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. 2019 Feb 14;4(2):3306–3313. doi: 10.1021/acsomega.8b02792

Unimolecular Decomposition Reactions of Propylamine and Protonated Propylamine

Mansour H Almatarneh †,‡,*, Ismael A Elayan †, Mazen Al-Sulaibi †, Ahmad Al Khawaldeh †, Sedeeqa O W Saber, Mahmood Al-Qaralleh †, Mohammednoor Altarawneh §,∥
PMCID: PMC6648381  PMID: 31459545

Abstract

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A detailed computational study of the decomposition reaction mechanisms of cis-propylamine (cis-PA), trans-propylamine (trans-PA), and the cis-isomer of its protonated form (cis-HPA) has been carried out. Fourteen major pathways with their kinetic and thermodynamic parameters are reported. All reported reactions have been located with a concerted transition state, leading to significant products that agree with previous theoretical and experimental studies. Among six decomposition pathways of trans-PA, the formation of propene and NH3 is the significant one, kinetically and thermodynamically, with an activation energy barrier of 281 kJ mol–1. The production of two carbenes is found via two different transition states, where the reactions are thermodynamically controlled and reversible. Furthermore, five decomposition pathways of cis-PA have been considered where the formation of ethene, methylimine, and H2 is the most plausible one with an activation energy barrier of 334 kJ mol–1. The results show that the formation of propene and NH4+ from the decomposition of cis-HPA is the most favorable reaction with an activation barrier of 184 kJ mol–1, that is, the lowest activation energy calculated for all decomposition pathways.

Introduction

n-Propylamine (PA) is an aliphatic primary amine with the chemical formula C3H9N. PA is incessantly included in organic syntheses, industrial formulation, and biological processes. In pharmacopeia, PA used as a synthetic precursor for antifungal,1 antimalarial,2 antibacterial,3 antivirals,4 and anticancer5 agents. In the processing of nanomaterials, PA is utilized in the synthesis,6,7 size8 and morphology9 control, and functionalization of nanosized products.10 Moreover, PA found applications in the preparation of catalysts,11−13 molecular sieves,14 corrosion inhibitors,15,16 and removal of liquid17 and gaseous18 pollutants. The combustion of PA with fuels leads to the formation of nitrogenized pollutants, that is, HCN, HCNO, and NH3.19,20 These pollutants, if emitted, react with carbonyl oxide (Criegee intermediate) and contribute to the secondary organic aerosol formation, thus contributing to climate change.21 The widespread applications of PA raise the environmental concern about the emitted species during environmental remediation of PA-containing products.

The decomposition of both PA and protonated n-propylamine (HPA) has been the focus of several studies.22−26 The decomposition of PA shows significant chemical characteristics that require further investigation. The reaction is homogeneous and apparently unimolecular;26 furthermore, it is the first order and probably a chain one.27 The order of skeletal bond scission probability is α-CC bond > CN bond >β-CC bond.28 PA has a peculiar property in that it dissociatively ionize with the production of alkyl-free radicals in addition to methylenimmonium ion CH2NH2+29 which is considerably stabilized by the formation of C=N π-bond,30 whereas the NH3 loss, that is analogous to the dehydration of n-propylalcohol, is unfavorable31 due to the electrostatic repulsion between the terminal heavy atoms.32 In the presence of kaolin,33 the PA is decomposed into hydrogen cyanide (6.6%), propylene (6.4%), H2 (6.3%), N2 (4.6%), and ethylene (3.0%). On the silicon Si surface, eight decomposition products were studied theoretically.25 The formation π-bond-containing products, alkyl cyanide (activation energy 242 kJ mol–1), alkene (334 kJ mol–1), and imine(343 kJ mol–1), were the most plausible dissociation pathways, whereas the formation of strained cyclic products, azetidine (467 kJ mol–1), cyclopropane (493 kJ mol–1), and aziridine (506 kJ mol–1), were kinetically less favorable. The formation of NH3 and H2 is within acceptable activation energy values of 384 and 400 kJ mol–1, respectively.25 In aqueous solution, the PA is in equilibrium with HPA. The proton affinity and the structural change of PA through protonation affect the dissociation products of HPA.34,35 Experimental studies showed that it follows three fragmentation patterns corresponding to propene loss, NH3 loss, and ethane elimination.22,24

