Unimolecular Decomposition Mechanism of 1,2-Dioxetanedione

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Unimolecular Decomposition Mechanism of 1,2Dioxetanedione; Concerted or Biradical? That is the Question! Pooria Farahani, and Wilhelm Josef Baader J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b10365 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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The Journal of Physical Chemistry

Unimolecular Decomposition Mechanism of 1,2-Dioxetanedione; Concerted or Biradical? That is the Question! Pooria Farahani,* and Wilhelm J. Baader* †

Instituto de Química, Departamento de Química Fundamental, Universidade de São Paulo,

C.P. 05508-000, São Paulo, SP, Brazil E-mail: [email protected]; [email protected] Abstract Determination of the ground and excited state unimolecular decomposition mechanisms of 1,2dioxetanedione gives a level of insight into bimolecular decomposition reactions of this kind for which some experimental results are reported. Although, a few studies have put some effort to describe a biradical mechanism of this decomposition, there is still no systematic study which proves an existence of a biradical character. In the present study, state-of-the-art high-level multistate multiconfigurational reference second-order perturbation theory calculations are performed to describe the reaction mechanism of 1,2-dioxetanedione, in detail. The calculations indicate that the decomposition of this four-membered ring peroxide containing two carbonyl carbon atoms occurs in concerted but not simultaneous fashion, so-called “merged”, contrarily to the case of unimolecular 1,2-dioxetane and 1,2-dioxetanones decomposition where biradical reaction pathways have been calculated. At the TS of the ground state surface, the system enters an entropic trapping region, where four singlet and four triplet manifolds are degenerated which can lead to the formation of triplet and singlet excited biradical species. However, these excited species have to overcome a second activation barrier for C-C bond cleavage for excited product formation, whereas the ground state energy surface possess only one TS. Thus, our calculations indicate that the unimolecular decomposition of 1,2-dioxetanedione should not lead to efficient excited state formation, in agreement with the lack of direct emission from the peroxyoxalate reaction.

1 Introduction Chemiluminescence (CL) reactions, chemical transformations which occur with the emission of light, are intimately linked to peroxide chemistry, more specifically to the chemistry of cyclic fourmembered peroxides. Although CL and bioluminescence phenomenon are known for a long time,1,2,3 mechanistic research on electronically excited state formation by chemical or enzymatic transformations actually started with the synthesis of these four-membered ring peroxides.4,5 The catalysed decomposition of 1,2-dioxetanedione, the high-energy intermediate occurring in the peroxyoxalate reaction, is proved to occur with very efficient formation of electronically excited state,6 which is different compared to the analogous reaction of 1,2-dioxetanones – the first model compounds to study the firefly bioluminescence that were shown to have lower quantum yields than initially measured.7

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Scheme 1. Mechanisms proposed in the literature for the decomposition of 1,2-dioxetane, 1,2-dioxetanone and 1,2dioxetanedione.

