J Mol Model (2014) 20:2304 DOI 10.1007/s00894-014-2304-7
ORIGINAL PAPER
Aromaticity of azines through dyotropic double hydrogen transfer reaction Maria & Muhammad Hanif & Tariq Mahmood & Ralf Ludwig & Khurshid Ayub
Received: 23 March 2014 / Accepted: 12 May 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Density functional theory calculations have been performed at B3LYP/6–31+G (d) level to quantify the aromaticities of mono- to triazines through dyotropic double hydrogen transfer (DDHT) reaction. The reaction was chosen such that the azines are products of double hydrogen dyotropic rearrangement, and activation barriers and energies of the reactions were functions of the aromaticities of azines. Small activation barriers and high energies of reactions were characteristic of the reactions delivering highly aromatic azines. Synchronicity, reaction energies and energies of activation have been analyzed, and the aromaticity values obtained thereof were compared with the aromaticity values from other geometric and magnetic criteria. Energies of activation were found superior to the energies of reaction for the determination of the aromaticities. Aromaticities of most of the azines were comparable to the aromaticity of benzene. Activation barriers
Electronic supplementary material The online version of this article (doi:10.1007/s00894-014-2304-7) contains supplementary material, which is available to authorized users. Maria : M. Hanif : T. Mahmood : K. Ayub COMSATS Institute of Information Technology, Abbottabad, KPK, Pakistan 22060 R. Ludwig Department of Physical Chemistry, University of Rostock, Dr.-Lorenz-Weg 1, 18059 Rostock, Germany R. Ludwig Leibniz-Institut für Katalyse an der Universität Rostock, e. V. Alebert-Einstein-Strasse 29a, 18059 Rostock, Germany K. Ayub (*) Department of Chemistry, College of Science, King Faisal University, Al-Ahsa 31982, Kingdom of Saudi Arabia e-mail:
[email protected] K. Ayub e-mail:
[email protected]
and reaction energies for the dyotropic reactions delivering contiguous or polynitrogeneous azines had thermodynamic contributions arising from the contiguous nature of azines, in addition to the aromaticity related thermodynamic contributions. Moreover, the aromaticity values of azines are also affected by the fusion of azine to the reaction center. When corrected for these factors, the aromaticities of azines using energies of activation for DDHT correlated nicely with the aromaticities of azines reported in the literature through NICS (0) πzz and some other energetic methods. Keywords Aromaticity . Azines . Dyotropic rearrangement . Thermodynamics
Introduction Aromaticity is a controversial but still useful concept in chemistry for theoretical and experimental research. Numerous criteria or definitions have been presented to estimate aromaticity since 1865 when Kekulé [1] proposed the cyclic structure of benzene, with alternating single and double bonds. However, no single method has received universal acceptance. This failure can be attributed to the multidimensional nature of aromaticity (composed of energetic, magnetic, and structural components) whereas many common aromaticity models rely on a single criterion [2]. As a consequence, the term “aromaticity” has become controversial. Binsch and Heilbronner [3], Labarre [4], and later again Binsch [5] even argued that aromaticity is an obsolete concept, and should be discarded along with other non-existing phenomena such as vital force and the all-pervading ether. However, many other chemists still believe aromaticity as a useful and essential concept for logical treatment of chemistry [6]. Three major categories to quantify aromaticity are energetic, structural, and magnetic. Among these criteria, energetic
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criterion is considered to be the principal one because it determines the chemical behavior and reactivity of molecules [7–10]. Several models have been developed in the last century to determine the “extra stabilization” but they varied considerably among themselves. As a result, an array of bewildering stabilization energies has resulted depending on the calculation and model used [11, 12]. A few common energetic criteria are Dewar resonance energy [13–16], Hückel resonance energy [17–19], Hess-Schaad resonance energy [20–25], Schleyer isomerization stabilization energies [26] and topological resonance energies [27–29]. These models have been used to develop aromaticity scales. Unfortunately, the order of aromaticity for compound depended mostly on the nature of the model and method used rather than the intrinisic property. For example, most