Microwave spectrum of the complex of 3,3,3-trifluoro-2-(trifluoromethyl)propanoic acid and formic acid

ABSTRACT

We report the first high resolution spectroscopic observation of the complex of 3,3,3-trifluoro-2-(trifluoromethyl)propanoic acid (TTPA) and formic acid. The rotational spectra were measured using a broadband chirped pulse and a narrow band cavity-based Fourier transform microwave spectrometer in the 8–10 GHz range. The conformational landscape of the TTPA-formic acid complex was explored at several levels of theory. The two most stable TTPA-formic acid conformers are of similar stability and feature the usual cyclic carboxylic double hydrogen bonded ring. Based on the broadband spectra obtained, only one stable heterodimer conformer was observed. We explain the absence of the second conformer to be a result of a double hydrogen tunnelling motion which converts the less stable heterodimer to the one observed in the jet expansion. Further CCSD(T) relative energy calculations confirm that the heterodimer conformer detected contains the most stable TTPA monomeric subunit. It is interesting to note that hydrogen bonding with formic acid offers a new path to effectively convert the less stable TTPA subunit to the most stable one in the jet expansion, while both TTPA monomeric conformers were detected in the same experiment.

GRAPHICAL ABSTRACT

KEYWORDS:

• Chirped pulse and cavity FTMW spectroscopy
• ab initio calculations
• structure
• conformational conversion
• 3,3,3-trifluoro-2-(trifluoromethyl)propanoic acid
• hydrogen bonding
• double hydrogen tunnelling motion

Acknowledgements

This research was funded by the University of Alberta and the Natural Sciences and Engineering Research Council (NSERC) of Canada. MC, AG, and WL gratefully acknowledge the support from the Welch Foundation # BG-0017 for financial support. MC and WL also thank the Undergraduate Research Initiative (URI) of The University of Texas Rio Grande Valley (UTRGV) for travel support to visit the University of Alberta to carry out experiments and to present the preliminary results at the 26^th Austin Symposium on Molecular Structure and Dynamics in Dallas. We acknowledge the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for providing HPC resources that have contributed to the research results reported within this paper. We gratefully acknowledge access to the computing facilities of the Shared Hierarchical Academic Research Computing Network (SHARCNET: www.sharcnet.ca), the Western Canada Research Grid (Westgrid), and Compute/Calcul Canada. YX holds a Tier I Canada Research Chair in Chirality and Chirality Recognition.

Disclosure statement

No potential conflict of interest was reported by the authors.