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The mechanism of ozonolysis on the surface of C70 fullerene. The free energy surface theoretical study
Journal of Molecular Structure 1185 (2019) 361e368
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The mechanism of ozonolysis on the surface of C70 fullerene. The free energy surface theoretical study Andrzej Bil Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383, Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2019 Received in revised form 1 March 2019 Accepted 3 March 2019 Available online 4 March 2019
Density functional theory based metadynamics simulations of the room temperature ground state free energy surfaces (FES) of a,a-C70O3/a,a-C70O, c,c-C70O3/c,c-C70O, d,d-C70O3/d,d-C70O and e,e-C70O3/e,eC70O systems have revealed several structures relevant for the thermal decomposition of the experimentally known c,c-C70O3 and hypothetical a,a-C70O3, d,d-C70O3 and e,e-C70O3 ozonides. Criegee intermediate, an open structure with carbonyl oxide and carbonyl groups, plays an important role in the possible mechanism of decomposition of all studied ozonides, while only for e,e-C70O3/e,e-C70O system it is related to the minimum on the underlying FES. The differences between the topography of FES and the previously reported related potential energy surface indicates the influence of a thermal factor on processes on C70 surface. Only c,c-C70O3 should decompose thermally to form c,c-C70O epoxide as the final product while for the other studied ozonides the preferred reaction channel seems to be the one leading to the structures with O2 moiety entrapped over the CeC bonds adjacent to the CeOeC group. Hypothetical oxidoannulene form of c,c-C70O is not related to a clear minimum on the free energy surface. © 2019 Elsevier B.V. All rights reserved.
Keywords: Ozonide Epoxide Oxidoannulene Carbonyl oxide DFT
1. Introduction Fullerene oxides and ozonides are examples of simple exohedral modifications of a parent molecule. C70 oxides were reported in 1991 [1], only a few years after C60 discovery by Kroto [2]. The chemistry of C70 is more challenging than of C60 because there are as many as eight non-equivalent types of bonds present in the former molecule, a consequence of five nonequivalent carbon atom types (Fig. 1). Of eight theoretically possible oxides, two experimentally know stable products i.e. a,b-C70O and c,c-C70O adopt epoxide structure, with O atom bridging a CeC bond. The six remaining isomers are expected to be oxidoannulenes, with the bond broken upon the oxide formation [3e5]. Fig. 1 illustrates the structures of four high symmetry isomers, c,c-C70O epoxide and a,a-C70O, d,d-C70O and e,e-C70O oxidoannulenes, which are the subject of this study. Low-pressure combustion of hydrocarbons in an oxygendeficient environment, the most common route to producing fullerenes commercially, is also a source of fullerenes oxides. On laboratory scale they can be obtained with the use of ozonolysis reaction, by bubbling ozone gas through fullerene solutions. The
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process, which involves complicated paths of thermolysis or photolysis, was the subject of the experimental comprehensive studies by Heymann [3]. The thermolysis paths were also researched with the use of computational simulations [5,6]. Carried out in the dark, the ozonolysis of C70 led to two monoozonides, assigned to be the a,b- and c,c-C70O3 isomers. Thermal decomposition of a,b-C70O3 molecule led to the previously known a,b-C70O epoxy fullerene. Interestingly, the experimental study of the thermolysis pathway of c,c-C70O3, which was a considerably slower reaction, quotes a surprising observation that the resulting direct product was a metastable structure, whose UVeVis spectrum shared features distinctive of oxidoannulenes. Because of this, the metastable product was assigned to be d,d-C70O [3], which as calculations suggest does adopt the oxidoannulene form. However, such a conclusion would require a complicated mechanism of the formation of the final c,c-C70O oxide, involving a transfer of an O atom between Cc-Cc and CdeCd bonds. Of all theoretically possible oxides, the two stable c,c-C70O and a,b-C70O isomers are not the most thermochemically preferable ones (Table 1, data from Ref. [5]). If the latter one serves as a reference, c,c-C70O is a minimum on the potential energy surface located at ca. þ 1.3 kcal mol1, while all symmetric oxides are lower in energy scale, with e,e-C70O being the lowest minimum. Note, suggested as an intermediate product d,d-C70O oxidoannulene has
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Fig. 1. The five different carbon atom environments in C70 (left); high symmetry isomers of C70O (right).
Table 1 Calculated total S0 energies [kcal mol1] of the C70O and C70O3 set of isomers, relative to the a,b-isomer; 0 K energies refer to the minima on the potential energy surface, 298 K to average potential energies from molecular dynamics trajectories [5]. ()* refers to a structure observed in the spontaneous ring-opening reaction in the course of molecular dynamics simulation. C70O
a,b c,c a,a d,d b,c c,d d,e e,e
C70O3
0K
298 K
0K
298 K
0 1.3 2.6 2.8 0.8 4.1 10.0 12.8
0 2.3 1.1 2.1 1.3 5.9 11.9 12.1
0 2.3 15.7 5.4 12.3 16.5 13.7 30.7 (19.2)*
0 2.0 15.4 4.6 12.2 16.0 13.9 29.3 (18.8)*
a lower energy than the final c,c-C70O epoxide. Interestingly, a,bC70O3 and c,c-C70O3, þ2.3 kcal mol1 higher in energy than the former one, are the most stable ozonides, which strongly suggests that the reaction pathway toward an oxide formation must proceed via the corresponding ozonide structure [5]. Conversely, the lowest
energy oxide would originate from the least stable ozonide e,eC70O3, þ30.7 kcal mol1 higher in energy than a,b-isomer, which seems to explain why this isomer has never been reported in a laboratory. The relative stability of isomers is practically not altered either by including a zero-point vibration correction or by a thermal boost introduced through room temperature molecular dynamics simulations [5]. The previously reported calculations have revealed that the crucial step towards the formation of c,c-C70O and a,b-C70O isomers from parent ozonides involves the opening of the ozone ring (Fig. 2, a) and formation of an open structure (Fig. 2, b) with a single oxygen atom attached to one carbon atom and an O2 moiety to the second one (carbonyl and carbonyl oxide groups, the last one known as the Criegee intermediate), which at this stage resembles a general mechanism for ozonolysis of alkenes proposed by Criegee [5,7,8]. On the basis of the resonant structure of (b), the mechanism of epoxide formation has been postulated (Fig. 2, c). However, it was indicated that a triplet channel can contribute to c,c-C70O formation, through an oxidoannulene structure, which is a minimum on its triplet potential energy surface (PES) and as such may be a metastable structure, converting finally into the ground state epoxide minimum [5]. The contribution of such an excited state
Fig. 2. The possible mechanism for the formation of C70O epoxide [5]. The pictures represent the frontier atoms of the epoxide.
