CONFORMATIONAL PROPERTIES OF 2-CYCLOPROPYLIDENEETHANOL AS STUDIED BY MICROWAVE, INFRARED AND RAMAN-SPECTROSCOPY AND BY AB-INITIO COMPUTATIONS

The microwave spectra of 2-cyclopropylideneethanol, (CH2)(2)C=C-CH2OH, and one deuterated species (hydroxyl group) have been investigated in the 23.0-39.0 GHz spectral region at 263 K. One conformer, denoted Sk ew I, was assigned. In this rotamer the C=C-C-O dihedral angle is 122 degrees from syn (0 degrees) and the C-C-O-H dihedral angle is 66 degr ees from syn. The hydrogen atom of the hydroxyl group forms a weak int ramolecular hydrogen bond with the pi electrons of the C=C double bond . The gas-phase IR spectrum in the OH stretching region revealed a bro ad and complex band at a rather high wavenumber. This absorption split into three bands, presumed to belong to two conformers with an intern al hydrogen bond and one without, when the compound was isolated in ar gon matrices at 5 K. Large shifts were observed between the IR spectra of the amorphous and crystalline solids at liquid nitrogen temperatur e due to hydrogen bonding. Certain weak IR and Raman bands present in the amorphous or liquid phases vanished in the crystal and are attribu ted to one or more additional conformers in the liquid. Some IR bands in the range 3670-3630 cm(-1), present in the argon matrix spectra at 5 K of a ca. 550 K vapour deposit, vanished after annealing and were t entatively attributed to high energy conformers. From temperature vari ations in the Raman spectra of the liquid Skew 1 was estimated to be 2 .5+/-0.6kJ mol(-1) lower in enthalpy than other conformations. The exp erimental work has been assisted by ab initio computations at the MP2/ 6-31G* level of theory.