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How and why do transition dipole moment orientations depend on conformer structure?
Christian Brand1, W Leo Meerts, Michael Schmitt
1Institut für Physikalische Chemie I, Heinrich-Heine-Universität, D-40225 Düsseldorf, Germany.
The Journal of Physical Chemistry. A
|April 20, 2011
Summary
Side chain orientation significantly impacts molecular electronic transitions in substituted benzenes. This study reveals how side chain interactions alter transition dipole moments, affecting excited-state properties and conformational analysis.
Area of Science:
- Quantum Chemistry
- Molecular Spectroscopy
- Computational Physics
Background:
- High-resolution electronic spectroscopy revealed side chain orientation influences transition dipole moments in monosubstituted benzenes.
- Non-Condon effects, where molecular vibrations affect electronic transition intensities and directions, are crucial for understanding molecular behavior.
Purpose of the Study:
- To generalize the understanding of non-Condon effects by investigating ethylamino-substituted indole (tryptamine) and benzene (2-phenylethylamine).
- To explore the influence of side chain conformation on electronic transitions and excited-state properties.
Main Methods:
- Ab initio quantum chemical calculations were employed to model molecular electronic structures.
- Rotationally resolved laser-induced fluorescence (LIF) measurements were used for experimental validation.
- Comparison of theoretical predictions with experimental spectroscopic data.
Main Results:
- In 2-phenylethylamine, the ethylamino side chain significantly alters the benzene chromophore's molecular orbital ordering, leading to state mixing and large changes in the excited-state transition dipole moment orientation.
- In tryptamine, these effects are less pronounced, with indole chromophore interactions and nitrogen atom Rydberg contributions influencing the transition dipole moment rotation.
- A strong dependence of oscillator strengths on side chain conformation was observed for 2-phenylethylamine's lowest singlet states.
Conclusions:
- The study highlights the significant role of side chain conformation in modulating electronic transitions and excited-state properties in aromatic molecules.
- Findings suggest that experimental vibronic intensities may be unreliable for assessing relative conformer stabilities in systems like phenylethylamine.
- The research provides a deeper understanding of non-Condon effects and their implications in molecular spectroscopy and computational chemistry.
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