302 300 308 302 296 294 285 ∆G‡, TSC3 309 277 303 302 310 303 298 294 285 ∆S‡, TSC3 -0.005 0.001 -0.005 -0.005 -0.005 -0.005 -0.005 0.001 0.001

Table 6: Relative Energies, Enthalpies, and Gibbs Free Energies for the Decomposition of Ethenamine (in kJ mol-1) at 298.15 K (Pathways C1→ C3).

B3LYP/ B3LYP/ B3LYP/ B3LYP/6 M06-2X/ M06-2X/ MP2/ MP2/ 6-31G 6-31+G 6-31G -311++G 6-31G 6-31G G4MP2 6-31G 6-31+G (d) (d) (2df,p) (3df,3pd) (d) (2df,p) (d) (d)

∆E, TSC1 110 116 108 112 129 124 121 108 115 ∆H, TSC1 117 124 115 121 135 129 129 115 123 ∆G, TSC1 96 99 95 92.4 121 116 108 93 97 ∆S, TSC1 0.072 0.084 0.068 0.096 0.047 0.046 0.068 0.072 0.089 ∆E, TSC2 171 170 160 158 179 164 163 150 153 ∆H, TSC2 181 179 169 169 186 171 173 161 165 ∆G, TSC2 156 158 146 141 168 154 149 133 137 ∆S, TSC2 0.083 0.070 0.080 0.092 0.060 0.056 0.080 0.091 0.094 ∆E, TSC3 144 108 131 119 145 133 110 88 84 ∆H, TSC3 151 115 138 126 151.1 140 118 97 92 ∆G, TSC3 136 99.1 121 109 137.1 124 100 78 75 ∆S, TSC3 0.052 0.054 0.057 0.059 0.047 0.051 0.058 0.062 0.060

Graphical Abstract

Highlights

 The decomposition of mathanimine, ethanimine, and ethenimine were investigated using several levels of theory.

 Investigated reactions characterise either H2 elimination or 1,3-proton shift.  The decomposition of ethanimine was the most plausible reaction with an activation energy of 302 kJ mol−1 at G4MP2.