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intermediate could offer an alternative interpretation of the experiment, involving a much simpler mechanism than the one originally suggested, through the oxidoannulene intermediate assumed to be a singlet d,d-C70O isomer [3]. It should be emphasized that the experiment by Heymann [3] was performed at room temperature. However, the reported earlier calculations refer to minima on the potential energy surface which may have a different topography that the related room temperature free energy surface (FES). Therefore, the role of a hypothetical oxidoannulene c,c-C70O isomer, which is not a minimum on the ground state PES, but might compete on a room temperature ground state FES cannot be excluded. CceCc and CaeCb bonds in C70 have dominant character expected for a double bond, whereas the remaining six types of bonds, can be considered more as single bonds with some contributions from the p-electron density network [9]. Due to the polar character of a CeO bond, the ozonide formation induces essential changes in the electron density distribution in the parent C70, not only in the vicinity of the directly involved CeC bond, but also in the adjacent bonds. It explains why the ozonide formation through a dipolar 1,3 cycloaddition reaction is theoretically possible not only over a formally double Cc-Cc and Ca-Cb bond, but also over formally single bonds, which does not lead to CeC bond cleavage [9]. Interestingly, the calculated relative stabilities of most ozonide isomers correlates well with the total perturbation of the bonding density in the molecule e the smaller perturbation, the more stable the isomer is [9]. All these facts suggest, that the oxidonallulene isomers formation, which would originate in ozonides other than the most stable a,b- and c,c-isomers, may be a more complex process than the one which leads to epoxides, and adjacent bonds may influence the course of the reaction. However, little has been known about the mechanism of the reactions taking place on the surface of C70 for ozonides other than a,b-C70O3 and c,c-C70O3. Only e,e-C70O3 is expected to undergo spontaneous ozone ring opening as such a process was observed in the course of a room temperature molecular dynamics simulation [5], however, the following course of the reaction remains unknown. Deeper understanding of the mechanism of ozonolysis of C70 is important as both ozone and fullerenes are the focus of attention of contemporary research [10e15]. Studying this issue can provide more information about the chemistry of carbonyl oxides (Criegee intermediate), which contributes to complicated oxidation patterns important for various branches of chemistry [10,16,17]. Ozonation is also a method of functionalization of carbon materials [18] such as carbon nanotubes and nanofibers [11]. Apart from that, recently the attention has been paid on reactions similar to the ozonolysis of C70, e.g. epoxy and oxidoannulene oxidation of fused-pentagon chlorofullerenes [12], reduction of SO2 on graphite [19], thiozonolysis of C70 [20] or ozonolysis of C60 [21]. This paper focuses on the mechanism of the ground state decomposition of C70O3 isomers at room temperature established on the basis of the calculated underlying free energy surfaces. Only four high symmetry isomers have been selected for this study (Fig. 1). For c,c-C70O3/cc-C70O system it is important to establish whether the oxide (epoxide) formation is preceded by the formation of the open oxidoannulene form and whether such a structure, not present as a minimum on the potential energy surface, is a true minimum on the room temperature ground state FES, which is the main research problem reported in this account. Disproving such a minimum would strengthened the hypothesis that the detected experimentally metastable intermediate may represent c,c-C70O oxidoannulene in its triplet electronic state [5]. Of the six ozonides being the parent structures for the oxidoannulene oxides, d,d-C70O3 has the lowest energy, therefore
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studying the mechanism of its decomposition seems important [5]. In turn, a,a-C70O3 may be a representative for high energy ozonides b,c-C70O3 and c,d-C70O3 [5,9]. Finally, e,e-C70O3/e,e-C70O is an interesting system consisted of the lowest energy oxide and the highest energy ozonide. e,e-C70O3 was also the isomer whose ozone ring opening was observed in the course of a room temperature molecular dynamics simulation [5]. Apart from the importance of high symmetry isomers, they are also easier to investigate because for the remaining (low symmetry) isomers the free energy subspace underlying the ozonide decomposition cannot be explored effectively with the use of two collective variables only. It should be emphasized that the expected product of the ozonide decomposition reaction consists of a pair of not linked molecules i.e. the oxide and O2. Thus, it is not possible to estimate credibly the free energy of the product with respect to the parent ozonide as the depth of the pertaining minimum on FES depends on how much space around the fullerene the O2 molecule is allowed to explore in the course of the simulation. Considering this limitation, the paper is aimed exclusively at the topography of FES and establishing the relevant structures and processes on the surface of the fullerene than at accurate prediction of the thermochemistry of the reaction.
2. Methods Metadynamics is a method which allows to study a free energy surface at a given temperature [22e24]. Thermal factor is often important for the course of a reaction and the method itself allows for exploring effectively structures on FES which are not accessible within regular molecular dynamics due to high energy barriers separating them from an initial structure and limited simulation time. A proper choice of collective variables enables one to define and explore a free energy subspace relevant for a given chemical process [25]. To represent the release of O2 moiety from the surface the first collective variable (CV1) has been built as a sum of coordination numbers [25] calculated from oxygen atoms in O2 moiety to the four nearest carbon atoms (Fig. 3, see also Figure S1 and further discussion in Supplementary Materials for details). The second collective variable (CV2) used to describe FES has been defined as a difference between C O and C C atomic distances (Fig. 3) and represents the process of the formation and decomposition of the bonds. All calculations have been performed using the BeckeeLeeeYangeParr exchangeecorrelation functional [26,27] with D3 semiempirical dispersion corrections proposed by Grimme [28], coupled to a Gaussian and plane-wave basis set [29], as
Fig. 3. The scheme illustrating the distances necessary to build CV1 and CV2 collective variable. Only frontier atoms of C70O3 relevant for the studied reaction have been included in the picture.
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implemented in the QUICKSTEP program [30], which is part of the CP2K suite of software [31]. The GoedeckereTetereHutter normconservative pseudo-potentials optimized for use against the BLYP functional have been used in this simulations [32]. Taking dispersion effects into account was demonstrated to affect chemical reactivity [33,34]. Such effects were employed previously to study the reactivity of fullerenes [9,35e38]. Valence orbitals of all elements were expanded using DZVPeGTHeBLYP double zeta quality basis sets. The energy cutoff of 350 Ry has been used to define an auxiliary basis set, together with the cubic periodic boundary conditions to create a simulation cell of length 20 Å. Metadynamics simulations with a time step of 0.7 fs have been performed in the canonical ensemble. A mean temperature of 298 K has been regulated using a chain of NoseeHoover thermostats. 3. Results and discussion The exploration of the free energy surfaces relevant for the ozonide to the oxide reaction has revealed a variety of structures, including these which are not minima on FES, some of them perhaps not obvious to be predicted a priori. The most relevant of them have been depicted symbolically, with the use of frontier atoms, in Figs. 4e9 and Figs. S9 and S10 (Supplementary Materials), while the full pictures of the representative structures with the most important bond lengths and numerical values of CVs have been reported in Supplementary Materials (Figs. S3-S7). Apart from the reactant (ozonide, labelled with ‘1’) and the expected product (the resultant epoxide, structure 3, or oxidoannulene, structure 4, both accompanied by O2), the structures 5 and 6 with O2 moiety entrapped over a CeC bond adjacent to the parent CeC bond to form a peroxide have turned out to be relevant for the analyzed mechanism. Note, that the choice of CVs makes structures 5 and 6 indistinguishable on FES, therefore the possibility of formation of any of these two structures contributes to the depth of the minimum at ca. (3.6, 0.9 Å). With the exception of the unstable e,eC70O3, deep minima on FES are related to the ozonide structure. On top of that, some other structures, not necessarily minima, have appeared along the possible reaction paths, structure 2 with the broken parent CeC bond and one of OeO bonds (Criegee
Fig. 5. Minimum free energy paths linking the most important minima on c,c-C70O3/ c,c-C70O FES. The relevant structures have been depicted schematically. The energies are expressed with respect to the minimum representing structure 1.
