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 argon-tagged 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(CHB₁₁Cl₁₁), which have predominant sp³ centres rather than sp² centres (and are therefore less supportive of hyperconjugation), also suggests the dominance of H-bonding over hyperconjugation.