Modulating Electrostatic Properties and Noncovalent Interactions via Structural Isomerism: The Microwave Spectra and Molecular Structures of (<i>E</i>)- and (<i>Z</i>)-1,2,3,3,3-Pentafluoropropene and Their Gas-Phase Heterodimers with the Argon Atom

Microwave spectra of both the <i>E</i> and <i>Z</i> isomers of 1,2,3,3,3-pentafluoropropene along with all three of the singly substituted ¹³C isotopologues for each are obtained using broadband chirped-pulse Fourier transform microwave spectroscopy from 2.0-18.1 GHz. Associated quantum chemistry calculations show that the barrier to internal rotation of the CF₃ group is significantly higher for the <i>Z</i> isomer, which is stabilized by an intramolecular hydrogen bond, although the barriers in both isomers are sufficiently high to prevent the observation of any effects due to internal rotation. The normal isotopologues of the argon heterodimers for both isomers are also observed in the broadband spectrum and a Balle-Flygare cavity Fourier transform microwave spectrometer is used to obtain the 5.0-20.6 GHz spectra of the corresponding ¹³C isotopologues. In each case, the argon atom locates so as to maximize its interactions with areas of significant electron density. However, mapped electrostatic potential surfaces indicate that the areas of greatest nucleophilicity are different for the two isomers, suggesting that they may interact differently in forming heterodimers with protic acids.