The mechanism of the gas-phase elimination kinetics

Molecular Physics An International Journal at the Interface Between Chemistry and Physics

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The mechanism of the gas-phase elimination kinetics of the β,γ-unsaturated aldehyde 2,2–dimethyl-3-butenal: a theoretical study Andrea Rodriguez, Loriett Cartaya, Alexis Maldonado, Edgar Marquez, José R. Mora, Tania Cordova & Gabriel Chuchani To cite this article: Andrea Rodriguez, Loriett Cartaya, Alexis Maldonado, Edgar Marquez, José R. Mora, Tania Cordova & Gabriel Chuchani (2017): The mechanism of the gas-phase elimination kinetics of the β,γ-unsaturated aldehyde 2,2–dimethyl-3-butenal: a theoretical study, Molecular Physics, DOI: 10.1080/00268976.2017.1310325 To link to this article: http://dx.doi.org/10.1080/00268976.2017.1310325

Published online: 03 Apr 2017.

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Date: 03 April 2017, At: 07:14

MOLECULAR PHYSICS,  https://doi.org/./..

RESEARCH ARTICLE

The mechanism of the gas-phase elimination kinetics of the β,γ-unsaturated aldehyde ,–dimethyl--butenal: a theoretical study Andrea Rodrigueza , Loriett Cartayab , Alexis Maldonadob , Edgar Marquezc , José R. Morad , Tania Cordovae and Gabriel Chuchanib a

Departamento de Química, Escuela de Ciencias, Universidad de Oriente Núcleo Sucre, Cumana, Venezuela; b Centro de Química, Instituto Venezolano de Investigaciones Científicas (IVIC), Apartado, Venezuela; c Departamento de Ciencias Naturales y Exactas, Universidad de la Costa, Barranquilla, Colombia; d Departamento de Ingeniería Química, Universidad de San Francisco de Quito, Quito, Ecuador; e Department of Chemistry, College of Science and Technology, Florida A&M University, Tallahassee, FL, USA

ABSTRACT

ARTICLE HISTORY

The study on the mechanism of the gas-phase elimination or thermal decomposition kinetics of 2, 2-dimethyl-3-butenal has been carried out by using theoretical calculation at MP2, combined ab initio CBSQB3 and DFT (B3LYP, B3PW91, MPW1PW91, PBEPBE, PBE1PBE, CAMB3LYP, M06, B97d) levels of theory. A good reasonable agreement between experimental and calculated parameters was obtained by using CAMB3LYP/6-311G(d,pd) calculations. The contrasted calculated parameters against experimental values suggested decarbonylation reaction to proceed through a concerted five-membered cyclic transition state type of mechanism, involving the hydrogen transfer from the carbonyl carbon to the gamma carbon, consistent with observed kinetic isotope effect. The breaking of alpha carbon– carbonyl carbon bond to produce carbon monoxide is 50% advanced in the transition state. The reaction mechanism may be described as a concerted moderately non-synchronous process. Examination of the Atoms in Molecules (AIM) analysis of electron density supports the suggested mechanism.

Received  November  Accepted  March 

1. Introduction Experimental works of the gas-phase thermal decomposition kinetics of α, β-unsaturated aldehydes are few [1–4]. These reactions proceed though a unimolecular decarbonylation with the formation of the corresponding olefin. The pyrolysis kinetic of 2-butenal in a flow system was demonstrated to be molecular in nature [1]. This investigation comprised the thermal decompositions of 2-furaldehyde and benzaldehyde. The former substrate was believed to undergo a biradical mechanism, while the latter aromatic aldehyde showed to be a free radical reaction. In addition of this observation, an interesting co-pyrolysis experiment of 2-butenal in a 1:3 mixture with 2-butenal-O-d, gave a negligible kinetic isotope effect. Therefore, this result implied the α, β-unsaturated CONTACT Gabriel Chuchani

[email protected]

©  Informa UK Limited, trading as Taylor & Francis Group

KEYWORDS

Theoretical calculation; kinetic; mechanism; elimination; ,-dimethyl--butenal

butanal to go through a concerted three-membered cyclic transition state (TS) type of mechanism as described in reaction (1). The thermal decompositions of furaldehyde and benzaldehyde were reexamined in a microtubular flow reactor [3]. Furaldehyde showed to be unimolecular producing furan and CO gas, while the aromatic aldehyde was found to be a complex free radical reaction. The intermediate furan suggested a rearrangement to α-carbene, which then proceeded to give HCࣕCH and CH2 =C=O. Moreover, the simultaneous reaction of the furan intermediate also appears to rearrange to a β-carbene decomposing to CO + CH3 CࣕCH or to the radical pair HCCCH2 + HCO. The gas-phase elimination kinetics of two α-βunsaturated aldehydes, E-2-butenal and E-3-phenyl-