There are different experimental factors that affect the fragmentation pattern of amines, including the activation or fragmentation regime,36 the applied power and time, the internal energies of the molecular ions,37 and the molecular weight.38 Collectively, these factors account for the experimental inconsistency in the reported products, fragmentation patterns, and intensities. Hence, further understanding is required on the decomposition reactions of PA (Scheme 1) and HPA (Scheme 2). This shall shed more light on the chemistry of the decomposition reactions, formed products, and decrease the uncertainties in the previous literature. Therefore, the goal of this computational study is to further investigate and study novel pathways of the dissociation of PA and HPA. Furthermore, the formed products through different pathways are compared with the reported experimental data to ensure the results of this work.

Scheme 1. Proposed Decomposition Reactions of trans-PA and cis-PA.

Scheme 1

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Scheme 2. Proposed Decomposition Reactions of cis-HPA.

Scheme 2

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Results and Discussion

Different gas-phase decomposition pathways of PA have been studied to provide more kinetic and mechanistic insights. The studied pathways are based on previous experimental findings26,27 and further computational consideration is built on chemical intuition and different discoveries. It is worth noting that all studied decomposition pathways occur in a concerted step as an endothermic process. The kinetic parameters (Ea, ΔH⧧, and ΔG⧧) for the investigated pathways are reported herein at different levels of theory. The plausible pathways that will be discussed are compared based on the calculated kinetics, the lower the energy the more plausible the reaction. The reaction coordinates (R: reactant, TS: transition state, and P: product) are depicted on a potential energy surface (PES) for related pathways to define the energies of the most plausible pathways. The α-, β-, and γ-carbons of PA with respect to the nitrogen atom are indicated in some pathways, upon discussion, whenever is needed.

Decomposition of trans-Propylamine

The cis and trans are conformational isomers; however, both isomers lead to the formation of different products. A connection between both isomers through the same pathways has been accounted for; however, the energy barrier is very negligible. For example, the experimentally detected cyclic forms of aziridine, azetidine, and cyclopropane are produced from the cis-isomer while other forms are produced more easily from the trans. The decomposition of trans-propylamine (trans-PA) leads to the formation of different products of propene, carbene (CH3CH2C̈H), imine, cyclopropanamine, NH3, H2, and CH4. This section examines the formation of these products through six separate pathways (see Scheme 1 and Figure 1).

Figure 1.

Figure 1

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Proposed reaction mechanisms for the decomposition of trans-PA (pathways A to C).

One of the most indicated and proposed pathways in the decomposition reactions is the formation of an alkene and NH3. Herein, this route of reaction is designated as pathway A, where TSA shows how the C–N and C–H bonds are elongated to 1.610 and 1.764 Å, respectively, while the C–C bond length is decreased to 1.483 Å. This dissociation step is a concerted one step that leads to the formation of propene and NH3 (PA), see Figure 1. The activation energy of TSA is lower compared to other pathways with a value of 297 kJ mol–1 at B3LYP/6-31G(d). Increasing the Gaussian functions to B3LYP/6-311++G(3df,3pd) decreased the energy barrier to 283 kJ mol–1, that is, in agreement with the lower calculated value of 281 kJ mol–1, at CBS-QB3. The thermodynamic parameters indicate that the reaction is endothermic by 39 kJ mol–1 and endergonic by 16 kJ mol–1 at CBS-QB3. The calculated activation energies are given in Table 1 at different levels of theory.

