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Direct Imaging of Laser-driven Ultrafast Molecular Rotation
Published on: February 4, 2017
Internal rotation in propionic acid: near-infrared-induced isomerization in solid argon
Ermelinda M S Maçôas1, Leonid Khriachtchev, Mika Pettersson
1Laboratory of Physical Chemistry, University of Helsinki, P.O. Box 55, FIN-00014 Helsinki, Finland. emacoas@qui.uc.pt
The Journal of Physical Chemistry. A
|July 15, 2006
Summary
Propionic acid exists in four stable forms, studied in solid argon. Light-induced rotations around chemical bonds convert these forms, with distinct decay rates aiding identification.
Area of Science:
- Physical Chemistry
- Spectroscopy
- Computational Chemistry
Background:
- Propionic acid (CH3CH2COOH) exhibits conformational isomerism due to rotations around its C-O and Calpha-C bonds.
- Understanding these conformers is crucial for predicting molecular behavior and reactivity.
- Previous studies suggested multiple stable conformers, but experimental characterization in solid matrices was limited.
Purpose of the Study:
- To experimentally investigate and characterize the stable conformers of propionic acid in a solid argon matrix.
- To elucidate the photoisomerization pathways between different conformers induced by vibrational excitation.
- To determine the decay kinetics of excited conformers back to the ground state.
Main Methods:
- Ab initio calculations were employed to predict the structures and vibrational spectra of propionic acid conformers.
- Infrared (IR) absorption spectroscopy was used to monitor conformational changes in solid argon at 8 K.
- Isotopically labeled propionic acid (CH3CH2COOH and CH3CH2COOD) was used to aid in spectral assignments and study site-dependent effects.
Main Results:
- Four stable conformers (Tt, Tg+/-, Ct, and Cg+/-) were identified, with Tt being the ground state.
- Selective excitation of the hydroxyl stretching overtone induced photoisomerization, converting Tt to Tg+/- and Ct conformers.
- Further excitation of Tg+/- led to Cg+/- formation, demonstrating controlled conformational switching via internal rotations.
Conclusions:
- The study experimentally confirms the existence of four distinct propionic acid conformers in solid argon.
- Photoisomerization via vibrational overtone excitation provides a method for controlled interconversion of conformers.
- IR spectroscopy, supported by ab initio calculations, is effective for identifying and characterizing these conformers based on their unique spectral signatures and decay kinetics.
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