2

A. RODRIGUEZ ET AL.

H

O H3CCH

H3CCH CH C

C

C

H 3C

O

CH=CH2 + CO

(1)

H H

especially in static reaction system. However, a few years ago Julio et al. [6] reported these decarbonylation reactions to occur at much lower temperatures in the presence of a hydrogen halide catalyst. This work consisted the experimental study of the homogeneous, unimolecular elimination kinetics of α-methyl-transcinamaldehyde and E-2-methyl-2-pentenal catalysed by HCl (370.0–438.7 °C and 44–165 Torr) [6 ]. The products formed were CO gas and the corresponding olefin, and the mechanism is revealed in reaction (3). Reaction (3) appears to support the uncatalysed decarbonylation of α-β-unsaturated aldehydes undergoing a concerted three-membered cyclic TS type of mechanism [1,4]. Further work of the HCl catalysed decomposition kinetics of α, β-unsaturated aldehydes were carried out with the substrates 2-methyl-2-propenal and E-2-pentenal [7]. The mechanisms were found to have similar TS as reported above [6], that is, a polar concerted five-membered cyclic structure (reaction (3)). These results led to establish a sequence of reactivity as a function of the kinetic and thermodynamic parameters of the α, β-unsaturated aldehydes in the presence of HCl catalysis as follow: 2-methyl-2-propenal > E-2-pentenal > E-2-methyl-2-pentenal> α-methyltrans-cinamaldehyde. This order was thought in terms of substituent effect on the substrate.

methylpropenal [4], were carried out in a static system with the reaction vessels deactivated by the product of decomposition of allyl bromide. The reactions were found to be homogeneous, unimolecular and obey a first-order law. The products of E-2-butenal are propene and CO gas, while E-3-phenyl-methylpropenal yielded α-methylstyrene, cis-trans-β-methylstyrene, indan and CO gas. The results of the kinetic and thermodynamic parameters have considered the mechanisms of these processes to undergo through a three-membered cyclic TS structure. As a result, this work supports the mechanism forwarded before [1] as described in reaction (1). Theoretical calculations of the unimolecular, homogeneous thermal decomposition of 2-butenal and 2-methyl3-phenyl-2-propenal [5] at MP2, MP4 and B3LYP levels of theories suggested two different mechanism of these unsaturated aldehydes. The first process proceeds as a concerted three-membered cyclic TS as depicted in reaction (1). The second mechanism was considered a stepwise process with the formation of a ketene by a fourmembered cyclic TS structure as shown in reaction (2). Yet, this work did not report agreement between experimental and theoretical results. The molecular gas-phase elimination of α-βunsaturated aldehydes required high temperatures,

H

CHO

R1

R2

H

C

O

O H

C

R1

R2

H R1

R2

H

C

O

H R1

R2 (2)

H

H

R1

R2

+ CO

R2

H

Cl H

R1 CH C β

α

C

O HCl

H

R1 = C6H5, R2 = H R1 = C2H5, R2 = CH3

R1

CH β

C

α

R2

R1 CH CH R2 + CO + HCl

C O

(3)

MOLECULAR PHYSICS

H

CH3

β

α

C CH3

C γ

C O

H2C R

CH3

β

α

C CH3

HC γ

C O

H2C R

α

β

CH3 + CO

C

HC γ

3

(4)