Table 1. Kinetic Parameters (Ea, ΔH⧧, and ΔG⧧) for the Decomposition of trans-PA (in kJ mol–1) at 298.15 K.

  levels of theory
  B3LYP/6-31G(d)
 B3LYP/6-311++G(3df,3pd)
 CBS-QB3
pathways (TS) Ea(0K) ΔH⧧ ΔG⧧ Ea(0K) ΔH⧧ ΔG⧧ Ea(0K) ΔH⧧ ΔG⧧
pathway A (TSA) 297 298 296 283 283 282 281 281 280
pathway B (TSB) 404 406 399 388 390 383 391 393 387
pathway C (TSC) 349 351 346 330 332 328 329 331 327
pathway D (TSD) 395 396 396 379 379 379 379 380 380
pathway E (TSE) 424 426 423 393 396 392 401 404 400
pathway F (TSF) 420 420 421 401 400 401 406 405 406
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Another significant route is producing CH4, which can be generated through two different reaction mechanisms. For example, pathway B occurs from the trans-PA while another different mechanism of pathway J (discussed in a further section) is initiated from the cis-propylamine (cis-PA), generating different coproducts. Pathway B involves the dissociation of the terminal γ-carbon through TSB with a bond length of 2.362 Å from the β-carbon, followed with a simultaneous proton transfer with a length of 1.202 and 1.392 Å approaching the γ and away from the β-carbon, respectively (Figure 1). Interestingly, this step produces a carbene (C̈HCH2NH2) and CH4 as final products (PB). However, compared to pathways B and C, the reaction is not kinetically favored because of the high calculated energy barriers of 404 kJ mol–1 at B3LYP/6-31G(d), 388 kJ mol–1 at B3LYP/6-311++G(3df,3pd), and 391 kJ mol–1 at CBS-QB3. The high energy barrier could be attributed to the formation of the carbene where the carbon is sp-hybridized. However, the lower activation energy values of TSC via pathway C mark that this is not the main case because another carbene of CH3CH2C̈H with NH3 as a coproduct (PC) is formed with respective values of 349, 330, and 329 kJ mol–1 at B3LYP/6-31G(d), B3LYP/6-311++G(3df,3pd), and CBS-QB3, respectively (Table 1). The carbene in pathway C is formed through the detachment of the C–N bond length of 1.684 Å with a simultaneous proton transfer from the α-carbon of 1.459 Å to the amino group forming PC. It is worth noting that based on the PES of Figure 3, pathways B and C are thermodynamically reversible. The carbenes most probably will react with other species in the atmosphere as they are very reactive and unstable.

Figure 3.

Figure 3

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PES of the decomposition of trans-PA, pathways A to F, calculated at CBS-QB3.

The formation of an imine and H2 (pathway D) through the decomposition of trans-PA via TSD occurs with a single step by the removal of H2 from PA with lengths of 1.560 and 1.437 Å. Simultaneously, the C=N imine bond is formed where the C–N bond decreased by about 0.8 Å to become 1.387 Å, see Figure 2. Likewise, pathway E signifies the formation of prop-2-en-1-amine and H2 (PE) through TSE. However, the kinetic scenario is different in favorability where the former (TSD) is more favorable than the latter (TSE) at different levels of theory. The activation energy values for TSD are lower than TSE by 29, 14, and 22 kJ mol–1 at B3LYP/6-31G(d), B3LYP/6-311++G(3df,3pd), and CBS-QB3, respectively. Moreover, enthalpies and Gibbs energies of activation are lower, significantly favoring pathway D than E, refer to Table1. Furthermore, thermodynamically, pathway D is more favorable with lower parameters (ΔE, ΔH, and ΔG) and by 71 kJ mol–1.

Figure 2.

Figure 2

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Proposed reaction mechanisms for the decomposition of trans-PA (pathways D to F).

Ultimately, the formation of cyclopropanamine and H2 through a different decomposition route, pathway F, undergoes TSF in a cyclization step. C–C bond with a length of 2.577 Å and hydrogens secession from the α- and γ-carbons of 2.735 and 2.109 Å, respectively. The hydrogen attached to the α-carbon is distinct from the γ with a length longer by about 0.6 Å, and this is attributed to its C–N bond that leads to a higher steric factor. However, in terms of energy, this reaction mechanism and formed products (PF) are not kinetically favored. Table 1 shows that for pathways F and E (producing H2), the activation energies are higher than the other investigated pathways at different levels of theory. For instance, the calculated energy barrier for the TSF value of 406 kJ mol–1 at CBS-QB3 is the highest among the other transition state energies. Interestingly, the thermodynamic calculations of this reaction mechanism are more favored than the other trans-PA H2-producing pathways D and E. The respective enthalpy and Gibbs energy values of 83 and 65 kJ mol–1 are more favored than for pathways D and E by about 7 and 74 kJ mol–1, respectively. This result sheds more light on the mechanism of how H2 is produced more favorably through pathway D and further helps in explaining why the cyclopropanamine and propenamine are not experimentally detected. The relative energies of a PES for the investigated decomposition pathways of trans-PA are given in Figure 3. It is worth noting that increasing the polarization and diffuse functions at B3LYP/6-311++G(3df,3pd) decreased the energies of all pathways, and the calculated energies for all TSs are consistent to CBS-QB3 by no more than 5 kJ mol–1 difference.