Fig. 6. Free energy [kcal mol1] surface for d,d-C70O3/d,d-C70O isomers together with three minimum free energy paths linking the most important minima. The relevant structures have been depicted schematically.
intermediate) being of the utmost importance.
3.1. Free energy surface for c,c-C70O3 decomposition
Fig. 4. Free energy [kcal mol1] surface for c,c-C70O3/c,c-C70O isomers together with three minimum free energy paths linking the most important minima. The relevant structures have been depicted schematically with the use of frontier atoms.
On the free energy surface calculated for c,c-C70O3 and related structures (Fig. 4) there are three clear minima which originate in the parent ozonide (structure 1), in the expected product, i.e. C70O epoxide (structure 3), and in structures 5 and 6, where O2 moiety bridges either Cc-Cb or Cc-Cd bond adjacent to the Cc-O-Cc group, respectively. The resulting structures therefore share structural motives typical of oxidoannulenes and peroxides. Two minimum energy paths originating in the minimum related to the ozonide have been detected on the FES, the first one links the ozonide and the epoxide (light blue curve in Figs. 4 and 5), while the second one (black curve) leads to products 5 and 6. The first stage of both processes involves the opening the ozone ring accompanied by the dissociation of Cc-Cc bond and the formation of
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Fig. 7. Minimum free energy paths linking the most important minima on d,d-C70O3/ d,d-C70O FES. The relevant structures have been depicted schematically. The energies are expressed with respect to the minimum representing structure 1.
Fig. 8. Free energy [kcal mol1] surface for e,e-C70O3/e,e-C70O isomers together with three minimum free energy paths linking the most important minima. The relevant structures have been depicted schematically.
structure 2. The structure, which is a minimum on the related potential energy surface [5], is not a minimum on the FES but is rather related to a shoulder some 8e13 kcal mol1 higher in free energy than the ozonide. Due to the dissociation of Cc-Cc bond and OeO bond, the resulting open structure is quite flexible, therefore a range of similar structures with somewhat different Cc-Cc and OeO distances can be explored, with CV2 between ca. 0e0.5 Å while CV1 does not deviate much from ca. 3.1e3.2. Structural parameters of a representative structure of type 2 can be found in Supplementary Materials, Fig. S3. The pertaining energy barriers for paths leading to product 3 and structures 5 and 6 are 16.2 and 18.6 kcal mol1, respectively. The fact that the barrier leading to 5/6 is higher than the former one seems to be in accord with the experimental findings that c,c-C70O is the final product of c,c-C70O3 decomposition. Moreover, the preferred structure of the product is an epoxide (structure 3 in Fig. 4), as the oxidoannulene form, also marked in Fig. 4 as structure 4, is not related to any clear minimum on FES. Although such a
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Fig. 9. Minimum free energy paths linking the most important minima on e,e-C70O3/ e,e-C70O FES. The relevant structures have been depicted schematically. The energies are expressed with respect to the minimum representing structure 2.
structure appears in the course of the simulation, it is considerably higher in free energy scale than the epoxide product (cf. Fig. S8 in Supplementary Materials). Importantly, the open structure 2 is linked to the oxide product via a path almost parallel to the horizontal axis, with CV2 ca. 0 Å. The path does not cross the area related to the oxidoannulene form but goes directly towards the epoxide minimum. Therefore the release of O2 moiety (represented by a gradual decrease in CV1) must be accompanied by the simultaneous formation of Cc-Cc and Cc-O bonds to form the epoxide group so that the difference r(Cc-O) - r(Cc-Cc) which defines CV1, can vary only little and remains close to zero. Contrary to that, the black curve designating the path towards 5 and 6 products first goes down, with CV2 changing from 0 to ca. 0.6 Å, with CV1 almost constant at ca. 3.1. Subsequently, CV1 increases to ca. 3.7. The evolution of the CVs along the reaction path illustrates that the process of trapping O2 moiety by the adjacent Cb-Cc or Cc-Cd bond must be preceded by the formation of Cc-O-Cc group (no Cc-Cc bond) to form oxidoannulene structure, which is related to the energy barrier. Although the formation of structures 5 and 6 seems thermochemically favorable, as the pertaining minimum is 9.4 kcal mol1 lower in energy than the one of the reactant, it seems that the heights of the energy barriers are a factor in favor of the path leading to the epoxide product. The path linking minimum 3 and 5/ 6 (red curve in Figs. 4 and 5) can be traced on the FES, but the free energy barrier limiting the possible trapping O2 by the epoxide to form structures 5 or 6 is higher than for the process leading back towards the ozonide. Structure 7 identified along the path has an oxidoannulene form, with O2 attached to Cc atom and is related to a shoulder rather than a clear minimum on FES.
3.2. Free energy surface for d,d-C70O3 decomposition The FES representing d,d-C70O3/d,d-C70O system resembles quantitatively the one for c,c-isomers, with minima related to the ozonide, oxide, and structures 5 and 6 with the O2 moiety entrapped over the adjacent Cd-Cc or CdeCe bond, and with a prominent shoulder related to the open structure 2 (Fig. 6). On closer inspection, the oxide minimum represents the open oxidoannulene product, as clearly indicated by the value of CV2 ca. 0.8 Å (cf. ca. 0.1 for c,c-C70O epoxide). The epoxide structure
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does not appear in the course of metadynamics simulations, which indicates that this hypothetical product would be high in free energy compared to the oxidoannulene. Fig. 7 reveals further differences with respect to the previous FES, the heights of the free energy barriers in particular. Although the barrier for the path leading to the oxide product is slightly lower than the one for c,c-isomers (15.3 kcal mol1 vs 16.2 kcal mol1) it is the same time higher than the barrier for the competing path leading to minimum 5/6 (14.3 kcal mol1). The minimum itself is deeper (11.2 kcal mol1) than in the case of c,cisomers (9.5 kcal mol1). Moreover, the barrier on the path linking the oxide product with structures 5 and 6 is essentially lower (10.5 kcal mol1, vs. 17.9 kcal mol1 for c,c isomers). All that facts suggest that for the decomposition of d,d-C70O3 isomer products 5 and 6 may be preferred both thermochemically and kinetically over the oxide product. The open structure 2 contributes to the both paths and similarly as for c,c-isomers is related to a shoulder (at ca. 6e10 kcal mol1) rather than a clear minimum on FES. Interestingly, the simulation has revealed a high energy shallow minimum on the FES (represented schematically by structure 8 in Fig. 6), with O atom attached to only one of the carbon atoms involved in the ozone ring in the parent ozonide. The structure is a product of simultaneous breaking of one of the polar CdeO bonds and the opposite OeO bond, with the retained CdeCd bond. Although the energy barrier for such a dissociation channel is only 13.2 kcal mol1, the resulting structure is as much as 11.2 kcal mol1 higher in free energy than the reference parent ozonide. One may guess that structure 8 might potentially lead to d,d-C70O or alternatively c,d-C70O or d,e-C70O (by bridging the adjacent CeC bond). However, the path towards d,d-C70O would have to involve the epoxide intermediate, which must be a high energy structure, as discussed earlier. The kinetic factor may therefore prevent the dissociation channel leading through structure 8. On the other hand, d,e-C70O or c,d-C70O were reported to be less favorable than d,d-C70O isomer (Table 1), therefore the thermodynamic factor does not support such a hypothetical reaction channel. Such alternative oxidoannulenes have not been observed in the course of this simulation; it must be emphasized, however, that the selected collective variables are not flexible enough to trace such a competing process. 3.3. Free energy surface for a,a-C70O3 decomposition As the topography of the free energy surface calculated for a,aC70O3/a,a-C70O isomers resembles well what has been revealed for d,d-isomers, with only some differences concerning the heights of the pertaining energy barriers, the detailed discussion has been moved to Supplementary Materials (Figs. S9 and S10 and the following paragraphs). The path leading to the products 5 and 6 seems to be preferred over the one leading to the oxidoannulene product due to both thermochemical and kinetic factors.