CH3

H2C R

R: H, D

From the works described above, the gas-phase thermal decomposition kinetics of α,β-unsaturated aldehydes appears to have been reasonably studied. However, just a single published work has been reported with regard to the study of β,γ -unsaturated aldehyde, i.e. 2,2dimethyl-3-butenal thermal decomposition [8]. The gasphase elimination kinetic of this substrate (282–302 °C) was found to produce 2-methyl-2-butene and carbon monoxide. The kinetic isotope effect of this reaction gave a kH / kD =2.8 at 296.9 °C (7.2 at 25 °C) implying the hydrogen attached to the C=O of the aldehyde migrated during the slow step of the reaction. These results suggested a unimolecular reaction and obeying a first-order rate law. Moreover, a five-membered cyclic TS type of mechanism was proposed (reaction (4)). Benson’s theory [9,10] postulated that a fivemembered cyclic structure is a more favoured TS than that of a three-membered cyclic structure in gas-phase elimination or decomposition reaction. As mentioned above, the experimental results of the single β,γ -unsaturated aldehyde was found to decompose at a significant lower temperatures and at a faster rate than those of the three-membered α,β-unsaturated aldehydes. From the reported experimental data of 2,2-dimethyl3-butenal thermal decomposition, the present study intents to verify or modify the proposed mechanism by means of quantum chemical calculations. This work presents a study of the minimum energy pathways of the β,γ -unsaturated aldehyde decomposition producing 2-methyl-2-butene and carbon monoxide at MP2, combined ab initio CBSQB3 and DFT (B3LYP, B3PW91, MPW1PW91, PBEPBE, PBE1PBE, CAM-B3LYP, M06, B97d) levels of theory. Additionally, the aim is to obtain the kinetic and thermodynamic parameters of this reaction and to compare them with the experimental values.

2. Theoretical methods and models The gas phase thermal decomposition of 2,2-dimethyl3-butenal has been estimated by Mǿller–Plesset perturbation theory (MP2) [11,12], combined ab initio CBS-QB3 [13,14] and density functional theory (DFT) methods B3LYP [15–17], B3PW91-CAMB3LYP [18–21],

MPW1PW91 [22], PBEPBE [23], PBE1PBE [24], M06 [25] and B97D [26]. The calculations were carried out using Gaussian 09 package [27]. In Berny analytical gradient optimisation algorithm required convergence on the density matrix was 10−9 atomic units, the thresh˚ old value for maximum displacement was 0.0018 A and maximum force 0.00045 Hartree/Bohr. To obtain the TS geometries, the Quadratic Synchronous Transit method was used. Calculations of the intrinsic reaction coordinate (IRC) were carried out to prove that the TS structures associate the reactant and products in the reaction path. Vibrational analysis permitted to attain thermodynamic quantities such as zero point vibrational energy (ZPVE), temperature corrections [E(T)] and absolute entropies [S(T)]. Corrections of the temperature and absolute entropies were gotten by assuming ideal gas behaviour from the harmonic frequencies and moments of inertia by standard methods [28] at average temperature and pressure values within the experimental range The Transition State Theory (TST) [10] was used to estimate the first-order rate coefficients k(T) and it was calculated assuming that the transmission coefficient is equal to 1, in the expression:   k (T ) = (kB T/h) exp −G# /RT where G# is the Gibbs free energy change between the reactant and the TS and kB , h are the Boltzmann and Planck constants, respectively. G# was calculated using the following relations: G# = H # − TS# and, H # = V # + ZPVE + E (T ) where V# is the potential energy barrier, ZPVE accounts for the differences of ZPVE between the TS and the reactant, and E (T) represents the contribution of thermal energy at a given temperature. Theoretical kinetic and thermodynamic parameters are to be compared with the reported experimental results in order to assume a reasonable mechanism. The consideration of this elimination process, the geometries of the structures of reactants and products, and the TSs

4

A. RODRIGUEZ ET AL.

CH3

H H3C

O

C H

H

CH3

H3C

CH3

CH3

+ CO

C O

H2C H

CH3

Scheme .

C

Mechanism of ,-dimethyl--butenal.

were estimated as described in most recent publications [29–31]. Topological analysis of the wave function [32] was employed for the determination of the electronic charge density. Default parameters were used to generate graphic contours and relief maps, as well as with the topological parameters and with the AIM charges.