of the methods used to calculate aromaticity through aromatic stabilization energy (ASE), theoretically or experimentally, are based on isodesmic reactions which are influenced by strain, hyperconjugation, proto-branching or syn-anti effects. These effects arise mainly because, aromaticities (through aromatic stabilization energies) are calculated with respect to a nonaromatic reference system; therefore, the calculated ASEs are not always reliable. A direct method (without the need of a reference system) delivering better estimate of aromatic stabilization energy is, therefore, required [26]. A protocol based on the dyotropic double hydrogen transfer (DDHT) reaction has been recently developed by Fernández and co-workers, and it has been shown to be independent of the reference compounds [30]. The dyotropic double hydrogen transfer reaction is also known as type II dyotropic rearrangement where two migrating groups move simultaneously to new bonding sites [31]. Type II dyotropic rearrangement is different than type I which is characterized by interchange of positions by migrating groups [31]. Dyotropic double hydrogen transfer method is shown to reliably estimate aromaticities (benzene, thiophene, furan, and pyridine) and antiaromaticities (cyclobutadiene) of certain conjugated cyclic compounds. A nice correlation exists between ASE and the activation barrier, or energy of reaction [32]. Since this method did not require any reference molecules, and is believed to deliver more reliable estimates of aromaticity, we became interested in the utilization of the method for the determination of aromaticities of azines. This interest is not only because of the existing controversies in the literature but also due to the role of azines in several fields [33–35]. Several methods in the literature show that benzene and pyridine (2), a prototype azine (see Scheme 1 for structures), exhibit comparable aromaticities. For example, comparison of resonance energies calculated by Wiberg [36] (benzene vs 2: 150.5 vs 142.3 kJ mol−1), by Bird [37–39] (benzene vs 2: 191.6 vs 181.2 kJ mol−1), and aromatic stabilization energies through homodesmotic equations [26] (benzene vs 2:120.5 vs
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Scheme 1 Structures of benzene, pyridine, and thiophene as representative aromatic compounds
129.7 kJ mol−1) reveal that both benzene and pyridine are almost equally aromatic. However, Mosquera [40] based on an N-Centre delocalization index (n-DI) pointed out that azines are less aromatic than benzene. Another inconsistency appears in the cases of contiguous azines; contiguous azines are more aromatic than benzene based on Mosquera’s n-DI [40–43] but less aromatic according to resonance energies (REs) derived from isodesmic reactions. Harmonic oscillator model of aromaiticty (HOMA) [42, 43], defined as a normalized sum of squared deviation of bond lengths from optimal value, has revealed quite divergent aromaticity order for di, tri, and tetrazene; however, Harmonic oscillator model for electron delocalization (HOMED) [44] indicated almost comparable aromaticities for azines to benzene. Very recently, Schleyer and coworkers [45], based on nucleus independent chemical shift (NICS(0)πzz) calculations, have also shown that azines are of comparable aromaticities to benzene regardless of the contiguous or non-contiguous nature. NICS(0)πzz is different than isotropic NICS because the former takes only the out of plane zz-tensor component which has contribution only from the π electron of the aromatic system. Isotropic NICS, on the other hand, has contribution from both sigma and π bonds. Isotropic NICS values are generally measured at some distance above the planar aromatic ring to minimize the contribution from sigma electron. Schleyer and co-workers [46] have shown that the NICS(0)πzz is the best NICS aromaticity index for aromatic π rings. In this work, we report aromaticity for a series of mono- to tri- azines relative to benzene through energy of activation and energy of reaction for dyotropic double hydrogen transfer reactions. Fernández and coworkers have shown the energy of activation is a more accurate parameter for the determination of aromaticities and antiaromaticities. Moreover two reactions (a and b, shown in Scheme 2) have been evaluated for the aromaticity estimation; however, intramolecular (b), although computationally expensive, delivers more reliable results therefore all calculations in this manuscript are based on (b, intramolecular) with slight modification in the structure. Methyl groups are present only at the olefin part. The reaction is shown in Scheme 2.