molecular dynamics simulation [5]. Due to the lack of a minimum related to the ozonide, structure 2 has been chosen as a reference to report the free energy of other structures. As illustrated in Fig. 8, the likely course of the reaction leads to the product 5(6). Note, that due to symmetry structures 5 and 6 are equivalent, with O2 moiety bridging CdeCe bond. The energy barrier for the path is only 5.7 kcal mol1 (Fig. 9), therefore the O2 unit attached to one of Ce atoms in structure 2 seems likely to get entrapped over one of the adjacent bonds. The minimum related to the product 5(6) is even deeper than in the case of d,d- and a,aisomers, 16.5 kcal mol1. There is a path linking it with e,e-C70O oxidoannullene product (accompanied as usual by O2 co-product), with a barrier of 9.0 kcal mol1. The alternative dissociation path leading to structure 8 has been detected, with the energy barrier of 13.0 kcal mol1, which is considerably higher than the value for the path leading to 5(6) product. The path is unlikely to continue towards the oxidoannulene e,e-C70O as it would require the dissociation of the pertaining CeeCe bond and therefore crossing another high energy barrier. The formation of d,e-C70O with O atom bridging the adjacent CdeCe bond is even less likely, as the product was reported to be the highest energy oxide of all possible isomers (Table 1). It seems, therefore, that the only possible dissociation path of e,e-C70O3 leads to the product 5(6) through the open structure 2. It seems that of the four studied ozonides, only c,c-C70O3 should decompose thermally to the related oxide (c,c-C70O) as the final product. The calculations performed for d,d-C70O3, a,a-C70O3 and e,e-C70O3 seem to suggest the reaction channel leading to structures 5 and 6 rather than to the related oxidoannulene. This opens the question about the further steps of the reaction, i.e. the evolution of these structures. Theoretically, the entrapped O2 unit could be released (through the intermediate represented by structure 7) as for all three cases the minimum energy path linking structures 5 and 6 with oxidoannulene 4 (accompanied by O2) has been detected. However, the height of the energy barrier with respect to minimum representing structures 5 and 6 is high (in a range ca. 20e22 kcal mol1). Due to the fact that the competing reaction paths originating in structures 5 and 6 are unknown, the question about the evolution of these structures cannot be addressed. One may suspect that such processes would lead to the degradation of the fullerene surface. Actually, such a possibility was suggested for the ozonolysis of C60 [21]. Also, the oxygentaion of C60 is known to lead to the cleavage of CeC bonds and to C60(O)3 product with a ketolactone moiety embedded into the fullerene skeleton [39]. It cannot be excluded that the further processes on the surface of C70 may lead to the formation of complicated dioligo- or polimeric structures e.g. by the agency of CeOeC bonds formed between the fullerene molecules present in the sample [40,41]. The polymerization of the sample was suggested to account for the unexpected insolubility of the traces of a,a-C70O obtained in the course of photolysis of a,b-C70O3 [3]. 4. Conclusions
3.4. Free energy surface for e,e-C70O3 decomposition Although the FES of e,e-C70O3/e,e-C70O system resembles roughly the ones of d,d-C70O3/d,d-C70O and a,a-C70O3/a,a-C70O, it differs in some important details. First of all, the ozonide appears to be unstable at room temperature as no minimum related to such a structure has been detected. Instead, the structure 2 gives rise to a broad minimum on FES. The result is an accord with the previous findings that e,e-C70O3 ozonide, which is a minimum on the related potential energy surface is essentially higher in energy than the open structure 2 and it undergoes a spontaneous dissociation to form the open structure in the course of the room temperature
Several structures relevant for the thermal decomposition of a,a-C70O3, c,c-C70O3, d,d-C70O3 and e,e-C70O3 ozonides have been revealed in the course of the metadynamics simulation of the room temperature free energy surfaces of a,a-C70O3/a,a-C70O, c,c-C70O3/ c,c-C70O, d,d-C70O3/d,d-C70O and e,e-C70O3/e,e-C70O systems. Although all four ozonides are known to be minima on potential energy surfaces, only the first three have turned out to be minima on the related free energy surfaces, which is in agreement with the previously reported room temperature molecular dynamics simulation indicating a spontaneous dissociation of CeeCe and one of OeO bonds in e,e-C70O3.
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The resulting open structure, with carbonyl and carbonyl oxide groups, plays an important role in the possible mechanism of decomposition of all studied ozonides, while only for e,e-C70O3/e,eC70O system it is related to the minimum on the underlying FES. For other isomers the open structure is represented by shoulders some 4e13 kcal mol1 higher in free energy than the parent ozonide. Interestingly, the open structure is a minimum on the PES of c,cC70O3 isomer and is separated from the ozonide by a considerable energy barrier. It clearly indicates the influence of a thermal factor on processes on C70 fullerene surface as the topographies of the related PES and FES of C70O3 are quantitatively different. For d,d-C70O3, a,a-C70O3 and e,e-C70O3 it seems that the preferred reaction channel is the one leading to the structures with O2 moiety entrapped over the CeC bonds adjacent to the CeOeC group. Only c,c-C70O3 should decompose thermally to the related c,c-C70O epoxide as the final product. Although the oxidoannulene form of c,c-C70O appears in the course of the metadynamics simulation of c,c-C70O3/c,c-C70O isomers as a high energy structure, it does not lead to any clear minimum on the underlying free energy surface. Moreover, the path linking the ozonide and the oxide does not cross the area related to oxidoannulene form but goes directly towards the epoxide minimum. These observations underpin the hypothesis that the oxidoannulene intermediate detected experimentally in the course of thermolysis of c,c-C70O3 may originate in a minimum on the triplet energy surface [5].
[12]
[13]
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[21]
Acknowledgements [22]
A grant of computer time from the Wrocław Center for Networking and Super- computing (WCSS) is gratefully acknowledged. I would like to thank the Ministry of Science and Higher Education, Republic of Poland, for supporting this work under the grant no. N N204 280738. Appendix A. Supplementary data
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Supplementary data to this article can be found online at https://doi.org/10.1016/j.molstruc.2019.03.005.
