Does the 2,6-didehydropyridinium cation exist?

FT-ICR mass spectrometry: Superconducting magnet, external ion source, ion-molecule reactions, and ion-ion traps

The world of Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry has witnessed, especially in the last 30 years significant advances in many fields of science, such as electronics, magnets, new ICR cell designs, developed ICR event sequences, modern external ionization sources, and linear ion beam guides, as well as modern vacuum technology. In this review, a brief account is given focusing especially on the studies performed in Wanczek's group and ICR research laboratory at the University of Bremen. An FT-ICR mass spectrometer has been developed with a high magnetic field superconducting magnet, operating at 4.7 T. At this magnetic field, a trapping time of 13.5 h was obtained with 30% efficiency. For the tetrachloromethane molecular ion, m/z 166, a mass-resolving power m/Δm = 1.5 × 10⁶ was measured at a pressure of 2 × 10^-8  Torr. The transition from magnet sweep to frequency sweep and the application of Fourier-transform has greatly enhanced the ICR technology. External ion sources were invented and differential pumping schemes were developed for enabling ultrahigh vacuum condition for ICR detection, while guiding ions at relatively higher pressures, during their flight to the ICR cell. With the external ion source, a time-of-flight ICR tandem instrument is built. A method to measure the ion flight time and to trap the ions in the ICR cell is described. Many ICR cell characteristics such as z-axis ion ejection and coupling of radial and axial ion motions in a superposed homogeneous magnetic and inhomogeneous trapping electric field were extensively studied. Gas-phase ion-molecule reactions of several reactive inorganic compounds with a focus on phosphorous and sulfur as well as silicon chemistry were also studied in great detail. The gas-phase ion chemistry of several trifluoromethyl-reagents such as trifluoromethyltrimethylsilane and tris(trifluoromethyl)phosphine were also investigated in ICR. Dual polarities multisegmented ICR cells were invented and deeply characterized. Sophisticated ICR pulse event programs were developed to enable long-range ion-ion interactions between simultaneously trapped positive and negative ions.

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

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.

Effects of the Distance between Radical Sites on the Reactivities of Aromatic Biradicals

Coupling of the radical sites in isomeric benzynes is known to hinder their radical reactivity. In order to determine how far apart the radical sites must be for them not to interact, the gas-phase reactivity of several isomeric protonated (iso)quinoline- and acridine-based biradicals was examined. All the (iso)quinolinium-based biradicals were found to react slower than the related monoradicals with similar vertical electron affinities (i.e., similar polar effects). In sharp contrast, the acridinium-based biradicals, most with the radical sites farther apart than in the (iso)quinolinium-based systems, showed greater reactivities than the relevant monoradicals with similar vertical electron affinities. The greater distances between the two radical sites in these biradicals lead to very little or no spin-spin coupling, and no suppression of radical reactivity was observed. Therefore, the radical sites can still interact if they are located on adjacent benzene rings and only after being separated further than that does no coupling occur. The most reactive radical site of each biradical was experimentally determined to be the one predicted to be more reactive based on the monoradical reactivity data. Therefore, the calculated vertical electron affinities of relevant monoradicals can be used to predict which radical site is most reactive in the biradicals.

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

The proton affinity (PA) of a neutral molecule is defined as the negative of the enthalpy change for the gas-phase reaction between a proton and the neutral molecule to produce the (charged) conjugate acid of the molecule. PA is a fundamental property that is related to the structure of a molecule and affects its reactivity. Very few PA values are available for basic organic monoradicals and none for biradicals. Here, the PA values for several σ-type carbon-centered pyridine-based monoradicals and biradicals have been experimentally determined by monitoring proton transfer from the protonated mono- and biradicals to reference bases with known proton affinities as a function of time in Fourier-transform ion cyclotron resonance (FT-ICR) and linear quadrupole ion trap (LQIT) mass spectrometers. A procedure was developed for both instruments that permits differentiation between exo- and endothermic proton transfer reactions. The PA values of all the (bi)radicals studied were found to be lower than that of pyridine. This is rationalized based on the electron-withdrawing nature of the radical site(s). Thus, the PA values decrease in the order: pyridine > monoradicals > biradicals. The PA values of the monoradicals were also found to increase (making the protonated radicals less acidic) as the distance between the basic nitrogen atom and the radical site increases. Similar behavior was found for the biradicals, with one exception: 3,5-didehydropyridine has a larger PA (215.3 ± 3.3 kcal mol^-1) than 3,4-didehydropyridine (PA = 213.4 ± 3.3 kcal mol^-1) even though the latter biradical has one radical site farther away from the basic nitrogen atom. Quantum chemical calculations of the PAs of the (bi)radicals are in reasonably good agreement with the experimentally determined values. At the DFT (B3LYP), CCSD(T), and CASPT2 levels of theory, the mean unsigned errors are 2.3, 1.7, and 2.1 kcal mol^-1.