Computational methods All calculations were performed with Gaussian 09 [47] suite of programs. Geometries of the structures were optimized
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Scheme 2 Double dyotropic hydrogen shift to generate (a) Benzene in a model reaction (b) benzene in fused polycyclic system
without any symmetry constraints at hybrid B3LYP method using 6–31+G* basis set. The B3LYP method, which consists of three parameter hybrid functional of Becke in conjunction with the correlation functional of Lee, Yang, and Parr, provides a nice balance between cost and accuracy and it is known to perform reasonably well for the prediction of geometries and activation/reaction energies [30, 32, 48–52]. The B3LYP/6–31+G* level is chosen because this level of theory is the best reported for studying the dyotropic double hydrogen transfer reactions [30]. Transition states were located through quadratic synchronous transition (QST) method implemented in Gaussian 09. For the ground states optimizations, structures were modified from the transition states and then completely optimized to ground states. Optimized geometries were obtained by gradient minimization at DFT method without any symmetry constraints and were considered complete when a stationary point was located. Each optimized structure was confirmed by frequency analysis at the same level (B3LYP/6–31+G*) as a true minimum (no imaginary frequency) or a transition state (one imaginary frequency). All reported energies are unscaled zero point corrected unless otherwise noted. Synchronicities of the reactions were calculated using the procedure already reported in the literature for the dyotropic double hydrogen transfer reaction [30, 53]. For a concerted reaction, the synchronicity (Sy) is calculated according to the procedure originally reported by Cossío and co-workers [53]. Synchronicity is defined as:
S y ¼ 1−
Σ ni¼1
jδBi −δBAV j δBAV ; 2n−2
ð1Þ
where n is the number of bond involved in the reaction (6 in this case), and δBi is the variation in the bond index (Bi) at the transition state, and can be calculated according to following equation. δBi ¼
R BTS i −Bi BPi −BRi
ð2Þ
BiTs, BiP and BiR are the Wiberg bond indices for transition state, products and reactants, respectively. The δBAV in Eq. (1) is calculated by using Eq. (3). δBAV ¼ n−1
n X
δBi
ð3Þ
i¼1
The Wiberg bond indices are calculated using natural bond orbital (NBO) method.
Results and discussion The dyotropic double hydrogen transfer reaction has been studied for a series of compounds where aromatic azine moieties are generated in the product. The Woodward-Hoffmann allowed dyotropic reaction is shown to proceed in a concerted fashion for all molecules. Based on the NICS values, transition states of these reactions are shown to be highly aromatic in nature [30]. Since aromatic azines are generated as a result of this dyotropic rearrangment, the activation barriers are quite low, and the reactions are highly exothermic. The exothermicity of the reactions and the activation barriers are strongly dependent on the aromaticity of the product. A more aromatic arene (azine in this case) moiety in the product will be formed
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through a small kinetic barrier and vice versa. Similarly, the dyotropic double hydrogen transfer will be more exergonic if the arene moiety in the product is highly aromatic. For the subsequent discussion, the numbering scheme shown in Table 1 will be used. Moreover, the label for transition state is based on the numbering of the starting material. For example, TS19 shown in Fig. 1 is a transition state from 19 (see Table 1 for details). Similarly, TS17, TS21, and TS23 (in the discussion below) are the transition states from 17, 19, and 23, respectively. Activation barriers, reaction energies, and the synchronicities for dyotropic double hydrogen transfer (type II dyotropic) reactions, generating azines moeities in the product (Table 1), are reported in Table 2. A number of isomeric azines with a maximum of three nitrogen in the ring are studied here. Synchronicities of the dyotropic double hydrogen transfer reactions were typically in the range of 0.86–0.89 except 0.82 for the formation 1,2,4 triazine moiety in the product 24 (Table 2 entry 9). One may realize from the data given in Table 2 that low synchronicity is observed generally when a nitrogen atom of azine is directly bonded to the reaction active site (Fig. 1). For example, a relatively low synchronicity value of 0.86 is observed for 19 → TS19 → 20 compared to 0.886 for the isomeric pyridazine product (18) formation (17 → TS17 → 18). A nitrogen atom in the pyridazine product 20 is directly attached to the active site whereas in 18, nitrogen Table 1 Numbering of starting materials (S.M.) and products