During the last 20 years, there have been significant theoretical and experimental efforts to better understand the mechanism of the cleavage of the four-membered ring peroxides. Based on these studies, two extreme pathways were suggested for the decomposition of these cyclic peroxides; a concerted mechanism, first proposed by Turro,8 as well as a biradical path initially formulated by Richardson.9,10 The concerted mechanism is a single-step reaction process in which the C-C and OO bonds dissociate, simultaneously. The two-step biradical mechanism, however, implies that once the O-O bond is cleaved, the system enters an entropic trapping region of biradical nature, where four singlet and four triplet manifolds are degenerated, then, the C-C bond breakage begins. Several of the obtained experimental data appear to be more compatible with the biradical mechanism. Although, others, like the efficiency of excited state formation, can be better rationalized with a concerted transformation. Consequently, a “merged” reaction pathway, in which the O-O elongation is not fully completed when the C-C cleavage occurs, has been suggested maintaining the biradical nature of the reaction pathway without formation of a biradical intermediate of this kind in a potential energy surface minimum, (see Scheme 1.).11 The state-of-the-art ab initio findings, on the other hand, confirm a step-wise biradical mechanism for unimolecular decomposition of 1,2-dioxetane and 1,2-dioxetanone.12,13 Recently, da Silva et al. performed theoretical studies employing density functional theory (DFT) methods to explore the unimolecular decomposition potential energy surfaces (PES) of the ground and lowest-lying excited states of 1,2-dioxetanedione.14 However, methods of higher accuracy are indispensable for understanding the state crossings in detail.15 It should be mentioned that for four-membered ring peroxide decompositions, like 1,2-dioxetanes and similar derivatives, the pathway always involves O-O and C-C bond cleavages, leading to the formation of two extremely stable carbonyl fragments.16 This behavior is different to that of endoperoxides, cyclic peroxides consisted of, at least, 5-membered rings, which are known to decompose predominantly by a pathway which involves C-O bond breakage, and the formation of singlet oxygen, as also indicated by recent theoretical calculations.17,18 Experimental studies on the chemiluminescence properties of 1,2-dioxetanedione are generally concerned with the activator (ACT) catalysed decomposition of this high-energy intermediate (HEI), supposed to be formed in the base-catalysed reaction of activated oxalic esters with hydrogen peroxide.19 Although the identity of this high-energy intermediate is not completely proven, there are spectroscopic evidences on its structure as 1,2-dioxetandione.20 Kinetic studies on the catalysed decomposition of this intermediate have been performed using an experimental design 2 ACS Paragon Plus Environment

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of the peroxyoxalate reaction, where the ACT was added after a delay period with respect to mixing of the other reagents.21 These studies allowed the direct determination of rate constants for the interaction of the HEI with different ACTs and thereby indicated the involvement of electron transfer processes in the chemiexcitation step of this transformation, as already suggested before due to indirect evidence.22 These studies also allowed to estimate the rate constant of the unimolecular decomposition of the HEI and to obtain approximate values for the activation energy of this process. The aim of the present article is to report a qualitative and quantitative study on the electronically excited state formation of unimolecular decomposition of 1,2-dioxetanedione as a prototype which could be applied in the future as a starting point for the bimolecular decomposition of this intermediate. The present results allow us to explain the reaction mechanism in detail. The article is, subsequently, structured as follows; a section with our computational details with regards to the electronic structure, a section of presentation and discussion of the results, and finally drawing some conclusions.

2 Computational Details Geometry optimizations of the stationary points along the ground state (S0), first singlet excited state (S1) and first triplet state (T1) as well as minimum energy path (MEP) and intrinsic reaction coordinate (IRC) searches were performed at multi-state (MS) complete active space second-order perturbation theory (CASPT2) method,23,24 with no symmetry constraints, in conjunction with the atomic natural orbital (ANO-L) basis set contracted to O,C[3s2p1d] (thereafter ANO-L-VDZP).25 Subsequent single point MS-CASPT2 energy calculations were carried out improving the basis set from double- to quadruple-ζ quality, O,C[5s4p3d2f] (thereafter ANO-L-VQZP).26 The MS- and SS-CASPT2 approaches give rise to energy values with differences of lower than 1 kcal/mol. This is caused by the small off-diagonal elements of the effective Hamiltonian matrix in the MS calculations. Throughout the CASPT2 calculations, freezing the core orbitals of nonhydrogen atoms as well as a modification of the zeroth-order Hamiltonian, excluding the standard ionization potential electron affinity (IPEA) (with 0.0 value) were applied.27 On the other hand, an imaginary level shift of 0.2 a.u was used, in order to solve weakly interacting intruder-states problems.28 The selection of a reasonable active space in the CASPT2 calculations was done based on the dissociation mechanism. The ideal amount of electrons and orbitals expected for the CL process of 1,2-dioxetanedione are 20 electrons distributed in 16 orbitals. This selection corresponds to C1-C2, O1-O2, C1-O1, C2-O2 σ bonding and σ* antibonding, C1=O´1 and C2=O´2 π bonding and π* antibonding as well as the four oxygen lone-pair orbitals. However, the geometry optimizations applying this active space would be time- and CPU-demanding and therefore unaffordable. On the other hand, not all of the above orbitals are significantly important throughout the reaction. For instance, the six orbitals corresponding to C1-O´1, C2-O´2 σ bonding and σ* antibonding as well as the O´1 and O´2 lone pairs are doubly occupied and, therefore, kept intact. Hence, for the geometry optimizations and the MEP calculations, it is safe to exclude these orbitals from the active space. Consequently, to achieve a standard in the results, the additional energy calculations for all the structures, were carried out using 16 electrons distributed in 14 orbitals (excluding only O´1 and O´2 lone pairs), see Figure 1. This active space was employed for computing the final energies of the stationary points at MS-CASPT2 approach (MS-CASPT2(16in14)). The validation of this approach 3 ACS Paragon Plus Environment


