Is 2-cyclopropylpropene really gauche?

Infrared spectra of gaseous and solid 2-cyclopropylpropene (2-CPP, c-C3H5C (CH3)CH2) have been recorded from 3500 to 40 cm-1, and Raman spectra (3200-150 cm-1) of the liquid as well as mid-infrared spectra of 2-CPP in liquid krypton solution (from -105 to -150 degrees C) were also obtained. Ab initio calculations, with basis sets up to 6-311+G(2df, 2pd), were carried out for this molecule, using the restricted Hartree-Fock (RHF) approach, with full electron correlation by the perturbation method to second order (MP2(full)) and density functional theory (DFT) by the B3LYP method. The combination of the experimental and computational results (particularly with the higher basis sets) unequivocally identifies the more stable conformer of 2-CPP as the trans form, with the gauche rotamer higher in energy, but also stable.