Hydrogen bonding versus hyperconjugation in condensed-phase carbocations

Abstract

Hyperconjugative stabilization of positive charge in tertiary carbocations is the textbook explanation for their stability and low frequency νCH bands in their IR spectra have long been taken as confirming evidence. While this is substantiated in the gas phase by the very close match of the IR spectrum of argontagged t-butyl cation with that calculated under C s symmetry, the situation in condensed phases is much less clear. The congruence of νCH max of t-Bu + in superacid media (2830 cm −1 ) with that in the gas phase (2834 cm −1 ) has recently been shown to be accidental. Rather, νCH max varies considerably as a function of counterion in a manner that reveals the presence of significant C-H⋯anion hydrogen bonding. This paper addresses the question of the relative importance of hyperconjugation versus H-bonding. We show by assigning IR spectra in the νCH region to specific C-H bonds in t-butyl cation that the low frequency νCH max band in the IR spectrum of t-butyl cation, long taken as direct evidence for hyperconjugation, appears to be due mostly to H-bonding. The appearance of similar low frequency νCH bands in the IR spectra of secondary alkyl carboranes such as i-Pr(CHB 11 Cl 11 ), which have predominant sp 3 centres rather than sp 2 centres (and are therefore less supportive of hyperconjugation), also suggests the dominance of H-bonding over hyperconjugation.

Vinyl Triflimides—A Case of Assisted Vinyl Cation Formation

“…After the protonation, this hyperconjugation intensifies (C 1 , C 2 )until it coalesces into the TS of the C À Nbond closure (D 1 , D 2 ). It occurs with abarrier of only 3.2 kcal mol À1 ,b ecause of as trong stabilization provided by ahydrogen bridge (D 3 ), [29] which is the remainder of the acidic OÀHbond, the protagonist of the first TS.T hus, both TSs mutually stabilize each other, which results in ar educed activation energy for the whole process.T his also explains the observed selectivities:the reaction only occurs if both TSs work in concert, which is only possible when the selectivity-inducing geometry is assumed in both TSs.…”

Myths about the Proton. The Nature of H+ in Condensed Media

“…13 Similarly, t -butyl cation (i.e. protonated iso -butene) salts are significantly stabilized by C-H⋯anion H-bonding 13 (possibly trumping hyperconjugation 14 ), again illustrating the propensity of acidic H atoms to become two-coordinate (or higher, when bifurcated CH hydrogen bonding is involved). See Fig.…”

The Strongest Brønsted Acid: Protonation of Alkanes by H(CHB11F11) at Room Temperature

“…[16] In a clear demonstration of the extremely high acid strength of this Brønsted-only acid, we find that H(CHB 11 F 11 ) reacts with alkanes at room temperature, like mixed Brønsted/Lewis acids such as "magic acid" (HFSO 3 /SbF 5 ). [18] When solid H(CHB 11 F 11 ) is stirred in suspension with nhexane, the appearance of a diagnostic low frequency nCH band at 2758 cm À1 in the IR spectrum of the product (the low frequency arising from hyperconjugation and CÀH···anion hydrogen bonding) [17,19] indicates that about 50 % of the solid acid is converted into a microcrystalline carbocation salt within 2 h (Supporting Information, Figure S14). This is the expected outcome of protonation of an alkane and elimination of H 2 [Eq.…”

“…1 Figure S3). 19 F NMR ([D 6 ]acetone, unreferenced): d = À251.59 (m, 1 F), À255.63 ppm (m, 10 F) (SI, Figure S2). The completeness of fluorination was initially judged by the absence of nBH bands near Angewandte Chemie 2550 cm À1 in the IR spectrum of the product (SI, Figure S1).…”