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was consequently confirmed by performing two test calculations using CAS(12in10) and CAS(16in14) in both reactant and TSS0, see Figure S1. Zero-Point vibrational energy (ZPVE) and Gibbs free energy corrections were computed numerically at the MS-CASPT2(12in10)/ANO-L-VDZP level of theory. Spin-orbit coupling (SOC) terms between singlet and triplet manifolds were calculated within the AMFI and CASSI frameworks29,30 at the MS-CASPT2(16in14)/ANO-L-VQZP wave function over four singlet (S0, S1, S2 and S3) and four triplet states (T1, T2, T3 and T4). A development version of the MOLCAS-8 quantum chemistry package suite was employed for all the calculations.31

Figure 1. Natural orbitals relevant in the unimolecular decomposition of 1,2-dioxetanedione.

3 Results and Discussions Geometry Optimization and Activation Energies Considering the previous studies on 1,2-dioxetane,32 1,2-dioxetanone,12 and Dewar 1,2-dioxetane,13 one can expect that these O-O bond breaking reactions occur via a two-step mechanism for most of the cases. However, our present findings for unimolecular decomposition of 1,2-dioxetanedione, speak to the contrary. The minimum energy path (MEP) calculations along the S0 surface confirms that first the O1-O2 cleavage takes place, while this cleavage is not terminated completely, the C1-C2 bond breakage begins. Therefore, there is only one transition state along the S0 surface that connects the reactant (dioxetanedione) and products (CO2). Taking into account the features of such mechanism, several structures can be characterized: the TS on the singlet ground state surface corresponding to the simultaneous O1-O2 and C1-C2 bond breakages (TSS0), from which the considered singlet and triplet excited states become degenerated with the S0; the TS on the first singlet excited state (TSS1), and the first triplet excited states (TST1) corresponding to the C1-C2 cleavage. Computed vertical energies and the corresponding electronic transitions at the FranckCondon (FC) geometry as well as the other relevant critical points are presented in Table 1. Since CL reaction is a thermal process the oscillator strength (ƒ) at the FC geometry is quite low (almost zero). As can be seen in the table, the nature of the lowest-lying states are ππ* at the FC geometry. However, immediately after the first transition state (TSS0), where at least 6 manifolds are degenerated, the nature of the lowest-lying states will be changed to nπ*. Table 1. CASPT2 vertical transition energies in gas phase, ∆E, in eV, computed for the important transition geometries. It is worth mentioning that the oscillator strength (ƒ) is almost zero for most of the energies. Reac S0

Electronic Transitions

-

TSS0 (eV)


0.00

-

∆E

TSS1 (eV)


0.00

-

∆E

MinS1 (eV)


0.00

-

∆E


TST1 (eV)


(eV)