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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc
The mechanism of ozonolysis on the surface of C70 fullerene. The free energy surface theoretical study Andrzej Bil Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383, Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2019 Received in revised form 1 March 2019 Accepted 3 March 2019 Available online 4 March 2019
Density functional theory based metadynamics simulations of the room temperature ground state free energy surfaces (FES) of a,a-C70O3/a,a-C70O, c,c-C70O3/c,c-C70O, d,d-C70O3/d,d-C70O and e,e-C70O3/e,eC70O systems have revealed several structures relevant for the thermal decomposition of the experimentally known c,c-C70O3 and hypothetical a,a-C70O3, d,d-C70O3 and e,e-C70O3 ozonides. Criegee intermediate, an open structure with carbonyl oxide and carbonyl groups, plays an important role in the possible mechanism of decomposition of all studied ozonides, while only for e,e-C70O3/e,e-C70O system it is related to the minimum on the underlying FES. The differences between the topography of FES and the previously reported related potential energy surface indicates the influence of a thermal factor on processes on C70 surface. Only c,c-C70O3 should decompose thermally to form c,c-C70O epoxide as the final product while for the other studied ozonides the preferred reaction channel seems to be the one leading to the structures with O2 moiety entrapped over the CeC bonds adjacent to the CeOeC group. Hypothetical oxidoannulene form of c,c-C70O is not related to a clear minimum on the free energy surface. © 2019 Elsevier B.V. All rights reserved.
Keywords: Ozonide Epoxide Oxidoannulene Carbonyl oxide DFT
1. Introduction Fullerene oxides and ozonides are examples of simple exohedral modifications of a parent molecule. C70 oxides were reported in 1991 [1], only a few years after C60 discovery by Kroto [2]. The chemistry of C70 is more challenging than of C60 because there are as many as eight non-equivalent types of bonds present in the former molecule, a consequence of five nonequivalent carbon atom types (Fig. 1). Of eight theoretically possible oxides, two experimentally know stable products i.e. a,b-C70O and c,c-C70O adopt epoxide structure, with O atom bridging a CeC bond. The six remaining isomers are expected to be oxidoannulenes, with the bond broken upon the oxide formation [3e5]. Fig. 1 illustrates the structures of four high symmetry isomers, c,c-C70O epoxide and a,a-C70O, d,d-C70O and e,e-C70O oxidoannulenes, which are the subject of this study. Low-pressure combustion of hydrocarbons in an oxygendeficient environment, the most common route to producing fullerenes commercially, is also a source of fullerenes oxides. On laboratory scale they can be obtained with the use of ozonolysis reaction, by bubbling ozone gas through fullerene solutions. The
E-mail address: [email protected]. https://doi.org/10.1016/j.molstruc.2019.03.005 0022-2860/© 2019 Elsevier B.V. All rights reserved.
process, which involves complicated paths of thermolysis or photolysis, was the subject of the experimental comprehensive studies by Heymann [3]. The thermolysis paths were also researched with the use of computational simulations [5,6]. Carried out in the dark, the ozonolysis of C70 led to two monoozonides, assigned to be the a,b- and c,c-C70O3 isomers. Thermal decomposition of a,b-C70O3 molecule led to the previously known a,b-C70O epoxy fullerene. Interestingly, the experimental study of the thermolysis pathway of c,c-C70O3, which was a considerably slower reaction, quotes a surprising observation that the resulting direct product was a metastable structure, whose UVeVis spectrum shared features distinctive of oxidoannulenes. Because of this, the metastable product was assigned to be d,d-C70O [3], which as calculations suggest does adopt the oxidoannulene form. However, such a conclusion would require a complicated mechanism of the formation of the final c,c-C70O oxide, involving a transfer of an O atom between Cc-Cc and CdeCd bonds. Of all theoretically possible oxides, the two stable c,c-C70O and a,b-C70O isomers are not the most thermochemically preferable ones (Table 1, data from Ref. [5]). If the latter one serves as a reference, c,c-C70O is a minimum on the potential energy surface located at ca. þ 1.3 kcal mol1, while all symmetric oxides are lower in energy scale, with e,e-C70O being the lowest minimum. Note, suggested as an intermediate product d,d-C70O oxidoannulene has
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Fig. 1. The five different carbon atom environments in C70 (left); high symmetry isomers of C70O (right).
Table 1 Calculated total S0 energies [kcal mol1] of the C70O and C70O3 set of isomers, relative to the a,b-isomer; 0 K energies refer to the minima on the potential energy surface, 298 K to average potential energies from molecular dynamics trajectories [5]. ()* refers to a structure observed in the spontaneous ring-opening reaction in the course of molecular dynamics simulation. C70O
a,b c,c a,a d,d b,c c,d d,e e,e
C70O3
0K
298 K
0K
298 K
0 1.3 2.6 2.8 0.8 4.1 10.0 12.8
0 2.3 1.1 2.1 1.3 5.9 11.9 12.1
0 2.3 15.7 5.4 12.3 16.5 13.7 30.7 (19.2)*
0 2.0 15.4 4.6 12.2 16.0 13.9 29.3 (18.8)*
a lower energy than the final c,c-C70O epoxide. Interestingly, a,bC70O3 and c,c-C70O3, þ2.3 kcal mol1 higher in energy than the former one, are the most stable ozonides, which strongly suggests that the reaction pathway toward an oxide formation must proceed via the corresponding ozonide structure [5]. Conversely, the lowest
energy oxide would originate from the least stable ozonide e,eC70O3, þ30.7 kcal mol1 higher in energy than a,b-isomer, which seems to explain why this isomer has never been reported in a laboratory. The relative stability of isomers is practically not altered either by including a zero-point vibration correction or by a thermal boost introduced through room temperature molecular dynamics simulations [5]. The previously reported calculations have revealed that the crucial step towards the formation of c,c-C70O and a,b-C70O isomers from parent ozonides involves the opening of the ozone ring (Fig. 2, a) and formation of an open structure (Fig. 2, b) with a single oxygen atom attached to one carbon atom and an O2 moiety to the second one (carbonyl and carbonyl oxide groups, the last one known as the Criegee intermediate), which at this stage resembles a general mechanism for ozonolysis of alkenes proposed by Criegee [5,7,8]. On the basis of the resonant structure of (b), the mechanism of epoxide formation has been postulated (Fig. 2, c). However, it was indicated that a triplet channel can contribute to c,c-C70O formation, through an oxidoannulene structure, which is a minimum on its triplet potential energy surface (PES) and as such may be a metastable structure, converting finally into the ground state epoxide minimum [5]. The contribution of such an excited state
Fig. 2. The possible mechanism for the formation of C70O epoxide [5]. The pictures represent the frontier atoms of the epoxide.