Decomposition of cis-PA

The decomposition of cis-PA, as other separate pathways occur, is due to the geometry of the PA; in which the trans-PA cannot undergo these reaction mechanisms and vice versa. Furthermore, the decomposition from both stereoisomers to form H2 proves that the yield of 6.3% cannot be neglected and might be more than expected.33 For example, TSG through pathway G is a different one that leads to the formation of prop-1-en-1-amine and H2 (similar to pathway E but with geometric isomerism). However, bond lengths and activation energies of TSG differ from TSE; this is attributed to the terminal alkene that is being formed in TSE which is less sterically hindered than TSG. The hydrogens dissociate from the α and γ-carbons with lengths of 2.445 and 1.186 Å with a forming C=C double bond of 1.448 Å, see Figure 4.

Figure 4.

Figure 4

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Proposed reaction mechanisms for the decomposition of cis-PA (pathways G, H, and I).

The more favorable formation of TSE rather than TSG is also confirmed through the activation energies, where it is higher in the latter by about 88 kJ mol–1at different levels of theory (Table 2). The activation energy value of 499 kJ mol–1 at B3LYP/6-31G(d) decreased to 481 kJ mol–1 at B3LYP/6-311++G(3df,3pd). Moreover, the calculations at CBS-QB3 are in agreement with both levels with a value of 489 kJ mol–1. However, it is worth noting that thermodynamically, pathway G is more plausible with lower enthalpy and Gibbs energies by about 34 and 40 kJ mol–1 at different levels of theory.

Table 2. Kinetic Parameters (Ea, ΔH⧧, and ΔG=⧧) for the Decomposition of cis-PA (in kJ mol–1) at 298.15 K.

  levels of theory
  B3LYP/6-31G(d)
 B3LYP/6-311++G(3df,3pd)
 CBS-QB3
pathways (TS) Ea(0K) ΔH⧧ ΔG⧧ Ea(0K) ΔH⧧ ΔG⧧ Ea(0K) ΔH⧧ ΔG⧧
pathway G (TSG) 499 501 498 481 482 480 489 490 487
pathway H (TSH) 500 498 503 494 492 496 492 490 494
pathway I (TSI) 435 436 434 414 416 413 416 417 414
pathway J (TSJ) 479 481 476 459 462 455 466 469 463
pathway K (TSK) 343 343 343 333 333 334 334 333 334
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As for the cyclic products, three significant pathways (H, I, and J) with different transition states were determined. Pathway H, where cis-PA decomposes to azetidine and H2 (PH), undergoes a reaction mechanism of C–N bond formation of 2.084 Å and H2 dissociation from the nitrogen atom and γ-carbon of 1.052 and 2.341 Å, respectively. However, the barrier heights of 500, 494, and 492 kJ mol–1 (Table 2) are higher than the previously reported value of 467 kJ mol–1, and this referred to the fact that Cho and Choi25 studied the decomposition pathways on a silicon surface. It should be noted that the cyclic formation pathways are indicated here according to their relevance. Moreover, in order to avoid repetition, the more plausible transition states are purely described while other ones are indicated by comparison and relevance.

Although the cyclopropane is more hindered than azetidine, it is found to be more favored in terms of energies, where the TSI barrier is lower than TSH, see Table 2. Furthermore, it is more thermodynamically favored with lower energies by about 100 kJ mol–1 at the reported levels. Thus, the presence of a nitrogen atom in PH alters the chemical reaction of electron delocalization from PI, which is also confirmed from the decomposition to aziridine and CH4 through TSJ (pathway J), see Figure 5. This indicates that TSJ is more plausible than TSH; hence, pathway J is more favored kinetically and thermodynamically than H.

