Hydrogen bond induced HF elimination from photoionized fluorophenol dimers in the gas phase

Abstract

In this paper, we report finding of a remarkable chemical effect of hydrogen bonding, elimination of hydrogen fluoride (HF) from the hydrogen bonded dimers of 2-fluorophenol (2-FP) and 3-fluorophenol (3-FP), in a supersonic jet expansion upon multi-photon ionization using 4th harmonic wavelength (266 nm) of a Q-switched Nd:YAG laser, and the reaction has been probed by time-of-flight mass spectrometry. No HF elimination is observed to occur by such means from the monomer of 3-FP, but it occurs with a small yield from the monomer of 2-FP. On the other hand, upon dimerization the reaction is triggered on for 3-FP, and for 2-FP it becomes so facile that no intact dimer cation survives and only the HF eliminated product ion appears in the mass spectra. Electronic structure calculation shows that in the cationic ground (D) state, although the reaction for 2-FP dimer is exothermic, the associated barrier is significantly high (2.75 eV) and for its occurrence, absorption of three photons (2+1 type) is required. However, the reaction is predicted barrierless in the intermediate S state of this dimer, and HF loss dimer cation mass peak could appear in the mass spectrum due to an effective two-photon (1+1) ionization process. In the case of 3-FP dimer, the energy barriers both in S (neutral) and D (ionic) states are high, and it is suggested that for occurrence of HF elimination, dimer cation needs to absorb an additional photon. For facilitation of HF loss from this dimer cation, a rearrangement of the geometry and formation of an intermediate adduct have been suggested, and it is argued that the latter could be produced by nucleophilic attack of the neutral moiety at the ortho site of the cationic counterpart.

Tracking Fluorine during Aqueous Photolysis and Advanced UV Treatment of Fluorinated Phenols and Pharmaceuticals Using a Combined 19F-NMR, Chromatography, and Mass Spectrometry Approach

“…It is known that the phenolate can undergo ring condensation as a photodegradation mechanism. Under basic conditions, the Ar–F motif can remain on the ring structure . Two degradation mechanisms involving ring-condensation form either fluoride or an organofluorine compound, both requiring the formation of phenolate, either by the fluorine removing the hydroxyl hydrogen, or deprotonation above the p K a value respectively (see Schemes S1 and S2).…”

“…52 Two degradation mechanisms involving ring-condensation form either fluoride or an organofluorine compound, both requiring the formation of phenolate, either by the fluorine removing the hydroxyl hydrogen, or deprotonation above the pK a value respectively (see Schemes S1 and S2). 52,53 The 19 F-NMR spectra of the fluorophenol model compounds only show fluoride formation at pH values even below the pK a , suggesting that, if ring-condensation occurs, it is through the mechanism of hydroxyl hydrogen abstraction by fluoride thereby forming hydrofluoric acid. 52 The difluorophenols (3a, 3b) were photolyzed to examine the effects of more diversely substituted fluorophenols.…”

“…52,53 The 19 F-NMR spectra of the fluorophenol model compounds only show fluoride formation at pH values even below the pK a , suggesting that, if ring-condensation occurs, it is through the mechanism of hydroxyl hydrogen abstraction by fluoride thereby forming hydrofluoric acid. 52 The difluorophenols (3a, 3b) were photolyzed to examine the effects of more diversely substituted fluorophenols. Photolysis rate constants were again highly pH dependent, increasing ∼10fold from pH 5 to 7 and from pH 7 to 10.…”

Barrierless Proton Transfer in the Weak C–H···O Hydrogen Bonded Methacrolein Dimer upon Nonresonant Multiphoton Ionization in the Gas Phase

“…The dimer and higher complexes of MC were generated by means of supersonic jet expansion method. Following laser ionization, the mass spectra of the fragment ions were recorded using a home-built time-of-flight (TOF) mass spectrometer, and details of the apparatus have been described earlier. , …”

Ultrafast Protonation of an Amide: Photoionization-Induced Proton Transfer in Phenol-Dimethylformamide Complex Cation

“…Our goal is to elucidate how the system evolves in the ionic collision complex from an initial nonproton-transferred (non-PT) to a final proton-transferred (PT) configuration, before the complex exits for the product channel. In spite of recent research interests on photoionization-induced PT in molecular complexes, − ultrafast time-resolved measurements of such reactions are still scarce. , …”