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intermediate could offer an alternative interpretation of the experiment, involving a much simpler mechanism than the one originally suggested, through the oxidoannulene intermediate assumed to be a singlet d,d-C70O isomer [3]. It should be emphasized that the experiment by Heymann [3] was performed at room temperature. However, the reported earlier calculations refer to minima on the potential energy surface which may have a different topography that the related room temperature free energy surface (FES). Therefore, the role of a hypothetical oxidoannulene c,c-C70O isomer, which is not a minimum on the ground state PES, but might compete on a room temperature ground state FES cannot be excluded. CceCc and CaeCb bonds in C70 have dominant character expected for a double bond, whereas the remaining six types of bonds, can be considered more as single bonds with some contributions from the p-electron density network [9]. Due to the polar character of a CeO bond, the ozonide formation induces essential changes in the electron density distribution in the parent C70, not only in the vicinity of the directly involved CeC bond, but also in the adjacent bonds. It explains why the ozonide formation through a dipolar 1,3 cycloaddition reaction is theoretically possible not only over a formally double Cc-Cc and Ca-Cb bond, but also over formally single bonds, which does not lead to CeC bond cleavage [9]. Interestingly, the calculated relative stabilities of most ozonide isomers correlates well with the total perturbation of the bonding density in the molecule e the smaller perturbation, the more stable the isomer is [9]. All these facts suggest, that the oxidonallulene isomers formation, which would originate in ozonides other than the most stable a,b- and c,c-isomers, may be a more complex process than the one which leads to epoxides, and adjacent bonds may influence the course of the reaction. However, little has been known about the mechanism of the reactions taking place on the surface of C70 for ozonides other than a,b-C70O3 and c,c-C70O3. Only e,e-C70O3 is expected to undergo spontaneous ozone ring opening as such a process was observed in the course of a room temperature molecular dynamics simulation [5], however, the following course of the reaction remains unknown. Deeper understanding of the mechanism of ozonolysis of C70 is important as both ozone and fullerenes are the focus of attention of contemporary research [10e15]. Studying this issue can provide more information about the chemistry of carbonyl oxides (Criegee intermediate), which contributes to complicated oxidation patterns important for various branches of chemistry [10,16,17]. Ozonation is also a method of functionalization of carbon materials [18] such as carbon nanotubes and nanofibers [11]. Apart from that, recently the attention has been paid on reactions similar to the ozonolysis of C70, e.g. epoxy and oxidoannulene oxidation of fused-pentagon chlorofullerenes [12], reduction of SO2 on graphite [19], thiozonolysis of C70 [20] or ozonolysis of C60 [21]. This paper focuses on the mechanism of the ground state decomposition of C70O3 isomers at room temperature established on the basis of the calculated underlying free energy surfaces. Only four high symmetry isomers have been selected for this study (Fig. 1). For c,c-C70O3/cc-C70O system it is important to establish whether the oxide (epoxide) formation is preceded by the formation of the open oxidoannulene form and whether such a structure, not present as a minimum on the potential energy surface, is a true minimum on the room temperature ground state FES, which is the main research problem reported in this account. Disproving such a minimum would strengthened the hypothesis that the detected experimentally metastable intermediate may represent c,c-C70O oxidoannulene in its triplet electronic state [5]. Of the six ozonides being the parent structures for the oxidoannulene oxides, d,d-C70O3 has the lowest energy, therefore
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studying the mechanism of its decomposition seems important [5]. In turn, a,a-C70O3 may be a representative for high energy ozonides b,c-C70O3 and c,d-C70O3 [5,9]. Finally, e,e-C70O3/e,e-C70O is an interesting system consisted of the lowest energy oxide and the highest energy ozonide. e,e-C70O3 was also the isomer whose ozone ring opening was observed in the course of a room temperature molecular dynamics simulation [5]. Apart from the importance of high symmetry isomers, they are also easier to investigate because for the remaining (low symmetry) isomers the free energy subspace underlying the ozonide decomposition cannot be explored effectively with the use of two collective variables only. It should be emphasized that the expected product of the ozonide decomposition reaction consists of a pair of not linked molecules i.e. the oxide and O2. Thus, it is not possible to estimate credibly the free energy of the product with respect to the parent ozonide as the depth of the pertaining minimum on FES depends on how much space around the fullerene the O2 molecule is allowed to explore in the course of the simulation. Considering this limitation, the paper is aimed exclusively at the topography of FES and establishing the relevant structures and processes on the surface of the fullerene than at accurate prediction of the thermochemistry of the reaction.
2. Methods Metadynamics is a method which allows to study a free energy surface at a given temperature [22e24]. Thermal factor is often important for the course of a reaction and the method itself allows for exploring effectively structures on FES which are not accessible within regular molecular dynamics due to high energy barriers separating them from an initial structure and limited simulation time. A proper choice of collective variables enables one to define and explore a free energy subspace relevant for a given chemical process [25]. To represent the release of O2 moiety from the surface the first collective variable (CV1) has been built as a sum of coordination numbers [25] calculated from oxygen atoms in O2 moiety to the four nearest carbon atoms (Fig. 3, see also Figure S1 and further discussion in Supplementary Materials for details). The second collective variable (CV2) used to describe FES has been defined as a difference between C O and C C atomic distances (Fig. 3) and represents the process of the formation and decomposition of the bonds. All calculations have been performed using the BeckeeLeeeYangeParr exchangeecorrelation functional [26,27] with D3 semiempirical dispersion corrections proposed by Grimme [28], coupled to a Gaussian and plane-wave basis set [29], as
Fig. 3. The scheme illustrating the distances necessary to build CV1 and CV2 collective variable. Only frontier atoms of C70O3 relevant for the studied reaction have been included in the picture.
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implemented in the QUICKSTEP program [30], which is part of the CP2K suite of software [31]. The GoedeckereTetereHutter normconservative pseudo-potentials optimized for use against the BLYP functional have been used in this simulations [32]. Taking dispersion effects into account was demonstrated to affect chemical reactivity [33,34]. Such effects were employed previously to study the reactivity of fullerenes [9,35e38]. Valence orbitals of all elements were expanded using DZVPeGTHeBLYP double zeta quality basis sets. The energy cutoff of 350 Ry has been used to define an auxiliary basis set, together with the cubic periodic boundary conditions to create a simulation cell of length 20 Å. Metadynamics simulations with a time step of 0.7 fs have been performed in the canonical ensemble. A mean temperature of 298 K has been regulated using a chain of NoseeHoover thermostats. 3. Results and discussion The exploration of the free energy surfaces relevant for the ozonide to the oxide reaction has revealed a variety of structures, including these which are not minima on FES, some of them perhaps not obvious to be predicted a priori. The most relevant of them have been depicted symbolically, with the use of frontier atoms, in Figs. 4e9 and Figs. S9 and S10 (Supplementary Materials), while the full pictures of the representative structures with the most important bond lengths and numerical values of CVs have been reported in Supplementary Materials (Figs. S3-S7). Apart from the reactant (ozonide, labelled with ‘1’) and the expected product (the resultant epoxide, structure 3, or oxidoannulene, structure 4, both accompanied by O2), the structures 5 and 6 with O2 moiety entrapped over a CeC bond adjacent to the parent CeC bond to form a peroxide have turned out to be relevant for the analyzed mechanism. Note, that the choice of CVs makes structures 5 and 6 indistinguishable on FES, therefore the possibility of formation of any of these two structures contributes to the depth of the minimum at ca. (3.6, 0.9 Å). With the exception of the unstable e,eC70O3, deep minima on FES are related to the ozonide structure. On top of that, some other structures, not necessarily minima, have appeared along the possible reaction paths, structure 2 with the broken parent CeC bond and one of OeO bonds (Criegee
Fig. 5. Minimum free energy paths linking the most important minima on c,c-C70O3/ c,c-C70O FES. The relevant structures have been depicted schematically. The energies are expressed with respect to the minimum representing structure 1.