3. Results and discussions The mechanism proposed in the thermal gas-phase decomposition reaction of 2,2-dimethyl-3-butenal producing 2-methyl-2-butene and carbon monoxide is depicted in Scheme 1. The calculated parameters at the several levels of theory used in this study are listed in Table 1. CBS-QB3 and CAMB3LYP level of theory gave reasonable values when related to the experimental results. The present work has chosen the CAMB3LYP/6-311G (d,pd) level of theory for discussion of the mechanism of gas-phase elimination of 2,2-dimethyl-3-butenal. The influence of the basis set in the activation energy obtained from the CAM-B3LYP

functional let to closer results using the addition of polarisation functions. Polarisation functions add flexibility within the basis set and permits molecular orbitals to be more asymmetric around the nucleus. Consequently, this means a more accurate description when a bond between atoms is distorted. Calculations adding diffuse functions tend to overestimate the energy. This fact may be because diffuse functions are necessary when negative species are present, which is not the case in our system, as shown in Table 3 [33]. The TS was confirmed using intrinsic reaction coordinate IRC calculation, connecting the reactant 2,2dimethyl-3-butenal and the products 2-methyl-2-butene, and carbon monoxide. The IRC plot is shown in Figure 1. The structural parameters for the bonds involved in 2,2-dimethyl-3-butenal thermal decarbonylation are revealed in Table 2. Atom numbering is shown in Scheme 2. Geometric parameters show the distance C3 –C4 has ˚ in the reactant increased significantly from 1.53 A ˚ in the TS, illustrating the breaking of the to 1.99 A

Table . Thermodynamic and Arrhenius parameters of ,-dimethyl--butenal at . ºC. Method Experimental BLYP/-G(d,p) BLYP/-++G(d,p) BPW/-G(d,p) BPW/-++G(d,p) MPWPW/-G(d,p) MPWPW/-++G(d,p) PBEPBE/-G(d,p) PBEPBE /-++G(d,p) PBEPBE/-G(d,p) PBEPBE/-G++(d,p) M/-G(d,p) M/-++G(d,p) Bd/-G(d,p) Bd/-G++(d,p) MP/-G (d,p) MP/-G++(d,p) CBS-QB CAMBLYP/-++G(d,p) CAMBLYP/-G(d,p) CAMBLYP/-G(d,pd) CAMBLYP/-G(d,p)

Ea (kJ/mol)

log A (s− )

Hࣔ (kJ/mol)

Sࣔ (J/mol K)

Gࣔ (kJ/mol)

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

− . − . − . − . − . − . − . − . − . − . − . − . − . − . − . − . − . − . − . − . − . − .

. . . . . . . . . . . . . . . . . . . . . .

MOLECULAR PHYSICS

5

Figure . IRC reaction profile for the decarbonylation of ,-dimethyl--butenal in gas phase at CAMBLYP/-G(d,pd) level of theory.

CH3

H C2

C3

C CH3

H C4 O

C1 H

H5

Scheme . Transition State of ,-dimethyl--butenal.

alpha carbon–carbonyl carbon bond leading to carbon monoxide formation. The distances C2 –C3 and C2 –C1 change from the reactant to the TS showing the C2 –C3 bond order alters from single to double, and C2 –C1 is converted from double to single. The variation in C4 –H5 and H5 –C1 bond distances in the TS shows the hydrogen transfer from the carbonyl carbon C4 to the gamma carbon C1 . It can be seen in the dihedrals angles that the five atoms involved in the TS are

not planar. The TS single imaginary frequency is associated with the vibration of the aldehyde hydrogen between the gamma carbon and the carbonyl carbon. The optimised structures of the reactant 2,2-dimethyl3-butenal, the TS and the products 2-methyl-2-butene and carbon monoxide are illustrated in Figure 1. Analysis of NBO charges is used to explain the change in electronic structure along the reaction pathway. NBO charges are reported in Table 3. Significant changes in NBO charges are observed in the TS. There is a decrease in electron density at the alpha carbon, i.e. C3 in the TS. There is also an increase in the negative charge at the gamma carbon C1, while hydrogen H5 becomes more positive in the TS as it transfers from the carbonyl carbon C4 to the gamma carbon C1 . Electron density variations are consistent with the changes in bond distances observed in the TS.

Table . Structural parameters of reactant (R), transition state (TS) and products (P), for the decarbonylation of ,-dimethyl--butenal in gas phase obtained from CAMBLYP/-G(d,pd) calculations. Atom distances (A˚ ) C -C R TS P

. . . C -C -C -C −.

C -C

C -C

. . . . . . Dihedral angles (degrees)

C -H

H -C

. . .

. . .

C -C -C -H C -C -H -C C -H -C -C . −. −. Imaginary frequency (cm− ): .

H -C -C -C .

6

A. RODRIGUEZ ET AL.

Table . NBO charges of reactant (R), transition state (TS) and products (P) for the decarbonylation of ,-dimethyl--butenal in gas phase obtained from CAMBLYP/-G(d,pd) calculations. NBO charges

R TS P

C

C

C

C

H

− . − . − .