0.00

-

0.00

∆E

∆E

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S1 S2 S3 T1 T2 T3 T4

σσ*/ππ*

4.18

ππ*/nπ*

0.31

nπ*

0.06

ππ*

4.81

ππ*/nπ*

0.35

ππ*/nπ*

0.89

ππ*/nπ*

6.50

nπ*

1.21

ππ*/nπ*

0.90

ππ*/ nπ*

0.03

ππ*/ nπ*

0.31

ππ*/ nπ*

0.34

ππ*/nπ*

0.34

ππ*/ nπ*

0.40

nπ*

1.23

ππ*

3.26

ππ*

0.06

ππ*

0.08

ππ*

0.01

nπ*

0.09

nπ*

4.84

nσ*/nπ*

0.31

ππ*

0.15

ππ*

0.03

ππ*

0.29

ππ*/nπ*

6.00

nπ*/ππ*

0.32

nπ*

0.90

nπ*

0.34

ππ*/ nπ*

0.33

ππ*

6.19

nσ*

1.23

nπ*

0.93

nπ*

0.49

nπ*

1.26

In the present study, MS-CASPT2(12in10)/ANO-L-VDZP geometry optimizations were carried out and the most relevant geometrical parameters for the unimolecular decomposition of 1,2dioxetanedione are compiled in Table 2. In contrast to 1,2-dioxetane32 and alkyl-substituted 1,2dioxetanes,33 where the O1-C1-C2-O2 dihedral angles are between 19 to 22 degrees, the starting geometry (Reac) has zero degree of the dihedral angle, whereas the C1-C2 and O1-O2 bond distances are more or less the same. The TSS0 has a close to zero dihedral angle, as well, which reveals a stretching of O1-O2 bond instead of a torsion of the O1-C1-C2-O2 dihedral angle for 1,2-dioxetanes that was stated to be responsible for the formation of an entropic trapping34,35 process and hence producing excited state species.36 This difference is due to the conjugation effects caused by the C1=O´1 and C2=O´2, that restrict the twisting around the C1-C2 bond. It is noteworthy that these findings are consistent with the study of 1,2-dioxetanone by Liu et al. that characterized the optimized geometries to be effectively planar.12

Table 2. Geometrical parameters of important transition geometries on the ground and excited states optimized at MS-CASPT2(12in10)/ANO-L-VDZP level of theory. Reac C1-C2 O1-O2 C1-O1 C2-O2 C1-O´1 C2-O´2

1.549 1.532 1.408 1.408 1.188 1.188

C1-C2-O2 O1-C1-C2 C1-C2-O´2 O´1-C1-C2 O1-C1-O´1 O2-C2-O´2

89.6 89.6 140.9 140.9 129.3 129.3

O1-C1-C2-O2

0.00

TSS0 TSS1 Bond Lengths (Å) 1.670 1.691 2.636 2.670 1.299 1.270 1.299 1.249 1.211 1.207 1.211 2.281 Angles (degree) 111.8 122.9 111.8 103.2 116.7 111.1 116.7 118.0 131.4 138.6 131.4 125.9 Dihedral Angle (degree) 1.58 0.65

MinS1

TST1

1.560 2.631 1.336 1.284 1.207 1.262

1.638 2.495 1.308 1.308 1.209 1.209

118.4 109.9 117.7 125.0 125.0 123.8

109.1 109.1 119.4 119.4 131.4 131.4

2.64

1.42

Table 3 compiles the relative energies of the optimized stationary points at the MS-CASPT2 level of increasing accuracy as well as ZPVE and Gibbs free energy correction. Unless stated otherwise, the energy values shown in this study are the MS-CASPT2(16in14)/ANO-L-VQZP. The ground state dissociation begins by surmounting a barrier height of 34 kcal/mol. Taking ZPVE corrections 5 ACS Paragon Plus Environment