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atom is displaced away from the active site. The active site refers to six bonds which are being broken and formed during the reaction (Fig. 1), and also used to calculate the synchronicity. Similar to pyridazine isomers 20 and 18 (vide supra), synchronicities for the formation of isomeric pyridine products 10 and 12 are 0.87 and 0.88, respectively. Structure 10 has the nitrogen atom of pyridine closer to the reaction site whereas in 12, the nitrogen atom is displaced by one more atom from the active site. A similar trend can also be seen for molecules bearing triazine moieties (0.875 for 22 vs 0.82 for 24). The drop in synchroncity for the formation of 24 was much higher compared to the other two pairs (20 vs 18, or 10 vs 12). A plausible reason for this may be the presence of nitrogen atoms at both ends of the reaction side. The triazine moiety generated in 24 is 1,2,4-triazine, in which nitrogen atoms are present at both ends of the reaction site. Dyotropic double hydrogen transfer reaction is shown to reliably estimate the aromaticity for a number of arenes including benzene, pyridine, thiophene, pyrrole, etc. However, a serious question arises; if a heteroaromatic arene is analyzed regarding aromaticity/antiaromaticity, what should be the fusion. Fernández et al. have chosen the molecule similar to 9 as the starting material in their comparative study of aromaticity of pyridine and benzene; however, there is another isomeric structure 11 that should have also been considered. Pyridine
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Fig. 1 Presentation of the reaction active site in TS19, a transition state for the conversion of 19 into 20
moiety can be generated as a result of dyotropic double hydrogen transfer (DDHT) reaction of both 9 and 11, but two isomeric products having pyridine moiety are possible theoretically, 10 and 12. One should expect the identical aromaticity values for pyridine through the dyotropic reactions generating isomeric pyridine moieties (Table 1, entries 1 & 2) provided; (a) all other effects are absent (b) the thermodynamics of the reaction depends only on the identity of arene. However, in reality, all those methods which are indirect and rely on the fusion of the arene with some other probe, depend on the fusion; at least to some extent [54]. We have studied the dyotropic double hydrogen transfer reaction in 9 and 11 which result in two isomeric pyridine species, and indeed activation barriers differ slightly. The activation barrier for dyotropic double hydrogen transfer from 11 is 94.6 kJ mol−1 compared to 96.4 kJ mol−1 from 9. Inductive effect of nitrogen right next to the reaction center in 1-pyridine 9 is believed not only to affect the activation barriers, but also has some influence on the reaction energies and synchronicities (entries 2 and 3 Table 2). Therefore 2Table 2 Synchronicities and zero point corrected activation and reaction energies of dyotropic double hydrogen transfer reactions Eact (ZPC) ER (ZPC) Synchronicity
Entry Arene
Reaction
1 2 3 4 5 6 7 8 9
7 →8 93.51 9 → 10 96.44 11 → 12 94.6 13 → 14 97.66 15 → 16 97.82 17 → 18 101.34 19 → 20 90.42 21 → 22 102.55 23 → 24 91.04
Benzene Pyridine Pyridine Pyrimidine Pyrazine Pyridazine Pyridazine Triazine Triazine
−135.64 −128.07 −126.57 −117.61 −121.0 −100.67 −134.01 −99.41 −125.90
0.88 0.87 0.88 0.86 0.86 0.886 0.865 0.875 0.82
pyridine is believed to deliver a better estimate of aromaticity because inductive and resonance effects (or any other anisotropic effect) of nitrogen seem to disappear, or to dilute at least, when the nitrogen is separated by an additional carbon atom. Comparison of activation energy for pyridine formation (94.6 kJ mol−1 for 12) to that of benzene (93.5 kJ mol−1 for 8) reveals that both pyridine and benzene are of comparable aromaticity, although the former is slightly less aromatic. This inference is based on the relatively high energy of activation for the DDHT reaction generating pyridine moiety. As stated above, the more aromatic arene is produced through a small activation barrier. The results are consistent with the findings of Schleyer and many others (vide supra) where both pyridine and benzene are shown to be comparable in aromaticities (see also Table 4 for comparison). Similar differences in the activation barriers and reaction energies are seen for DDHT reaction generating isomeric pyridazine products (18 and 20). Pyridazine, based on the activation barrier for the dyotropic reaction may be considered more or less aromatic than benzene depending on the fusion. Activation barrier for the formation of 20 from 19 is 90.4 kJ mol−1, about 3 kJ mol−1 smaller than that for the generation of benzene moiety. One may infer from the low activation barrier that pyridazine is more aromatic than benzene. However, the activation barrier for the formation of the isomeric 18 is much higher (101.3 kJ mol−1). In this case as well, we believe that the reaction for the formation of 18 will lead to a better estimate of aromaticity, because the presence of nitrogen atom close to the reaction active site (19 into 20) probably influences the activation barriers. Comparison of activation energies for a series of reactions reveals that all azines are comparable in aromaticity to benzene, but slightly on the lower side except for 1,2,4 triazine. A relatively low activation barrier of 91.0 kJ mol−1 (for 23 to 24) is observed which would force one to think that the 1,2.4triazine is more aromatic than benzene but it is believed that some inductive effects are in operation here for the low activation