A fourier transform-ion cyclotron resonance study of steric effects on proton transfer reactions of polyalkyl benzenes

Abstract

The rates of deprotonation, MH+ + B + M + BH+ (M = 1, 2), and the rates of protonation, M + BH+ -t MH+ + B (M = 2,3,4), have been measured by Fourier transform-ion cyclotron resonance (FT-ICR) spectrometry for the increasingly sterically crowded alkyl benzenes ethylbenzene, 1; 1,3,5-trimethylbenzene (mesitylene), 2; 3,5-di-(t-butyl)toluene, 3; 1,3,5-tri(t-butyl) benzene, 4; and a series of n-bases B. In addition, the gas phase basicities of 3 and 4 were determined by proton transfer equilibrium measurements using FT-ICR.The efficiencies of deprotonation and protonation are always large (> 50%) for exoergic reactions and increase with increasing exoergicity, but never reach unit efficiency. For each of the alkylated benzenes l-4 the efficiency decreases steeply at the border between exoergic and endoergic reactions. This is the "normal" behaviour of proton transfer reactions and shows the absence of steric effects for proton transfer to and from alkylated benzenes even in the case of 3 and 4 substituted by bulky r-butyl groups, and is in complete contrast to the behaviour of t-butylated pyridines. This difference is explained by different geometries of the transition state for the proton transfer reactions in the case of the alkylated pyridines and alkylated benzenes, and by the critical role of x-complexes in proton transfer reactions of benzene derivatives.

Evaluation of proton-binding capabilities of polyether and pyridyl ligands

“…The guideline to di †erentiate an endoergic vs. exoergic reaction is typically deÐned at a proton transfer reaction efficiency of 10%. 23 The kinetic method involves the formation and dissociation of proton-bound dimers containing two bases of interest. The loosely bound dimer typically dissociates by cleavage across the proton bridge, resulting in the production of protonated bases.…”

Charge permutation reactions in tandem mass spectrometry

“…191 In both the equilibrium and bracketing methods, a proton is transferred from a monocation, MH C , to a neutral base species, B, with known gas-phase basicity, according to the Reaction (2.2). The free-energy change for this reaction, G rxn , is the difference in gas-phase basicities of M and B, 196,197 Other tools, such as triple-quadrupole instruments, 198 quadrupole ion traps 199,200 and guided ion beam mass spectrometers, 201 have also been employed. 'Flow reactor' or 'flow tube' techniques, including flowing afterglow (FA) and selected ion flow drift tube (SIFDT), have been used for the determination of thermodynamic values associated with proton transfer.…”

A chemical ionization study of deuteron transfer initiated propene loss from propoxypyridines

“…The site of protonation depends not only on the proton affinity of the distinct sites in a molecule, but also on the existence of possible energy barriers for the proton transfer reaction [7][8][9][10]. In particular, proton transfer to the aromatic ring can be hindered by a kinetic barrier even though this entity of a substituted benzene is often associated with a higher proton affinity than a substituent such as an OH or OR group [11][12][13][14][15][16][17][18].…”