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

Abstract

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.

A Portable Reagent Inlet System Designed to Diminish the Impact of Air and Water to Ion–Molecule Reactions Studied in a Linear Quadrupole Ion Trap

“…In addition to the reactions of the 4-hydroxyphenylcarbyne anion with acetonitrile, reactions of several other ions with various reagents were explored (Table , see Figures S2–S8). These systems were selected because the mass spectra obtained previously by using the traditional manifold revealed abundant, unexpected, and undesirable H 2 O adducts. , As observed for the 4-hydroxyphenylcarbyne anion/acetonitrile system discussed above, the relative abundance of H 2 O adducts of the reacting ions were found to be substantially lower for all the reactions studied when using the portable system than when using the traditional manifold. Based on these results, the portable system introduced 10–60% of the H 2 O into the ion trap that was introduced using the traditional manifold.…”

“…They were purchased from Alfa Aesar and used as received. 2,6-Diiodopyridine (purity ≥95%) was the precursor for the 2,6-didehydropyridinium cation, which was generated in the mass spectrometer . The reagent was purchased from Sigma-Aldrich.…”

“…1,3-Diiodoisoquinoline was the precursor for 1,3-didehydroisoquinolinium cation, which was generated in the mass spectrometer. This reagent was synthesized using procedures described in the literature . 2-Iodo-5-nitroquinoline, 2-iodo-6-nitroquinoline, 4,6-diiodoquinoline, and 5-nitro-8-iodoquinoline were used to generate 2,5-didehydroquinolinium cation, 2,6-didehydroquinolinium cation, 4,6-didehydroquinolinium cation, and 5,8-didehydroquinolinium cation, respectively, in the mass spectrometer .…”

“…2,6-Diiodopyridine (purity ≥95%) was the precursor for the 2,6-didehydropyridinium cation, which was generated in the mass spectrometer. 23 The reagent was purchased from Sigma-Aldrich. 1,3-Diiodoisoquinoline was the precursor for 1,3-didehydroisoquinolinium cation, which was generated in the mass spectrometer.…”

“…This reagent was synthesized using procedures described in the literature. 23 2-Iodo-5-nitroquinoline, 2-iodo-6-nitroquinoline, 4,6-diiodoquinoline, and 5-nitro-8-iodoquinoline were used to generate 2,5-didehydroquinolinium cation, 2,6-didehydroquinolinium cation, 4,6-didehydroquinolinium cation, and 5,8-didehydroquinolinium cation, respectively, in the mass spectrometer. 24 These precursors were synthesized as described previously.…”