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. 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.