Reactivity of an Aromatic sigma, sigma, sigma-Triradical: The 2, 4, 6-Tridehydropyridinium Cation (original) (raw)
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The Journal of Physical Chemistry A
Electronic structure of 2,4,6-tridehydropyridine and isoelectronic 1,3,5-tridehydrobenzene is characterized by the equation-of-motion spin-flip coupled-cluster calculations with single and double substitutions and including perturbative triple corrections. Equilibrium geometries of the three lowest electronic states, vertical and adiabatic states ordering, and triradical stabilization energies are reported for both triradicals. In 1,3,5tridehydrobenzene, the ground 2 A 1 state is 0.016 eV below the 2 B 2 state, whereas in 2,4,6-tridehydropyridine the heteroatom reverses adiabatic state ordering bringing 2 B 2 below 2 A 1 by 0.613 eV. The doublet-quartet gap is also larger in 2,4,6-tridehydropyridine as compared to 1,3,5-tridehydrobenzene; the respective adiabatic values are 1.223 and 0.277 eV. Moreover, the heteroatom reduces bonding interactions between the C 2 and C 6 radical centers, which results in the increased stabilizing interactions between C 4 and C 2 /C 6. Triradical stabilization energies corresponding to the separation of C 4 and C 2 are 19.7 and-0.2 kcal/mol, respectively, in contrast to 2.8 kcal/mol in 1,3,5-tridehydrobenzene. Similarly weak interactions between C 2 and C 6 are also observed in 2,6-didehydropyridine resulting in a nearly zero singlet-triplet energy gap, in contrast to m-benzyne and 2,4-didehydropyridine. The total interaction energy of the three radical centers is very similar in 1,3,5-tridehydrobenzene and 2,4,6-tridehydropyridine and is 19.5 and 20.1 kcal/mol, respectively.
Reactions of an aromatic σ,σ-biradical with amino acids and dipeptides in the gas phase
Journal of the American Society for Mass Spectrometry, 2010
Gas-phase reactivity of a positively charged aromatic ,-biradical (N-methyl-6,8-didehydroquinolinium) was examined toward six aliphatic amino acids and 15 dipeptides by using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR) and laser-induced acoustic desorption (LIAD). While previous studies have revealed that H-atom and NH 2 abstractions dominate the reactions of related monoradicals with aliphatic amino acids and small peptides, several additional, unprecedented reaction pathways were observed for the reactions of the biradical. For amino acids, these are 2H-atom abstraction, H 2 O abstraction, addition-CO 2 , addition-HCOOH, and formation of a stable adduct. The biradical reacts with aliphatic dipeptides similarly as with aliphatic amino acids, but undergoes also one additional reaction pathway, addition/C-terminal amino acid elimination (addition-CO-NHCHR C). These reactions are initiated by H-atom abstraction by the biradical from the amino acid or peptide, or nucleophilic addition of an NH 2 or a HO group of the amino acid or peptide at the radical site at C-6 in the biradical. Reactions of the unquenched C-8 radical site then yield the products not observed for related monoradicals. The biradical reacts with aromatic dipeptides with an aromatic ring in N-terminus (i.e., Tyr-Leu, Phe-Val, and Phe-Pro) similarly as with aliphatic dipeptides. However, for those aromatic dipeptides that contain an aromatic ring in the C-terminus (i.e., Leu-Tyr and Ala-Phe), one additional pathway, addition/N-terminal amino acid elimination (addition-CO-NHCHR N), was observed. This reaction is likely initiated by radical addition of the biradical at the aromatic ring in the C-terminus. Related monoradicals add to aromatic amino acids and small peptides, which is followed by C␣-C bond cleavage, resulting in side-chain abstraction by the radical. For biradicals, with one unquenched radical site after the initial addition, the reaction ultimately results in the loss of the N-terminal amino acid. Similar to monoradicals, the C-S bond in amino acids and dipeptides was found to be especially susceptible to biradical attack.
