Infrared Spectroscopy of thetert-Butyl Cation in the Gas Phase

Abstract

We report the first observation of the infrared spectrum of the free tert-butyl cation in the gas phase. This ion is produced in a pulsed supersonic expansion equipped with a pulsed electrical discharge. C4H9 + is detected with a time-of-flight mass spectrometer. Its infrared spectrum is recorded using infrared photodissociation spectroscopy and the method of rare gas tagging with argon. For this, the C4H9 +Ar complex is produced, mass selected, and studied with resonance-enhanced IR photodissociation spectroscopy using a tunable optical parametric oscillator laser system. Ab initio computations are employed at the MP2(fc)/6-311+G(2d,2p) level for various isomeric structures of C4H9 + and C4H9 +Ar, and for various configurations of the tert-butyl cation, to elucidate the spectra. We find that argon has a negligible effect on the IR spectra and that the patterns in the spectrum are consistent only with the tert-butyl cation in a C 1 or C s configuration.

Hydrogen bonding versus hyperconjugation in condensed-phase carbocations

“…1 Following Olah's pioneering 1963 characterization of t-butyl cation by NMR in superacid media, 2 five decades of research on its stability and energetics has prompted the question: the final answer or a never ending story? 3 The remarkably close coincidence of the experimental gas phase IR spectrum 4 of argon-tagged t-Bu + with that calculated 3,5,6 under C s symmetry (Fig. 1) suggests that we have indeed arrived at the final answer -at least for the gas phase.…”

“…Assignment of νCH in t-Bu + The interpreted IR spectrum of t-Bu + in the gas phase 4 under C s symmetry is the point of departure for assigning the spectrum in the solid phase. This is justified by the finding that the high resolution X-ray structure of [t-Bu + ][CHB 11 17 shows the cation to have near C s symmetry (Fig.…”

“…8,11,12,18 In the gas phase spectrum of t-Bu + , the νCH vibrations give rise to three observed bands at 3036, 2965 and 2834 cm −1 . 4 The two highest frequency bands can be assigned to ν asym and ν sym arising from the six minimally-hyperconjugated C-H groups. The lowest frequency band is assigned to the three maximally-hyperconjugated C-H groups.…”

“…Although the low frequency of νCH max in the IR spectrum of t-Bu + in superacid media (2830 cm −1 ), long taken as evidence of hyperconjugation, 7 nearly coincides with that in the gas phase (2834 cm −1 ), 4 this coincidence has recently been shown to be accidental. 8 In fact, the experimental value of νCH max varies considerably as a function of counterion, by up to 90 cm −1 .…”

“…Indeed, there is considerable evidence of C-H hydrogen bonding in compounds having various degrees of acidity in their C-H bonds, [10][11][12][13] including arenium ions. 8 So, while the low frequency of νCH max has been taken as prima facie 7 and continuing 4 evidence for C-H bond hyperconjugation in condensed media, C-H hydrogen bonding to the environment presents an alternative explanation.…”