tert-Butyl Carbocation in Condensed Phases: Stabilization via Hyperconjugation, Polarization, and Hydrogen Bonding

Despite the seeming similarity of the infrared (IR) spectra between tert-butyl cations (t-Bu) in gaseous and condensed phases, there are important but so far unrecognized differences. The IR spectroscopic investigation of the hydrogen (H)-bonding of t-Bu+ with the immediate environment together with the X-ray crystallographic data shows that one CH, group of t-Bu+ differs from the other two. In the Ar-tagged t-Bu in vacuum, this group is predominantly polarized, showing three C-H stretch vibrations at 2913, 2965, and 3036 cm(-1) whereas the other two methyls are predominantly involved in strong h-yperconjugation, yielding an intense triple IR band with a maximum at 2839 cm(-1). In a condensed phase, the bulk solvent effect promoted participation of the polarized CH3 group in additional hyperconjugation, decreasing its vCH(3) frequencies by approximately 120 cm(-1), whereas frequencies of the other CH, groups decreased by only ca. 4-10 cm(-1). This observation indicates that the influence of the condensed phase on t-Bu+ stabilization is substantial. Thus, enhancement of H-bonding between t-Bu and Anion-strengthens h-yperconjugation and promotes further cation stabilization.