Investigation of a polar form of fluoroethylene dimer, (C2H3F)2, by microwave spectroscopy

Abstract

The structure of fluoroethylene dimer ((C2H3F)2, (FE)2) has been determined using chirped-pulse Fourier-transform microwave (CP-FTMW) spectroscopy. Although the most stable structure was expected to be nonpolar, a planar, polar configuration was observed with sufficient intensity to measure spectra for four unique 13C substituted isotopologues in natural abundance. This allowed a least-squares fit of the structural parameters of the dimer to observed moments of inertia. The resulting structure has a T-shaped arrangement of CC bonds, similar to the recently studied FE⋯1,1-difluoroethylene complex. Both dimers contain a cyclic arrangement of CH⋯F contacts between the H–C–F end of one monomer and the H–CC–F side of the second monomer. ωB97X-D/6-31+G(d,p), MP2/6–311++G(2d,2p) and MP2/aug-cc-pVDZ calculations give the expected nonpolar end-to-end structure, ranging from 9 cm−1 (DFT) to 63 cm−1 and 57 cm−1 (MP2/6–311++G(2d,2p) and aug-cc-pVDZ, respectively) more stable than the experimentally observed orientation. The ωB97X-D/6-31+G(d,p) calculations also indicate a nonplanar, polar configuration that is over 60 cm−1 more favorable than the nonpolar structure; however, MP2 calculations predict significantly higher energy, and spectroscopic results provide no evidence of that structure’s existence. The rotational constants of the observed T-shaped (FE)2 structure are A = 6582.8323(12) MHz, B = 1256.6203(4) MHz, C = 1060.3474(3) MHz, and the dipole moment components, determined via Stark effect measurements, are μa = 0.683 (3) D, μb = 0.301 (5) D, μtot = 0.746 (5) D.