− . − . − .

− . − . .

. . .

. . .

Table . Wiberg bond index of reactant (R), transition state (TS) and products (P) for the decarbonylation of ,-dimethyl-butenal in gas phase obtained from CAMBLYP/-G(d,pd) calculations. C -C

C -C

C -C

C -H

H -C

Sy

BRi BTS i BPi

. . .

. . .

. . .

. . .

. . .

.

%Ev

.

.

.

.

.

Wiberg bond indexes [34] were calculated to study the evolution of the reaction along the coordinates involved in products formation. The natural bond orbital (NBO) program [35] implemented in Gaussian 09 [27] was used. Calculated Wiberg bond indexes are given in Table 4. The most advanced reaction coordinate is the bond order change between the gamma and the beta carbon atoms, C1 –C2 , 60% progress. The bond between the alpha carbon C3 and the carbonyl carbon C4 is half broken in the TS (50% progress). The change in bond order C4 – H5 and H5 –C1 show the transfer of the hydrogen from the carbonyl carbon C4 to the gamma carbon C1 . Other reaction coordinates show less progress in the TS. The synchronous character of concerted reaction mechanism were estimated as detailed in recent works [29–31]. In an additional verification of the mechanism described in Scheme 1, a topological analysis of the electron density was carried out by using the AIM theory [36]. To perform this study, the obtained wave function at the CAMB3LYP/ 6-311G(d,pd) level was employed. This analysis can examine bond properties by determination

of bond critical points (BCPs). The Poincaré–Hopf relationship [37] was satisfied from the results of the critical point distribution. Figure 2 shows the molecular graph with the corresponding critical points. Table 5 describes bond distances and topological properties such as electron densities (ρ b ), the Laplacian (࢟2 ρ b ), the relationship between the potential energy density and the kinetic energy density (|Vb |/Gb ), together with the ellipticity (ε) of the atoms involved in the decarbonylation of 2,2-dimethyl-3-butenal in gas phase. In the TS, the C4 –H5 bond shows a decrease of ρ b and ࢟2 ρ b . Consequently, a bond breaking occurs and, therefore a decrease in the electron density of the bond pathway belonging to these pair of atoms. Formation of H5 –C1 single bond leads to an increase in the ρ b . Furthermore, C1 –C2 and C2 –C3 bonds in the TS, ra and rb are symmetrical, as well as with the other properties. This fact suggests charge delocalisation from resonance. The C1 –C2 bond in the product reveals a decrease of ρ b , and the bond ellipticity tends toward zero consistent with sp2 hybridisation change to sp3 . Thus far, the effect of the properties in C2 –C3 bond is opposite because of the formation of double bond in the product (ρ b = 0.3435au and Є=0.4118). However, C3 –C4 bond is weakened, thus leading to bond breaking in agreement with the strong decrease of electron density (ρ b ) in the TS. The increase in the Laplacian (࢟2 ρ b > 0) has been attributed to the polarisation in the bond because of the presence of the carbonyl group. The ࢯVb ࢯ/Gb relationship always is greater than cero, which indicates that the contribution of the potential energy density is greater than kinetic energy density. This fact is characteristic of a shared interaction in the BCP. Among the breaking bonds in the TS, there is the C3 – C bond showing a decrease in electron density which is greater than a C4 –H5 bond. Likewise, it happens with the Laplacian electron density as described below. This tendency suggests an advance bond breaking C3 –C4 , which agrees with the evolution of bond indicated in Wiberg bond index (Table 4) for C3 –C4 (50.80%) and C4 –H5 (43.12%).

Figure . (Colour online) Molecular graphs for (R) ,-dimethyl--butenal, (TS) transition state and (P) -methyl--butene and carbon monoxide. Magenta, orange and yellow circles correspond to (,−), (, −) and (,+) critical points, brown lines indicate bond paths

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7

Table . Distance (in A˚ ) from BCP to atoms A and B, and topological properties (in au.) of the selected BCP in reactants (R), transition state (TS) and products (P). A-B bond A

B

H

C

C

C

C

C

C

C

C

H

R TS P R TS P R TS P R TS P R TS P

ra

rb

ρb

࢟ ρ b

ࢯVb ࢯ/Gb

Є

. . – . . – . . . . . . – . .

