The infrared spectra (3500-50 cm - 1 ) of the gas and solid and the Raman spectrum (3500-30 cm - 1 ) of solid 2-pentyne, CH 3 CH 2 CCCH 3 , have been recorded. Additionally, the infrared spectrum (3500-400 cm - 1 ) of a xenon solution has been recorded. A complete vibrational assignment is proposed based on infrared band contours, relative intensities, depolarization values, and group frequencies. The assignment is supported by normal coordinate calculations utilizing ab initio force constants. The internal rotational barrier for the CH 3 rotor of the ethyl group was determined to be 1285 cm - 1 from the torsional transitions whereas that for the CH 3 rotor attached to the carbon of the triple bond has nearly free rotation. Complete equilibrium geometries have been determined employing several basis sets at the levels of restricted Hartree-Fock (HF), and/or with full electron correlation by the perturbation method to second order (MP2) as well as with a hybrid density functional theory (B3LYP). The results are discussed and compared to those obtained for some similar molecules.