Electronic absorptions of the benzylium cation

Area of Science:

• Physical Chemistry
• Spectroscopy
• Quantum Chemistry

Background:

• The benzylium cation (Bz(+)) is a key aromatic carbocation with significant implications in organic chemistry.
• Understanding its electronic transitions is crucial for interpreting its reactivity and spectral properties.

Purpose of the Study:

• To investigate the electronic transitions of the benzylium cation (Bz(+)) in the 250-550 nm range.
• To characterize the spectral features and vibrational structure of Bz(+) absorption bands.

Main Methods:

• Photodissociation of mass-selected C(7)H(7)(+)-Ar(n) complexes (n=1, 2) in a tandem mass spectrometer.
• Analysis of spectral band systems, vibrational progressions, and electronic origins using time-dependent density functional theory and Franck-Condon simulations.

Main Results:

• Two distinct band systems for Bz(+)-Ar were observed: S(1)←S(0) (370-530 nm) and S(3)←S(0) (270-320 nm).
• The S(1)←S(0) system showed resolved vibrational structure, reflecting ring deformation modes.
• The S(3)←S(0) system was broad and structureless.
• No evidence for tropylium cation absorption in the UV range was found.

Conclusions:

• The study provides detailed characterization of the electronic transitions of the benzylium cation.
• Vibronic structure analysis offers insights into the molecular dynamics and symmetry of Bz(+).
• The findings contribute to a better understanding of aromatic carbocation spectroscopy.