Rotational spectroscopy of hydrogen-bonded binary trifluoro-propanol conformers: conformational diversity, preference and abundances in a jet expansion

Abstract

The rich conformational landscape including the associated conformational conversion paths of the hydrogen-bonded binary 3,3,3-trifluoropropanol (TFP) aggregate was explored using chirped pulse Fourier transform microwave spectroscopy and computational chemistry. To appropriately identify the binary TFP conformers responsible for the five sets of candidate rotational transitions assigned, we established a set of important conformational assignment criteria. These include an extensive conformational search, good agreement between the experimental and theoretical rotational constants, relative magnitude of the three dipole moment components, and quartic centrifugal distortion constants, and observation and non-observation of the predicted conformers. Extensive conformational searches were carried out using CREST, a conformational search tool, producing hundreds of structural candidates. The CREST candidates were screened using a multitier approach and subsequently the low energy conformers (<25 kJ mol⁻¹) were optimized at the B3LYP-D3BJ/def2-TZVP level, leading to 62 minima within an energy window of 10 kJ mol⁻¹. Good agreement with the predicted spectroscopic properties mentioned above allowed us to clearly identify five binary TFP conformers as the molecular carriers. Particularly, a combined kinetic and thermodynamic model was developed, which provides a satisfactory explanation for the observation and non-observation of the low energy conformers predicted. The role of the intra- and intermolecular hydrogen bonding interactions in the stability ordering of the binary conformers is discussed.