. . – . . – . . . . . . – . .

. . – . . – . . . . . . – . .

− . − . – − . . – − . − . − . − . − . − . – − . − .

. . – . . – . . . . . . – . .

. . – . . – . . . . . . – . .

Figure 3 shows the contour maps of the Laplacian (࢟2 ρ b ), where the plane containing allyl carbon atoms C1 –C2 –C3 (left) describes the symmetry mentioned above. The centre contour map displays H5 transfer from C4 to C1 . In here, which involves BCP depicts a share shell interaction (࢟2 ρ b < 0). On the right side, the plane containing H5 –C4 –C3 in the TS, the C4 –C3 reveals a decrease of the electronic density, that is, in the region of ࢟2 ρ b > 0 related with a closed shell interaction. Figure 4 displays the relief maps with the number of local maxima in −࢟2 ρ b and the valence shell state for the TS of 2,2-dimethyl-3-butenal, containing the C1 –C4 – H5 and C1 –C2 –C3 atoms in a plane. The core and valence shell charge concentration can be seen at the position of each carbon atom. Also, in the left figure, there is one maximum corresponding to the position of the hydrogen atom. Also, it displays (3, −3) CP from the C1 atom in

Table . AIM charges (qb) of the atoms of interest in reactants (R), transition state (TS) and products (P) for the decarbonylation of ,-dimethyl--butenal in gas phase. AIM charges

R TS P

C

C

C

C

H

− . − . .

− . − . − .

. − . − .

. . .

. . .

the direction of the H5 atom. The relief map on the right exhibits evidence two (3, −3) CPs of carbon atoms, each corresponding to valence shell (VS) highly symmetrical. The AIM charges of the atoms of interest were obtained through identification of the atomic basins and their integration. The evolution of these charges is given in Table 6. The atomic charge of C1 reaches a

Figure . Contour maps of the Laplacian distribution (࢟ ρ b ) in the transition state, the planes contain atoms C, C, C (left), C, H, C (centre) and H, C, C (right).

8

A. RODRIGUEZ ET AL.

Figure . Relief map of the −࢟ ρ b for the transition state in a plane containing the C-C-H and C-C-C atoms.

minimum value near the TS (−0.1482), a result of abstracting the electrons from the neighbouring atom C2 . Moreover, there is an increase towards the end of the reaction due to the electronic density which is transferred to the basin of the H5 atom. The C1 –C2 –C3 atoms are negatively charged in the TS through electron delocalisation. Yet, the negative charge of the product is found in the atoms C2 –C3 . The H5 charge decrease, from reagent to product, when is transferred from the carbonyl to give a CH3 . Both NBO and AIM charges give negative values in the TS of allylic carbons. The first one is associated with orbitals delocalisation of p-electrons in C1 –C2 –C3 . With regard to AIM charges interaction of sharing shell in the electron density redistribution between these three carbon atoms are demonstrated.

the carbonyl carbon to the gamma carbon. This is consistent with the hydrogen isotope effect observed. The reaction shows the most advanced reaction coordinate in the TS where the bond order change between the gamma and beta carbon atoms (60%). The aldehyde hydrogen is halfway transfer between the carbonyl carbon and the gamma carbon in the TS. Other reaction coordinates show less progress in the TS. Overall, the reaction can be described as a concerted moderately non-synchronous process. The AIM analysis of electron density confirms the elimination reaction undergoing a five-membered cyclic TS type of mechanism.

Disclosure statement No potential conflict of interest was reported by the authors.

References 4. Conclusions The reaction mechanism of the gas-phase thermal decarbonylation of a β,γ -unsaturated aldehyde, i.e. 2,2dimethyl-3-butenal, was studied by means of theoretical calculations. The structures of the reactant, TS and product along the minimum energy pathway were optimised and analysed using various levels of theory. Calculated kinetic and thermodynamic parameters were contrasted with experimental values. Good results of kinetics and thermodynamic parameters were obtained using CAMB3LYP/6-311G(d,pd) level of theory. Geometrical parameters, NBO charges and bond orders were used to analyse the progress of the reaction along the different bonds involved in the reaction changes and to study the nature of the TS. Bond distances changes, NBO charges and bond orders are consistent with the proposed reaction mechanism through a five-membered cyclic TS, in which the aldehyde hydrogen is being transferred from

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