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into account, however, decreases this value to 30.6, due to bond dissociation. The computed CL activation energy (TSS1) is 37.3 kcal/mol, which means that formation of electronically excited singlet states in the unimolecular 1,2-dioxetanedione decomposition should be of very low efficiency. This is in agreement with the fact that no experimental report exists where chemiluminescence emission in the peroxyoxalate reaction has been unequivocally proven in the absence of an ACT, which would correspond to the unimolecular decomposition of 1,2dioxetanedione.16,19 Additionally, in a study on the catalyzed decomposition of the HEI in the peroxyoxalate reaction, accumulated in the absence of the ACT, no light emission, which would correspond to the unimolecular decomposition, could be observed.21 The calculated activation energy of 30.6 kcal/mol for the TSS0 is substantially higher than the experimentally determined free activation enthalpy for the unimolecular decomposition of 1,2dioxetanedione, obtained from the delayed ACT addition experiments from our group,21 which is 18 kcal /mol. However, the obtained experimental data also indicate the participation of dark catalysis in the decomposition of the HEI, as the experimental activation entropy values showed to be highly negative.21 On the other hand, this energy difference can be also explained by the errors involved in the CASPT2 excitation energies using the IPEA correction, reported, very recently, by J. P. Zobel et al.37 According to that study, the IPEA correction deviates the excitation energies and hence must not be applied for open-shell electronic states, especially using large basis sets, since it is strongly dependent to the size of the basis set. However, employing the correction shall not change the qualitative concept of the present study. Table 3. Relative energies (kcal mol-1) of transition structures of the unimolecular decomposition of 1,2-dioxetanedione optimized at MS-CASPT2 level of theory. DZ and QZ refer to the ANO-LVDZP and ANO-L-VQZP basis sets, respectively.a MS-CASPT2(16in14)/QZ //MS-CASPT2(12in10)/DZ

ZPVE

Gibbs (298 K)

0.0 0.0 0.0 Reac 34.05 30.58 30.49 TSS0 43.45 37.38 37.27 TSS1 31.61 28.65 28.71 TST1 -117.58 -117.82 -118.93 Prod a Vertical excitation energies for the other singlet and triplet states at these geometries are compiled in Table S2,3 and 4.

Ground State decomposition The TSS0 on the ground state potential energy surface (PES) was located and verified by vibrational frequency analysis at the MS-CASPT2 method. An imaginary frequency (1103 cm-1) corresponding to the O1-O2 and C1-C2 stretching modes was discovered for the TS. A downward MEP at the MSCASPT2/ANO-L-VQZP//MS-CASPT2/ANO-L-VDZP level of theory, starting from the optimized TSS0, connects the reactant (Reac) and the product (Prod). Based on the MEP, the mechanism for the thermal decomposition can be described. First, stretching the O1-O2 bond leads the molecule from the reactant to formation of the TSS0. Before the O1-O2 bond breaking is fully completed, the C1-C2 bond undergoes some stretching and the molecule is decomposed to the products (merged mechanism) see Figure 2. At this TS on the MEP, the MS-CASPT2 energy gaps between S0, S1 and T1 manifolds are less than 3 kcal/mol, implying that there is a crossing where the adiabatic states strongly interact. The energetics at the TSS0 reveals that the T1 is slightly lower in energy than the S0 6 ACS Paragon Plus Environment

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(2 kcal/mol), which indicates the presence of an intersystem crossing (ISC) on top of the internal conical intersection (CIx) between S0 and S1. The triplet population likelihood is proven by the large SOC values obtained at MS-CASPT2(16in14) level, see Table S7. The averaged value of the couplings between S0 and T2, where the nature of the state is 3(n,π*), surfaces at the TSS0 geometry increases up to ~60 cm-1. From these crossing points adiabatic transformations take place on the MEPs from 1(σ,σ*) to 1,3(n,σ*). However, this transformation does not break the planar symmetry and after the crossing point, the 1,3(n,σ*) states more or less maintain the planar structure.

Figure 2. Computed MS-CASPT2(16in14)/ANO-L-VQZP//MS-CASPT2(12in10)/ANO-L-VDZP energy profile on the S0 surface started from the TSS0. The crossing point in which all the states are degenerated or nearly-degenerated are clearly shown at this TS structure.

