Laser-Induced Fluorescence Spectroscopy of Large Secondary Alkoxy Radicals: Part II. Rotational and Fine Structure

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Secondary alkoxy radicals like 2-pentoxy and 2-hexoxy possess closely spaced electronic states (X̃ and Ã) with energy differences around 100 cm⁻¹.
• Understanding the electronic structure and spectral properties of these radicals is crucial for various chemical applications.

Purpose of the Study:

• To experimentally measure and theoretically analyze the laser-induced fluorescence (LIF) spectra of jet-cooled 2-pentoxy and 2-hexoxy.
• To determine precise molecular constants, including rotational and spin-rotation parameters, for the X̃ and B̃ electronic states.
• To elucidate the electronic configurations of the nearly degenerate X̃ and à states and determine spin-orbit coupling constants.

Main Methods:

• Acquisition of rotationally resolved LIF spectra for selected vibronic bands (origin and CO-stretch) of 2-pentoxy and 2-hexoxy.
• Analysis using an 'isolated-states model' with an effective Hamiltonian including rotational and spin-rotation terms.
• Simulation employing a genetic algorithm with parameters derived from electronic structure theory and transferable spin-rotation constants.
• Application of a 'coupled-states model' to simultaneously treat the nearly degenerate X̃ and à states.

Main Results:

• Successful simulation of experimental LIF spectra using both isolated-states and coupled-states models.
• Determination of rotational constants for X̃ and B̃ states, spin-rotation constants for the X̃ state, and transition dipole moments.
• Determination of effective spin-orbit constants and the spin-orbit-free energy separation between the à and X̃ states.
• Unambiguous assignment of observed vibronic bands to specific conformers of 2-pentoxy and 2-hexoxy.

Conclusions:

• The study successfully characterized the electronic and spectral properties of 2-pentoxy and 2-hexoxy radicals.
• The combined experimental and computational approach provided detailed molecular constants and electronic state information.
• The findings enable accurate identification of radical conformers through LIF spectroscopy.