Figure 5.

Figure 5

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Proposed reaction mechanisms for the decomposition of cis-PA (pathways J and K).

The more favored TSI forms the cyclopropane and NH3 (PI) through the dissociation of the amino group from the α-carbon of 2.104 Å with simultaneous proton transfer from the γ-carbon of 2.003 Å. This is accompanied by a C–C bond formation of 2.292 Å, corresponding to the cyclopropane formation, through TSI, with an activation energy of 435 kJ mol–1 at B3LYP/6-31G(d). The calculated results at B3LYP/6-311++G(3df,3pd) and CBS-QB3 levels of theory are consistent with energy barriers of 414 and 416 kJ mol–1, respectively. Furthermore, Table 2 shows that the energy values of decomposition to aziridine with CH4 (TSJ, Figure 5) and cyclopropane with NH3 (TSI) are lower than the previously reported values25 of 506 and 493 kJ mol–1, respectively.

The calculated activation energies of TSI are lower than TSH and TSJ, see Table 2. Thus, among the cyclic formation pathway, pathway I is the most plausible one. However, this does not mean that the other products are not formed because the decomposition reactions occur at very high temperatures that aid in the formation of different unexpected products. Moreover, studying the decomposition reaction and its modeling on other cluster surfaces increases the expectation of forming other products by acting as a catalyst surface for these reactions, considering the fact that also some products undergo further unimolecular or bimolecular reactions, leading to the formation of other products.

An interesting novel decomposition reaction, pathway K, that leads to the formation of ethane, methanimine, and H2 is found to be kinetically more favored than other pathways. The reaction parameters shown in Figure 5 depict the concerted step, TSK, of C–C and H2 dissociation. The former belongs to an α- and β-carbon length of 2.308 Å, while the latter belongs to 1.447 and 1.602 Å from the nitrogen and γ-carbon, respectively. This change in bond lengths is accompanied by the formation of imine with 1.319 Å and C=C double bond of 1.381 Å (PK). Extensive efforts to find a step-wise mechanism of this reaction to check with the bond lengths and energy barriers were considered; however, all efforts lead to the concerted one. It should be noted that the product geometries of pathway K are obtained at B3LYP/6-31G(d) because we were unable to locate the final products (PK) at B3LYP/6-311++G(3df,3pd) and CBS-QB3 levels of theory.

The kinetic scenario fulfills a reputable reaction barrier of the investigated decomposition reaction of cis-PA, where the calculated activation energies are the lowest compared to other pathways, see Table 2. The activation energy value of 343 kJ mol–1 at B3LYP/6-31G(d) is lower than range off other pathways and further improvement of basis sets conformable to the B3LYP/6-311++G(3df,3pd) level of theory decreased the barrier by 10 kJ mol–1. This value is also consistent with CBS-QB3 calculated energy barrier value of 334 kJ mol–1. Thus, pathway K is considered the most plausible pathway with lower activation energies at different levels of theory. This is also given in Figure 6 as a comparative PES of the investigated pathways at CBS-QB3. This reaction mechanism may also be the indicated sink reaction for the formation of H2 because it has a yield of 6.3%, along with the experimentally detected ethylene with its yield of 3.0%.36 This also shows the probabilities of H2 and other product formation through different decomposition reactions. It should be noted here that the barriers at B3LYP/6-311++G(3df,3pd) are comparable to CBS-QB3, differing by no more than 8 kJ mol–1.

Figure 6.

Figure 6

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PES of the decomposition of cis-PA, pathways G to J, calculated at CBS-QB3.

Decomposition of cis-HPA

The selection of the cis-isomerism of HPA rather than the trans is based on the investigated pathways (L, M, and N), in which it is possible to locate the desired transition states and products from the cis. This is based on our efforts to locate other transition states to find other pathways from the trans-isomer, which all leads to the investigated pathways of L, M, and N. The protonation is on the amino group (NH2) of PA that leads to the formation of an ammonia (NH3+) group, that is, a significant moiety for proton-related reactions as it will be discussed in this section.