barrier. Moreover, this is the reaction which has very low synchronicity therefore, no conclusion can be drawn safely for the higher aromaticity of 1,2,4 triazine than benzene. Next, the reaction energies were analyzed for the quantification of aromaticities. Fernández et al. have shown that activation energies are a better indicator for aromaticity than reaction energies. Indeed, the results obtained here through reaction energies are somewhat different, and more scattered than obtained through activation energies. For example the order of aromaticities obtained through reaction energies is benzene > pyridine >1,2,4 triazine > pyrazine > pyrimidine > pyridazine >1,2,3 triazine whereas the order of aromaticities obtained through activation barriers is 1,2,4 triazine > benzne > pyridine > pyrimidine > pyrazine > pyridazine >1,2,3 triazine. Based on the energy of reaction, aromaticity of azines
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vary from 73 % for 1,2,3 triazine to 99 % for 1-fused pyridazine relative to benzene. This variation ins aromaticity is much higher than the variation in relative aromaticities calculated through activation barriers. Moreover, the relative aromaticities of azines to benzene calculated through the activation barrier are much in line with the values reported by Schleyer [45] through NICS (0) πzz, (Table 4); however, the estimate of aromaticities through energies of reactions is much divergent. According to NICS (0) πzz calculations, all azines are 90–100 % aromatic relative to benzene. This is consistent with the observation of Fernández and co-workers that the aromaticity estimates from the dyotropic double hydrogen transfer reaction show nice correlation with ASE from energy decomposition analysis (EDA) which in turn shows nice correlation with NICS values. After evaluating that activation barriers show better estimation of aromaticities of azines, attention was directed to find the discrepancies in relative aromaticities of pyridazine (17 vs 19) and triazines (21 vs 23) when two isomeric species were used. For example, the activation energy for the dyotropic double hydrogen transfer is 101.34 kJ mol−1 from 17 compared to 90.42 kJ mol−1 from 19. It is believed that the differences in activation barriers (and energies of reaction) for two isomeric pyridazines or triazines are due to the relative stabilities of reactant, products and transition states, therefore, absolute energies of starting material, product and transition states for each pair are analyzed, and given in Table 3. In this comparison, energy of reactant 17 is described relative to the isomeric reactant 19. Similarly, the energy of TS17 is described relative to the energy of TS19, and the energy of product 18 is relative to 20. A similar comparison is adapted for the isomeric triazines reactions where reactant 21 is the reference for reactant 23. Reactant 17 is 21.8 kJ mol−1 more stable than the isomeric reactant 19 (Table 3, entries 1 and 2), and this difference in energies drops to 12.7 kJ mol−1 in the transition states (TS17 is 12.7 kJ mol−1 more stable than TS19). However, the stability order gets reversed for the products. Compound 20 (product from 19) is 9.4 kJ mol−1 more stable than the isomeric pyridazine 18 (product from 17). The instability of 19 could be attributed to the azo type Table 3 Absolute energies of reactants, products and transition states (hartree units) for two isomeric pyridazines and triazines and their effect on the energies of activation and reactions, relative energies (kJ mol−1) for each pair are given in the parenthesis
structure compared to bis-imine type structure for 17 (see the Fig. 2 for bond lengths). Structure bearing N=N (azo) bond is generally of higher energy than C=N bearing structure. This difference definitely leads to difference in activation energies and reaction energies which, in turn, affects the aromaticity values. This difference in energies gets further intensified in the case of triazines where starting material for 1,2,3-triazine formation (21) is 47.7 kJ mol−1 less stable than 23 (Table 3, entries 3 and 4) and this difference keeps on increasing on going from starting material → TS → Products. This implies that the method is not absolutely free from errors, and its reliability to estimate the aromaticities of triazines (and higher azines if possible) is questionable because it involves a considerable amount of contribution from thermodynamics factors other than those related to aromaticity. All attempts to optimize the tetrazine substituted structures led to the elimination of N2 molecules, thereby limiting the application of this method to triazines. Azines beyond tetrazines cannot be studied by this method since it would require the incorporation of nitrogen atom(s) inside the active site of the reaction. A comparison of aromaticity estimates from the dyotropic double hydrogen transfer reaction with aromaticity values from other methods [45, 55] is given in the Table 4. One can realize from the table that the aromaticity values from the DDHT reaction are very similar to NICS, extra cyclic resonance energy (ECRE) and BLW (block localized wavefunction methods. ECRE is the difference in resonance energy between a cyclic conjugated compound and that of corresponding acyclic conjugated polyene with the same number and type of double bonds. BLW resonance energy, on the other hand, is the difference in total energy between the completely delocalized planar molecule and its most stable resonance contributor. In brief, we have shown that the dyotropic double hydrogen transfer reaction can successfully be applied to quantify the aromaticities of azines through activation barriers. Since NICS(0)πzz only takes into account the contribution of the out of plane (zz) tensor components of the π MOs directly relevant to aromaticity. Comparable estimate of aromaticities from DDHT and through NICS(0)πzz reveal that both energetic and magnetic criteria may provide similar