Dissociation on the S1 and T1 Manifolds Due to the small energy gap between S0, S1 and T1 structures, located at the TSS0, this structure has been taken as the starting geometry to optimize the TSS1 and TST1. The TSS1 on the first lowestlying singlet excited state (S1) and the TST1 on the first lowest-lying triplet excited state (T1) were located and verified by vibrational frequency analysis at the MS-CASPT2 level of theory. Imaginary frequencies of (920 cm-1) and (952 cm-1) corresponding to the C1-C2 stretching mode were discovered for TSS1 and TST1, respectively. After finding the TS structures, downward MEPs along these states were computed at MS-CASPT2/ANO-L-VQZP//MS-CASPT2/ANO-L-VDZP. The probability of populating the triplet manifolds along the MEPs are evaluated by computing the SOC at both TSS1 and TST1 geometries along the PES. The average values of the couplings at the TSS1 increases up to ~40 cm-1. Our results indicate that the excited state population takes place at the TSS0 where at least six manifolds are degenerated and a strong SOC was discovered. After that, the system enters a biradical-like region. In order to form the excited carbon dioxide, a second barrier corresponding to the C-C rupture must be, eventually, surmounted. This most accessible pathway to the excited carbon dioxide has been found to be in perpendicular direction to the ground state dissociation path. The IRC search on the second TS in both S1 (Figure 3a) and T1 (Figure 3b) states, connects the minimum on the entropic trapping region (MinS1&T1) to an excited and a ground state carbon dioxides. The first points along the MEPs indicate that most of the computed states are 7 ACS Paragon Plus Environment


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nearly degenerated until the molecule arrives to the transition structure. The nature of the states in the TSS1&T1 region (2.81 au) is (n,σ*) and consequently, no change in the nature takes place after the TS along the IRC. The MEPs finally terminate in the excited CO2. As depicted in the Figures, the energy profile for the T1 manifold is essentially identical to the S1 manifold, except that the T1 is lower in energy than the S1. The MS-CASPT2/ANO-L-VQZP approach is the most accurate method one can apply to describe this reaction mechanism in detail, and hence it is quite safe to qualitatively interpret its reaction mechanism and to some extent quantitatively.

Figure 3. Computed MS-CASPT2(16in14)/ANO-L-VQZP//MS-CASPT2(12in10)/ANO-L-VDZP energy profile on the S1 (a) and T1 (b) states started from the TST1 and TSS1, respectively.

4 Conclusion The chemiluminescence reaction of the unimolecular 1,2-dioxetanedione is studied in the present contribution. High-level MS-CASPT2 calculations have been carried out in order to understand the reaction mechanism in detail. The findings confirm a one-step concerted mechanism for the ground state decomposition, which can be explained as follows. Before the O1-O2 bond is fully cleaved, the C1-C2 rupture comes into action and the molecule dissociates to the ground state fragments. In the next step, the system enters an extended biradical-like region in which computed singlet and triplet manifolds are degenerated. The non-adiabatic reaction can be feasible in singlet as well as the triplet states, as proved by the SOC calculations. However, in order to produce CL, a second energy barrier must be surmounted in the lowest-lying singlet and triplet excited manifolds. Consequently, our calculations clearly indicate that no triplet or singlet excited states should be formed in the unimolecular decomposition of 1,2-dioxetanedione. This can explain why no clear-cut evidence on a direct CL emission from the unimolecular 1,2-dioxetanedione decomposition (peroxyoxalate reaction in the absence of an ACT) has been presented till now. The reaction mechanism for the decomposition of this cyclic peroxide containing two carbonyl carbon atoms has been shown to be a concerted one (initially called merged mechanism to indicate this process as a mixture between the biradical and concerted / simultaneous mechanism which had been proposed for 1,2-dioxetane decomposition by Adam and Baader11), without formation of a OO biradical intermediate in a potential energy minimum on the ground state energy surface.

Acknowledgements 8 ACS Paragon Plus Environment

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PF acknowledges Prof. Antonio Carlos Borin for his helpful comments and sharing his experiences in doing this research. We are grateful to the Fundação de Amparo á Pesquisa do Estado de São Paulo (FAPESP) for the financial support under the 2015/02314-8 and 2014/22136-4 project numbers. Supporting Information Available. Cartesian coordinates for the involved structures and their corresponding relative electronic energies. Spin-orbit coupling terms between singlet and triplet manifolds. Pseudo-natural (average) orbitals as well as their corresponding occupation numbers. 2D-PES of the unimolecular decomposition of 1,2-dioxetanedione.

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Unimolecular Decomposition Mechanism of 1,2-Dioxetanedione

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