Infrared and Raman spectral data and conformations of 1-pentyne

The infrared spectra of 1-pentyne (CH3CH2CH2-C equivalent to CH) have been studied as a vapour and as a crystalline solid at 78 K. Raman spectra of the liquid were recorded at 293 K and polarization data were obtained. Additional Raman spectra of the liquid were recorded at various temperatures between 295 and 183 K. Several Raman bands vanished in the low temperature solid at 160 K, and both the IR and Raman spectra revealed that only one conformer was present in the crystal lattice. The compound exists a priori in two conformers, anti and gauche, and from bands belonging to these conformers, treated as peak heights or integrated areas at variable temperatures, a value of AN equal to 0.75 kJ mol(-1) was found in the neat liquid, leading to 59% of the gauche conformer at room temperature. Below freezing temperatures, anti was the only conformer present in the crystal lattice. The observed vibrational spectra were interpreted, supported by previous calculations and in good agreement with earlier studies. (c) 2011 Elsevier B.V. All rights reserved.