Proton transfer to the fluorine atom in fluorobenzene: a dramatic temperature dependence in the gas phase

Abstract

Collisionally induced decomposition mass spectrometry has shown that during the gas phase reaction of CH5+ with fluorobenzene at temperatures < 300 K, a proton transfers preferentially to the F atom even though its proton affinity is ca. 180 kJ mol-1 less than the ring.Protonated species postulated as transients in solution can often be prepared and studied by mass spectrometry (m.s.) in the gas phase.1 Usually, however, structures can be assigned only by indirect means.2 The best method for determining the thermodynamically most favourable protonation site is to match molecular orbital (M.O.) calculations on postulated structures against relative proton affinity (P. A.) measurements (provided the molecule is not too large).' P.A. correlation methods have also been used.4 None of these methods takes into account kinetic effects2 during proton transfer.Cooks et al.2 have investigated the use of collisionally induced decomposition (c.i.d.) m.s. to determine protonation sites more directly. Of special interest for substituted aromatic compounds is the competition for the proton between the ring and a substituent X. It is thought that a peak in the c.i.d. spectrum corresponding to the loss of HX from the protonated molecule ought to be a measure of the degree of protonation on the substrate itself. Unfortunately, conclusions based on this premise are usually ambiguous2 because the possibility always remains that, under the influence of collisional activation, proton migraton occurs prior to decomposition. Even deuterium labelling c.i.d. experiments can be ambivalent because of possible isotope exchange, in parallel with proton transfer, in the ion source.We report here interesting observations from a c.i.d. study of proton transfer reactions involving fluorobenzene. The

Kinetic and DFT Studies on the Ag/TiO2‐Photocatalyzed Selective Reduction of Nitrobenzene to Aniline

“…9, 11, 12 The ratio of both isomers strongly depends on the experimental conditions (e.g., pressure, temperature, protonating agent). For example, 5 is produced in significant abundance using CH 5 + for protonation, because of the similar PAs of CH 4 (544 kJ mol −1 )12 and C 6 H 5 F for F‐protonation (577±24 kJ mol −1 ) 11a. In contrast, the production of 5 is suppressed when Brønsted acids with much smaller or larger PAs are used for protonation, leading to the selective preparation of C ‐C 6 H 6 F + isomers only 9.…”

Getrennter spektroskopischer Nachweis von Carbenium‐ und Fluoronium‐Isomeren von protoniertem Fluorbenzol

“…B. Druck, Temperatur, Protonierungsreagens). Unter Einwirkung von CH 5 + beispielsweise wird 5 in relativ hohen Konzentrationen erhalten, da die Protonenaffinitäten von CH 4 (544 kJ mol −1 )12 und C 6 H 5 F für F‐Protonierung (577±24 kJ mol −1 ) ähnlich sind 11a. Brønsted‐Säuren mit weitaus höherer oder niedrigerer Protonenaffinität unterdrücken dagegen die Bildung von 5 und ermöglichen die selektive Erzeugung von C ‐C 6 H 6 F + ‐Isomeren 9.…”

The Protonation Site of para-Dimethylaminobenzoic Acid Using Atmospheric Pressure Ionization Methods

“…However, the chemical environment such as solvent effect can change the preferred site of protonation [6,7]. It is also interesting that a molecule can be protonated at energetically less favored position when different ionization methods are used, reflecting a kinetic influence over the protonation site [8][9][10][11][12][13][14]. In the case of aniline, it has no objection that the amino group is the protonation site in aqueous solution because of its good stability from solvation.…”