Fig. 6. Free energy [kcal mol1] surface for d,d-C70O3/d,d-C70O isomers together with three minimum free energy paths linking the most important minima. The relevant structures have been depicted schematically.
intermediate) being of the utmost importance.
3.1. Free energy surface for c,c-C70O3 decomposition
Fig. 4. Free energy [kcal mol1] surface for c,c-C70O3/c,c-C70O isomers together with three minimum free energy paths linking the most important minima. The relevant structures have been depicted schematically with the use of frontier atoms.
On the free energy surface calculated for c,c-C70O3 and related structures (Fig. 4) there are three clear minima which originate in the parent ozonide (structure 1), in the expected product, i.e. C70O epoxide (structure 3), and in structures 5 and 6, where O2 moiety bridges either Cc-Cb or Cc-Cd bond adjacent to the Cc-O-Cc group, respectively. The resulting structures therefore share structural motives typical of oxidoannulenes and peroxides. Two minimum energy paths originating in the minimum related to the ozonide have been detected on the FES, the first one links the ozonide and the epoxide (light blue curve in Figs. 4 and 5), while the second one (black curve) leads to products 5 and 6. The first stage of both processes involves the opening the ozone ring accompanied by the dissociation of Cc-Cc bond and the formation of
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Fig. 7. Minimum free energy paths linking the most important minima on d,d-C70O3/ d,d-C70O FES. The relevant structures have been depicted schematically. The energies are expressed with respect to the minimum representing structure 1.
Fig. 8. Free energy [kcal mol1] surface for e,e-C70O3/e,e-C70O isomers together with three minimum free energy paths linking the most important minima. The relevant structures have been depicted schematically.
structure 2. The structure, which is a minimum on the related potential energy surface [5], is not a minimum on the FES but is rather related to a shoulder some 8e13 kcal mol1 higher in free energy than the ozonide. Due to the dissociation of Cc-Cc bond and OeO bond, the resulting open structure is quite flexible, therefore a range of similar structures with somewhat different Cc-Cc and OeO distances can be explored, with CV2 between ca. 0e0.5 Å while CV1 does not deviate much from ca. 3.1e3.2. Structural parameters of a representative structure of type 2 can be found in Supplementary Materials, Fig. S3. The pertaining energy barriers for paths leading to product 3 and structures 5 and 6 are 16.2 and 18.6 kcal mol1, respectively. The fact that the barrier leading to 5/6 is higher than the former one seems to be in accord with the experimental findings that c,c-C70O is the final product of c,c-C70O3 decomposition. Moreover, the preferred structure of the product is an epoxide (structure 3 in Fig. 4), as the oxidoannulene form, also marked in Fig. 4 as structure 4, is not related to any clear minimum on FES. Although such a
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Fig. 9. Minimum free energy paths linking the most important minima on e,e-C70O3/ e,e-C70O FES. The relevant structures have been depicted schematically. The energies are expressed with respect to the minimum representing structure 2.
structure appears in the course of the simulation, it is considerably higher in free energy scale than the epoxide product (cf. Fig. S8 in Supplementary Materials). Importantly, the open structure 2 is linked to the oxide product via a path almost parallel to the horizontal axis, with CV2 ca. 0 Å. The path does not cross the area related to the oxidoannulene form but goes directly towards the epoxide minimum. Therefore the release of O2 moiety (represented by a gradual decrease in CV1) must be accompanied by the simultaneous formation of Cc-Cc and Cc-O bonds to form the epoxide group so that the difference r(Cc-O) - r(Cc-Cc) which defines CV1, can vary only little and remains close to zero. Contrary to that, the black curve designating the path towards 5 and 6 products first goes down, with CV2 changing from 0 to ca. 0.6 Å, with CV1 almost constant at ca. 3.1. Subsequently, CV1 increases to ca. 3.7. The evolution of the CVs along the reaction path illustrates that the process of trapping O2 moiety by the adjacent Cb-Cc or Cc-Cd bond must be preceded by the formation of Cc-O-Cc group (no Cc-Cc bond) to form oxidoannulene structure, which is related to the energy barrier. Although the formation of structures 5 and 6 seems thermochemically favorable, as the pertaining minimum is 9.4 kcal mol1 lower in energy than the one of the reactant, it seems that the heights of the energy barriers are a factor in favor of the path leading to the epoxide product. The path linking minimum 3 and 5/ 6 (red curve in Figs. 4 and 5) can be traced on the FES, but the free energy barrier limiting the possible trapping O2 by the epoxide to form structures 5 or 6 is higher than for the process leading back towards the ozonide. Structure 7 identified along the path has an oxidoannulene form, with O2 attached to Cc atom and is related to a shoulder rather than a clear minimum on FES.
3.2. Free energy surface for d,d-C70O3 decomposition The FES representing d,d-C70O3/d,d-C70O system resembles quantitatively the one for c,c-isomers, with minima related to the ozonide, oxide, and structures 5 and 6 with the O2 moiety entrapped over the adjacent Cd-Cc or CdeCe bond, and with a prominent shoulder related to the open structure 2 (Fig. 6). On closer inspection, the oxide minimum represents the open oxidoannulene product, as clearly indicated by the value of CV2 ca. 0.8 Å (cf. ca. 0.1 for c,c-C70O epoxide). The epoxide structure
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does not appear in the course of metadynamics simulations, which indicates that this hypothetical product would be high in free energy compared to the oxidoannulene. Fig. 7 reveals further differences with respect to the previous FES, the heights of the free energy barriers in particular. Although the barrier for the path leading to the oxide product is slightly lower than the one for c,c-isomers (15.3 kcal mol1 vs 16.2 kcal mol1) it is the same time higher than the barrier for the competing path leading to minimum 5/6 (14.3 kcal mol1). The minimum itself is deeper (11.2 kcal mol1) than in the case of c,cisomers (9.5 kcal mol1). Moreover, the barrier on the path linking the oxide product with structures 5 and 6 is essentially lower (10.5 kcal mol1, vs. 17.9 kcal mol1 for c,c isomers). All that facts suggest that for the decomposition of d,d-C70O3 isomer products 5 and 6 may be preferred both thermochemically and kinetically over the oxide product. The open structure 2 contributes to the both paths and similarly as for c,c-isomers is related to a shoulder (at ca. 6e10 kcal mol1) rather than a clear minimum on FES. Interestingly, the simulation has revealed a high energy shallow minimum on the FES (represented schematically by structure 8 in Fig. 6), with O atom attached to only one of the carbon atoms involved in the ozone ring in the parent ozonide. The structure is a product of simultaneous breaking of one of the polar CdeO bonds and the opposite OeO bond, with the retained CdeCd bond. Although the energy barrier for such a dissociation channel is only 13.2 kcal mol1, the resulting structure is as much as 11.2 kcal mol1 higher in free energy than the reference parent ozonide. One may guess that structure 8 might potentially lead to d,d-C70O or alternatively c,d-C70O or d,e-C70O (by bridging the adjacent CeC bond). However, the path towards d,d-C70O would have to involve the epoxide intermediate, which must be a high energy structure, as discussed earlier. The kinetic factor may therefore prevent the dissociation channel leading through structure 8. On the other hand, d,e-C70O or c,d-C70O were reported to be less favorable than d,d-C70O isomer (Table 1), therefore the thermodynamic factor does not support such a hypothetical reaction channel. Such alternative oxidoannulenes have not been observed in the course of this simulation; it must be emphasized, however, that the selected collective variables are not flexible enough to trace such a competing process. 3.3. Free energy surface for a,a-C70O3 decomposition As the topography of the free energy surface calculated for a,aC70O3/a,a-C70O isomers resembles well what has been revealed for d,d-isomers, with only some differences concerning the heights of the pertaining energy barriers, the detailed discussion has been moved to Supplementary Materials (Figs. S9 and S10 and the following paragraphs). The path leading to the products 5 and 6 seems to be preferred over the one leading to the oxidoannulene product due to both thermochemical and kinetic factors.
