The Raman and infrared spectra, ab initio calculations and spectral assignments of 1,1-dicyclopropyl-2,2-dimethylethene (c-C3H5)2C[dbnd]C(CH3)2

Abstract

Raman spectra of 1,1-dicyclopropyl-2,2-dimethylethene (c-C3H5)2C = C(CH3)2) as a liquid were obtained at 293 K and polarization data recorded. Additional Raman spectra were observed at various temperatures between 293 and 143 K, and intensity changes of certain bands with temperature detected. When the sample was cooled to 153 K an amorphous solid appeared; upon slow warming it turned crystalline at 188 K and melted at higher temperatures. Various predominantly weak bands observed in the liquid and amorphous solid vanished in the Raman spectrum of the crystalline phase indicating the occurrence of two or more conformers in the liquid and amorphous phases. Van't Hoff plots of selected band pairs revealed an enthalpy difference of 2.0 ± 0.2 kJ mol−1 between the two most abundant conformers in the liquid. The vapour infrared spectrum was studied in the 4000−400 cm−1 region.Quantum chemical calculations were carried out to identify possible stable conformers and their vibrational wavenumbers, IR and Raman intensities and depolarization properties. A potential energy surface scan, obtained by rotating the geminal cyclopropyl-groups independently, disclosed occurrence of two dominant conformers of symmetries C2 and C1 with an enthalpy difference of around 4 kJ mol−1 and separated by a low barrier of ca. 6 kJ mol−1, indicating a flexible molecule where the cyclopropyl groups may rotate partially. Three additional local minima on the PES were found with relative enthalpies above 15 kJ mol−1, suggesting near absence of these conformers at ambient temperature and pressure.