The momentum of the ammonia group (NH3+) of HPA is proton releasing to α-, β-, or γ-carbons, which results in different kinetics, thermodynamics, and bond parameters. Pathway L shows the proton transfer from NH3+to γ-carbon with a length of 1.816 Å, followed by a simultaneous proton transfer of 1.553 Å from the γ- to β-carbon via TSL, see Figure 7. Interestingly, this step led to the formation of a hydrogen bond bridge between the β- and γ-carbons (PL), where the positive charge is located on the CH2 group (as shown in Scheme 2). Despite our extensive efforts, we were unable to locate a TS of the dissociation of the hydrogen bound complex that produces the desired products. Therefore, we believe that, at this state, the reaction proceeds to the hydrogen bound complex (the located product on the PES) with a higher energy barrier which makes it a kinetically unfavorable step and it is not worth investigating more. On the other hand, the barrier heights for TSL are relatively lower than the investigated decomposition pathways of cis- and trans-PA, where the barriers are 277, 265, and 269 kJ mol–1 at the B3LYP/6-31G(d), B3LYP/6-311++G(3df,3pd), and CBS-QB3 levels of theory (Table 3), respectively, particularly because these energies are significantly lower than other CH4-forming pathways of B and J by about 197 kJ mol–1. However, for pathway L, the PES in Figure 8 shows that the reaction is thermodynamically reversible with low product energies relative to the transition states.

Figure 7.

Figure 7

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Proposed reaction mechanisms for the decomposition of cis-PA (pathways G, H, and I).

Table 3. Kinetic Parameters (Ea, ΔH⧧, and ΔG⧧) for the Decomposition of cis-HPA (in kJ mol–1) at 298.15 K.

  levels of theory
  B3LYP/6-31G(d)
 B3LYP/6-311++G(3df,3pd)
 CBS-QB3
pathways (TS) Ea(0K) ΔH⧧ ΔG⧧ Ea(0K) ΔH⧧ ΔG⧧ Ea(0K) ΔH⧧ ΔG⧧
pathway L (TSL) 277 277 278 265 265 266 269 269 270
pathway M (TSM) 332 334 329 311 314 309 317 320 315
pathway N (TSN) 197 201 191 181 185 174 184 188 178
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Figure 8.

Figure 8

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PES of the decomposition of cis-HPA and pathways L, M, and N, calculated at CBS-QB3.

As mentioned earlier, there are considerable pathways of H2 production. For example, pathway M describes the formation of H2 through dissociation of the hydrogen bonds from the NH3+ and β-carbon with bond lengths in the range of 1.519 Å through TSM (Figure 7). Furthermore, this step of TSM leads to the formation of protonated propanimine (PM) with an imine consistent with other pathway bond length of 1.387 Å. However, the calculated energy barrier of TSM is exceedingly high, in comparison to other steps, with a value of 332 kJ mol–1, at B3LYP/6-31G(d). Further calculations at B3LYP/6-311++G(3df,3pd) and CBS-QB3 lower the energy barriers to 311 and 317 kJ mol–1 (Table 3), respectively. Relative energies of this pathway compared to other ones are given in a PES of Figure 8, where the reaction is endothermic and exergonic by 77 kJ mol–1.

Essentially, the production of ammonium cation (NH4+) along with propene is the most anticipated reaction mechanism of cis-HPA because of the presence of the galvanizing nitrogen moiety toward having the additional hydrogen. The reaction is initialized via TSN by the nitrogen dissociation followed with the hydrogen abstraction from the β-carbon with bond lengths of 2.291 and 1.521 Å, see Figure 8. The latter bond (N–H) formation has a bond length of 1.473 Å, that is, higher than the other N–H bond formation by about 0.4 Å. Furthermore, this concerted step is accompanied with the C=C double-bond formation with a bond length of 1.378 Å. The reaction is considerable with the lowest activation energies of all investigated pathways, making TSN the most plausible transition structure with an activation energy of 197 kJ mol–1 at B3LYP/6-31G(d). Further addition of polarization and diffuse functions led to a decrease in the energy barrier by 16 kJ mol–1. This is in agreement with the complete basis set method, CBS-QB3, the value of 184 kJ mol–1, refer to Table 3. Thus, based on the calculated energies and relative to other pathways (Figure 8), the reaction is kinetically favored and the most plausible pathway, forming propene and NH4+ (PN). It should be mentioned that the barriers at B3LYP/6-311++G(3df,3pd) are comparable to CBS-QB3 differing by no more than 6 kJ mol–1.

