Infrared Spectroscopy of the tert -Butyl Cation in the Gas Phase

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. C 4 H 9 + 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 C 4 H 9 + 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 C 4 H 9 + and C 4 H 9 + 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.