Energetics, electronic structures and geometries of didehydroazines

Ciosłowski, Jerzy; Szarecka, Agnieszka; Moncrieff, David

doi:10.1080/0026897021000034512

Cited by 18 publications

(63 citation statements)

References 51 publications

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“… Rate constants from the ST/LS experiments for 1,3,5‐C 3 H 3 N 3 dissociation showing a comparison with the results of RRKM calculations with Dyakov's barrier E 0 = 81 kcal/mol 30 and a lower 〈Δ E 〉 down = 126(T/298) 0.9 cm −1 . ST/LS experiments: 5% 1,3,5‐C 3 H 3 N 3 /Kr: (▪) 115–174 Torr, (◯) 307–355 Torr, (▴) 405–586 Torr.…”

Section: Dissociationmentioning

confidence: 95%

“…10 indicates a high 〈Δ E 〉 down of 1200 cm −1 . However, both Dyakov et al 30 and Pai et al 13 have proposed a lower barrier of 81 kcal/mol: Using this, a new RRKM calculation was attempted altering only the barrier and 〈Δ E 〉 down , and this is compared to experiment in Fig. 11, where the fit is seen to be slightly improved.…”

Section: Dissociationmentioning

confidence: 99%

“…Considering the increase in entropy associated with multiple products, it seems possible that there may have been some contribution from molecular reaction in the earlier dissociation experiments. The bond energy issue may also be affected by some uncertainties recently uncovered in the heats of formation for pyridazine and pyrazine [30].…”

Section: Introductionmentioning

confidence: 98%

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Shock tube study of 1,3,5‐triazine dissociation and relaxation and relaxation of pyrazine

Kiefer

2010

Int J of Chemical Kinetics

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The three-body dissociation of 1,3,5-triazine (s-triazine, s-C 3 H 3 N 3 → 3HCN) has been observed in incident shock waves with the laser-schlieren technique. The experiments use 5% triazine/Kr and cover 1630-2350 K for 100-600 Torr. These experiments show dissociation rates with strong falloff and a slight but fully expected pressure dependence. The dissociation is without secondary reaction save for a possible, but rather unlikely, contribution from the isomerization HCN → HNC. Electronic structure calculations of the transitionstate properties (G3B3, HL1, E o = 84.6 kcal/mol) are used to construct a Rice-RamspergerKassel-Marcus (RRKM) model whose fit to the rate measurements suggests a E down of 1200 cm −1 . However, a seemingly better fit is achieved using the barrier of 81 kcal/mol proposed by Dyakov et al. (J. Phys. Chem. A 2007, 111, 9591-9599). With this barrier k ∞ (s −1 ) = 5.3 × 10 16 exp(−86.6(kcal/mol)/RT), and the fit now accepts the more routine E down = 126(T /298) 0.9 . It seems the dissociation most likely occurs by a direct, one-step, "triple" dissociation to 3HCN, although the present experiments cannot rule out a multistep process. Vibrational relaxation of the triazine was also examined in 5% and 20% mixtures with Kr over 770-1500 K for pressures between 6 and 14 Torr. Relaxation is very fast, with a slight inverse temperature dependence, P τ rising from 100 to 200 ns-atm over the full temperature range. Integrated gradients are in good accord with calculated total changes in density, indicating a single exponential relaxation. A separate investigation of relaxation in the related molecule pyrazine (500-1300 K, in 1% and 5% in Kr, between 13 and 66 Torr) is included. Again relaxation is rapid, but here the temperature dependence seems more normal, the relaxation times decreasing slightly with temperature. C

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Section: Dissociationmentioning

confidence: 95%

Section: Dissociationmentioning

confidence: 99%

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Shock tube study of 1,3,5‐triazine dissociation and relaxation and relaxation of pyrazine

Kiefer

2010

Int J of Chemical Kinetics

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“…163,164 The values of the other two isomers were the first to be recorded: ΔH f,298 (42) = 116 ± 3 kcal mol −1 and ΔH f,298 (43) = 128 ± 3 kcal mol −1 . 169 CASSCF(8,8) provides good agreement for 42 but underestimates the energy of 43. Over the following years, a few additional computational assessments of the relative energies of the benzynes have appeared.…”

