Measurement of the Proton Affinities of a Series of Mono- and Biradicals of Pyridine

Li, Guannan; Gallardo, Vanessa A.; Li, Anyin; Milton, Jacob; Nash, John J.; Kenttämaa, Hilkka I.

doi:10.1021/jacs.0c00202

Cited by 12 publications

(23 citation statements)

References 39 publications

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“…Compared to the PA of allyl iodide (193.4 kcal mol −1 ), 16 proton transfer from the singlet states of all biradicals except two to allyl iodide would be highly endothermic (by at least 13 kcal mol −1 ) and thus would not be observable under the experimental conditions used. 2,9,17 The two exceptions are the singlet states of biradicals 4 and 6, whose conjugate bases have PAs 196.2 and 197.4 kcal mol −1 , respectively. These biradicals may be able to protonate allyl iodide from their singlet ground states via slightly endothermic reactions (note that we have previously reported 17 an uncertainty of ca.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…2,9,17 The two exceptions are the singlet states of biradicals 4 and 6, whose conjugate bases have PAs 196.2 and 197.4 kcal mol −1 , respectively. These biradicals may be able to protonate allyl iodide from their singlet ground states via slightly endothermic reactions (note that we have previously reported 17 an uncertainty of ca. ±3 kcal mol −1 for PAs of several related neutral mono-and biradicals calculated at the level of theory used here).…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…All values are corrected for thermal (298 K) and zero-point vibrational energy differences by using (U)B3LYP/cc-pVTZ calculated frequencies. Electronic states that appear to be able to transfer a proton to allyl iodide are indicated in bold. c Calculated by using an isodesmic equation involving proton transfer from the protonated biradical to pyridine to produce the pyridinium cation and the (neutral) conjugate base of the biradical; estimated uncertainty: ±3 kcal mol –1 . d Calculated at the CASPT2/CASSCF( m,n )/cc-pVTZ//(U)B3LYP/cc-pVTZ level of theory. e Based on CASPT2/CASSCF( m,n )/cc-pVTZ//(U)B3LYP/cc-pVTZ calculations for biradicals 3 – 6 , we estimate this value to be 4–8 kcal mol –1 smaller due to the use of (U)B3LYP geometries. f Reference; in kcal mol –1 . g Calculated; ref . h abs = abstraction. i 2° indicates a secondary product of the primary product listed above. j Reaction efficiency (% of collisions leading to reaction) = k exp / k coll × 100; precision ±10%; accuracy ±50%. k trn = transfer. l The product ion ( m / z 295) is formed upon substitution of a proton with an iodine atom in protonated allyl iodide ( m / z 169). m sub = substitution; the product ion ( m / z 81) is formed by substitution of the iodine atom in the protonated allyl iodide ( m / z 169) with an allene molecule. …”

Section: Resultsmentioning

confidence: 99%

“…4−8 kcal mol −1 compared to those calculated using the coupled-cluster geometries. As noted 17 previously, we estimate an uncertainty of ±3 kcal mol −1 for the proton affinity values calculated at the CASPT2/CASSCF(m,n)/cc-pVTZ level of theory using coupled-cluster geometries.…”

Section: T H Imentioning

confidence: 99%

“…10 kcal mol −1 ) cannot take place. 17 It appears that both the singlet ground and triplet excited states are generated upon formation of these biradicals via energetic collisions in the mass spectrometer. Once the triplet state of the biradical is formed, the rate of proton transfer is faster than the rate of intersystem crossing (except for biradicals 7 (containing a chlorine atom) and the 3-bromo-2,6-didehydropyridium cation due to a heavy-atom effect facilitating intersystem crossing).…”

Section: ■ Conclusionmentioning

confidence: 99%

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Protonated Ground-State Singlet meta-Pyridynes React from an Excited Triplet State