molecular dynamics simulation [5]. Due to the lack of a minimum related to the ozonide, structure 2 has been chosen as a reference to report the free energy of other structures. As illustrated in Fig. 8, the likely course of the reaction leads to the product 5(6). Note, that due to symmetry structures 5 and 6 are equivalent, with O2 moiety bridging CdeCe bond. The energy barrier for the path is only 5.7 kcal mol1 (Fig. 9), therefore the O2 unit attached to one of Ce atoms in structure 2 seems likely to get entrapped over one of the adjacent bonds. The minimum related to the product 5(6) is even deeper than in the case of d,d- and a,aisomers, 16.5 kcal mol1. There is a path linking it with e,e-C70O oxidoannullene product (accompanied as usual by O2 co-product), with a barrier of 9.0 kcal mol1. The alternative dissociation path leading to structure 8 has been detected, with the energy barrier of 13.0 kcal mol1, which is considerably higher than the value for the path leading to 5(6) product. The path is unlikely to continue towards the oxidoannulene e,e-C70O as it would require the dissociation of the pertaining CeeCe bond and therefore crossing another high energy barrier. The formation of d,e-C70O with O atom bridging the adjacent CdeCe bond is even less likely, as the product was reported to be the highest energy oxide of all possible isomers (Table 1). It seems, therefore, that the only possible dissociation path of e,e-C70O3 leads to the product 5(6) through the open structure 2. It seems that of the four studied ozonides, only c,c-C70O3 should decompose thermally to the related oxide (c,c-C70O) as the final product. The calculations performed for d,d-C70O3, a,a-C70O3 and e,e-C70O3 seem to suggest the reaction channel leading to structures 5 and 6 rather than to the related oxidoannulene. This opens the question about the further steps of the reaction, i.e. the evolution of these structures. Theoretically, the entrapped O2 unit could be released (through the intermediate represented by structure 7) as for all three cases the minimum energy path linking structures 5 and 6 with oxidoannulene 4 (accompanied by O2) has been detected. However, the height of the energy barrier with respect to minimum representing structures 5 and 6 is high (in a range ca. 20e22 kcal mol1). Due to the fact that the competing reaction paths originating in structures 5 and 6 are unknown, the question about the evolution of these structures cannot be addressed. One may suspect that such processes would lead to the degradation of the fullerene surface. Actually, such a possibility was suggested for the ozonolysis of C60 [21]. Also, the oxygentaion of C60 is known to lead to the cleavage of CeC bonds and to C60(O)3 product with a ketolactone moiety embedded into the fullerene skeleton [39]. It cannot be excluded that the further processes on the surface of C70 may lead to the formation of complicated dioligo- or polimeric structures e.g. by the agency of CeOeC bonds formed between the fullerene molecules present in the sample [40,41]. The polymerization of the sample was suggested to account for the unexpected insolubility of the traces of a,a-C70O obtained in the course of photolysis of a,b-C70O3 [3]. 4. Conclusions
3.4. Free energy surface for e,e-C70O3 decomposition Although the FES of e,e-C70O3/e,e-C70O system resembles roughly the ones of d,d-C70O3/d,d-C70O and a,a-C70O3/a,a-C70O, it differs in some important details. First of all, the ozonide appears to be unstable at room temperature as no minimum related to such a structure has been detected. Instead, the structure 2 gives rise to a broad minimum on FES. The result is an accord with the previous findings that e,e-C70O3 ozonide, which is a minimum on the related potential energy surface is essentially higher in energy than the open structure 2 and it undergoes a spontaneous dissociation to form the open structure in the course of the room temperature
Several structures relevant for the thermal decomposition of a,a-C70O3, c,c-C70O3, d,d-C70O3 and e,e-C70O3 ozonides have been revealed in the course of the metadynamics simulation of the room temperature free energy surfaces of a,a-C70O3/a,a-C70O, c,c-C70O3/ c,c-C70O, d,d-C70O3/d,d-C70O and e,e-C70O3/e,e-C70O systems. Although all four ozonides are known to be minima on potential energy surfaces, only the first three have turned out to be minima on the related free energy surfaces, which is in agreement with the previously reported room temperature molecular dynamics simulation indicating a spontaneous dissociation of CeeCe and one of OeO bonds in e,e-C70O3.
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The resulting open structure, with carbonyl and carbonyl oxide groups, plays an important role in the possible mechanism of decomposition of all studied ozonides, while only for e,e-C70O3/e,eC70O system it is related to the minimum on the underlying FES. For other isomers the open structure is represented by shoulders some 4e13 kcal mol1 higher in free energy than the parent ozonide. Interestingly, the open structure is a minimum on the PES of c,cC70O3 isomer and is separated from the ozonide by a considerable energy barrier. It clearly indicates the influence of a thermal factor on processes on C70 fullerene surface as the topographies of the related PES and FES of C70O3 are quantitatively different. For d,d-C70O3, a,a-C70O3 and e,e-C70O3 it seems that the preferred reaction channel is the one leading to the structures with O2 moiety entrapped over the CeC bonds adjacent to the CeOeC group. Only c,c-C70O3 should decompose thermally to the related c,c-C70O epoxide as the final product. Although the oxidoannulene form of c,c-C70O appears in the course of the metadynamics simulation of c,c-C70O3/c,c-C70O isomers as a high energy structure, it does not lead to any clear minimum on the underlying free energy surface. Moreover, the path linking the ozonide and the oxide does not cross the area related to oxidoannulene form but goes directly towards the epoxide minimum. These observations underpin the hypothesis that the oxidoannulene intermediate detected experimentally in the course of thermolysis of c,c-C70O3 may originate in a minimum on the triplet energy surface [5].
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A grant of computer time from the Wrocław Center for Networking and Super- computing (WCSS) is gratefully acknowledged. I would like to thank the Ministry of Science and Higher Education, Republic of Poland, for supporting this work under the grant no. N N204 280738. Appendix A. Supplementary data
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