Infrared Spectroscopy of Protonated Ethane in Helium Droplets

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Protonated saturated hydrocarbons, such as protonated ethane (C2H7+), possess complex structures and isomers.
• Studying these species is challenging due to their transient nature and propensity for fragmentation.
• Previous gas-phase studies suggested a bridged isomer for protonated ethane.

Purpose of the Study:

• To investigate the infrared spectrum of protonated ethane (C2H7+) and the C2H5+-H2 complex.
• To elucidate the structural properties and isomeric forms of protonated ethane.
• To compare experimental findings with theoretical calculations for accurate spectral assignments.

Main Methods:

• Infrared spectroscopy of protonated ethane (C2H7+) within helium droplets.
• Formation of C2H7+ via H3+ protonation of C2H6 and ionization of methane dimers.
• Anharmonic vibrational frequency calculations using second-order vibrational perturbation theory (VPT2) with B2PLYP-D3/aug-cc-pVTZ level of theory.

Main Results:

• The experimental infrared spectrum aligns with the bridged isomer of protonated ethane.
• The classical isomer with a pentacoordinated carbon atom was not identified.
• VPT2 calculations support spectral assignments, indicating broad C2H7+ bands may arise from overlapping fundamental and combination bands.

Conclusions:

• The study confirms the prevalence of the bridged isomer of protonated ethane (C2H7+) under the experimental conditions.
• The classical isomer remains elusive, suggesting potential stability differences or detection limitations.
• Refined spectral analysis reveals complex band structures previously misinterpreted as simple stretches.