Conclusions

A thorough computational study of the gas-phase decomposition of PA and its protonated form has been detailed using density functional theory (DFT) and CBS-QB3 methods. Fourteen different pathways have been investigated; six for trans-PA, five for the cis-PA, and three for the cis-HPA. The kinetic and thermodynamic parameters have been reported at different levels of theory. Each desired reactant, transition structure, and product have been optimized and ensured its connection to the related pathway at different levels of theory. Two pathways have been considered to lead to the carbine formation with different reaction energies. Different decomposition pathways that lead to the formation of H2 along with significant products of propene, CH4, NH4+, and imine have been studied, compared, and reported, kinetically and thermodynamically. The most favored (energetically) pathways in the investigated decomposition reactions are A (281 kJ mol–1), K (334 kJ mol–1), and N (184 kJ mol–1) of trans-PA, cis-PA, and cis-HPA, respectively. Furthermore, three different pathways (B, C, and L) with thermodynamic reversibility were located and studied. Further unimolecular and bimolecular reactions are expected to occur and affect the energy barrier of the decomposition of PA, along with the need for further understanding of HPA decomposition chemistry because it leads to different products.

Computational Methods

All of the electronic structure calculations have been considered using the Gaussian 09 quantum package.39 Geometries were initially optimized by employing the hybrid method of Becke’s three-parameter using the LYP functional (B3LYP),40,41 utilizing the 6-31G(d) basis set.42 Further calculations by increasing the Gaussian functions corresponding to B3LYP/6-311++G(3df,3pd) were considered; this is to test the effects of polarization and diffuse functions on the energy barrier and ensure the reliability of calculations. Moreover, single-point energy calculations were employed using the more accurate method, CBS-QB3.43 The selected methods are also based on our previous studies of the decomposition pathways of different amines.19,44−47

Minimum energy stationary points were located and defined by first optimizing the geometries of the minima and saddle points. Analysis of the harmonic vibrational frequency of minima showed no imaginary frequency, while the transition state shows only one negative imaginary frequency in the Hessian matrix for the desired reaction pathways. Further analysis of transition states via the intrinsic reaction coordinate48 at B3LYP/6-31G(d) ensured the connection of the desired points for each proposed mechanism. Each reaction pathway is depicted on the PES along with the reacting points. The PES formulates the calculated activation energies relative to the initial reactive complex, which draws more conclusions on the investigated reaction pathways. All relative energies of optimized geometries were corrected with zero-point vibrational energies. The calculated activation energies (Ea) are reported at 0 K, while enthalpies of activation (ΔH⧧) and Gibbs energies of activation (ΔG⧧) are at 298.15 K.

B3LYP/6-31G(d) is the most commonly used functional, however, it still misses the long-range dispersion effect and van der Waals interactions. Furthermore, the accurate prediction of energy barriers, as well as the kinetics, is still a challenge to the B3LYP functional and DFT methods. Hence, the utilization of the composite method, CBS-QB3, comes in handy; due to the accurate description of energy barriers. Increasing the Gaussian functionals, corresponding to B3LYP/6-311++G(3df,3pd), produces energy values in qualitative agreement with the CBS-QB3 calculations, within no more than 8 kJ mol–1. This indicates that the B3LYP/6-311++G(3df,3pd) level of theory performs well to this system; however, further and different functionals should be used in future studies to describe the pyrolysis/decomposition reactions.

Acknowledgments

We are gratefully acknowledge the Atlantic Computational Excellence Network (ACENET) and Compute Canada for the computer time.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02792.

  • Cartesian coordinates of all optimized geometries for all proposed mechanisms along with the vibrational frequencies (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao8b02792_si_001.pdf (617.2KB, pdf)

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