Substituent Effects on the Reactivity of the 2,4,6-Tridehydropyridinium Cation, an Aromatic σ,σ,σ-Triradical

2,4,6-Tridehydropyridinium cation (7) undergoes three consecutive atom or atom group abstractions from reagent molecules in the gas phase. By placing a π-electron-donating hydroxyl group between two radical sites, their reactivity can be quenched by enhancing their through-space coupling via a favorable resonance structure. Indeed, 3-hydroxy-2,4,6-tridehydropyridinium cation (8) abstracts only one atom or group of atoms from reagents. On the other hand, an electron-withdrawing cyano group between two of the radical sites (9) destabilizes the analogous resonance structure and diminishes through-space coupling between the radical sites, resulting in abstraction of three atoms, just like 7. However, the cyano-substituent also increases acidity to the point that 9 reacts pre-dominantly via proton transfer instead of undergoing radical reactions. Therefore, acidic triradicals may undergo nonradical, barrierless proton transfer reactions faster than radical reactions, which are usually accompanied by barriers. Examination of the analogous cyano-substituted mono-and biradicals revealed behavior similar to that of the corresponding unsubstituted species, with the exception of substantially greater reactivities due to their greater (calculated) vertical electron affinities. Finally, the 3-cyano-2,6-didehydropyridinium cation with a singlet ground state (S-T splitting: -11.9 kcal mol-1) was found to react exclusively from the lowest-energy triplet state by fast proton transfer reactions.

Polar Effects Control the Gas-Phase Reactivity of para-Benzyne Analogs

We report herein a gas-phase reactivity study on a para-benzyne cation and its three cyano-substituted, isomeric derivatives performed using a dual-linear quadrupole ion trap mass spectrometer. All four biradicals were found to undergo primary and secondary radical reactions analogous to those observed for the related monoradicals, indicating the presence of two reactive radical sites. The reactivity of all biradicals is substantially lower than that of the related monoradicals, as expected based on the singlet ground states of the biradicals. The cyano-substituted biradicals show substantially greater reactivity than the analogous unsubstituted biradical. The greater reactivity is rationalized by the substantially greater (calculated) electron affinity of the radical sites of the cyano-substituted biradicals, which results in stabilization of their transition states through polar effects. This finding is in contrast to the long-standing thinking that the magnitude of the singlet-triplet splitting controls the reactivity of para-benzynes.

Polar Effects Control the Gas-phase Reactivity of Charged para-Benzyne Analogs

The gas-phase reactivity of charged para-benzynes is entirely unexplored as they and/or their precursors tend to undergo ring-opening upon their generation. We report here a gas-phase reactivity study of two such benzynes, the 2,5-didehydropyridinium and 5,8-didehydroisoquinolinium cations, generated in a modified dual-linear quadrupole ion trap (DLQIT) mass spectrometer. Both biradicals were found to form diagnostic products with organic molecules, indicating the presence of two radical sites. As opposed to earlier predictions that the singlet-triplet (S-T) splitting controls the radical reactivity of such species, the 2,5-didehydropyridinium cation reacts much faster in spite of its larger S-T splitting. Calculated vertical electron affinities of the radical sites of the para-benzynes, a parameter related to the polarity of the transition states of their reactions, appears to be the most important reactivity controlling factor.