Feng

Jiang

et al. 2021

J. Org. Chem.

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The gaseous 2,6-didehydropyridinium cation and its derivatives transfer a proton to reagents for which the reaction for their singlet ground states is too endothermic to be observed. These reactions occur from the lowest-energy excited triplet states, which has not been observed (or reported) for other meta-benzyne analogues. Quantum chemical calculations indicate that the (excited) triplet states are stronger Brønsted acids than their (ground) singlet states, likely due to unfavorable three-center, four-electron interactions in the singlet-state conjugate bases. The cations have substantially smaller (calculated) singlet–triplet (S–T) splittings (ranging from ca. −11 to −17 kcal mol–1) than other related meta-benzyne analogues (e.g., −23.4 kcal mol–1 for the 3,5-isomer). This is rationalized by the destabilization of the singlet states (relative to the triplet states) by reduced (spatial) overlap of the nonbonding molecular orbitals due to the presence of the nitrogen atom between the radical sites (making the ring more rigid). Both the singlet and triplet states are believed to be generated upon formation of these biradicals via energetic collisions due to their small S–T splittings. It appears that once the triplet states are formed, the rate of proton transfer is faster than the rate of intersystem crossing unless the biradicals contain heavy atoms.

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Section: ■ Results and Discussionmentioning

confidence: 99%

Section: ■ Results and Discussionmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: T H Imentioning

confidence: 99%

Section: ■ Conclusionmentioning

confidence: 99%

See 3 more Smart Citations

Protonated Ground-State Singlet meta-Pyridynes React from an Excited Triplet State

Feng

Jiang

et al. 2021

J. Org. Chem.

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Initiation Step in the Condensed Phase Decomposition Process of Ammonium Perchlorate

Patel,

Panchal,

Chowdhury

et al. 2023

ChemistrySelect

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Ammonium perchlorate (AP) is a commonly used oxidizer in solid propellant compositions. As a result, numerous experimental and theoretical studies have been carried out in order to better understand the behavior of its decomposition in both the liquid and gas phases. In this work, the first step of Ammonium perchlorate (AP) decomposition in the condensed phase has been investigated using quantum mechanics‐based calculations. The process of proton transfer from ammonium ion (NH4+) to perchlorate ion (ClO4−) was investigated in‐depth as it is thought to be the preferred reaction for the initiation of AP decomposition in the condensed phase. The calculations were carried out using density functional theory (DFT) at the B3LYP level in conjunction with the 6‐311G++(d,p) basis set. The current study revealed that proton transfer from ammonium cation to perchlorate anion takes place in two steps. The first step involves the decomposition of ClO4− to ClO2− and O2 followed by the second step which is H+ transfer from NH4+ to ClO2− giving NH3 and HClO2. In order to better understand protonation and deprotonation, it is essential to know the proton affinity (PA) of the involved species. In this study, the CBS‐QB3 method is used to determine the PA of various energetic ionic compounds. These compounds include guanidinium azotetrazolate, triaminoguanidinium azotetrazolate, guanidinium nitrate, ammonium nitrate, and ammonium perchlorate. Based on the results of the calculations, it is concluded that direct proton transfer process does not occur in the condensed phase because the proton affinities of all cations are significantly higher than those of the corresponding anions. In conjunction with experimental studies, this modelling study can help in the development of a better understanding of AP decomposition in the condensed phase.

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Unraveling redox pathways of the disulfide bond in dimethyl disulfide: Ab initio modeling

Ould Mohamed,

Abtouche,

Ghoualem

et al. 2024

J Mol Model

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Measurement of the Proton Affinities of a Series of Mono- and Biradicals of Pyridine

Cited by 12 publications

References 39 publications

Protonated Ground-State Singlet meta-Pyridynes React from an Excited Triplet State

Protonated Ground-State Singlet meta-Pyridynes React from an Excited Triplet State

Initiation Step in the Condensed Phase Decomposition Process of Ammonium Perchlorate

Unraveling redox pathways of the disulfide bond in dimethyl disulfide: Ab initio modeling

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