Calculated Vibrational Intensities in the Ã-&Xtilde; Electronic Transition of Acetylene

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Acetylene's electronic transitions are crucial for understanding its photochemistry.
• Accurate vibrational intensity calculations require accounting for geometric and coordinate changes between electronic states.
• The Dushinskií effect and geometric distortions complicate vibrational intensity predictions.

Purpose of the Study:

• To calculate relative intensities in the vibrational structure of acetylene's Ã(1)A(u)-&Xtilde;(1)Sigma(+)(g) electronic transition.
• To assess the validity of existing theoretical models for acetylene's spectroscopy.
• To investigate the impact of geometric changes and the Dushinskií effect on vibrational intensities.

Main Methods:

• Quantum chemical calculations of vibrational structure.
• Inclusion of the Dushinskií effect (change in normal coordinates).
• Approximation of vibrational integrals due to geometric changes.
• Calculations performed with and without a bending factor to account for the forbidden transition at linearity.

Main Results:

• Good agreement with experimental absorption spectra for low vibrational quanta.
• Support for the harmonic potential of the Ã-state proposed by Tobiason et al.
• Qualitative agreement with emission spectra when anharmonicity in the &Xtilde; state is considered.

Conclusions:

• The theoretical approach provides a good description of acetylene's absorption spectrum.
• Further improvements in the &Xtilde;-state potential energy surface are needed for quantitative agreement with emission spectra.
• The study validates computational methods for predicting vibrational intensities in electronic transitions with significant geometric changes.