Protonation of fused aromatic systems: Ab initio study of some model Wheland intermediates

Abstract

Structure and properties of Wheland's intermediates are studied in some model systems. The effect of the annelated small ring is simulated by bending two vicinal CH bonds in benzene toward each other. The extent of bending describes the ring-size effect of a small fused fragment molecule. The electrophilic substituent is represented by a proton. Calculations at the HF/6-31G* and MP2//HF/6-31G* levels of sophistication confirm the Mills-Nixon (MN) hypothesis. It is conclusively shown that P-complexes are more stable than are a-protonated species. The results are interpreted in terms of the ground-state density distribution dictated by rehybridization and of the transition structure ?r-electron redistribution triggered by protonation. Their effects are additive to a large extent as far as CC bond distances are concerned. Energetic properties are determined by the interplay of two characteristic ?r-electron localization patterns, one caused by the initial angular strain and the accompanying rehybridization related to the ground state and the other one occurring during formation of the proton a-complex. This interplay destabilizes a-intermediates relative to P-intermediates.

Additivity of the Proton Affinity in Aromatics: Fluorinated Naphthalenes⊥

“…A reason behind the more pronounced susceptibility toward the proton attack of position 1 relative to position 2 is very instructive. It is well-known that a substantial bond fixation takes place in benzenonium ion exhibiting a quite characteristic pattern, which is reflected inter alia in the corresponding L ( d ) index ( vide supra ). Careful examination of the B3-LYP/6-31G* and MP2(fc)/6-31G* geometries shows that a dominant mode of π-bond localization of the benzenonium ion is preserved to some extent in the protonated forms 1 1 and 1 2 leading to slightly predominant pairing schemes shown in Figure .…”

“…Combination of these PA values with thermodynamic data measured in solution provides a valuable source of information on solvent effects thus enabling a clear-cut distinction to be made between intrinsic and external (intermolecular) interactions. Additionally, the proton is the simplest example of the electrophilic substituent group, which has proved very useful in developing a general picture of the electrophilic reactivity of aromatics, particularly in elucidating the effect of annealation of small strained rings. , We note in passing that the angular strain considerably affects basicity through rehybridization as confirmed by a meticulous theoretical analysis by Yáñez et al recently. Experimental determination of PAs encounters some difficulties since they are estimated usually by measuring gas-phase equilibrium constants for reactions implying that, strictly speaking, only the relative values for bases B 1 and B 2 could be obtained. , Choice of different anchor (gauge) base(s) leads to different PA ladders.…”

Advances in Determining the Absolute Proton Affinities of Neutral Organic Molecules in the Gas Phase and Their Interpretation: A Theoretical Account

“…This qualitative conjecture is confirmed by calculations for X + = H + , CH 3 + . Let us consider the protonation of a model system, where two vicinal CH bonds are bent toward each other, mimicking a small fused ring . Angles ε of 110° and 94° simulate five- and four-membered carbocycles, respectively, thereby representing two characteristic MN systems.…”

Proton affinity of substituted naphthalenes

“…For instance, we have conclusively shown that the PA values reproduce well the selectivity in the electrophilic reactions of benzene fused to small strained rings (Mills-Nixon effect), in good accordance with numerous experimental findings. [19][20][21] We shall show in the forthcoming discussion that the proton affinity is closely related to the orientational properties of various substituents, attached to the We commence discussion with the methyl substituent. It is useful to give PA increments of toluene for the sake of comparison; they are 6.3, 3.0 and 7.4 kcal mol À1 for ortho, meta and para protonation, respectively, 11 and represent a close similarity with the corresponding values in a-methylnaphthalene and for the C(1) and C(3) positions in b-methylnaphthalene (Table 2).…”