Section: Relative Energies Of the Benzynesmentioning

confidence: 98%

“…Nevertheless, CISD provides reasonable results, and the CCCI results are in excellent agreement with experiment. 169 The energies listed in Table 5.10 do not tell the whole story. 159,169 -171 DFT is also a single-configuration method and again raises doubt about its applicability to the benzynes.…”

Section: Relative Energies Of the Benzynesmentioning

confidence: 99%

Diradicals and Carbenes

Bachrach

2014

Computational Organic Chemistry

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Reactive intermediates play a critical role in the chemist's understanding of many reaction mechanisms. 1 Given their often fleeting appearance, experimental observation, let alone acquiring any property data on them, can be a great challenge. It is not, therefore, surprising that computational chemistry has been a strong partner with experiment in gaining knowledge of a broad spectrum of reactive intermediates.In this chapter, we focus on the class of reactive intermediates that bear at least two unpaired electrons: diradicals and carbenes. The exact definition of a diradical is somewhat in the eye of the beholder. Salem and Rowland 2 provided perhaps the most general, yet effective, definition-a diradical is a molecule that has two degenerate or nearly degenerate orbitals occupied by two electrons. With this definition, carbenes can be considered as a subcategory of diradicals. In a carbene, the two degenerate molecular orbitals are localized about a single carbon atom.The chemistry of radicals has been described many times. 3 -6 Likewise, the chemistry of diradicals and carbenes has been the subject of many reviews. 7 -16 Two issues will guide our presentation in this chapter. First and foremost, we discuss examples of diradicals and carbenes, where computational chemistry has greatly aided in understanding the properties, structure, and chemistry of these intermediates. A theme that will emerge here is the strong collaborative relationship between experimental and computational chemists that greatly aided in resolving the controversies and discrepancies that arose in trying to understand these unusual species. The second aspect of this chapter is that computational studies of diradicals and (especially) carbenes helped to establish the discipline of computational chemistry. In fact, the first example we discuss in this chapter is the nature of methylene, which is the topic that many claim to have single handedly won over skeptics to the advantages and power of computational chemistry.Following the section on methylene, we present the chemistry of phenylcarbene and phenylnitrene and describe how computational chemistry helped detail why these two closely related molecules behave so differently. A discussion of tetramethyleneethane (TME) and oxyallyl diradicals explores how theories of apparently simple molecules may be quite complicated. Next, we discuss the chemistry of Computational Organic Chemistry, Second Edition. Steven M. Bachrach

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Thermal unimolecular reactivity pathways in dehydro‐diazines radicals

Saraswat

Venkataramani

2020

J of Physical Organic Chem

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In this work, we investigated the detailed reactivity of dehydro‐diazine radicals through unimolecular dissociation pathways. Aromatic six‐membered rings with two nitrogens as heteroatoms are classified as diazines, which include pyrimidine 1, pyridazine 2, and pyrazine 3. The C–H bond scission in pyrimidine, pyridazine, and pyrazine leads to three (1a–1c), two (2a and 2b), and one (3a) radical isomers, respectively. Through quantum chemical calculations, reactivity aspects of all the six radical isomers have been investigated by considering different modes of unimolecular reactions. On the basis of the computations at (U)B3LYP/cc‐pVTZ, (U)M06‐2X/cc‐pVTZ, and (U)CCSD(T)/cc‐pVTZ//(U)B3LYP/cc‐pVTZ levels of theory and analysis, we found out that ring‐opening pathways were kinetically more favorable compared with isomerization (1,2‐H shift) pathways or C–H bond dissociation leading to biradicals. In all the cases, ring‐opening via C–N bond cleavage has a lower energy barrier compared with C–C bond cleavage owing to the stabilizing interaction between radical electron and the nitrogen lone pair. Moreover, the unimolecular decomposition channels can lead to several fragmentation products, which are interesting in the astrochemical context. On the basis of these investigations and also the impact of three centered–five electrons (3c–5e) interactions on the reactivity aspects, we determined the kinetic stability order among the six isomeric dehydrodiazine radicals.

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Energetics, electronic structures and geometries of didehydroazines

Cited by 18 publications

References 51 publications

Shock tube study of 1,3,5‐triazine dissociation and relaxation and relaxation of pyrazine

Shock tube study of 1,3,5‐triazine dissociation and relaxation and relaxation of pyrazine

Diradicals and Carbenes

Thermal unimolecular reactivity pathways in dehydro‐diazines radicals

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