Azine
Reactants
Products
TS
Eact
ER
1
Pyridine
−730.81681
−730.867846
−730.782377
90.42
−134.01
2
(19→ TS19 →20) Pyridine
(0) −730.825742
(0) −730.864083
(0) −730.787137
101.34
−100.67
3
(17→ TS17 →18) Triazine
(−21.8) −746.852602
(9.4) −746.890463
(−12.7) −746.813541
102.55
−99.41
4
(21→ TS21 →22) Triazine
(0) −746.870356
(0) −746.918303
(0) −746.83568
91.04
−125.90
(23→ TS23 →24)
(−47.7)
(−73.05)
(−58.1)
Entry
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Fig. 2 Optimized geometries and selected bond lengths (angstroms) for 17, 18, 19, and 20. Hydrogen atoms are omitted for clarity
estimate of aromaticity provided that a suitable system is carefully chosen. Similarly, the extra cyclic resonance energy (ECRE) values calculated through bond localized wavefunction (BLW) also give similar results. A main reason for the similar results through ECRE lies in the fact that BLW separates all electrons and basis functions into sets of localized MOs which ultimately disables intramolecular interaction among selected subgroups. Absence of any intramolecular interaction in subgroups delivers a better estimate of resonance energies. In short we have shown that with a suitable
model, magnetic (NICS(0)πzz), energetic (ECRE through BLW) give comparable estimates of aromaticities. Moreover, we have shown that the dimethyldihydropyrene nucleus can reliably be used for the quantification of aromaticity of azines provided a suitable saturated reference model and nonaromatic model are available. In brief, we have shown that the dyotropic double hydrogen transfer reaction can successfully be applied to quantify the aromaticities of azines through activation barriers.
Conclusions Table 4 Comparison of aromaticity related thermodynamic values obtained from DDHT with NICS (0) πzz, ECRE, BLW [55] and ASE Arene
Eact
ER
NICS (0) πzz
−135.65 −128.07 −126.57 −117.61 −121.0
−36.12 −35.94 −35.94 −35.15 −36.11
ECRE
122.55 124.18 124.18 123.26 135.98 108.7 Pyridazine 101.34 −100.67 −36.11 135.98 108.7 Pyrazine 90.42 −134.01 −34.75 125.23 1,2,3-Triazine 102.55 −99.41 −36.34 113.85
BLW
Benzene Pyridine Pyridine Pyrimidine Pyridazine
93.51 96.44 94.6 97.66 97.82
1,2,4-Triazine
91.04 −125.90 −35.88 132.34 254.60 119.0 −36.36 124.68 221.84
Tetrazine
ASE
256.86 145.73 256.48 90.50 256.48 90.50 254.68 81.13 267.02 92.76 223.34 267.02 92.76 223.34 249.32 97.45 224.85 – – –
Density functional theory calculations have been performed at B3LYP/6–31+G(d) level to quantify the aromaticities of monoto triazines through dyotropic double hydrogen transfer reaction (DDHT). The exothermicity of the reactions and the activation barriers are strongly dependent on the aromaticity of the product. A more aromatic arene moiety in the product will be formed through a small kinetic barrier and vice versa. Synchronicity, reaction energies and energies of activation have been analyzed and the aromaticity values obtained thereof were compared with the aromaticity values from other geometric and magnetic criteria. Energies of activation were found superior to the energies of reaction for the determination of the aromaticities. Aromaticities of most of the azines were comparable to the aromaticity of benzene, albeit on the lower sides. Activation barriers and reaction energies for the dyotropic reactions delivering contiguous (pyridazines and triazines) or
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polynitrogeneous azines (triazines) had thermodynamic contributions arising from the contiguous nature of azines in addition to the aromaticity related thermodynamic contributions. Moreover, the aromaticity values of azines are also affected by the fusion of azine to the reaction center. Presence of nitrogen atom(s) in the neighbor of the reaction center affected the estimation of aromaticity values due to inductive and resonance effects. When corrected for these factors, the aromaticities of azines using energies of activation correlated nicely with the aromaticities of azines reported in the literature. Acknowledgments K.A. acknowledges the Higher Education Commission (HEC) of Pakistan (Grant No.20-1899/R & D/10/8863-), COMSATS Institute of Information Technology and King Faisal University for financial support to the project. R.L acknowledges the support to this work by the project “Light2Hydrogen” of the BMBF and the project “Nano4Hydrogen” of the ESF and the state of Mecklenburg-Vorpommern.
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