Energetics and chemical bonding of the 1,3,5-tridehydrobenzene triradical and its protonated form
Chemical Physics, 2005
Quantum chemical calculations were applied to investigate the electronic structure of the parent 1,3,5-tridehydrobenzene triradical (C6H 3, TDB) and its anion (C6H3-), cation (C6H3+) and protonated form (C6H4+). Our results obtained using the state-averaged complete active space self-consistent-field (CASSCF) followed by second-order multi-state multi-configuration perturbation theory, MS-CASPT2, and MRMP2 in conjunction with the large ANO-L and 6-311++G(3df,2p) basis set, confirm and reveal the followings: (i) TDB has a doublet 2A1 ground state with a 4B2-2A1 energy gap of 29 kcal/mol, (ii) the ground state of the C6H3-anion in the triplet 3B2 being 4 kcal/mol below the 1A 1 state. (iii) the electron affinity (EA), ionization energy (IE) and proton affinity (PA) are computed to be: EA = 1.6 eV, IE = 7.2 eV, PA = 227 kcal/mol using UB3LYP/6-311++G(3df,2p) + ZPE; standard heat of formation ΔHf(298 K, 1 atm)(TDB) = 179 ± 2 kcal/mol was calculated with CBS-QB3 method. An atoms-in-molecules (AIM) analysis of the structure reveals that the topology of the electron density is similar in all compounds: hydrogens connect to a six-membered ring, except for the case of the 2A2 state of C6H4+ (MBZ+) which is bicyclic with fused five-and three-membered rings. Properties of the chemical bonds were characterized with Electron Localization Function (ELF) analysis, as well as Wiberg indices, Laplacian and spin density maps. We found that the radicals form separate monosynaptic basins on the ELF space, however its pair character remains high. In the 2A1 state of TDB, the radical center is mainly localized on the C1 atom, while in the 2B2 state it is equally distributed between the C3 and C5 atoms and, due to the symmetry, in the 4B2 state the C1, C2 and C3 atoms have the same radical character. There is no C3-C5 bond in the 2A1 state of TDB, but the interaction between these atoms is strong. The ground state of cation C6H3+ (DHP), 1A1, is not a diradical and has a doubly aromatic character. Aromaticity of the different compounds was studied within the ELF framework and the standard deviation of the bond lengths and bond orders. The Jahn-Teller distorted 2A1 and 2B2(C2v) states of TDB were found to exhibit an aromaticity comparable to that of benzene. Overall protonation of the TDB reinforces the stability of the low-spin doublet states, the classical Hund's rule is not obeyed. In a view, these species could better be regarded as radicals than triradicals.
Quinoline Triradicals: A Reactivity Study
Journal of the American Chemical Society, 2019
The gas-phase reactivities of several protonated quinoline-based σ-type (carbon-centered) mono-, bi-and triradicals towards dimethyl disulfide (DMDS) were studied by using a linear quadrupole ion trap mass spectrometer. The mono-and biradicals produce abundant thiomethyl abstraction products and small amounts of DMDS radical cation, as expected. Surprisingly, all triradicals produce very abundant DMDS radical cations. A single-step mechanism involving electron transfer from DMDS to the triradicals is highly unlikely because the (experimental) adiabatic ionization energy of DMDS is almost 3 eV greater than the (calculated) adiabatic electron affinities of the triradicals. The unexpected reactivity can be explained based on an unprecedented two-step mechanism wherein the protonated triradical first transfers a proton to DMDS, which is then followed by hydrogen atom abstraction from the protonated sulfur atom in DMDS by the radical site in the benzene ring of the deprotonated triradical to generate the conventional DMDS radical cation and a neutral biradical. Quantum chemical calculations as well as examination of deuterated and methylated triradicals provide support for this mechanism. The proton affinities of the neutral triradicals (and DMDS) influence the first step of the reaction while the vertical electron affinities and spin-spin coupling of the neutral triradicals influence the second step. The calculated total reaction exothermicities for the triradicals studied range from 27.6 up to 29.9 kcal mol-1 .
Gas-Phase Reactivity of Protonated 2-, 3-, and 4-Dehydropyridine Radicals Toward Organic Reagents
Journal of Physical Chemistry A, 2009
To explore the effects of the electronic nature of charged phenyl radicals on their reactivity, reactions of the three distonic isomers of n-dehydropyridinium cation (n = 2, 3, or 4) have been investigated in the gas phase by using Fourier-transform ion cyclotron resonance mass spectrometry. All three isomers react with cyclohexane, methanol, ethanol, and 1-pentanol exclusively via hydrogen atom abstraction and with allyl iodide mainly via iodine atom abstraction, with a reaction efficiency ordering of 2 > 3 > 4. The observed reactivity ordering correlates well with the calculated vertical electron affinities of the charged radicals (i.e., the higher the vertical electron affinity, the faster the reaction). Charged radicals 2 and 3 also react with tetrahydrofuran exclusively via hydrogen atom abstraction, but the reaction of 4 with tetrahydrofuran yields products arising from nonradical reactivity. The unusual reactivity of 4 is likely to result from the contribution of an ionized carbene-type resonance structure that facilitates nucleophilic addition to the most electrophilic carbon atom (C-4) in this charged radical. The influence of such a resonance structure on the reactivity of 2 is not obvious, and this may be due to stabilizing hydrogen-bonding interactions in the transition states for this molecule. Charged radicals 2 and 3 abstract a hydrogen atom from the substituent in both phenol and toluene, but 4 abstracts a hydrogen atom from the phenyl ring, a reaction that is unprecedented for phenyl radicals. Charged radical 4 reacts with tert-butyl isocyanide mainly by hydrogen cyanide (HCN) abstraction, whereas CN abstraction is the principal reaction for 2 and 3. The different reactivity observed for 4 (as compared to 2 and 3) is likely to result from different charge and spin distributions of the reaction intermediates for these charged radicals.